Edward I. Solomon

All Publications

• Spectroscopic Investigation of the Role of Water in Copper Zeolite Methane Oxidation. Journal of the American Chemical Society Heyer, A. J., Ma, J., Plessers, D., Braun, A., Bols, M. L., Rhoda, H. M., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2024

  Abstract

  Methane is one of the most potent greenhouse gases; developing technology for its abatement is essential for combating climate change. Copper zeolites can activate methane at low temperatures and pressures, demonstrating promise for this technology. However, a barrier to industrial implementation is the inability to recycle the Cu(II) active site. Anaerobic active site regeneration has been reported for copper-loaded mordenite, where it is proposed that water oxidizes Cu(I) formed from the methane reaction, producing H2 gas as a byproduct. However, this result has been met with skepticism given the overall reaction is thermodynamically unfavorable. In this study, we use X-ray absorption and electron paramagnetic resonance spectroscopies to study the role of water in copper zeolite methane oxidation. We find that water does not oxidize Cu(I) to Cu(II) in CH4-reacted Cu-MOR. Further, using isotope label mass spectrometry, we detail an alternate source of the hydrogen byproduct. We uncover that, although water does not oxidize Cu(I), it has the potential to facilitate low temperature methane abatement through promotion of product decomposition to carbon dioxide and H2.

  View details for DOI 10.1021/jacs.4c06010

  View details for PubMedID 39046226

• Spectroscopic definition of ferrous active sites in non-heme iron enzymes. Methods in enzymology Solomon, E. I., Gipson, R. R. 2024; 703: 29-49

  Abstract

  Non-heme iron enzymes play key roles in antibiotic, neurotransmitter, and natural product biosynthesis, DNA repair, hypoxia regulation, and disease states. These enzymes had been refractory to traditional bioinorganic spectroscopic methods. Thus, we developed variable-temperature variable-field magnetic circular dichroism (VTVH MCD) spectroscopy to experimentally define the excited and ground ligand field states of non-heme ferrous enzymes (Solomon et al., 1995). This method provides detailed geometric and electronic structure insight and thus enables a molecular level understanding of catalytic mechanisms. Application of this method across the five classes of non-heme ferrous enzymes has defined that a general mechanistic strategy is utilized where O2 activation is controlled to occur only in the presence of all cosubstrates.

  View details for DOI 10.1016/bs.mie.2024.05.019

  View details for PubMedID 39261000

  View details for PubMedCentralID PMC11391101

• Intramolecular Phenolic H-Atom Abstraction by a N3ArOH Ligand-Supported (mu-eta2:eta2-Peroxo)dicopper(II) Species Relevant to the Active Site Function of oxy-Tyrosinase. Journal of the American Chemical Society Panda, S., Phan, H., Dunietz, E. M., Brueggemeyer, M. T., Hota, P. K., Siegler, M. A., Jose, A., Bhadra, M., Solomon, E. I., Karlin, K. D. 2024

  Abstract

  Synthetic side-on peroxide-bound dicopper(II) (SP) complexes are important for understanding the active site structure/function of many copper-containing enzymes. This work highlights the formation of new {CuII(mu-eta2:eta2-O22-)CuII} complexes (with electronic absorption and resonance Raman (rR) spectroscopic characterization) using tripodal N3ArOH ligands at -135 °C, which spontaneously participate in intramolecular phenolic H-atom abstraction (HAA). This results in the generation of bis(phenoxyl radical)bis(mu-OH)dicopper(II) intermediates, substantiated by their EPR/UV-vis/rR spectroscopic signatures and crystal structural determination of a diphenoquinone dicopper(I) complex derived from ligand para-C═C coupling. The newly observed chemistry in these ligand-Cu systems is discussed with respect to (a) our Cu-MeAN (tridentate N,N,N',N',N-pentamethyldipropylenetriamine)-derived model SP species, which was unreactive toward exogenous monophenol addition (J. Am. Chem. Soc. 2012, 134, 8513-8524), emphasizing the impact of intramolecularly tethered ArOH groups, and (b) recent advances in understanding the mechanism of action of the tyrosinase (Ty) enzyme.

  View details for DOI 10.1021/jacs.4c04402

  View details for PubMedID 38775712

• Coordination Variations within Binuclear Copper Dioxygen-Derived (Hydro)Peroxo and Superoxo Species; Influences upon Thermodynamic and Electronic Properties. Journal of the American Chemical Society Hota, P. K., Jose, A., Panda, S., Dunietz, E. M., Herzog, A. E., Wojcik, L., Le Poul, N., Belle, C., Solomon, E. I., Karlin, K. D. 2024

  Abstract

  Copper ion is a versatile and ubiquitous facilitator of redox chemical and biochemical processes. These include the binding of molecular oxygen to copper(I) complexes where it undergoes stepwise reduction-protonation. A detailed understanding of thermodynamic relationships between such reduced/protonated states is key to elucidate the fundamentals of the chemical/biochemical processes involved. The dicopper(I) complex [CuI2(BPMPO-)]1+ {BPMPOH = 2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol)} undergoes cryogenic dioxygen addition; further manipulations in 2-methyltetrahydrofuran generate dicopper(II) peroxo [CuII2(BPMPO-)(O22-)]1+, hydroperoxo [CuII2(BPMPO-)(-OOH)]2+, and superoxo [CuII2(BPMPO-)(O2•-)]2+ species, characterized by UV-vis, resonance Raman and electron paramagnetic resonance (EPR) spectroscopies, and cold spray ionization mass spectrometry. An unexpected EPR spectrum for [CuII2(BPMPO-)(O2•-)]2+ is explained by the analysis of its exchange-coupled three-spin frustrated system and DFT calculations. A redox equilibrium, [CuII2(BPMPO-)(O22-)]1+ ⇄ [CuII2(BPMPO-)(O2•-)]2+, is established utilizing Me8Fc+/Cr(η6-C6H6)2, allowing for [CuII2(BPMPO-)(O2•-)]2+/[CuII2(BPMPO-)(O22-)]1+ reduction potential calculation, E°' = -0.44 ± 0.01 V vs Fc+/0, also confirmed by cryoelectrochemical measurements (E°' = -0.40 ± 0.01 V). 2,6-Lutidinium triflate addition to [CuII2(BPMPO-)(O22-)]1+ produces [CuII2(BPMPO-)(-OOH)]2+; using a phosphazene base, an acid-base equilibrium was achieved, pKa = 22.3 ± 0.7 for [CuII2(BPMPO-)(-OOH)]2+. The BDFEOO-H = 80.3 ± 1.2 kcal/mol, as calculated for [CuII2(BPMPO-)(-OOH)]2+; this is further substantiated by H atom abstraction from O-H substrates by [CuII2(BPMPO-)(O2•-)]2+ forming [CuII2(BPMPO-)(-OOH)]2+. In comparison to known analogues, the thermodynamic and spectroscopic properties of [CuII2(BPMPO-)] O2-derived adducts can be accounted for based on chelate ring size variations built into the BPMPO- framework and the resulting enhanced CuII-ion Lewis acidity.

  View details for DOI 10.1021/jacs.3c14422

  View details for PubMedID 38688016

• In SituUV-Vis-NIR Absorption Spectroscopy and Catalysis. Chemical reviews Bols, M. L., Ma, J., Rammal, F., Plessers, D., Wu, X., Navarro-Jaen, S., Heyer, A. J., Sels, B. F., Solomon, E. I., Schoonheydt, R. A. 2024

  Abstract

  This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

  View details for DOI 10.1021/acs.chemrev.3c00602

  View details for PubMedID 38408190

• Magnetic Exchange Coupling in Zeolite Copper Dimers and Its Contribution to Methane Activation. Journal of the American Chemical Society Heyer, A. J., Plessers, D., Ma, J., Snyder, B. E., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2024

  Abstract

  The highly reactive binuclear [Cu2O]2+ active site in copper zeolites activates the inert C-H bond of methane at low temperatures, offering a potential solution to reduce methane flaring and mitigate atmospheric methane levels. While substantial progress has been made in understanding the activation of methane by this core, one critical aspect, the active site's spin, has remained undetermined. In this study, we use variable-temperature, variable-field magnetic circular dichroism spectroscopy to define the ground state spin of the [Cu2O]2+ active sites in Cu-CHA and Cu-MFI. This novel approach allows for site-selective determination of the magnetic exchange coupling between the two copper centers of specific [Cu2O]2+ cores in a heterogeneous mixture, circumventing the drawbacks of bulk magnetic techniques. These experimental findings are coupled to density functional theory calculations to elucidate magnetostructural correlations in copper zeolites that are different from those of homogeneous binuclear Cu(II) complexes. The different spin states for the [Cu2O]2+ cores have different reactivities governed by how methane approaches the active site. This introduces a new understanding of zeolite topological control on active site reactivity.

  View details for DOI 10.1021/jacs.3c13295

  View details for PubMedID 38385349

• Experimental Evidence and Mechanistic Description of the Phenolic H-Transfer to the Cu2O2 Active Site of oxy-Tyrosinase. Journal of the American Chemical Society Kipouros, I., Stańczak, A., Dunietz, E. M., Ginsbach, J. W., Srnec, M., Rulíšek, L., Solomon, E. I. 2023

  Abstract

  Tyrosinase is a ubiquitous coupled binuclear copper enzyme that activates O2 toward the regioselective monooxygenation of monophenols to catechols via a mechanism that remains only partially defined. Here, we present new mechanistic insights into the initial steps of this monooxygenation reaction by employing a pre-steady-state, stopped-flow kinetics approach that allows for the direct measurement of the monooxygenation rates for a series of para-substituted monophenols by oxy-tyrosinase. The obtained biphasic Hammett plot and the associated solvent kinetic isotope effect values provide direct evidence for an initial H-transfer from the protonated phenolic substrate to the Cu2O2 core of oxy-tyrosinase. The correlation of these experimental results to quantum mechanics/molecular mechanics calculations provides a detailed mechanistic description of this H-transfer step. These new mechanistic insights revise and expand our fundamental understanding of Cu2O2 active sites in biology.

  View details for DOI 10.1021/jacs.3c07450

  View details for PubMedID 37844210

• Primary and Secondary Coordination Sphere Effects on the Structure and Function of S-Nitrosylating Azurin. Journal of the American Chemical Society Van Stappen, C., Dai, H., Jose, A., Tian, S., Solomon, E. I., Lu, Y. 2023

  Abstract

  Much progress has been made in understanding the roles of the secondary coordination sphere (SCS) in tuning redox potentials of metalloproteins. In contrast, the impact of SCS on reactivity is much less understood. A primary example is how copper proteins can promote S-nitrosylation (SNO), which is one of the most important dynamic post-translational modifications, and is crucial in regulating nitric oxide storage and transportation. Specifically, the factors that instill CuII with S-nitrosylating capabilities and modulate activity are not well understood. To address this issue, we investigated the influence of the primary and secondary coordination sphere on CuII-catalyzed S-nitrosylation by developing a series of azurin variants with varying catalytic capabilities. We have employed a multidimensional approach involving electronic absorption, S and Cu K-edge XAS, EPR, and resonance Raman spectroscopies together with QM/MM computational analysis to examine the relationships between structure and molecular mechanism in this reaction. Our findings have revealed that kinetic competency is correlated with three balancing factors, namely Cu-S bond strength, Cu spin localization, and relative S(ps) vs S(pp) contributions to the ground state. Together, these results support a reaction pathway that proceeds through the attack of the Cu-S bond rather than electrophilic addition to CuII or radical attack of SCys. The insights gained from this work provide not only a deeper understanding of SNO in biology but also a basis for designing artificial and tunable SNO enzymes to regulate NO and prevent diseases due to SNO dysregulation.

  View details for DOI 10.1021/jacs.3c07399

  View details for PubMedID 37696009

• Stabilizing Au2+ in a mixed-valence 3D halide perovskite NATURE CHEMISTRY Lindquist, K. P., Eghdami, A., Deschene, C. R., Heyer, A. J., Wen, J., Smith, A. G., Solomon, E. I., Lee, Y. S., Neaton, J. B., Ryan, D. H., Karunadasa, H. I. 2023

  View details for DOI 10.1038/s41557-023-01305

  View details for Web of Science ID 001064800000001

• Stabilizing Au2+ in a mixed-valence 3D halide perovskite. Nature chemistry Lindquist, K. P., Eghdami, A., Deschene, C. R., Heyer, A. J., Wen, J., Smith, A. G., Solomon, E. I., Lee, Y. S., Neaton, J. B., Ryan, D. H., Karunadasa, H. I. 2023

  Abstract

  Although Cu2+ is ubiquitous, the relativistic destabilization of the 5d orbitals makes the isoelectronic Au2+ exceedingly rare, typically stabilized only through Au-Au bonding or by using redox non-innocent ligands. Here we report the perovskite Cs4AuIIAuIII2Cl12, an extended solid with mononuclear Au2+ sites, which is stable to ambient conditions and characterized by single-crystal X-ray diffraction. The 2+ oxidation state of Au was assigned using 197Au Mössbauer spectroscopy, electron paramagnetic resonance, and magnetic susceptibility measurements, with comparison to paramagnetic and diamagnetic analogues with Cu2+ and Pd2+, respectively, as well as to density functional theory calculations. This gold perovskite offers an opportunity to study the optical and electronic transport of the uncommon Au2+/3+ mixed-valence state and the characteristics of the elusive Au2+ ion coordinated to simple ligands. Compared with the perovskite Cs2AuIAuIIICl6, which has been studied since the 1920s, Cs4AuIIAuIII2Cl12 exhibits a 0.7 eV reduction in optical absorption onset and a 103-fold increase in electronic conductivity.

  View details for DOI 10.1038/s41557-023-01305-y

  View details for PubMedID 37640854

  View details for PubMedCentralID 8246118

• X-ray Spectroscopic Study of the Electronic Structure of a Trigonal High-Spin Fe(IV)═O Complex Modeling Non-Heme Enzyme Intermediates and Their Reactivity. Journal of the American Chemical Society Braun, A., Gee, L. B., Mara, M. W., Hill, E. A., Kroll, T., Nordlund, D., Sokaras, D., Glatzel, P., Hedman, B., Hodgson, K. O., Borovik, A. S., Baker, M. L., Solomon, E. I. 2023

  Abstract

  Fe K-edge X-ray absorption spectroscopy (XAS) has long been used for the study of high-valent iron intermediates in biological and artificial catalysts. 4p-mixing into the 3d orbitals complicates the pre-edge analysis but when correctly understood via 1s2p resonant inelastic X-ray scattering and Fe L-edge XAS, it enables deeper insight into the geometric structure and correlates with the electronic structure and reactivity. This study shows that in addition to the 4p-mixing into the 3dz2 orbital due to the short iron-oxo bond, the loss of inversion in the equatorial plane leads to 4p mixing into the 3dx2-y2,xy, providing structural insight and allowing the distinction of 6- vs 5-coordinate active sites as shown through application to the Fe(IV)═O intermediate of taurine dioxygenase. Combined with O K-edge XAS, this study gives an unprecedented experimental insight into the electronic structure of Fe(IV)═O active sites and their selectivity for reactivity enabled by the π-pathway involving the 3dxz/yz orbitals. Finally, the large effect of spin polarization is experimentally assigned in the pre-edge (i.e., the α/β splitting) and found to be better modeled by multiplet simulations rather than by commonly used time-dependent density functional theory.

  View details for DOI 10.1021/jacs.3c06181

  View details for PubMedID 37590931

• Kβ X-ray Emission Spectroscopy of Cu(I)-Lytic Polysaccharide Monooxygenase: Direct Observation of the Frontier Molecular Orbital for H2O2 Activation. Journal of the American Chemical Society Lim, H., Brueggemeyer, M. T., Transue, W. J., Meier, K. K., Jones, S. M., Kroll, T., Sokaras, D., Kelemen, B., Hedman, B., Hodgson, K. O., Solomon, E. I. 2023

  Abstract

  Lytic polysaccharide monooxygenases (LPMOs) catalyze the degradation of recalcitrant carbohydrate polysaccharide substrates. These enzymes are characterized by a mononuclear Cu(I) active site with a three-coordinate T-shaped "His-brace" configuration including the N-terminal histidine and its amine group as ligands. This study explicitly investigates the electronic structure of the d10 Cu(I) active site in a LPMO using Kβ X-ray emission spectroscopy (XES). The lack of inversion symmetry in the His-brace site enables the 3d/p mixing required for intensity in the Kβ valence-to-core (VtC) XES spectrum of Cu(I)-LPMO. These Kβ XES data are correlated to density functional theory (DFT) calculations to define the bonding, and in particular, the frontier molecular orbital (FMO) of the Cu(I) site. These experimentally validated DFT calculations are used to evaluate the reaction coordinate for homolytic cleavage of the H2O2 O-O bond and understand the contribution of this FMO to the low barrier of this reaction and how the geometric and electronic structure of the Cu(I)-LPMO site is activated for rapid reactivity with H2O2.

  View details for DOI 10.1021/jacs.3c04048

  View details for PubMedID 37441786

• Nuclear Resonance Vibrational Spectroscopy Definition of Peroxy Intermediates in Catechol Dioxygenases: Factors that Determine Extra- versus Intradiol Cleavage. Journal of the American Chemical Society Babicz, J. T., Rogers, M. S., DeWeese, D. E., Sutherlin, K. D., Banerjee, R., Bottger, L. H., Yoda, Y., Nagasawa, N., Saito, M., Kitao, S., Kurokuzu, M., Kobayashi, Y., Tamasaku, K., Seto, M., Lipscomb, J. D., Solomon, E. I. 2023

  Abstract

  The extradiol dioxygenases (EDOs) and intradiol dioxygenases (IDOs) are nonheme iron enzymes that catalyze the oxidative aromatic ring cleavage of catechol substrates, playing an essential role in the carbon cycle. The EDOs and IDOs utilize very different FeII and FeIII active sites to catalyze the regiospecificity in their catechol ring cleavage products. The factors governing this difference in cleavage have remained undefined. The EDO homoprotocatechuate 2,3-dioxygenase (HPCD) and IDO protocatechuate 3,4-dioxygenase (PCD) provide an opportunity to understand this selectivity, as key O2 intermediates have been trapped for both enzymes. Nuclear resonance vibrational spectroscopy (in conjunction with density functional theory calculations) is used to define the geometric and electronic structures of these intermediates as FeII-alkylhydroperoxo (HPCD) and FeIII-alkylperoxo (PCD) species. Critically, in both intermediates, the initial peroxo bond orientation is directed toward extradiol product formation. Reaction coordinate calculations were thus performed to evaluate both the extra- and intradiol O-O cleavage for the simple organic alkylhydroperoxo and for the FeII and FeIII metal catalyzed reactions. These results show the FeII-alkylhydroperoxo (EDO) intermediate undergoes facile extradiol O-O bond homolysis due to its extra e-, while for the FeIII-alkylperoxo (IDO) intermediate the extradiol cleavage involves a large barrier and would yield the incorrect extradiol product. This prompted our evaluation of a viable mechanism to rearrange the FeIII-alkylperoxo IDO intermediate for intradiol cleavage, revealing a key role in the rebinding of the displaced Tyr447 ligand in this rearrangement, driven by the proton delivery necessary for O-O bond cleavage.

  View details for DOI 10.1021/jacs.3c02242

  View details for PubMedID 37414058

• Tuning the Type 1 Reduction Potential of Multicopper Oxidases: Uncoupling the Effects of Electrostatics and H-Bonding to Histidine Ligands. Journal of the American Chemical Society Singha, A., Sekretareva, A., Tao, L., Lim, H., Ha, Y., Braun, A., Jones, S. M., Hedman, B., Hodgson, K. O., Britt, R. D., Kosman, D. J., Solomon, E. I. 2023

  Abstract

  In multicopper oxidases (MCOs), the type 1 (T1) Cu accepts electrons from the substrate and transfers these to the trinuclear Cu cluster (TNC) where O2 is reduced to H2O. The T1 potential in MCOs varies from 340 to 780 mV, a range not explained by the existing literature. This study focused on the ∼350 mV difference in potential of the T1 center in Fet3p and Trametes versicolor laccase (TvL) that have the same 2His1Cys ligand set. A range of spectroscopies performed on the oxidized and reduced T1 sites in these MCOs shows that they have equivalent geometric and electronic structures. However, the two His ligands of the T1 Cu in Fet3p are H-bonded to carboxylate residues, while in TvL they are H-bonded to noncharged groups. Electron spin echo envelope modulation spectroscopy shows that there are significant differences in the second-sphere H-bonding interactions in the two T1 centers. Redox titrations on type 2-depleted derivatives of Fet3p and its D409A and E185A variants reveal that the two carboxylates (D409 and E185) lower the T1 potential by 110 and 255-285 mV, respectively. Density functional theory calculations uncouple the effects of the charge of the carboxylates and their difference in H-bonding interactions with the His ligands on the T1 potential, indicating 90-150 mV for anionic charge and ∼100 mV for a strong H-bond. Finally, this study provides an explanation for the generally low potentials of metallooxidases relative to the wide range of potentials of the organic oxidases in terms of different oxidized states of their TNCs involved in catalytic turnover.

  View details for DOI 10.1021/jacs.3c03241

  View details for PubMedID 37294874

• Fenton-like Chemistry by a Copper(I) Complex and H2O2 Relevant to Enzyme Peroxygenase C-H Hydroxylation. Journal of the American Chemical Society Kim, B., Brueggemeyer, M. T., Transue, W. J., Park, Y., Cho, J., Siegler, M. A., Solomon, E. I., Karlin, K. D. 2023

  Abstract

  Lytic polysaccharide monooxygenases have received significant attention as catalytic convertors of biomass to biofuel. Recent studies suggest that its peroxygenase activity (i.e., using H2O2 as an oxidant) is more important than its monooxygenase functionality. Here, we describe new insights into peroxygenase activity, with a copper(I) complex reacting with H2O2 leading to site-specific ligand-substrate C-H hydroxylation. [CuI(TMG3tren)]+ (1) (TMG3tren = 1,1,1-Tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine) and a dry source of hydrogen peroxide, (o-Tol3P═O·H2O2)2 react in the stoichiometry, [CuI(TMG3tren)]+ + H2O2 → [CuI(TMG3tren-OH)]+ + H2O, wherein a ligand N-methyl group undergoes hydroxylation giving TMG3tren-OH. Furthermore, Fenton-type chemistry (CuI + H2O2 → CuII-OH + ·OH) is displayed, in which (i) a Cu(II)-OH complex could be detected during the reaction and it could be separately isolated and characterized crystallographically and (ii) hydroxyl radical (·OH) scavengers either quenched the ligand hydroxylation reaction and/or (iii) captured the ·OH produced.

  View details for DOI 10.1021/jacs.3c02273

  View details for PubMedID 37195014

• Tuning Copper Active Site Composition in Cu-MOR through Co-Cation Modification for Methane Activation. ACS catalysis Plessers, D., Heyer, A. J., Rhoda, H. M., Bols, M. L., Solomon, E. I., Schoonheydt, R. A., Sels, B. F. 2023; 13 (3): 1906-1915

  Abstract

  The industrial implementation of a direct methane to methanol process would lead to environmental and economic benefits. Copper zeolites successfully execute this reaction at relatively low temperatures, and mordenite zeolites in particular enable high methanol production. When loaded to a Cu/Al ratio of 0.45, mordenite (Si/Al 5 to 9) has been shown to host three active sites: two [CuOCu]2+ sites labeled MOR1 and MOR2, and a mononuclear [CuOH]+ site. Also at low copper loadings (Cu/Al < 0.20), mordenite has been demonstrated to activate methane, but its active site has never been reported. Here, we investigate Na+ mordenite with varying copper loadings to better understand copper speciation in mordenite. At low copper loadings, we uncover an unidentified active site ('MOR3') with a strong overlap with the [CuOH]+ site's spectroscopic signal. By changing the co-cation, we selectively speciate more MOR3 relative to [CuOH]+, allowing its identification as a [CuOCu]2+ site. Active site identification in heterogeneous catalysts is a frequent problem due to signal overlap. By changing cation composition, we introduce an innovative method for simplifying a material to allow better analysis. This has implications for the study of Cu zeolites for methane to methanol and NOx catalysis, but also for studying and tuning heterogeneous catalysts in general.

  View details for DOI 10.1021/acscatal.2c05271

  View details for PubMedID 37377676

  View details for PubMedCentralID PMC10299574

• Tuning Copper Active Site Composition in Cu-MOR through Co-Cation Modification for Methane Activation ACS CATALYSIS Plessers, D., Heyer, A. J., Rhoda, H. M., Bols, M. L., Solomon, E. I., Schoonheydt, R. A., Sels, B. F. 2023: 1906-1915

  View details for DOI 10.1021/acscatal.2c05271

  View details for Web of Science ID 000922845700001

• Boronated Cyanometallates. Inorganic chemistry McNicholas, B. J., Nie, C., Jose, A., Oyala, P. H., Takase, M. K., Henling, L. M., Barth, A. T., Amaolo, A., Hadt, R. G., Solomon, E. I., Winkler, J. R., Gray, H. B., Despagnet-Ayoub, E. 2022

  Abstract

  Thirteen boronated cyanometallates [M(CN-BR3)6]3/4/5- [M = Cr, Mn, Fe, Ru, Os; BR3 = BPh3, B(2,4,6,-F3C6H2)3, B(C6F5)3] and one metalloboratonitrile [Cr(NC-BPh3)6]3- have been characterized by X-ray crystallography and spectroscopy [UV-vis-near-IR, NMR, IR, spectroelectrochemistry, and magnetic circular dichroism (MCD)]; CASSCF+NEVPT2 methods were employed in calculations of electronic structures. For (t2g)5 electronic configurations, the lowest-energy ligand-to-metal charge-transfer (LMCT) absorptions and MCD C-terms in the spectra of boronated species have been assigned to transitions from cyanide pi + B-C borane sigma orbitals. CASSCF+NEVPT2 calculations including t1u and t2u orbitals reproduced t1u/t2u t2g excitation energies. Many [M(CN-BR3)6]3/4- complexes exhibited highly electrochemically reversible redox couples. Notably, the reduction formal potentials of all five [M(CN-B(C6F5)3)6]3- anions scale with the LMCT energies, and Mn(I) and Cr(II) compounds, [K(18-crown-6)]5[Mn(CN-B(C6F5)3)6] and [K(18-crown-6)]4[Cr(CN-B(C6F5)3)6], are surprisingly stable. Continuous-wave and pulsed electron paramagnetic resonance (EPR; hyperfine sublevel correlation) spectra were collected for all Cr(III) complexes; as expected, 14N hyperfine splittings are greater for (Ph4As)3[Cr(NC-BPh3)6] than for (Ph4As)3[Cr(CN-BPh3)6].

  View details for DOI 10.1021/acs.inorgchem.2c03066

  View details for PubMedID 36534001

• Particle Swarm Fitting of Spin Hamiltonians: Magnetic Circular Dichroism of Reduced and NO-Bound Flavodiiron Protein. Inorganic chemistry Transue, W. J., Snyder, R. A., Caranto, J. D., Kurtz, D. M., Solomon, E. I. 2022

  Abstract

  A particle swarm optimization (PSO) algorithm is described for the fitting of ground-state spin Hamiltonian parameters from variable-temperature/variable-field (VTVH) magnetic circular dichroism (MCD) data. This PSO algorithm is employed to define the ground state of two catalytic intermediates from a flavodiiron protein (FDP), a class of enzymes with nitric oxide reductase activity. The bimetallic iron active site of this enzyme proceeds through a biferrous intermediate and a mixed ferrous-{FeNO}7 intermediate during the catalytic cycle, and the MCD spectra of these intermediates are presented and analyzed. The fits of the spin Hamiltonians are shown to provide important geometric and electronic insight into these species that is compared and contrasted with previous reports.

  View details for DOI 10.1021/acs.inorgchem.2c02234

  View details for PubMedID 36223761

• Methane Activation by a Mononuclear Copper Active Site in the Zeolite Mordenite: Effect of Metal Nuclearity on Reactivity. Journal of the American Chemical Society Heyer, A. J., Plessers, D., Braun, A., Rhoda, H. M., Bols, M. L., Hedman, B., Hodgson, K. O., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2022

  Abstract

  The direct conversion of methane to methanol would have a wide reaching environmental and industrial impact. Copper-containing zeolites can perform this reaction at low temperatures and pressures at a previously defined O2-activated [Cu2O]2+ site. However, after autoreduction of the copper-containing zeolite mordenite and removal of the [Cu2O]2+ active site, the zeolite is still methane reactive. In this study, we use diffuse reflectance UV-vis spectroscopy, magnetic circular dichroism, resonance Raman spectroscopy, electron paramagnetic resonance, and X-ray absorption spectroscopy to unambiguously define a mononuclear [CuOH]+ as the CH4 reactive active site of the autoreduced zeolite. The rigorous identification of a mononuclear active site allows a reactivity comparison to the previously defined [Cu2O]2+ active site. We perform kinetic experiments to compare the reactivity of the [CuOH]+ and [Cu2O]2+ sites and find that the binuclear site is significantly more reactive. From the analysis of density functional theory calculations, we elucidate that this increased reactivity is a direct result of stabilization of the [Cu2OH]2+ H-atom abstraction product by electron delocalization over the two Cu cations via the bridging ligand. This significant increase in reactivity from electron delocalization over a binuclear active site provides new insights for the design of highly reactive oxidative catalysts.

  View details for DOI 10.1021/jacs.2c06269

  View details for PubMedID 36219763

• New mechanistic insights into coupled binuclear copper monooxygenases from the recent elucidation of the ternary intermediate of tyrosinase. FEBS letters Kipouros, I., Solomon, E. I. 2022

  Abstract

  Tyrosinase is the most predominant member of the coupled binuclear copper (CBC) protein family. The recent trapping and spectroscopic definition of the elusive catalytic ternary intermediate (enzyme/O2 /monophenol) of tyrosinase dictates a monooxygenation mechanism that revises previous proposals and involves cleavage of the μ-η2 :η2 -peroxide dicopper(II) O-O bond to accept the phenolic proton, followed by monophenolate coordination to copper concomitant with aromatic hydroxylation by the non-protonated μ-oxo. Here, we compare and contrast previously proposed and current mechanistic models for monophenol monooxygenation of tyrosinase. Next, we discuss how these recent insights provide new opportunities towards uncovering structure-function relationships in CBC enzymes, as well as understanding fundamental principles for O2 activation and reactivity by bioinorganic active sites.

  View details for DOI 10.1002/1873-3468.14503

  View details for PubMedID 36178078

• Elucidation of the tyrosinase/O2/monophenol ternary intermediate that dictates the monooxygenation mechanism in melanin biosynthesis. Proceedings of the National Academy of Sciences of the United States of America Kipouros, I., Stańczak, A., Ginsbach, J. W., Andrikopoulos, P. C., Rulíšek, L., Solomon, E. I. 2022; 119 (33): e2205619119

  Abstract

  Melanins are highly conjugated biopolymer pigments that provide photoprotection in a wide array of organisms, from bacteria to humans. The rate-limiting step in melanin biosynthesis, which is the ortho-hydroxylation of the amino acid L-tyrosine to L-DOPA, is catalyzed by the ubiquitous enzyme tyrosinase (Ty). Ty contains a coupled binuclear copper active site that binds O2 to form a μ:η2:η2-peroxide dicopper(II) intermediate (oxy-Ty), capable of performing the regioselective monooxygenation of para-substituted monophenols to catechols. The mechanism of this critical monooxygenation reaction remains poorly understood despite extensive efforts. In this study, we have employed a combination of spectroscopic, kinetic, and computational methods to trap and characterize the elusive catalytic ternary intermediate (Ty/O2/monophenol) under single-turnover conditions and obtain molecular-level mechanistic insights into its monooxygenation reactivity. Our experimental results, coupled with quantum-mechanics/molecular-mechanics calculations, reveal that the monophenol substrate docks in the active-site pocket of oxy-Ty fully protonated, without coordination to a copper or cleavage of the μ:η2:η2-peroxide O-O bond. Formation of this ternary intermediate involves the displacement of active-site water molecules by the substrate and replacement of their H bonds to the μ:η2:η2-peroxide by a single H bond from the substrate hydroxyl group. This H-bonding interaction in the ternary intermediate enables the unprecedented monooxygenation mechanism, where the μ-η2:η2-peroxide O-O bond is cleaved to accept the phenolic proton, followed by substrate phenolate coordination to a copper site concomitant with its aromatic ortho-hydroxylation by the nonprotonated μ-oxo. This study provides insights into O2 activation and reactivity by coupled binuclear copper active sites with fundamental implications in biocatalysis.

  View details for DOI 10.1073/pnas.2205619119

  View details for PubMedID 35939688

• Millisecond timescale reactions observed via X-ray spectroscopy in a 3D microfabricated fused silica mixer. Corrigendum. Journal of synchrotron radiation Huyke, D. A., Ramachandran, A., Ramirez-Neri, O., Guerrero-Cruz, J. A., Gee, L. B., Braun, A., Sokaras, D., Garcia-Estrada, B., Solomon, E. I., Hedman, B., Delgado-Jaime, M. U., DePonte, D. P., Kroll, T., Santiago, J. G. 2022; 29 (Pt 3): 930

  Abstract

  A figure in the article by Huyke et al. [(2021), J. Synchrotron Rad. 28, 1100-1113] is corrected.

  View details for DOI 10.1107/S1600577522002806

  View details for PubMedID 35511027

• Evidence for H-bonding interactions to the mu-eta2:eta2-peroxide of oxy-tyrosinase that activate its coupled binuclear copper site. Chemical communications (Cambridge, England) Kipouros, I., Stanczak, A., Culka, M., Andris, E., Machonkin, T. R., Rulisek, L., Solomon, E. I. 2022

  Abstract

  The factors that control the diverse reactivity of the mu-eta2:eta2-peroxo dicopper(II) oxy-intermediates in the coupled binuclear copper proteins remain elusive. Here, spectroscopic and computational methods reveal H-bonding interactions between active-site waters and the mu-eta2:eta2-peroxide of oxy-tyrosinase, and define their effects on the Cu(II)2O2 electronic structure and O2 activation.

  View details for DOI 10.1039/d2cc00750a

  View details for PubMedID 35237779

• Spiers Memorial Lecture: activating metal sites for biological electron transfer. Faraday discussions Solomon, E. I., Jose, A. 2022

  Abstract

  Metal sites in biology often exhibit unique spectroscopic features that reflect novel geometric and electronic structures imposed by the protein that are key to reactivity. The blue copper active site involved in long range, rapid biological electron transfer is a classic example. This review presents an overview of both traditional and synchrotron based spectroscopic methods and their coupling to electronic structure calculations to understand the unique features of the blue copper active site, their contributions to function and the role of the protein in determining the geometric and electronic structure of the active site (called the "entatic state"). The relation of this active site to other biological electron transfer sites is further developed. In particular, ultrafast XFEL spectroscopy is used to evaluate the methionine-S-Fe bond in cytochrome c, and its entatic control by the protein in determining function (electron transfer vs. apoptosis).

  View details for DOI 10.1039/d2fd00001f

  View details for PubMedID 35133385

• Second-Sphere Lattice Effects in Copper and Iron Zeolite Catalysis. Chemical reviews Rhoda, H. M., Heyer, A. J., Snyder, B. E., Plessers, D., Bols, M. L., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 1800

  Abstract

  Transition-metal-exchanged zeolites perform remarkable chemical reactions from low-temperature methane to methanol oxidation to selective reduction of NOx pollutants. As with metalloenzymes, metallozeolites have impressive reactivities that are controlled in part by interactions outside the immediate coordination sphere. These second-sphere effects include activating a metal site through enforcing an "entatic" state, controlling binding and access to the metal site with pockets and channels, and directing radical rebound vs cage escape. This review explores these effects with emphasis placed on but not limited to the selective oxidation of methane to methanol with a focus on copper and iron active sites, although other transition-metal-ion zeolite reactions are also explored. While the actual active-site geometric and electronic structures are different in the copper and iron metallozeolites compared to the metalloenzymes, their second-sphere interactions with the lattice or the protein environments are found to have strong parallels that contribute to their high activity and selectivity.

  View details for DOI 10.1021/acs.chemrev.1c00915

  View details for PubMedID 35077641

• S K-edge XAS of CuII, CuI, and ZnII oxidized Dithiolene complexes: Covalent contributions to structure and the Jahn-Teller effect. Journal of inorganic biochemistry Ha, Y., Dille, S. A., Braun, A., Colston, K., Hedman, B., Hodgson, K. O., Basu, P., Solomon, E. I. 2022; 230: 111752

  Abstract

  Reduced dithiolene ligands are bound to high valent Mo centers in the active site of the oxotransferase family of enzymes. Related model complexes have been studied with great insight by Prof. Holm and his colleagues. This study focuses on the other limit of dithiolene chemistry: an investigation of the 2-electron oxidized dithiolene bound to low-valent late transition metal (TM) ions (ZnII, CuI, and CuII). The bonding descriptions of the oxidized dithiolene [N,N-dimethyl piperazine 2,3-dithione (Me2Dt0)] complexes are probed using S K-edge X-ray absorption spectroscopy (XAS) and the results are correlated to density functional theory (DFT) calculations. These experimentally supported calculations are then extended to explain the different geometric structures of the three complexes. The ZnII(Me2Dt0)2 complex has only ligand-ligand repulsion so it is stabilized at the D2d symmetry limit. The CuI(Me2Dt0)2 complex has additional weak backbonding thus distorts somewhat from D2d toward D2h symmetry. The CuII(Me2Dt0)2 complex has a strong σ donor bond that leads to both a large Jahn-Teller stabilization to D2h and an additional covalent contribution to the geometry. The combined strong stabilization results in the square planar, D2h structure. This study quantifies the competition between the ligand-ligand repulsion and the change in electronic structures in determining the final geometric structures of the oxidized dithiolene complexes, and provides quantitative insights into the Jahn-Teller stabilization energy and its origin.

  View details for DOI 10.1016/j.jinorgbio.2022.111752

  View details for PubMedID 35202982

• Electron Transfer to the Trinuclear Copper Cluster in Electrocatalysis by the Multicopper Oxidases. Journal of the American Chemical Society Sekretareva, A., Tian, S., Gounel, S., Mano, N., Solomon, E. I. 2021

  Abstract

  High-potential multicopper oxidases (MCOs) are excellent catalysts able to perform the oxygen reduction reaction (ORR) at remarkably low overpotentials. Moreover, MCOs are able to interact directly with the electrode surfaces via direct electron transfer (DET), that makes them the most commonly used electrocatalysts for oxygen reduction in biofuel cells. The central question in MCO electrocatalysis is whether the type 1 (T1) Cu is the primary electron acceptor site from the electrode, or whether electrons can be transferred directly to the trinuclear copper cluster (TNC), bypassing the rate-limiting intramolecular electron transfer step from the T1 site. Here, using site-directed mutagenesis and electrochemical methods combined with data modeling of electrode kinetics, we have found that there is no preferential superexchange pathway for DET to the T1 site. However, due to the high reorganization energy of the fully oxidized TNC, electron transfer from the electrode to the TNC does occur primarily through the T1 site. We have further demonstrated that the lower reorganization energy of the TNC in its two-electron reduced, alternative resting, form enables DET to the TNC, but this only occurs in the first turnover. This study provides insight into the factors that control the kinetics of electrocatalysis by the MCOs and a guide for the design of more efficient biocathodes for the ORR.

  View details for DOI 10.1021/jacs.1c08456

  View details for PubMedID 34633193

• Thermally stable manganese(III) peroxido complexes with hindered N3 tripodal ligands: Structures and their physicochemical properties. Journal of inorganic biochemistry Fujisawa, K., Sakuma, S., Ikarugi, R., Jose, A., Solomon, E. I. 2021; 225: 111597

  Abstract

  Mononuclear manganese(III) peroxido complexes are candidates for the reaction intermediates in manganese containing proteins, such as manganese superoxide dismutase (Mn-SOD) etc. In this study, manganese(III) peroxido complexes [Mn(O2)(L3)] and [Mn(O2)(L10)] ligated by anionic N3 type ligands with sterically hindered substituents, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (L3-) and hydrotris(3-adamantyl-5-isopropyl-1-pyrazolyl)borate (L10-), respectively, were structurally characterized. These complexes are the first examples of structurally characterized five-coordinate manganese(III) peroxido complexes. Their characteristic nu(OO) and nu(MnO) stretchings were determined by using H218O2 for the first time. Theoretical calculations were performed to obtain further insight into their structural parameters. The decomposed products were obtained as [{MnIII(mu-O)(L3)}2MnIV] and [MnIII(OH){L10(O)}] from [Mn(O2)(L3)] and [Mn(O2)(L10)], respectively.

  View details for DOI 10.1016/j.jinorgbio.2021.111597

  View details for PubMedID 34547605

• Cage effects control the mechanism of methane hydroxylation in zeolites SCIENCE Snyder, B. R., Bols, M. L., Rhoda, H. M., Plessers, D., Schoonheydt, R. A., Sels, B. F., Solomon, E. 2021; 373 (6552): 327-+

  View details for DOI 10.1126/science.abd5803

  View details for Web of Science ID 000679214500034

• Cage effects control the mechanism of methane hydroxylation in zeolites. Science (New York, N.Y.) Snyder, B. E., Bols, M. L., Rhoda, H. M., Plessers, D., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2021; 373 (6552): 327-331

  Abstract

  Catalytic conversion of methane to methanol remains an economically tantalizing but fundamentally challenging goal. Current technologies based on zeolites deactivate too rapidly for practical application. We found that similar active sites hosted in different zeolite lattices can exhibit markedly different reactivity with methane, depending on the size of the zeolite pore apertures. Whereas zeolite with large pore apertures deactivates completely after a single turnover, 40% of active sites in zeolite with small pore apertures are regenerated, enabling a catalytic cycle. Detailed spectroscopic characterization of reaction intermediates and density functional theory calculations show that hindered diffusion through small pore apertures disfavors premature release of CH3 radicals from the active site after C-H activation, thereby promoting radical recombination to form methanol rather than deactivated Fe-OCH3 centers elsewhere in the lattice.

  View details for DOI 10.1126/science.abd5803

  View details for PubMedID 34437151

• Mechanisms of O2 Activation by Mononuclear Non-Heme Iron Enzymes. Biochemistry Solomon, E. I., DeWeese, D. E., Babicz, J. T. 2021

  Abstract

  Two major subclasses of mononuclear non-heme ferrous enzymes use two electron-donating organic cofactors (alpha-ketoglutarate or pterin) to activate O2 to form FeIV═O intermediates that further react with their substrates through hydrogen atom abstraction or electrophilic aromatic substitution. New spectroscopic methodologies have been developed, enabling the study of the active sites in these enzymes and their oxygen intermediates. Coupled to electronic structure calculations, the results of these spectroscopies provide fundamental insight into mechanism. This Perspective summarizes the results of these studies in elucidating the mechanism of dioxygen activation to form the FeIV═O intermediate and the geometric and electronic structure of this intermediate that enables its high reactivity and selectivity in product formation.

  View details for DOI 10.1021/acs.biochem.1c00370

  View details for PubMedID 34266238

• Millisecond timescale reactions observed via X-ray spectroscopy in a 3D microfabricated fused silica mixer. Journal of synchrotron radiation Huyke, D. A., Ramachandran, A., Ramirez-Neri, O., Guerrero-Cruz, J. A., Gee, L. B., Braun, A., Sokaras, D., Garcia-Estrada, B., Solomon, E. I., Hedman, B., Delgado-Jaime, M. U., DePonte, D. P., Kroll, T., Santiago, J. G. 2021; 28 (Pt 4): 1100-1113

  Abstract

  Determination of electronic structures during chemical reactions remains challenging in studies which involve reactions in the millisecond timescale, toxic chemicals, and/or anaerobic conditions. In this study, a three-dimensionally (3D) microfabricated microfluidic mixer platform that is compatible with time-resolved X-ray absorption and emission spectroscopy (XAS and XES, respectively) is presented. This platform, to initiate reactions and study their progression, mixes a high flow rate (0.50-1.5 ml min-1) sheath stream with a low-flow-rate (5-90 l min-1) sample stream within a monolithic fused silica chip. The chip geometry enables hydrodynamic focusing of the sample stream in3D and sample widths as small as 5 m. The chip is also connected to a polyimide capillary downstream to enable sample stream deceleration, expansion, and X-ray detection. In this capillary, sample widths of 50 m are demonstrated. Further, convection-diffusion-reaction models of the mixer are presented. The models are experimentally validated using confocal epifluorescence microscopy and XAS/XES measurements of a ferricyanide and ascorbic acid reaction. The models additionally enable prediction of the residence time and residence time uncertainty of reactive species as well as mixing times. Residence times (from initiation of mixing to the point of X-ray detection) during sample stream expansion as small as 2.1 ± 0.3 ms are also demonstrated. Importantly, an exploration of the mixer operational space reveals a theoretical minimum mixing time of 0.91 ms. The proposed platform is applicable to the determination of the electronic structure of conventionally inaccessible reaction intermediates.

  View details for DOI 10.1107/S1600577521003830

  View details for PubMedID 34212873

• Spectroscopic Definition of a Highly Reactive Site in Cu-CHA for Selective Methane Oxidation: Tuning a Mono-mu-Oxo Dicopper(II) Active Site for Reactivity. Journal of the American Chemical Society Rhoda, H. M., Plessers, D., Heyer, A. J., Bols, M. L., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2021

  Abstract

  Using UV-vis and resonance Raman spectroscopy, we identify a [Cu2O]2+ active site in O2 and N2O activated Cu-CHA that reacts with methane to form methanol at low temperature. The Cu-O-Cu angle (120°) is smaller than that for the [Cu2O]2+ core on Cu-MFI (140°), and its coordination geometry to the zeolite lattice is different. Site-selective kinetics obtained by operando UV-vis show that the [Cu2O]2+ core on Cu-CHA is more reactive than the [Cu2O]2+ site in Cu-MFI. From DFT calculations, we find that the increased reactivity of Cu-CHA is a direct reflection of the strong [Cu2OH]2+ bond formed along the H atom abstraction reaction coordinate. A systematic evaluation of these [Cu2O]2+ cores reveals that the higher O-H bond strength in Cu-CHA is due to the relative orientation of the two planes of the coordinating bidentate O-Al-O T-sites that connect the [Cu2O]2+ core to the zeolite lattice. This work along with our earlier study ( J. Am. Chem. Soc, 2018, 140, 9236-9243) elucidates how zeolite lattice constraints can influence active site reactivity.

  View details for DOI 10.1021/jacs.1c02835

  View details for PubMedID 33970624

• Direct coordination of pterin to FeII enables neurotransmitter biosynthesis in the pterin-dependent hydroxylases. Proceedings of the National Academy of Sciences of the United States of America Iyer, S. R., Tidemand, K. D., Babicz, J. T., Jacobs, A. B., Gee, L. B., Haahr, L. T., Yoda, Y., Kurokuzu, M., Kitao, S., Saito, M., Seto, M., Christensen, H. E., Peters, G. H., Solomon, E. I. 2021; 118 (15)

  Abstract

  The pterin-dependent nonheme iron enzymes hydroxylate aromatic amino acids to perform the biosynthesis of neurotransmitters to maintain proper brain function. These enzymes activate oxygen using a pterin cofactor and an aromatic amino acid substrate bound to the FeII active site to form a highly reactive FeIV = O species that initiates substrate oxidation. In this study, using tryptophan hydroxylase, we have kinetically generated a pre-FeIV = O intermediate and characterized its structure as a FeII-peroxy-pterin species using absorption, Mossbauer, resonance Raman, and nuclear resonance vibrational spectroscopies. From parallel characterization of the pterin cofactor and tryptophan substrate-bound ternary FeII active site before the O2 reaction (including magnetic circular dichroism spectroscopy), these studies both experimentally define the mechanism of FeIV = O formation and demonstrate that the carbonyl functional group on the pterin is directly coordinated to the FeII site in both the ternary complex and the peroxo intermediate. Reaction coordinate calculations predict a 14 kcal/mol reduction in the oxygen activation barrier due to the direct binding of the pterin carbonyl to the FeII site, as this interaction provides an orbital pathway for efficient electron transfer from the pterin cofactor to the iron center. This direct coordination of the pterin cofactor enables the biological function of the pterin-dependent hydroxylases and demonstrates a unified mechanism for oxygen activation by the cofactor-dependent nonheme iron enzymes.

  View details for DOI 10.1073/pnas.2022379118

  View details for PubMedID 33876764

• Coordination and activation of nitrous oxide by iron zeolites NATURE CATALYSIS Bols, M. L., Snyder, B. R., Rhoda, H. M., Cnudde, P., Fayad, G., Schoonheydt, R. A., Van Speybroeck, V., Solomon, E. I., Sels, B. F. 2021; 4 (4): 332-+

  View details for DOI 10.1038/s41929-021-00602-4

  View details for Web of Science ID 000642497000012

• Effect of 3d/4p Mixing on 1s2p Resonant Inelastic X-ray Scattering: Electronic Structure of Oxo-Bridged Iron Dimers. Journal of the American Chemical Society Kroll, T., Baker, M. L., Wilson, S. A., Lundberg, M., Juhin, A., Arrio, M., Yan, J. J., Gee, L. B., Braun, A., Weng, T., Sokaras, D., Hedman, B., Hodgson, K. O., Solomon, E. I. 2021

  Abstract

  1s2p resonant inelastic X-ray scattering (1s2p RIXS) has proven successful in the determination of the differential orbital covalency (DOC, the amount of metal vs ligand character in each d molecular orbital) of highly covalent centrosymmetric iron environments including heme models and enzymes. However, many reactive intermediates have noncentrosymmetric environments, e.g., the presence of strong metal-oxo bonds, which results in the mixing of metal 4p character into the 3d orbitals. This leads to significant intensity enhancement in the metal K-pre-edge and as shown here, the associated 1s2p RIXS features, which impact their insight into electronic structure. Binuclear oxo bridged high spin Fe(III) complexes are used to determine the effects of 4p mixing on 1s2p RIXS spectra. In addition to developing the analysis of 4p mixing on K-edge XAS and 1s2p RIXS data, this study explains the selective nature of the 4p mixing that also enhances the analysis of L-edge XAS intensity in terms of DOC. These 1s2p RIXS biferric model studies enable new structural insight from related data on peroxo bridged biferric enzyme intermediates. The dimeric nature of the oxo bridged Fe(III) complexes further results in ligand-to-ligand interactions between the Fe(III) sites and angle dependent features just above the pre-edge that reflect the superexchange pathway of the oxo bridge. Finally, we present a methodology that enables DOC to be obtained when L-edge XAS is inaccessible and only 1s2p RIXS experiments can be performed as in many metalloenzyme intermediates in solution.

  View details for DOI 10.1021/jacs.0c11193

  View details for PubMedID 33730507

• A Thioether-Ligated Cupric Superoxide Model with Hydrogen Atom Abstraction Reactivity. Journal of the American Chemical Society Bhadra, M. n., Transue, W. J., Lim, H. n., Cowley, R. E., Lee, J. Y., Siegler, M. A., Josephs, P. n., Henkel, G. n., Lerch, M. n., Schindler, S. n., Neuba, A. n., Hodgson, K. O., Hedman, B. n., Solomon, E. I., Karlin, K. D. 2021

  Abstract

  The central role of cupric superoxide intermediates proposed in hormone and neurotransmitter biosynthesis by noncoupled binuclear copper monooxygenases like dopamine-β-monooxygenase has drawn significant attention to the unusual methionine ligation of the CuM ("CuB") active site characteristic of this class of enzymes. The copper-sulfur interaction has proven critical for turnover, raising still-unresolved questions concerning Nature's selection of an oxidizable Met residue to facilitate C-H oxygenation. We describe herein a model for CuM, [(TMGN3S)CuI]+ ([1]+), and its O2-bound analog [(TMGN3S)CuII(O2•-)]+ ([1·O2]+). The latter is the first reported cupric superoxide with an experimentally proven Cu-S bond which also possesses demonstrated hydrogen atom abstraction (HAA) reactivity. Introduction of O2 to a precooled solution of the cuprous precursor [1]B(C6F5)4 (-135 °C, 2-methyltetrahydrofuran (2-MeTHF)) reversibly forms [1·O2]B(C6F5)4 (UV/vis spectroscopy: λmax 442, 642, 742 nm). Resonance Raman studies (413 nm) using 16O2 [18O2] corroborated the identity of [1·O2]+ by revealing Cu-O (446 [425] cm-1) and O-O (1105 [1042] cm-1) stretches, and extended X-ray absorption fine structure (EXAFS) spectroscopy showed a Cu-S interatomic distance of 2.55 Å. HAA reactivity between [1·O2]+ and TEMPO-H proceeds rapidly (1.28 × 10-1 M-1 s-1, -135 °C, 2-MeTHF) with a primary kinetic isotope effect of kH/kD = 5.4. Comparisons of the O2-binding behavior and redox activity of [1]+ vs [2]+, the latter a close analog of [1]+ but with all N atom ligation (i.e., N3S vs N4), are presented.

  View details for DOI 10.1021/jacs.1c00260

  View details for PubMedID 33684290

• Nuclear Resonance Vibrational Spectroscopic Definition of the Fe(IV)2 Intermediate Q in Methane Monooxygenase and Its Reactivity. Journal of the American Chemical Society Jacobs, A. B., Banerjee, R., Deweese, D. E., Braun, A., Babicz, J. T., Gee, L. B., Sutherlin, K. D., Böttger, L. H., Yoda, Y., Saito, M., Kitao, S., Kobayashi, Y., Seto, M., Tamasaku, K., Lipscomb, J. D., Park, K., Solomon, E. I. 2021

  Abstract

  Methanotrophic bacteria utilize the nonheme diiron enzyme soluble methane monooxygenase (sMMO) to convert methane to methanol in the first step of their metabolic cycle under copper-limiting conditions. The structure of the sMMO Fe(IV)2 intermediate Q responsible for activating the inert C-H bond of methane (BDE = 104 kcal/mol) remains controversial, with recent studies suggesting both "open" and "closed" core geometries for its active site. In this study, we employ nuclear resonance vibrational spectroscopy (NRVS) to probe the geometric and electronic structure of intermediate Q at cryogenic temperatures. These data demonstrate that Q decays rapidly during the NRVS experiment. Combining data from several years of measurements, we derive the NRVS vibrational features of intermediate Q as well as its cryoreduced decay product. A library of 90 open and closed core models of intermediate Q is generated using density functional theory to analyze the NRVS data of Q and its cryoreduced product as well as prior spectroscopic data on Q. Our analysis reveals that a subset of closed core models reproduce these newly acquired NRVS data as well as prior data. The reaction coordinate with methane is also evaluated using both closed and open core models of Q. These studies show that the potent reactivity of Q toward methane resides in the "spectator oxo" of its Fe(IV)2O2 core, in contrast to nonheme mononuclear Fe(IV)═O enzyme intermediates that H atoms abstract from weaker C-H bonds.

  View details for DOI 10.1021/jacs.1c05436

  View details for PubMedID 34570980

• The three-spin intermediate at the O-O cleavage and proton-pumping junction in heme-Cu oxidases. Science (New York, N.Y.) Jose, A., Schaefer, A. W., Roveda, A. C., Transue, W. J., Choi, S. K., Ding, Z., Gennis, R. B., Solomon, E. I. 2021; 373 (6560): 1225-1229

  Abstract

  [Figure: see text].

  View details for DOI 10.1126/science.abh3209

  View details for PubMedID 34516790

• Selective Formation of α-Fe(II) Sites on Fe-Zeolites through One-Pot Synthesis. Journal of the American Chemical Society Bols, M. L., Devos, J., Rhoda, H. M., Plessers, D., Solomon, E. I., Schoonheydt, R. A., Sels, B. F., Dusselier, M. 2021

  Abstract

  α-Fe(II) active sites in iron zeolites catalyze N2O decomposition and form highly reactive α-O that selectively oxidizes unreactive hydrocarbons, such as methane. How these α-Fe(II) sites are formed remains unclear. Here different methods of iron introduction into zeolites are compared to derive the limiting factors of Fe speciation to α-Fe(II). Postsynthetic iron introduction procedures on small pore zeolites suffer from limited iron diffusion and dispersion leading to iron oxides. In contrast, by introducing Fe(III) in the hydrothermal synthesis mixture of the zeolite (one-pot synthesis) and the right treatment, crystalline CHA can be prepared with >1.6 wt % Fe, of which >70% is α-Fe(II). The effect of iron on the crystallization is investigated, and the intermediate Fe species are tracked using UV-vis-NIR, FT-IR, and Mössbauer spectroscopy. These data are supplemented with online mass spectrometry in each step, with reactivity tests in α-O formation and with methanol yields in stoichiometric methane activation at room temperature and pressure. We recover up to 134 μmol methanol per gram in a single cycle through H2O/CH3CN extraction and 183 μmol/g through steam desorption, a record yield for iron zeolites. A general scheme is proposed for iron speciation in zeolites through the steps of drying, calcination, and activation. The formation of two cohorts of α-Fe(II) is discovered, one before and one after high temperature activation. We propose the latter cohort depends on the reshuffling of aluminum in the zeolite lattice to accommodate thermodynamically favored α-Fe(II).

  View details for DOI 10.1021/jacs.1c07590

  View details for PubMedID 34570975

• Atmospheric Methane Removal: A Research Agenda Philosophical Transactions of the Royal Society A Jackson, R. B., et al 2021; 379: 20200454

  View details for DOI 10.1098/rsta.2020.0454

• Short-lived metal-centered excited state initiates iron-methionine photodissociation in ferrous cytochrome c. Nature communications Reinhard, M. E., Mara, M. W., Kroll, T., Lim, H., Hadt, R. G., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Kunnus, K., Driel, T. B., Hartsock, R. W., Kjaer, K. S., Weninger, C., Biasin, E., Gee, L. B., Hodgson, K. O., Hedman, B., Bergmann, U., Solomon, E. I., Gaffney, K. J. 2021; 12 (1): 1086

  Abstract

  The dynamics of photodissociation and recombination in heme proteins represent an archetypical photochemical reaction widely used to understand the interplay between chemical dynamics and reaction environment. We report a study of the photodissociation mechanism for the Fe(II)-S bond between the heme iron and methionine sulfur of ferrous cytochrome c. This bond dissociation is an essential step in the conversion of cytochrome c from an electron transfer protein to a peroxidase enzyme. We use ultrafast X-ray solution scattering to follow the dynamics of Fe(II)-S bond dissociation and 1s3p (Kbeta) X-ray emission spectroscopy to follow the dynamics of the iron charge and spin multiplicity during bond dissociation. From these measurements, we conclude that the formation of a triplet metal-centered excited state with anti-bonding Fe(II)-S interactions triggers the bond dissociation and precedes the formation of the metastable Fe high-spin quintet state.

  View details for DOI 10.1038/s41467-021-21423-w

  View details for PubMedID 33597529

• Ferric Heme Superoxide Reductive Transformations to Ferric Heme (Hydro)Peroxide Species: Spectroscopic Characterization and Thermodynamic Implications for H-atom Transfer (HAT). Angewandte Chemie (International ed. in English) Karlin, K. D., Kim, H., Rogler, P. J., Sharma, S. K., Schaefer, A. W., Solomon, E. I. 2020

  Abstract

  A new end-on low-spin ferric heme peroxide, [(P Im )Fe III -(O 2 2- )] - ( P Im - P ), and subsequently formed hydroperoxide species, [(P Im )Fe III -(OOH)] ( P Im - HP ) are generated utilizing the iron-porphyrinate P Im with its tethered axial base imidazolyl group. Measured thermodynamic parameters, the ferric heme superoxide [(P Im )Fe III -(O 2 -)] ( P Im - S ) reduction potential ( E °') and the P Im - HP p K a value, lead to the finding of the OO-H bond dissociation free energy (BDFE) of P Im - HP as 69.5 kcal/mol, using a thermodynamic square scheme and Bordwell relationship. The results are validated by the observed oxidizing ability of P Im - S via hydrogen atom transfer (HAT) compared to that of the F 8 superoxide complex, [(F 8 )Fe III -(O 2 -)] ( S ) (F 8 = tetrakis(2,6-difluorophenyl)porphyrinate, without an internally appended axial base imidazolyl), as determined from reactivity comparison of superoxide complexes P Im - S and S with the hydroxylamine (O-H) substrates TEMPO-H and ABNO-H.

  View details for DOI 10.1002/anie.202013791

  View details for PubMedID 33348450

• Valence-Dependent Electrical Conductivity in a 3D Tetrahydroxyquinone-Based Metal-Organic Framework. Journal of the American Chemical Society Chen, G., Gee, L. B., Xu, W., Zhu, Y., Lezama-Pacheco, J. S., Huang, Z., Li, Z., Babicz, J. T., Choudhury, S., Chang, T., Reed, E., Solomon, E. I., Bao, Z. 2020

  Abstract

  Electrically conductive metal-organic frameworks (cMOFs) have become a topic of intense interest in recent years because of their great potential in electrochemical energy storage, electrocatalysis, and sensing applications. Most of the cMOFs reported hitherto are 2D structures, and 3D cMOFs remain rare. Herein we report FeTHQ, a 3D cMOF synthesized from tetrahydroxy-1,4-quinone (THQ) and iron(II) sulfate salt. FeTHQ exhibited a conductivity of 3.3 ± 0.55 mS cm-1 at 300 K, which is high for 3D cMOFs. The conductivity of FeTHQ is valence-dependent. A higher conductivity was measured with the as-prepared FeTHQ than with the air-oxidized and sodium naphthalenide-reduced samples.

  View details for DOI 10.1021/jacs.0c09379

  View details for PubMedID 33315385

• Advances in the synthesis, characterisation, and mechanistic understanding of active sites in Fezeolites for redox catalysts DALTON TRANSACTIONS Bols, M. L., Rhoda, H. M., Snyder, B. R., Solomon, E., Pierloot, K., Schoonheydt, R. A., Sels, B. F. 2020; 49 (42): 14749–57

  Abstract

  The recent research developments on the active sites in Fe-zeolites for redox catalysis are discussed. Building on the characterisation of the α-Fe/α-O active sites in the beta and chabazite zeolites, we demonstrate a bottom-up approach to successfully understand and develop Fe-zeolite catalysts. We use the room temperature benzene to phenol reaction as a relevant example. We then suggest how the spectroscopic identification of other monomeric and dimeric iron sites could be tackled. The challenges in the characterisation of active sites and intermediates in NOX selective catalytic reduction catalysts and further development of catalysts for mild partial methane oxidation are briefly discussed.

  View details for DOI 10.1039/d0dt01857k

  View details for Web of Science ID 000589506700038

  View details for PubMedID 33140781

• Kbeta X-ray Emission Spectroscopy as a Probe of Cu(I) Sites: Application to the Cu(I) Site in Preprocessed Galactose Oxidase. Inorganic chemistry Lim, H., Baker, M. L., Cowley, R. E., Kim, S., Bhadra, M., Siegler, M. A., Kroll, T., Sokaras, D., Weng, T., Biswas, D. R., Dooley, D. M., Karlin, K. D., Hedman, B., Hodgson, K. O., Solomon, E. I. 2020

  Abstract

  Cu(I) active sites in metalloproteins are involved in O2 activation, but their O2 reactivity is difficult to study due to the Cu(I) d10 closed shell which precludes the use of conventional spectroscopic methods. Kbeta X-ray emission spectroscopy (XES) is a promising technique for investigating Cu(I) sites as it detects photons emitted by electronic transitions from occupied orbitals. Here, we demonstrate the utility of Kbeta XES in probing Cu(I) sites in model complexes and a metalloprotein. Using Cu(I)Cl, emission features from double-ionization (DI) states are identified using varying incident X-ray photon energies, and a reasonable method to correct the data to remove DI contributions is presented. Kbeta XES spectra of Cu(I) model complexes, having biologically relevant N/S ligands and different coordination numbers, are compared and analyzed, with the aid of density functional theory (DFT) calculations, to evaluate the sensitivity of the spectral features to the ligand environment. While the low-energy Kbeta2,5 emission feature reflects the ionization energy of ligand np valence orbitals, the high-energy Kbeta2,5 emission feature corresponds to transitions from molecular orbitals (MOs) having mainly Cu 3d character with the intensities determined by ligand-mediated d-p mixing. A Kbeta XES spectrum of the Cu(I) site in preprocessed galactose oxidase (GOpre) supports the 1Tyr/2His structural model that was determined by our previous X-ray absorption spectroscopy and DFT study. The high-energy Kbeta2,5 emission feature in the Cu(I)-GOpre data has information about the MO containing mostly Cu 3dx2-y2 character that is the frontier molecular orbital (FMO) for O2 activation, which shows the potential of Kbeta XES in probing the Cu(I) FMO associated with small-molecule activation in metalloproteins.

  View details for DOI 10.1021/acs.inorgchem.0c02495

  View details for PubMedID 33136386

• Nuclear Resonance Vibrational Spectroscopic Definition of the Facial Triad FeIV═O Intermediate in Taurine Dioxygenase: Evaluation of Structural Contributions to Hydrogen Atom Abstraction. Journal of the American Chemical Society Srnec, M., Iyer, S. R., Dassama, L. M., Park, K., Wong, S. D., Sutherlin, K. D., Yoda, Y., Kobayashi, Y., Kurokuzu, M., Saito, M., Seto, M., Krebs, C., Bollinger, J. M., Solomon, E. I. 2020

  Abstract

  The alpha-ketoglutarate (alphaKG)-dependent oxygenases catalyze a diverse range of chemical reactions using a common high-spin FeIV═O intermediate that, in most reactions, abstract a hydrogen atom from the substrate. Previously, the FeIV═O intermediate in the alphaKG-dependent halogenase SyrB2 was characterized by nuclear resonance vibrational spectroscopy (NRVS) and density functional theory (DFT) calculations, which demonstrated that it has a trigonal-pyramidal geometry with the scissile C-H bond of the substrate calculated to be perpendicular to the Fe-O bond. Here, we have used NRVS and DFT calculations to show that the FeIV═O complex in taurine dioxygenase (TauD), the alphaKG-dependent hydroxylase in which this intermediate was first characterized, also has a trigonal bipyramidal geometry but with an aspartate residue replacing the equatorial halide of the SyrB2 intermediate. Computational analysis of hydrogen atom abstraction by square pyramidal, trigonal bipyramidal, and six-coordinate FeIV═O complexes in two different substrate orientations (one more along [sigma channel] and another more perpendicular [pi channel] to the Fe-O bond) reveals similar activation barriers. Thus, both substrate approaches to all three geometries are competent in hydrogen atom abstraction. The equivalence in reactivity between the two substrate orientations arises from compensation of the promotion energy (electronic excitation within the d manifold) required to access the pi channel by the significantly larger oxyl character present in the ppi orbital oriented toward the substrate, which leads to an earlier transition state along the C-H coordinate.

  View details for DOI 10.1021/jacs.0c08903

  View details for PubMedID 33103886

• Reply to: Practical constraints on atmospheric methane removal NATURE SUSTAINABILITY Jackson, R. B., Solomon, E. I., Canadell, J. G., Cargnello, M., Field, C. B., Abernethy, S. 2020

  View details for DOI 10.1038/s41893-020-0498-5

  View details for Web of Science ID 000521528300002

• Heme-FeIII Superoxide, Peroxide and Hydroperoxide Thermodynamic Relationships: FeIII-O2•- Complex H-Atom Abstraction Reactivity. Journal of the American Chemical Society Kim, H. n., Rogler, P. J., Sharma, S. K., Schaefer, A. W., Solomon, E. I., Karlin, K. D. 2020

  Abstract

  Establishing redox and thermodynamic relationships between metal-ion-bound O2 and its reduced (and protonated) derivatives is critically important for a full understanding of (bio)chemical processes involving dioxygen processing. Here, a ferric heme peroxide complex, [(F8)FeIII-(O22-)]- ( P ) (F8 = tetrakis(2,6-difluorophenyl)porphyrinate), and a superoxide complex, [(F8)FeIII-(O2•-)] ( S ), are shown to be redox interconvertible. Using Cr(η-C6H6)2, an equilibrium state where S and P are present is established in tetrahydrofuran (THF) at -80 °C, allowing determination of the reduction potential of S as -1.17 V vs Fc+/0. P could be protonated with 2,6-lutidinium triflate, yielding the low-spin ferric hydroperoxide species, [(F8)FeIII-(OOH)] ( HP ). Partial conversion of HP back to P using a derivatized phosphazene base gave a P / HP equilibrium mixture, leading to the determination of pKa = 28.8 for HP (THF, -80 °C). With the measured reduction potential and pKa, the O-H bond dissociation free energy (BDFE) of hydroperoxide species HP was calculated to be 73.5 kcal/mol, employing the thermodynamic square scheme and Bordwell relationship. This calculated O-H BDFE of HP , in fact, lines up with an experimental demonstration of the oxidizing ability of S via hydrogen atom transfer (HAT) from TEMPO-H (2,2,6,6-tetramethylpiperdine-N-hydroxide, BDFE = 66.5 kcal/mol in THF), forming the hydroperoxide species HP and TEMPO radical. Kinetic studies carried out with TEMPO-H(D) reveal second-order behavior, kH = 0.5, kD = 0.08 M-1 s-1 (THF, -80 °C); thus, the hydrogen/deuterium kinetic isotope effect (KIE) = 6, consistent with H-atom abstraction by S being the rate-determining step. This appears to be the first case where experimentally derived thermodynamics lead to a ferric heme hydroperoxide OO-H BDFE determination, that FeIII-OOH species being formed via HAT reactivity of the partner ferric heme superoxide complex.

  View details for DOI 10.1021/jacs.9b12571

  View details for PubMedID 31913628

• Rapid Decay of the Native Intermediate in the Metallooxidase Fet3p Enables Controlled FeII Oxidation for Efficient Metabolism. Journal of the American Chemical Society Jones, S. M., Heppner, D. E., Vu, K. n., Kosman, D. J., Solomon, E. I. 2020

  Abstract

  The multicopper oxidases (MCOs) couple four 1e- oxidations of substrate to the 4e- reduction of O2 to H2O. These divide into two groups: those that oxidize organic substrates with high turnover frequencies (TOFs) up to 560 s-1 and those that oxidize metal ions with low TOFs, ∼1 s-1 or less. The catalytic mechanism of the organic oxidases has been elucidated, and the high TOF is achieved through rapid intramolecular electron transfer (IET) to the native intermediate (NI), which only slowly decays to the resting form. Here, we uncover the factors that govern the low TOF in Fet3p, a prototypical metallooxidase, in the context of the MCO mechanism. We determine that the NI decays rapidly under optimal turnover conditions, and the mechanism thereby becomes rate-limited by slow IET to the resting enzyme. Development of a catalytic model leads to the important conclusions that proton delivery to the NI controls the mechanism and enables the slow turnover in Fet3p that is functionally significant in Fe metabolism enabling efficient ferroxidase activity while avoiding ROS generation.

  View details for DOI 10.1021/jacs.0c02384

  View details for PubMedID 32379440

• Evaluation of a concerted vs. sequential oxygen activation mechanism in α-ketoglutarate-dependent nonheme ferrous enzymes. Proceedings of the National Academy of Sciences of the United States of America Goudarzi, S. n., Iyer, S. R., Babicz, J. T., Yan, J. J., Peters, G. H., Christensen, H. E., Hedman, B. n., Hodgson, K. O., Solomon, E. I. 2020

  Abstract

  Determining the requirements for efficient oxygen (O2) activation is key to understanding how enzymes maintain efficacy and mitigate unproductive, often detrimental reactivity. For the α-ketoglutarate (αKG)-dependent nonheme iron enzymes, both a concerted mechanism (both cofactor and substrate binding prior to reaction with O2) and a sequential mechanism (cofactor binding and reaction with O2 precede substrate binding) have been proposed. Deacetoxycephalosporin C synthase (DAOCS) is an αKG-dependent nonheme iron enzyme for which both of these mechanisms have been invoked to generate an intermediate that catalyzes oxidative ring expansion of penicillin substrates in cephalosporin biosynthesis. Spectroscopy shows that, in contrast to other αKG-dependent enzymes (which are six coordinate when only αKG is bound to the FeII), αKG binding to FeII-DAOCS results in ∼45% five-coordinate sites that selectively react with O2 relative to the remaining six-coordinate sites. However, this reaction produces an FeIII species that does not catalyze productive ring expansion. Alternatively, simultaneous αKG and substrate binding to FeII-DAOCS produces five-coordinate sites that rapidly react with O2 to form an FeIV=O intermediate that then reacts with substrate to produce cephalosporin product. These results demonstrate that the concerted mechanism is operative in DAOCS and by extension, other nonheme iron enzymes.

  View details for DOI 10.1073/pnas.1922484117

  View details for PubMedID 32094179

• Kinetic analysis of amino acid radicals formed in H2O2-driven CuI LPMO reoxidation implicates dominant homolytic reactivity. Proceedings of the National Academy of Sciences of the United States of America Jones, S. M., Transue, W. J., Meier, K. K., Kelemen, B. n., Solomon, E. I. 2020

  Abstract

  Lytic polysaccharide monooxygenases (LPMOs) have been proposed to react with both [Formula: see text] and [Formula: see text] as cosubstrates. In this study, the [Formula: see text] reaction with reduced Hypocrea jecorina LPMO9A (CuI-HjLPMO9A) is demonstrated to be 1,000-fold faster than the [Formula: see text] reaction while producing the same oxidized oligosaccharide products. Analysis of the reactivity in the absence of polysaccharide substrate by stopped-flow absorption and rapid freeze-quench (RFQ) electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) yields two intermediates corresponding to neutral tyrosyl and tryptophanyl radicals that are formed along minor reaction pathways. The dominant reaction pathway is characterized by RFQ EPR and kinetic modeling to directly produce CuII-HjLPMO9A and indicates homolytic O-O cleavage. Both optical intermediates exhibit magnetic exchange coupling with the CuII sites reflecting facile electron transfer (ET) pathways, which may be protective against uncoupled turnover or provide an ET pathway to the active site with substrate bound. The reactivities of nonnative organic peroxide cosubstrates effectively exclude the possibility of a ping-pong mechanism.

  View details for DOI 10.1073/pnas.1922499117

  View details for PubMedID 32414932

• Proton-Electron Transfer to the Active Site Is Essential for the Reaction Mechanism of Soluble Δ9-Desaturase. Journal of the American Chemical Society Bím, D. n., Chalupský, J. n., Culka, M. n., Solomon, E. I., Rulíšek, L. n., Srnec, M. n. 2020

  Abstract

  A full understanding of the catalytic action of non-heme iron (NHFe) and non-heme diiron (NHFe2) enzymes is still beyond the grasp of contemporary computational and experimental techniques. Many of these enzymes exhibit fascinating chemo-, regio-, and stereoselectivity, in spite of employing highly reactive intermediates which are necessary for activations of most stable chemical bonds. Herein, we study in detail one intriguing representative of the NHFe2 family of enzymes: soluble Δ9 desaturase (Δ9D), which desaturates rather than performing the thermodynamically favorable hydroxylation of substrate. Its catalytic mechanism has been explored in great detail by using QM(DFT)/MM and multireference wave function methods. Starting from the spectroscopically observed 1,2-μ-peroxo diferric P intermediate, the proton-electron uptake by P is the favored mechanism for catalytic activation, since it allows a significant reduction of the barrier of the initial (and rate-determining) H-atom abstraction from the stearoyl substrate as compared to the "proton-only activated" pathway. Also, we ruled out that a Q-like intermediate (high-valent diamond-core bis-μ-oxo-[FeIV]2 unit) is involved in the reaction mechanism. Our mechanistic picture is consistent with the experimental data available for Δ9D and satisfies fairly stringent conditions required by Nature: the chemo-, stereo-, and regioselectivity of the desaturation of stearic acid. Finally, the mechanisms evaluated are placed into a broader context of NHFe2 chemistry, provided by an amino acid sequence analysis through the families of the NHFe2 enzymes. Our study thus represents an important contribution toward understanding the catalytic action of the NHFe2 enzymes and may inspire further work in NHFe(2) biomimetic chemistry.

  View details for DOI 10.1021/jacs.0c01786

  View details for PubMedID 32406236

• A Binuclear CuA Center Designed in an All α-Helical Protein Scaffold. Journal of the American Chemical Society Mirts, E. N., Dikanov, S. A., Jose, A. n., Solomon, E. I., Lu, Y. n. 2020; 142 (32): 13779–94

  Abstract

  The primary and secondary coordination spheres of metal binding sites in metalloproteins have been investigated extensively, leading to the creation of high-performing functional metalloproteins; however, the impact of the overall structure of the protein scaffold on the unique properties of metalloproteins has rarely been studied. A primary example is the binuclear CuA center, an electron transfer cupredoxin domain of photosynthetic and respiratory complexes and, recently, a protein coregulated with particulate methane and ammonia monooxygenases. The redox potential, Cu-Cu spectroscopic features, and a valence delocalized state of CuA are difficult to reproduce in synthetic models, and every artificial protein CuA center to-date has used a modified cupredoxin. Here, we present a fully functional CuA center designed in a structurally nonhomologous protein, cytochrome c peroxidase (CcP), by only two mutations (CuACcP). We demonstrate with UV-visible absorption, resonance Raman, and magnetic circular dichroism spectroscopy that CuACcP is valence delocalized. Continuous wave and pulsed (HYSCORE) X-band EPR show it has a highly compact g z area and small A z hyperfine principal value with g and A tensors that resemble axially perturbed CuA. Stopped-flow kinetics found that CuA formation proceeds through a single T2Cu intermediate. The reduction potential of CuACcP is comparable to native CuA and can transfer electrons to a physiological redox partner. We built a structural model of the designed Cu binding site from extended X-ray absorption fine structure spectroscopy and validated it by mutation of coordinating Cys and His residues, revealing that a triad of residues (R48C, W51C, and His52) rigidly arranged on one α-helix is responsible for chelating the first Cu(II) and that His175 stabilizes the binuclear complex by rearrangement of the CcP heme-coordinating helix. This design is a demonstration that a highly conserved protein fold is not uniquely necessary to induce certain characteristic physical and chemical properties in a metal redox center.

  View details for DOI 10.1021/jacs.0c04226

  View details for PubMedID 32662996

• Role of a Tyrosine Radical in Human Ceruloplasmin Catalysis. ACS central science Tian, S. n., Jones, S. M., Solomon, E. I. 2020; 6 (10): 1835–43

  Abstract

  Multicopper oxidases (MCOs) are a large family of diverse enzymes found in both eukaryotes and prokaryotes that couple one-electron oxidations of various substrates to the four-electron reduction of O2 to H2O, functioning through a set of metallocofactors consisting of one type 1 copper (T1 Cu) and one trinuclear copper cluster (TNC). Human serum ceruloplasmin (Cp) is a unique member of MCOs composed of six cupredoxin domains and harbors six Cu ions arranged as three T1 Cu and one TNC. The native substrate of Cp is Fe2+. It is an essential ferroxidase critical for iron homeostasis and is closely associated with metal-mediated diseases and metal neurotoxicity. In human serum, Cp operates under substrate-limiting low [Fe2+] but high [O2] conditions, implying the possible involvement of partially reduced intermediates in Cp catalysis. In this work, we studied for the first time Cp reactivities at defined partially reduced states and discovered a tyrosine radical weakly magnetically coupled to the native intermediate (NI) of the TNC via a hydrogen bond. Our results lead to a new hypothesis that human iron transport is regulated as the paired transfer of iron from ferroportin to Cp to transferrin, and the tyrosine residue in Cp acts as a gate to avoid reactive oxygen species (ROS) formation when Fe2+ delivery is dysregulated.

  View details for DOI 10.1021/acscentsci.0c00953

  View details for PubMedID 33145420

  View details for PubMedCentralID PMC7596862

• Oxygen intermediates in Cu and Fe zeolites: Correlations to metalloenzymes Solomon, E., Snyder, B., Rhoda, H. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000525061501374

• O2 Reduction to Water by High Potential Multicopper Oxidases: Contributions of the T1 Copper Site Potential and the Local Environment of the Trinuclear Copper Cluster. Journal of the American Chemical Society Sekretaryova, A., Jones, S. M., Solomon, E. I. 2019

  Abstract

  High potential multicopper oxidases (MCOs) have T1 reduction potentials >600 mV (vs normal hydrogen electrode), making them important catalysts for O2 reduction in various biotechnological applications. The oxygen reduction mechanism for the low potential MCOs is well-characterized; however, O2 reactivity of high potential MCOs is not well understood. In this study, we have shown that laccase from Trametes versicolor, where the T1 redox potential is increased by 350 mV over that of the low potential MCOs corresponding to an 8 kcal/mol decrease in the driving force, exhibits a slower intramolecular electron transfer (IET) rate to the trinuclear Cu cluster (TNC) in the native intermediate (NI), relative to the low potential MCO from Rhus vernicifera laccase. This IET rate is, however, >102 times faster than the decay rate of the NI, demonstrating that this intermediate form of the enzyme is catalytically relevant enabling fast turnover. However, in contrast to the low potential MCOs where T1 reduction by substrate is rate limiting, the rate limiting step in turnover of high potential MCOs is the first IET to NI. Part of the reduction potential difference of the T1 sites in high vs low potential MCOs is balanced by an 100 mV higher reduction potential of NI due to the more positive protein environment in the vicinity of the TNC.

  View details for DOI 10.1021/jacs.9b05230

  View details for PubMedID 31260290

• Chloride Control of the Mechanism of Human Serum Ceruloplasmin (Cp) Catalysis. Journal of the American Chemical Society Tian, S., Jones, S. M., Jose, A., Solomon, E. I. 2019

  Abstract

  Unraveling the mechanism of ceruloplasmin (Cp) is fundamentally important toward understanding the pathogenesis of metal-mediated diseases and metal neurotoxicity. Here we report that Cl-, the most abundant anion in blood plasma, is a key component of Cp catalysis. Based on detailed spectroscopic analyses, Cl- preferentially interacts with the partially reduced trinuclear Cu cluster (TNC) in Cp under physiological conditions and shifts the electron equilibrium distribution among the two redox active type 1 (T1) Cu sites and the TNC. This shift in potential enables the intramolecular electron transfer (IET) from the T1 Cu to the native intermediate (NI) and accelerates the IET from the T1 Cu to the TNC, resulting in faster turnover in Cp catalysis.

  View details for DOI 10.1021/jacs.9b03661

  View details for PubMedID 31203609

• X-ray Absorption Spectroscopy as a Probe of Ligand Noninnocence in Metallocorroles: The Case of Copper Corroles INORGANIC CHEMISTRY Lim, H., Thomas, K. E., Hedman, B., Hodgson, K. O., Ghosh, A., Solomon, E. I. 2019; 58 (10): 6722–30

  View details for DOI 10.1021/acs.inorgchem.9b00128

  View details for Web of Science ID 000469304700018

• Spin Interconversion of Heme-Peroxo-Copper Complexes Facilitated by Intramolecular Hydrogen-Bonding Interactions JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Schaefer, A. W., Ehudin, M. A., Quist, D. A., Tang, J. A., Karlin, K. D., Solomon, E. I. 2019; 141 (12): 4936–51

  View details for DOI 10.1021/jacs.9b00118

  View details for Web of Science ID 000462950800021

• Formylglycine-generating enzyme binds substrate directly at a mononuclear Cu(I) center to initiate O-2 activation PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Appel, M. J., Meier, K. K., Lafrance-Vanasse, J., Lim, H., Tsai, C., Hedman, B., Hodgson, K. O., Tainer, J. A., Solomon, E. I., Bertozzi, C. R. 2019; 116 (12): 5370–75

  View details for DOI 10.1073/pnas.1818274116

  View details for Web of Science ID 000461679000031

• Influence of intramolecular secondary sphere hydrogen-bonding interactions on cytochrome c oxidase inspired low-spin heme-peroxo-copper complexes. Chemical science Ehudin, M. A., Schaefer, A. W., Adam, S. M., Quist, D. A., Diaz, D. E., Tang, J. A., Solomon, E. I., Karlin, K. D. 2019; 10 (10): 2893-2905

  Abstract

  Dioxygen reduction by heme-copper oxidases is a critical biochemical process, wherein hydrogen bonding is hypothesized to participate in the critical step involving the active-site reductive cleavage of the O-O bond. Sixteen novel synthetic heme-(μ-O22-)-Cu(XTMPA) complexes, whose design is inspired by the cytochrome c oxidase active site structure, were generated in an attempt to form the first intramolecular H-bonded complexes. Derivatives of the "parent" ligand (XTMPA, TMPA = (tris((2-pyridyl)methyl)amine)) possessing one or two amine pendants preferentially form an H-bond with the copper-bound O-atom of the peroxide bridge. This is evidenced by a characteristic blue shift in the ligand-to-metal charge transfer (LMCT) bands observed in UV-vis spectroscopy (consistent with lowering of the peroxo π* relative to the iron orbitals) and a weakening of the O-O bond determined by resonance Raman spectroscopy (rR), with support from Density Functional Theory (DFT) calculations. Remarkably, with the TMPA-based infrastructure (versus similar heme-peroxo-copper complexes with different copper ligands), the typically undetected Cu-O stretch for these complexes was observed via rR, affording critical insights into the nature of the O-O peroxo core for the complexes studied. While amido functionalities have been shown to have greater H-bonding capabilities than their amino counterparts, in these heme-peroxo-copper complexes amido substituents distort the local geometry such that H-bonding with the peroxo core only imparts a weak electronic effect; optimal H-bonding interactions are observed by employing two amino groups on the copper ligand. The amino-substituted systems presented in this work reveal a key orientational anisotropy in H-bonding to the peroxo core for activating the O-O bond, offering critical insights into effective O-O cleavage chemistry. These findings indirectly support computational and protein structural studies suggesting the presence of an interstitial H-bonding water molecule in the CcO active site, which is critical for the desired reactivity. The results are evaluated with appropriate controls and discussed with respect to potential O2-reduction capabilities.

  View details for DOI 10.1039/c8sc05165h

  View details for PubMedID 30996867

  View details for PubMedCentralID PMC6431958

• Influence of intramolecular secondary sphere hydrogen-bonding interactions on cytochrome c oxidase inspired low-spin heme-peroxo-copper complexes CHEMICAL SCIENCE Ehudin, M. A., Schaefer, A. W., Adam, S. M., Quist, D. A., Diaz, D. E., Tang, J. A., Solomon, E. I., Karlin, K. D. 2019; 10 (10): 2893–2905

  View details for DOI 10.1039/c8sc05165h

  View details for Web of Science ID 000461509800001

• Spin Interconversion of Heme-Peroxo-Copper Complexes Facilitated by Intramolecular Hydrogen-Bonding Interactions. Journal of the American Chemical Society Schaefer, A. W., Ehudin, M. A., Quist, D. A., Tang, J. A., Karlin, K. D., Solomon, E. I. 2019

  Abstract

  Synthetic peroxo-bridged high-spin (HS) heme-(mu-eta2:eta1-O22-)-Cu(L) complexes incorporating (as part of the copper ligand) intramolecular hydrogen-bond (H-bond) capabilities and/or steric effects are herein demonstrated to affect the complex's electronic and geometric structure, notably impacting the spin state. An H-bonding interaction with the peroxo core favors a low-spin (LS) heme-(mu-eta1:eta1-O22-)-Cu(L) structure, resulting in a reversible temperature-dependent interconversion of spin state (5 coordinate HS to 6 coordinate LS). The LS state dominates at low temperatures, even in the absence of a strong trans-axial heme ligand. Lewis base addition inhibits the H-bond facilitated spin interconversion by competition for the H-bond donor, illustrating the precise H-bonding interaction required to induce spin-crossover (SCO). Resonance Raman spectroscopy (rR) shows that the H-bonding pendant interacts with the bridging peroxide ligand to stabilize the LS but not the HS state. The H-bond (to the Cu-bound O atom) acts to weaken the O-O bond and strengthen the Fe-O bond, exhibiting nu(M-O) and nu(O-O) values comparable to analogous known LS complexes with a strong donating trans-axial ligand, 1,5-dicyclohexylimidazole, (DCHIm)heme-(mu-eta1:eta1-O22-)-Cu(L). Variable-temperature (-90 to -130 °C) UV-vis and 2H NMR spectroscopies confirm the SCO process and implicate the involvement of solvent binding. Examining a case of solvent binding without SCO, thermodynamic parameters were obtained from a van't Hoff analysis, accounting for its contribution in SCO. Taken together, these data provide evidence for the H-bond group facilitating a core geometry change and allowing solvent to bind, stabilizing a LS state. The rR data, complemented by DFT analysis, reveal a stronger H-bonding interaction with the peroxo core in the LS compared to the HS complexes, which enthalpically favors the LS state. These insights enhance our fundamental understanding of secondary coordination sphere influences in metalloenzymes.

  View details for PubMedID 30836005

• Formylglycine-generating enzyme binds substrate directly at a mononuclear Cu(I) center to initiate O2 activation. Proceedings of the National Academy of Sciences of the United States of America Appel, M. J., Meier, K. K., Lafrance-Vanasse, J., Lim, H., Tsai, C., Hedman, B., Hodgson, K. O., Tainer, J. A., Solomon, E. I., Bertozzi, C. R. 2019

  Abstract

  The formylglycine-generating enzyme (FGE) is required for the posttranslational activation of type I sulfatases by oxidation of an active-site cysteine to Calpha-formylglycine. FGE has emerged as an enabling biotechnology tool due to the robust utility of the aldehyde product as a bioconjugation handle in recombinant proteins. Here, we show that Cu(I)-FGE is functional in O2 activation and reveal a high-resolution X-ray crystal structure of FGE in complex with its catalytic copper cofactor. We establish that the copper atom is coordinated by two active-site cysteine residues in a nearly linear geometry, supporting and extending prior biochemical and structural data. The active cuprous FGE complex was interrogated directly by X-ray absorption spectroscopy. These data unambiguously establish the configuration of the resting enzyme metal center and, importantly, reveal the formation of a three-coordinate tris(thiolate) trigonal planar complex upon substrate binding as furthermore supported by density functional theory (DFT) calculations. Critically, inner-sphere substrate coordination turns on O2 activation at the copper center. These collective results provide a detailed mechanistic framework for understanding why nature chose this structurally unique monocopper active site to catalyze oxidase chemistry for sulfatase activation.

  View details for PubMedID 30824597

• Resonant inelastic X-ray scattering determination of the electronic structure of oxyhemoglobin and its model complex PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Yan, J. J., Kroll, T., Baker, M. L., Wilson, S. A., Decreau, R., Lundberg, M., Sokaras, D., Glatzel, P., Hedman, B., Hodgson, K. O., Solomon, E. I. 2019; 116 (8): 2854–59

  View details for DOI 10.1073/pnas.1815981116

  View details for Web of Science ID 000459074400018

• Resonant inelastic X-ray scattering determination of the electronic structure of oxyhemoglobin and its model complex. Proceedings of the National Academy of Sciences of the United States of America Yan, J. J., Kroll, T., Baker, M. L., Wilson, S. A., Decreau, R., Lundberg, M., Sokaras, D., Glatzel, P., Hedman, B., Hodgson, K. O., Solomon, E. I. 2019

  Abstract

  Hemoglobin and myoglobin are oxygen-binding proteins with S = 0 heme {FeO2}8 active sites. The electronic structure of these sites has been the subject of much debate. This study utilizes Fe K-edge X-ray absorption spectroscopy (XAS) and 1s2p resonant inelastic X-ray scattering (RIXS) to study oxyhemoglobin and a related heme {FeO2}8 model compound, [(pfp)Fe(1-MeIm)(O2)] (pfp = meso-tetra(alpha,alpha,alpha,alpha-o-pivalamido-phenyl)porphyrin, or TpivPP, 1-MeIm = 1-methylimidazole) (pfpO2), which was previously analyzed using L-edge XAS. The K-edge XAS and RIXS data of pfpO2 and oxyhemoglobin are compared with the data for low-spin FeII and FeIII [Fe(tpp)(Im)2]0/+ (tpp = tetra-phenyl porphyrin) compounds, which serve as heme references. The X-ray data show that pfpO2 is similar to FeII, while oxyhemoglobin is qualitatively similar to FeIII, but with significant quantitative differences. Density-functional theory (DFT) calculations show that the difference between pfpO2 and oxyhemoglobin is due to a distal histidine H bond to O2 and the less hydrophobic environment in the protein, which lead to more backbonding into the O2 A valence bond configuration interaction multiplet model is used to analyze the RIXS data and show that pfpO2 is dominantly FeII with 6-8% FeIII character, while oxyhemoglobin has a very mixed wave function that has 50-77% FeIII character and a partially polarized Fe-O2 pi-bond.

  View details for PubMedID 30718404

• X-ray Absorption Spectroscopy as a Probe of Ligand Noninnocence in Metallocorroles: The Case of Copper Corroles. Inorganic chemistry Lim, H. n., Thomas, K. E., Hedman, B. n., Hodgson, K. O., Ghosh, A. n., Solomon, E. I. 2019

  Abstract

  The question of ligand noninnocence in Cu corroles has long been a topic of discussion. Presented herein is a Cu K-edge X-ray absorption spectroscopy (XAS) study, which provides a direct probe of the metal oxidation state, of three Cu corroles, Cu[TPC], Cu[Br8TPC], and Cu[(CF3)8TPC] (TPC = meso-triphenylcorrole), and the analogous Cu(II) porphyrins, Cu[TPP], Cu[Br8TPP], and Cu[(CF3)8TPP] (TPP = meso-tetraphenylporphyrin). The Cu K rising-edges of the Cu corroles were found to be about 0-1 eV upshifted relative to the analogous porphyrins, which is substantially lower than the 1-2 eV shifts typically exhibited by authentic Cu(II)/Cu(III) model complex pairs. In an unusual twist, the Cu K pre-edge regions of both the Cu corroles and the Cu porphyrins exhibit two peaks split by 0.8-1.3 eV. Based on time-dependent density functional theory calculations, the lower- and higher-energy peaks were assigned to a Cu 1s → 3d x2- y2 transition and a Cu 1s → corrole/porphyrin π* transition, respectively. From the Cu(II) porphyrins to the corresponding Cu corroles, the energy of the Cu 1s → 3d x2- y2 transition peak was found to upshift by 0.6-0.8 eV. This shift is approximately half that observed between Cu(II) to Cu(III) states for well-defined complexes. The Cu K-edge XAS spectra thus show that although the metal sites in the Cu corroles are more oxidized relative to those in their Cu(II) porphyrin analogues, they are not oxidized to the Cu(III) level, consistent with the notion of a noninnocent corrole. The relative importance of σ-donation versus corrole π-radical character is discussed.

  View details for PubMedID 31046257

• Geometric and Electronic Structural Contributions to Fe/O2 Reactivity. Bulletin of Japan Society of Coordination Chemistry Solomon, E. I., Iyer, S. R. 2019; 73: 3–14

  Abstract

  While two classes of non-heme iron enzymes use ferric centers to activate singlet organic substrates for the spin forbidden reaction with 3O2, most classes use high spin ferrous sites to activate dioxygen. These FeII active sites do not exhibit intense absorption bands and have an integer spin ground state thus are mostly EPR inactive. We have developed new spectroscopic methodologies that provide geometric and electronic structural insight into the ferrous centers and their interactions with cosubstrates for dioxygen activation and into the nature of the intermediates generated in these reactions. First, we present our variable-temperature variable-field magnetic circular dichroism (VTVH MCD) methodology to experimentally define the geometric and electronic structure of the high spin ferrous active site. Then, we focus on using Nuclear Resonance Vibrational Spectroscopy (NRVS, performed at SPring-8) to define geometric structure and VTVH MCD to define the electronic structure of the FeIII-OOH and FeIV=O intermediates generated in O2 activation and the spin state dependence of their frontier molecular orbitals (FMOs) in controlling reactivity. Experimentally validated reaction coordinates are derived for the anticancer drug bleomycin in its cleavage of DNA and for an alpha- ketoglutarate dependent dioxygenase in its selective halogenation over the thermodynamically favored hydroxylation of substrate.

  View details for DOI 10.4019/bjscc.73.3

  View details for PubMedID 32391114

• Heme-Cu Binucleating Ligand Supports Heme/O2 and FeII-CuI/O2 Reactivity Providing High- and Low-Spin FeIII-Peroxo-CuII Complexes. Inorganic chemistry Kim, H. n., Sharma, S. K., Schaefer, A. W., Solomon, E. I., Karlin, K. D. 2019

  Abstract

  The focus of this study is in the description of synthetic heme/copper/O2 chemistry employing a heme-containing binucleating ligand which provides a tridentate chelate for copper ion binding. The addition of O2 (-80 °C, tetrahydrofuran (THF) solvent) to the reduced heme compound (PImH)FeII (1), gives the oxy-heme adduct, formally a heme-superoxide complex FeIII-(O2•-) (2) (resonance Raman spectroscopy (rR): νO-O, 1171 cm-1 (Δ18O2, -61 cm-1); νFe-O, 575 cm-1 (Δ18O2, -24 cm-1)). Simple warming of 2 to room temperature regenerates reduced complex 1; this reaction is reversible, as followed by UV-vis spectroscopy. Complex 2 is electron paramagnetic resonance (EPR)-silent and exhibits upfield-shifted pyrrole resonances (δ 9.12 ppm) in 2H NMR spectroscopy, indicative of a six-coordinate low-spin heme. The coordination of the tethered imidazolyl arm to the heme-superoxide complex as an axial base ligand is suggested. We also report the new fully reduced heme-copper complex [(PImH)FeIICuI]+ (3), where the copper ion is bound to the tethered tridentate portion of PImH. This reacts with O2 to give a distinctive low-temperature-stable, high-spin (S = 2, overall) peroxo-bridged complex [(PImH)FeIII-(O22-)-CuII]+ (3a): λmax, 420 (Soret), 545, 565 nm; δpyrr, 93 ppm; νO-O, 799 cm-1 (Δ18O2, -48 cm-1); νFe-O, 524 cm-1 (Δ18O2, -23 cm-1). To 3a, the addition of dicyclohexylimidazole (DCHIm), which serves as a heme axial base, leads to low-spin (S = 0 overall) species complex [(DCHIm)(PImH)FeIII-(O22-)-CuII]+ (3b): λmax, 425 (Soret), 538 nm; δpyrr, 10.2 ppm; νO-O, 817 cm-1 (Δ18O2, -55 cm-1); νFe-O, 610 cm-1 (Δ18O2, -26 cm-1). These investigations into the characterization of the O2-adducts from (PImH)FeII (1) with/without additional copper chelation advance our understanding of the dioxygen reactivity of heme-only and heme/Cu-ligand heterobinuclear system, thus potentially relevant to O2 reduction in heme-copper oxidases or fuel-cell chemistry.

  View details for DOI 10.1021/acs.inorgchem.9b02521

  View details for PubMedID 31657921

• The Electronic Structure of the Metal Active Site Determines the Geometric Structure and Function of the Metalloregulator NikR. Biochemistry Ha, Y. n., Hu, H. n., Higgins, K. n., Maroney, M. n., Hedman, B. n., Hodgson, K. n., Solomon, E. n. 2019

  Abstract

  NikR is a nickel-responsive metalloregulator protein that controls the level of Ni2+ ions in living cells. Previous studies have shown that NikR can bind a series of first-row transition metal ions but binds to DNA with high affinity only as a Ni2+ complex. To understand this metal selectivity, S K-edge X-ray absorption spectroscopy of NikR bound to different metal ions was used to evaluate the different electronic structures. The experimental results are coupled with density functional theory calculations on relevant models. This study shows that both the Zeff of the metal ion and the donor nature of the ligands determine the electronic structure of the metal site. This impacts the geometric structure of the metal site and thus the conformation of the protein. This contribution of electronic structure to geometric structure can be extended to other metal selective metalloregulators.

  View details for DOI 10.1021/acs.biochem.9b00542

  View details for PubMedID 31339709

• Characterization of the Preprocessed Copper Site Equilibrium in Amine Oxidase and Assignment of the Reactive Copper Site in Topaquinone Biogenesis. Journal of the American Chemical Society Adelson, C. N., Johnston, E. M., Hilmer, K. M., Watts, H. n., Dey, S. G., Brown, D. E., Broderick, J. B., Shepard, E. M., Dooley, D. M., Solomon, E. I. 2019

  Abstract

  Copper-dependent amine oxidases produce their redox active cofactor, 2,4,5-trihydroxyphenylalanine quinone (TPQ), via the CuII-catalyzed oxygenation of an active site tyrosine. This study addresses possible mechanisms for this biogenesis process by presenting the geometric and electronic structure characterization of the CuII-bound, prebiogenesis (preprocessed) active site of the enzyme Arthrobacter globiformis amine oxidase (AGAO). CuII-loading into the preprocessed AGAO active site is slow ( kobs = 0.13 h-1), and is preceded by CuII binding in a separate kinetically favored site that is distinct from the active site. Preprocessed active site CuII is in a thermal equilibrium between two species, an entropically favored form with tyrosine protonated and unbound from the CuII site, and an enthalpically favored form with tyrosine bound deprotonated to the CuII active site. It is shown that the CuII-tyrosinate bound form is directly active in biogenesis. The electronic structure determined for the reactive form of the preprocessed CuII active site is inconsistent with a biogenesis pathway that proceeds through a CuI-tyrosyl radical intermediate, but consistent with a pathway that overcomes the spin forbidden reaction of 3O2 with the bound singlet substrate via a three-electron concerted charge-transfer mechanism.

  View details for DOI 10.1021/jacs.9b01922

  View details for PubMedID 31060358

• Geometric and Electronic Structure Contributions to O-O Cleavage and the Resultant Intermediate Generated in Heme-Copper Oxidases. Journal of the American Chemical Society Schaefer, A. W., Roveda, A. C., Jose, A. n., Solomon, E. I. 2019; 141 (25): 10068–81

  Abstract

  This study investigates the mechanism of O-O bond cleavage in heme-copper oxidase (HCO) enzymes, combining experimental and computational insights from enzyme intermediates and synthetic models. It is determined that HCOs undergo a proton-initiated O-O cleavage mechanism where a single water molecule in the active site enables proton transfer (PT) from the cross-linked tyrosine to a peroxo ligand bridging the heme FeIII and CuII, and multiple H-bonding interactions lower the tyrosine p Ka. Due to sterics within the active site, the proton must either transfer initially to the O(Fe) (a high-energy intermediate), or from another residue over a ∼10 Å distance to reach the O(Cu) atom directly. While the distance between the H+ donor (Tyr) and acceptor (O(Cu)) results in a barrier to PT, this separation is critical for the low barrier to O-O cleavage as it enhances backbonding from Fe into the O22- σ* orbital. Thus, PT from Tyr precedes O-O elongation and is rate-limiting, consistent with available kinetic data. The electron transfers from tyrosinate after the barrier via a superexchange pathway provided by the cross-link, generating intermediate PM. PM is evaluated using available experimental data. The geometric structure contains an FeIV═O that is H-bonded to the CuII-OH. The electronic structure is a singlet, where the FeIV and CuII are antiferromagnetically coupled through the H-bond between the oxo(Fe) and hydroxo(Cu) ligands, while the CuII and Tyr• are ferromagnetically coupled due their delocalization into orthogonal magnetic orbitals on the cross-linked His residue. These findings provide critical insights into the mechanism of efficient O2 reduction in HCOs, and the nature of the PM intermediate that couples this reaction to proton pumping.

  View details for DOI 10.1021/jacs.9b04271

  View details for PubMedID 31146528

• Ligand Identity-Induced Generation of Enhanced Oxidative Hydrogen Atom Transfer Reactivity for a CuII2(O2•-) Complex Driven by Formation of a CuII2(-OOH) Compound with a Strong O-H Bond. Journal of the American Chemical Society Quist, D. A., Ehudin, M. A., Schaefer, A. W., Schneider, G. L., Solomon, E. I., Karlin, K. D. 2019

  Abstract

  A superoxide-bridged dicopper(II) complex, [CuII2(XYLO)(O2•-)]2+ (1) (XYLO = binucleating m-xylyl derivative with a bridging phenolate ligand donor and two bis(2-{2-pyridyl}ethyl)amine arms), was generated from chemical oxidation of the peroxide-bridged dicopper(II) complex [CuII2(XYLO)(O22-)]+ (2), using ferrocenium (Fc+) derivatives, in 2-methyltetrahydrofuran (MeTHF) at -125 °C. Using Me10Fc+, a 1 ⇆ 2 equilibrium was established, allowing for calculation of the reduction potential of 1 as -0.525 ± 0.01 V vs Fc+/0. Addition of 1 equiv of strong acid to 2 afforded the hydroperoxide-bridged dicopper(II) species [CuII2(XYLO)(OOH)]2+ (3). An acid-base equilibrium between 3 and 2 was achieved through spectral titrations using a derivatized phosphazene base. The pKa of 3 was thus determined to be 24 ± 0.6 in MeTHF at -125 °C. Using a thermodynamic square scheme and the Bordwell relationship, the hydroperoxo complex (3) O-H bond dissociation free energy (BDFE) was calculated as 81.8 ± 1.5 (BDE = 86.8) kcal/mol. The observed oxidizing capability of [CuII2(XYLO)(O2•-)]2+ (1), as demonstrated in H atom abstraction reactions with certain phenolic ArO-H and hydrocarbon C-H substrates, provides direct support for this experimentally determined O-H BDFE. A kinetic study reveals a very fast reaction of TEMPO-H with 1 in MeTHF, with k (-100 °C) = 5.6 M-1 s-1. Density functional theory (DFT) calculations reveal how the structure of 1 may minimize stabilization of the superoxide moiety, resulting in its enhanced reactivity. The thermodynamic insights obtained herein highlight the importance of the interplay between ligand design and the generation and properties of copper (or other metal ion) bound O2-derived reduced species, such as pKa, reduction potential, and BDFE; these may be relevant to the capabilities (i.e., oxidizing power) of reactive oxygen intermediates in metalloenzyme chemical system mediated oxidative processes.

  View details for DOI 10.1021/jacs.9b05277

  View details for PubMedID 31299154

• Impact of Intramolecular Hydrogen Bonding on the Reactivity of Cupric Superoxide Complexes with O-H and C-H Substrates. Angewandte Chemie (International ed. in English) Diaz, D. E., Quist, D. A., Herzog, A. E., Schaefer, A. W., Kipouros, I. n., Bhadra, M. n., Solomon, E. I., Karlin, K. D. 2019

  Abstract

  A series of TMPA-based copper(I) complexes (TMPA ≡ tris(2-pyridylmethyl)amine), with and without secondary coordination sphere hydrogen-bonding moieties, were prepared and reactivity with O2 at -135 °C in 2-methyltetrahydrofuran (MeTHF) was studied. H-bonding moieties are demonstrated to play a crucial role in the kinetic stabilization of [((X1)(X2)TMPA)CuII(O2•-)]+ cupric superoxide species, sustaining these primary copper-dioxygen compounds rather than subsequent secondary dicopper-dioxygen adducts. Support for the presence of H-bonding to the Cu-O-O•- superoxide O-atom(s) comes from resonance Raman (rR) spectroscopy, analogy to azido analogues [((X1)(X2)TMPA)CuII(N3-)]+, and the alternative O2 reactivity behavior of ligand-CuI complexes when an H-bonding modality is replaced by a methyl group. A dramatic enhancement in cupric superoxide reactivity towards phenolic substrates, as well as oxidation of substrates possessing moderate C-H BDEs is observed, correlating with the number and strength of the H-bonding groups.

  View details for DOI 10.1002/anie.201908471

  View details for PubMedID 31469942

• Tuning the Geometric and Electronic Structure of Synthetic High-Valent Heme Iron(IV)-Oxo Models in the Presence of a Lewis Acid and Various Axial Ligands. Journal of the American Chemical Society Ehudin, M. A., Gee, L. B., Sabuncu, S. n., Braun, A. n., Moënne-Loccoz, P. n., Hedman, B. n., Hodgson, K. O., Solomon, E. I., Karlin, K. D. 2019; 141 (14): 5942–60

  Abstract

  High-valent ferryl species (e.g., (Por)FeIV═O, Cmpd-II) are observed or proposed key oxidizing intermediates in the catalytic cycles of heme-containing enzymes (P-450s, peroxidases, catalases, and cytochrome c oxidase) involved in biological respiration and oxidative metabolism. Herein, various axially ligated iron(IV)-oxo complexes were prepared to examine the influence of the identity of the base. These were generated by addition of various axial ligands (1,5-dicyclohexylimidazole (DCHIm), a tethered-imidazole system, and sodium derivatives of 3,5-dimethoxyphenolate and imidazolate). Characterization was carried out via UV-vis, electron paramagnetic resonance (EPR), 57Fe Mössbauer, Fe X-ray absorption (XAS), and 54/57Fe resonance Raman (rR) spectroscopies to confirm their formation and compare the axial ligand perturbation on the electronic and geometric structures of these heme iron(IV)-oxo species. Mössbauer studies confirmed that the axially ligated derivatives were iron(IV) and six-coordinate complexes. XAS and 54/57Fe rR data correlated with slight elongation of the iron-oxo bond with increasing donation from the axial ligands. The first reported synthetic H-bonded iron(IV)-oxo heme systems were made in the presence of the protic Lewis acid, 2,6-lutidinium triflate (LutH+), with (or without) DCHIm. Mössbauer, rR, and XAS spectroscopic data indicated the formation of molecular Lewis acid ferryl adducts (rather than full protonation). The reduction potentials of these novel Lewis acid adducts were bracketed through addition of outer-sphere reductants. The oxidizing capabilities of the ferryl species with or without Lewis acid vary drastically; addition of LutH+ to F8Cmpd-II (F8 = tetrakis(2,6-difluorophenyl)porphyrinate) increased its reduction potential by more than 890 mV, experimentally confirming that H-bonding interactions can increase the reactivity of ferryl species.

  View details for PubMedID 30860832

• Nuclear Resonance Vibrational Spectroscopy Definition of O-2 Intermediates in an Extradiol Dioxygenase: Correlation to Crystallography and Reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sutherlin, K. D., Wasada-Tsutsui, Y., Mbughuni, M. M., Rogers, M. S., Park, K., Liu, L., Kwak, Y., Srnec, M., Bottger, L. H., Frenette, M., Yoda, Y., Kobayashi, Y., Kurokuzu, M., Saito, M., Seto, M., Hu, M., Zhao, J., Alp, E., Lipscomb, J. D., Solomon, E. 2018; 140 (48): 16495–513

  View details for DOI 10.1021/jacs.8b06517

  View details for Web of Science ID 000452693800021

• Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Snyder, B. R., Bols, M. L., Rhoda, H. M., Vanelderen, P., Bottger, L. H., Braun, A., Yan, J. J., Hadt, R. G., Babicz, J. T., Hu, M. Y., Zhao, J., Alp, E., Hedman, B., Hodgson, K. O., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2018; 115 (48): 12124–29

  View details for DOI 10.1073/pnas.1813849115

  View details for Web of Science ID 000451351000043

• Nuclear Resonance Vibrational Spectroscopy Definition of O2 Intermediates in an Extradiol Dioxygenase: Correlation to Crystallography and Reactivity. Journal of the American Chemical Society Sutherlin, K. D., Wasada-Tsutsui, Y., Mbughuni, M. M., Rogers, M. S., Park, K., Liu, L. V., Kwak, Y., Srnec, M., Bottger, L. H., Frenette, M., Yoda, Y., Kobayashi, Y., Kurokuzu, M., Saito, M., Seto, M., Hu, M., Zhao, J., Alp, E. E., Lipscomb, J. D., Solomon, E. I. 2018

  Abstract

  The extradiol dioxygenases are a large subclass of mononuclear nonheme Fe enzymes that catalyze the oxidative cleavage of catechols distal to their OH groups. These enzymes are important in bioremediation, and there has been significant interest in understanding how they activate O2. The extradiol dioxygenase homoprotocatechuate 2,3-dioxygenase (HPCD) provides an opportunity to study this process, as two O2 intermediates have been trapped and crystallographically defined using the slow substrate 4-nitrocatechol (4NC): a side-on Fe-O2-4NC species and a Fe-O2-4NC peroxy bridged species. Also with 4NC, two solution intermediates have been trapped in the H200N variant, where H200 provides a second-sphere hydrogen bond in the wild-type enzyme. While the electronic structure of these solution intermediates has been defined previously as FeIII-superoxo-catecholate and FeIII-peroxy-semiquinone, their geometric structures are unknown. Nuclear resonance vibrational spectroscopy (NRVS) is an important tool for structural definition of nonheme Fe-O2 intermediates, as all normal modes with Fe displacement have intensity in the NRVS spectrum. In this study, NRVS is used to define the geometric structure of the H200N-4NC solution intermediates in HPCD as an end-on FeIII-superoxo-catecholate and an end-on FeIII-hydroperoxo-semiquinone. Parallel calculations are performed to define the electronic structures and protonation states of the crystallographically defined wild-type HPCD-4NC intermediates, where the side-on intermediate is found to be a FeIII-hydroperoxo-semiquinone. The assignment of this crystallographic intermediate is validated by correlation to the NRVS data through computational removal of H200. While the side-on hydroperoxo semiquinone intermediate is computationally found to be nonreactive in peroxide bridge formation, it is isoenergetic with a superoxo catecholate species that is competent in performing this reaction. This study provides insight into the relative reactivities of FeIII-superoxo and FeIII-hydroperoxo intermediates in nonheme Fe enzymes and into the role H200 plays in facilitating extradiol catalysis.

  View details for PubMedID 30418018

• Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites. Proceedings of the National Academy of Sciences of the United States of America Snyder, B. E., Bols, M. L., Rhoda, H. M., Vanelderen, P., Bottger, L. H., Braun, A., Yan, J. J., Hadt, R. G., Babicz, J. T., Hu, M. Y., Zhao, J., Alp, E. E., Hedman, B., Hodgson, K. O., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2018

  Abstract

  A direct, catalytic conversion of benzene to phenol would have wide-reaching economic impacts. Fe zeolites exhibit a remarkable combination of high activity and selectivity in this conversion, leading to their past implementation at the pilot plant level. There were, however, issues related to catalyst deactivation for this process. Mechanistic insight could resolve these issues, and also provide a blueprint for achieving high performance in selective oxidation catalysis. Recently, we demonstrated that the active site of selective hydrocarbon oxidation in Fe zeolites, named alpha-O, is an unusually reactive Fe(IV)=O species. Here, we apply advanced spectroscopic techniques to determine that the reaction of this Fe(IV)=O intermediate with benzene in fact regenerates the reduced Fe(II) active site, enabling catalytic turnover. At the same time, a small fraction of Fe(III)-phenolate poisoned active sites form, defining a mechanism for catalyst deactivation. Density-functional theory calculations provide further insight into the experimentally defined mechanism. The extreme reactivity of alpha-O significantly tunes down (eliminates) the rate-limiting barrier for aromatic hydroxylation, leading to a diffusion-limited reaction coordinate. This favors hydroxylation of the rapidly diffusing benzene substrate over the slowly diffusing (but more reactive) oxygenated product, thereby enhancing selectivity. This defines a mechanism to simultaneously attain high activity (conversion) and selectivity, enabling the efficient oxidative upgrading of inert hydrocarbon substrates.

  View details for PubMedID 30429333

• Spectroscopic and Electronic Structure Study of ETHE1: Elucidating the Factors Influencing Sulfur Oxidation and Oxygenation in Mononuclear Nonheme Iron Enzymes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Goudarzi, S., Babicz, J. T., Kabil, O., Banerjee, R., Solomon, E. I. 2018; 140 (44): 14887–902

  View details for DOI 10.1021/jacs.8b09022

  View details for Web of Science ID 000449887800049

• Spectroscopic and Electronic Structure Study of ETHE1: Elucidating the Factors Influencing Sulfur Oxidation and Oxygenation in Mononuclear Nonheme Iron Enzymes. Journal of the American Chemical Society Goudarzi, S., Babicz, J. T., Kabil, O., Banerjee, R., Solomon, E. I. 2018

  Abstract

  ETHE1 is a member of a growing subclass of nonheme Fe enzymes that catalyzes transformations of sulfur-containing substrates without a cofactor. ETHE1 dioxygenates glutathione persulfide (GSSH) to glutathione (GSH) and sulfite in a reaction which is similar to that of cysteine dioxygenase (CDO), but with monodentate (vs bidentate) substrate coordination and a 2-His/1-Asp (vs 3-His) ligand set. In this study, we demonstrate that GSS- binds directly to the iron active site, causing coordination unsaturation to prime the site for O2 activation. Nitrosyl complexes without and with GSSH were generated and spectroscopically characterized as unreactive analogues for the invoked ferric superoxide intermediate. New spectral features from persulfide binding to the FeIII include the appearance of a low-energy FeIII ligand field transition, an energy shift of a NO- to FeIII CT transition, and two new GSS- to FeIII CT transitions. Time-dependent density functional theory calculations were used to simulate the experimental spectra to determine the persulfide orientation. Correlation of these spectral features with those of monodentate cysteine binding in isopenicillin N synthase (IPNS) shows that the persulfide is a poorer donor but still results in an equivalent frontier molecular orbital for reactivity. The ETHE1 persulfide dioxygenation reaction coordinate was calculated, and while the initial steps are similar to the reaction coordinate of CDO, an additional hydrolysis step is required in ETHE1 to break the S-S bond. Unlike ETHE1 and CDO, which both oxygenate sulfur, IPNS oxidizes sulfur through an initial H atom abstraction. Thus, factors that determine oxygenase vs oxidase reactivity were evaluated. In general, sulfur oxygenation is thermodynamically favored and has a lower barrier for reactivity. However, in IPNS, second-sphere residues in the active site pocket constrain the substrate, raising the barrier for sulfur oxygenation relative to oxidation via H atom abstraction.

  View details for PubMedID 30362717

• Spectroscopic Identification of the alpha-Fe/alpha-O Active Site in Fe-CHA Zeolite for the Low-Temperature Activation of the Methane C-H Bond JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Bols, M. L., Hallaert, S. D., Snyder, B. R., Devos, J., Plessers, D., Rhoda, H. M., Dusselier, M., Schoonheydt, R. A., Pierloot, K., Solomon, E., Sels, B. F. 2018; 140 (38): 12021–32

  Abstract

  The formation of single-site α-Fe in the CHA zeolite topology is demonstrated. The site is shown to be active in oxygen atom abstraction from N2O to form a highly reactive α-O, capable of methane activation at room temperature to form methanol. The methanol product can subsequently be desorbed by online steaming at 200 °C. For the intermediate steps of the reaction cycle, the evolution of the Fe active site is monitored by UV-vis-NIR and Mössbauer spectroscopy. A B3LYP-DFT model of the α-Fe site in CHA is constructed, and the ligand field transitions are calculated by CASPT2. The model is experimentally substantiated by the preferential formation of α-Fe over other Fe species, the requirement of paired framework aluminum and a MeOH/Fe ratio indicating a mononuclear active site. The simple CHA topology is shown to mitigate the heterogeneity of iron speciation found on other Fe-zeolites, with Fe2O3 being the only identifiable phase other than α-Fe formed in Fe-CHA. The α-Fe site is formed in the d6r composite building unit, which occurs frequently across synthetic and natural zeolites. Finally, through a comparison between α-Fe in Fe-CHA and Fe-*BEA, the topology's 6MR geometry is found to influence the structure, the ligand field, and consequently the spectroscopy of the α-Fe site in a predictable manner. Variations in zeolite topology can thus be used to rationally tune the active site properties.

  View details for PubMedID 30169036

• O-2 Activation by Nonheme Fe-II alpha-Ketoglutarate-Dependent Enzyme Variants: Elucidating the Role of the Facial Triad Carboxylate in FIH JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Iyer, S. R., Chaplin, V. D., Knapp, M. J., Solomon, E. I. 2018; 140 (37): 11777–83

  View details for DOI 10.1021/jacs.8b07277

  View details for Web of Science ID 000445439700035

• A mononuclear nonheme {FeNO}(6) complex: synthesis and structural and spectroscopic characterization CHEMICAL SCIENCE Hong, S., Yan, J. J., Karmalkar, D. G., Sutherlin, K. D., Kim, J., Lee, Y., Goo, Y., Mascharak, P. K., Hedman, B., Hodgson, K. O., Karlin, K. D., Solomon, E. I., Nam, W. 2018; 9 (34): 6952–60

  Abstract

  While the synthesis and characterization of {FeNO}7,8,9 complexes have been well documented in heme and nonheme iron models, {FeNO}6 complexes have been less clearly understood. Herein, we report the synthesis and structural and spectroscopic characterization of mononuclear nonheme {FeNO}6 and iron(iii)-nitrito complexes bearing a tetraamido macrocyclic ligand (TAML), such as [(TAML)FeIII(NO)]- and [(TAML)FeIII(NO2)]2-, respectively. First, direct addition of NO(g) to [FeIII(TAML)]- results in the formation of [(TAML)FeIII(NO)]-, which is sensitive to moisture and air. The spectroscopic data of [(TAML)FeIII(NO)]-, such as 1H nuclear magnetic resonance and X-ray absorption spectroscopies, combined with computational study suggest the neutral nature of nitric oxide with a diamagnetic Fe center (S = 0). We also provide alternative pathways for the generation of [(TAML)FeIII(NO)]-, such as the iron-nitrite reduction triggered by protonation in the presence of ferrocene, which acts as an electron donor, and the photochemical iron-nitrite reduction. To the best of our knowledge, the present study reports the first photochemical nitrite reduction in nonheme iron models.

  View details for PubMedID 30210769

  View details for PubMedCentralID PMC6124912

• O2 Activation by Nonheme FeII alpha-Ketoglutarate-Dependent Enzyme Variants: Elucidating the Role of the Facial Triad Carboxylate in FIH. Journal of the American Chemical Society Iyer, S. R., Chaplin, V. D., Knapp, M. J., Solomon, E. I. 2018

  Abstract

  FIH [factor inhibiting HIF (hypoxia inducible factor)] is an alpha-ketoglutarate (alphaKG)-dependent nonheme iron enzyme that catalyzes the hydroxylation of the C-terminal transactivation domain (CAD) asparagine residue in HIF-1alpha to regulate cellular oxygen levels. The role of the facial triad carboxylate ligand in O2 activation and catalysis was evaluated by replacing the Asp201 residue with Gly (D201G), Ala (D201A), and Glu (D201E). Magnetic circular dichroism (MCD) spectroscopy showed that the (FeII)FIH variants were all 6-coordinate (6C) and the alphaKG plus CAD bound FIH variants were all 5-coordinate (5C), mirroring the behavior of the wild-type ( wt) enzyme. When only alphaKG is bound, all FIH variants exhibited weaker FeII-OH2 bonds for the sixth ligand compared to wt, and for alphaKG-bound D201E this is either extremely weak or the site is 5C, demonstrating that the Asp201 residue plays an important role in the wt enzyme in ensuring that the (FeII/alphaKG)FIH site remains 6C. Variable-temperature, variable-field (VTVH) MCD spectroscopy showed that all of the alphaKG- and CAD-bound FIH variants, though 5C, have different ground-state geometric and electronic structures, which impair their oxygen activation rates. Comparison of O2 consumption to substrate hydroxylation kinetics revealed uncoupling between the two half reactions in the variants. Thus, the Asp201 residue also ensures fidelity between CAD substrate binding and oxygen activation, enabling tightly coupled turnover.

  View details for PubMedID 30148961

• An intravascular magnetic wire for the high-throughput retrieval of circulating tumour cells in vivo NATURE BIOMEDICAL ENGINEERING Vermesh, O., Aalipour, A., Ge, T., Saenz, Y., Guo, Y., Alam, I. S., Park, S., Adelson, C. N., Mitsutake, Y., Vilches-Moure, J., Godoy, E., Bachmann, M. H., Ooi, C., Lyons, J. K., Mueller, K., Arami, H., Green, A., Solomon, E., Wang, S. X., Gambhir, S. S. 2018; 2 (9): 696–705

  View details for DOI 10.1038/s41551-018-0257-3

  View details for Web of Science ID 000444282800014

• An intravascular magnetic wire for the high-throughput retrieval of circulating tumour cells in vivo. Nature biomedical engineering Vermesh, O., Aalipour, A., Ge, T. J., Saenz, Y., Guo, Y., Alam, I. S., Park, S. M., Adelson, C. N., Mitsutake, Y., Vilches-Moure, J., Godoy, E., Bachmann, M. H., Ooi, C. C., Lyons, J. K., Mueller, K., Arami, H., Green, A., Solomon, E. I., Wang, S. X., Gambhir, S. S. 2018; 2 (9): 696-705

  Abstract

  The detection and analysis of rare blood biomarkers is necessary for early diagnosis of cancer and to facilitate the development of tailored therapies. However, current methods for the isolation of circulating tumour cells (CTCs) or nucleic acids present in a standard clinical sample of only 5-10 ml of blood provide inadequate yields for early cancer detection and comprehensive molecular profiling. Here, we report the development of a flexible magnetic wire that can retrieve rare biomarkers from the subject's blood in vivo at a much higher yield. The wire is inserted and removed through a standard intravenous catheter and captures biomarkers that have been previously labelled with injected magnetic particles. In a proof-of-concept experiment in a live porcine model, we demonstrate the in vivo labelling and single-pass capture of viable model CTCs in less than 10 s. The wire achieves capture efficiencies that correspond to enrichments of 10-80 times the amount of CTCs in a 5-ml blood draw, and 500-5,000 times the enrichments achieved using the commercially available Gilupi CellCollector.

  View details for DOI 10.1038/s41551-018-0257-3

  View details for PubMedID 30505627

  View details for PubMedCentralID PMC6261517

• Activating metal sites for biological electron transfer Solomon, E. AMER CHEMICAL SOC. 2018

  View details for Web of Science ID 000447609103504

• Kinetic and spectroscopic investigation of oxygen activation at a single iron center via Gibbs free energy coupling: Generation of an active alkane oxidation catalyst Cunningham, L., Babicz, J., Tucker, W., McCracken, J., Rybak-Akimova, E., Solomon, E., Caradonna, J. AMER CHEMICAL SOC. 2018

  View details for Web of Science ID 000447609101690

• Intramolecular Hydrogen Bonding Enhances Stability and Reactivity of Mononuclear Cupric Superoxide Complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Bhadra, M., Lee, J. C., Cowley, R. E., Kim, S., Siegler, M. A., Solomon, E., Karlin, K. D. 2018; 140 (29): 9042–45

  Abstract

  [(L)CuII(O2•-)]+ (i.e., cupric-superoxo) complexes, as the first and/or key reactive intermediates in (bio)chemical Cu-oxidative processes, including in the monooxygenases PHM and DβM, have been systematically stabilized by intramolecular hydrogen bonding within a TMPA ligand-based framework. Also, gradual strengthening of ligand-derived H-bonding dramatically enhances the [(L)CuII(O2•-)]+ reactivity toward hydrogen-atom abstraction (HAA) of phenolic O-H bonds. Spectroscopic properties of the superoxo complexes and their azido analogues, [(L)CuII(N3-)]+, also systematically change as a function of ligand H-bonding capability.

  View details for PubMedID 29957998

• Second-Sphere Effects on Methane Hydroxylation in Cu-Zeolites JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Snyder, B. R., Vanelderen, P., Schoonheydt, R. A., Sels, B. F., Solomon, E. 2018; 140 (29): 9236–43

  View details for DOI 10.1021/jacs.8b05320

  View details for Web of Science ID 000440515200038

• Second-Sphere Effects on Methane Hydroxylation in Cu-Zeolites. Journal of the American Chemical Society Snyder, B. E., Vanelderen, P., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2018

  Abstract

  Two [Cu2O]2+ cores have been identified as the active sites of low temperature methane hydroxylation in the zeolite Cu-MOR. These cores have similar geometric and electronic structures, yet different reactivity with CH4: one reacts with a much lower activation enthalpy. In the present study, we couple experimental reactivity and spectroscopy studies to DFT calculations to arrive at structural models of the Cu-MOR active sites. We find that the more reactive core is located in a constricted region of the zeolite lattice. This leads to close van der Waals contact between the substrate and the zeolite lattice in the vicinity of the active site. The resulting enthalpy of substrate adsorption drives the subsequent H atom abstraction step-a manifestation of the "nest" effect seen in hydrocarbon cracking on acid zeolites. This defines a mechanism to tune the reactivity of metal active sites in microporous materials.

  View details for PubMedID 29954176

• Oxidation of Naphthalene with a Manganese(IV) Bis(hydroxo) Complex in the Presence of Acid ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Jeong, D., Yan, J. J., Noh, H., Hedman, B., Hodgson, K. O., Solomon, E. I., Cho, J. 2018; 57 (26): 7764–68

  Abstract

  Naphthalene oxidation with metal-oxygen intermediates is a difficult reaction in environmental and biological chemistry. Herein, we report that a MnIV bis(hydroxo) complex, which was fully characterized by various physicochemical methods, such as ESI-MS, UV/Vis, and EPR analysis, X-ray diffraction, and XAS, can be employed for the oxidation of naphthalene in the presence of acid to afford 1,4-naphthoquinone. Redox titration of the MnIV bis(hydroxo) complex gave a one-electron reduction potential of 1.09 V, which is the most positive potential for all reported nonheme MnIV bis(hydroxo) species as well as MnIV oxo analogues. Kinetic studies, including kinetic isotope effect analysis, suggest that the naphthalene oxidation occurs through a rate-determining electron transfer process.

  View details for PubMedID 29701293

  View details for PubMedCentralID PMC6013404

• Structural characterization of a non-heme iron active site in zeolites that hydroxylates methane PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Snyder, B. R., Bottger, L. H., Bols, M. L., Yan, J. J., Rhoda, H. M., Jacobs, A. B., Hu, M. Y., Zhao, J., Alp, E., Hedman, B., Hodgson, K. O., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2018; 115 (18): 4565–70

  Abstract

  Iron-containing zeolites exhibit unprecedented reactivity in the low-temperature hydroxylation of methane to form methanol. Reactivity occurs at a mononuclear ferrous active site, α-Fe(II), that is activated by N2O to form the reactive intermediate α-O. This has been defined as an Fe(IV)=O species. Using nuclear resonance vibrational spectroscopy coupled to X-ray absorption spectroscopy, we probe the bonding interaction between the iron center, its zeolite lattice-derived ligands, and the reactive oxygen. α-O is found to contain an unusually strong Fe(IV)=O bond resulting from a constrained coordination geometry enforced by the zeolite lattice. Density functional theory calculations clarify how the experimentally determined geometric structure of the active site leads to an electronic structure that is highly activated to perform H-atom abstraction.

  View details for PubMedID 29610304

• NRVS Studies of the Peroxide Shunt Intermediate in a Rieske Dioxygenase and Its Relation to the Native Fe-II O-2 Reaction JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sutherlin, K. D., Rivard, B. S., Bottger, L. H., Liu, L. V., Rogers, M. S., Srnec, M., Park, K., Yoda, Y., Kitao, S., Kobayashi, Y., Saito, M., Seto, M., Hu, M., Zhao, J., Lipscomb, J. D., Solomon, E. I. 2018; 140 (16): 5544–59

  Abstract

  The Rieske dioxygenases are a major subclass of mononuclear nonheme iron enzymes that play an important role in bioremediation. Recently, a high-spin FeIII-(hydro)peroxy intermediate (BZDOp) has been trapped in the peroxide shunt reaction of benzoate 1,2-dioxygenase. Defining the structure of this intermediate is essential to understanding the reactivity of these enzymes. Nuclear resonance vibrational spectroscopy (NRVS) is a recently developed synchrotron technique that is ideal for obtaining vibrational, and thus structural, information on Fe sites, as it gives complete information on all vibrational normal modes containing Fe displacement. In this study, we present NRVS data on BZDOp and assign its structure using these data coupled to experimentally calibrated density functional theory calculations. From this NRVS structure, we define the mechanism for the peroxide shunt reaction. The relevance of the peroxide shunt to the native FeII/O2 reaction is evaluated. For the native FeII/O2 reaction, an FeIII-superoxo intermediate is found to react directly with substrate. This process, while uphill thermodynamically, is found to be driven by the highly favorable thermodynamics of proton-coupled electron transfer with an electron provided by the Rieske [2Fe-2S] center at a later step in the reaction. These results offer important insight into the relative reactivities of FeIII-superoxo and FeIII-hydroperoxo species in nonheme Fe biochemistry.

  View details for PubMedID 29618204

• Introduction: Oxygen Reduction and Activation in Catalysis CHEMICAL REVIEWS Solomon, E. L., Stahl, S. S. 2018; 118 (5): 2299–2301

  View details for PubMedID 29534575

• Oxygen Activation by Cu LPMOs in Recalcitrant Carbohydrate Polysaccharide Conversion to Monomer Sugars CHEMICAL REVIEWS Meier, K. K., Jones, S. M., Kaper, T., Hansson, H., Koetsier, M. J., Karkehabadi, S., Solomon, E. I., Sandgren, M., Kelemen, B. 2018; 118 (5): 2593–2635

  Abstract

  Natural carbohydrate polymers such as starch, cellulose, and chitin provide renewable alternatives to fossil fuels as a source for fuels and materials. As such, there is considerable interest in their conversion for industrial purposes, which is evidenced by the established and emerging markets for products derived from these natural polymers. In many cases, this is achieved via industrial processes that use enzymes to break down carbohydrates to monomer sugars. One of the major challenges facing large-scale industrial applications utilizing natural carbohydrate polymers is rooted in the fact that naturally occurring forms of starch, cellulose, and chitin can have tightly packed organizations of polymer chains with low hydration levels, giving rise to crystalline structures that are highly recalcitrant to enzymatic degradation. The topic of this review is oxidative cleavage of carbohydrate polymers by lytic polysaccharide mono-oxygenases (LPMOs). LPMOs are copper-dependent enzymes (EC 1.14.99.53-56) that, with glycoside hydrolases, participate in the degradation of recalcitrant carbohydrate polymers. Their activity and structural underpinnings provide insights into biological mechanisms of polysaccharide degradation.

  View details for PubMedID 29155571

  View details for PubMedCentralID PMC5982588

• Iron and Copper Active Sites in Zeolites and Their Correlation to Metalloenzymes CHEMICAL REVIEWS Snyder, B. R., Bols, M. L., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2018; 118 (5): 2718–68

  Abstract

  Metal-exchanged zeolites are a class of heterogeneous catalysts that perform important functions ranging from selective hydrocarbon oxidation to remediation of NO x pollutants. Among these, copper and iron zeolites are remarkably reactive, hydroxylating methane and benzene selectively at low temperature to form methanol and phenol, respectively. In these systems, reactivity occurs at well-defined molecular transition metal active sites, and in this review we discuss recent advances in the spectroscopic characterization of these active sites and their reactive intermediates. Site-selective spectroscopy continues to play a key role, making it possible to focus on active sites that exist within a distribution of inactive spectator metal centers. The definition of the geometric and electronic structures of metallozeolites has advanced to the level of bioinorganic chemistry, enabling direct comparison of metallozeolite active sites to functionally analogous Fe and Cu sites in biology. We identify significant parallels and differences in the strategies used by each to achieve high reactivity, highlighting potentially interesting mechanisms to tune the performance of synthetic catalysts.

  View details for PubMedID 29256242

• An intravascular magnetic wire for the high-throughput retrieval of circulating tumour cells in vivo. Nature biomedical engineering Vermesh, O., Aalipour, A., Ge, T. J., Saenz, Y., Guo, Y., Alam, I. S., Park, S., Adelson, C. N., Mitsutake, Y., Vilches-Moure, J., Godoy, E., Bachmann, M., Ooi, C. C., Lyons, J. K., Mueller, K., Arami, H., Green, A., Solomon, E. I., Wang, S. X., Gambhir, S. S. 2018; 2: 696–705

  Abstract

  The detection and analysis of rare blood biomarkers is necessary for early cancer diagnosis and to facilitate the development of tailored therapies. However, current methods for the isolation of circulating tumor cells (CTCs) or nucleic acids present in a standard clinical sample of only 5-10 mL of blood provide inadequate yields for early cancer detection and comprehensive molecular profiling. We have developed a flexible magnetic wire that can retrieve rare biomarkers from the subject's blood in vivo at a much higher yield. The wire is inserted and removed through a standard intravenous catheter and captures biomarkers that have been previously labeled with injected magnetic particles. In a proof-of-concept experiment in a live porcine model, we demonstrate the in vivo labeling and single-pass capture of viable model CTCs in less than 10 seconds. The wire achieves capture efficiencies that correspond to enrichments of 10-80 times the amount of CTCs in a 5-mL blood draw, and to 500-5,000 times the enrichments achieved by the commercially available Gilupi CellCollector.

  View details for PubMedID 30524876

• L-edge spectroscopy of dilute, radiation-sensitive systems using a transition-edge-sensor array JOURNAL OF CHEMICAL PHYSICS Titus, C. J., Baker, M. L., Lee, S., Cho, H., Doriese, W. B., Fowler, J. W., Gaffney, K., Gard, J. D., Hilton, G. C., Kenney, C., Knight, J., Li, D., Marks, R., Minitti, M. P., Morgan, K. M., O'Neil, G. C., Reintsema, C. D., Schmidt, D. R., Sokaras, D., Swetz, D. S., Ullom, J. N., Weng, T., Williams, C., Young, B. A., Irwin, K. D., Solomon, E. I., Nordlund, D. 2017; 147 (21): 214201

  Abstract

  We present X-ray absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) measurements on the iron L-edge of 0.5 mM aqueous ferricyanide. These measurements demonstrate the ability of high-throughput transition-edge-sensor (TES) spectrometers to access the rich soft X-ray (100-2000 eV) spectroscopy regime for dilute and radiation-sensitive samples. Our low-concentration data are in agreement with high-concentration measurements recorded by grating spectrometers. These results show that soft-X-ray RIXS spectroscopy acquired by high-throughput TES spectrometers can be used to study the local electronic structure of dilute metal-centered complexes relevant to biology, chemistry, and catalysis. In particular, TES spectrometers have a unique ability to characterize frozen solutions of radiation- and temperature-sensitive samples.

  View details for PubMedID 29221417

  View details for PubMedCentralID PMC5720893

• A Six-Coordinate Peroxynitrite Low-Spin Iron(III) Porphyrinate Complex-The Product of the Reaction of Nitrogen Monoxide (center dot NO(g)) with a Ferric-Superoxide Species JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sharma, S. K., Schaefer, A. W., Lim, H., Matsumura, H., Moenne-Loccoz, P., Hedman, B., Hodgson, K. O., Solomon, E. I., Karlin, K. D. 2017; 139 (48): 17421–30

  Abstract

  Peroxynitrite (-OON═O, PN) is a reactive nitrogen species (RNS) which can effect deleterious nitrative or oxidative (bio)chemistry. It may derive from reaction of superoxide anion (O2•-) with nitric oxide (·NO) and has been suggested to form an as-yet unobserved bound heme-iron-PN intermediate in the catalytic cycle of nitric oxide dioxygenase (NOD) enzymes, which facilitate a ·NO homeostatic process, i.e., its oxidation to the nitrate anion. Here, a discrete six-coordinate low-spin porphyrinate-FeIII complex [(PIm)FeIII(-OON═O)] (3) (PIm; a porphyrin moiety with a covalently tethered imidazole axial "base" donor ligand) has been identified and characterized by various spectroscopies (UV-vis, NMR, EPR, XAS, resonance Raman) and DFT calculations, following its formation at -80 °C by addition of ·NO(g) to the heme-superoxo species, [(PIm)FeIII(O2•-)] (2). DFT calculations confirm that 3 is a six-coordinate low-spin species with the PN ligand coordinated to iron via its terminal peroxidic anionic O atom with the overall geometry being in a cis-configuration. Complex 3 thermally transforms to its isomeric low-spin nitrato form [(PIm)FeIII(NO3-)] (4a). While previous (bio)chemical studies show that phenolic substrates undergo nitration in the presence of PN or PN-metal complexes, in the present system, addition of 2,4-di-tert-butylphenol (2,4DTBP) to complex 3 does not lead to nitrated phenol; the nitrate complex 4a still forms. DFT calculations reveal that the phenolic H atom approaches the terminal PN O atom (farthest from the metal center and ring core), effecting O-O cleavage, giving nitrogen dioxide (·NO2) plus a ferryl compound [(PIm)FeIV═O] (7); this rebounds to give [(PIm)FeIII(NO3-)] (4a).The generation and characterization of the long sought after ferriheme peroxynitrite complex has been accomplished.

  View details for PubMedID 29091732

• High-resolution structure of a lytic polysaccharide monooxygenase from Hypocrea jecorina reveals a predicted linker as an integral part of the catalytic domain JOURNAL OF BIOLOGICAL CHEMISTRY Hansson, H., Karkehabadi, S., Mikkelsen, N., Douglas, N. R., Kim, S., Lam, A., Kaper, T., Kelemen, B., Meier, K. K., Jones, S. M., Solomon, E. I., Sandgren, M. 2017; 292 (46): 19099–109

  Abstract

  For decades, the enzymes of the fungus Hypocrea jecorina have served as a model system for the breakdown of cellulose. Three-dimensional structures for almost all H. jecorina cellulose-degrading enzymes are available, except for HjLPMO9A, belonging to the AA9 family of lytic polysaccharide monooxygenases (LPMOs). These enzymes enhance the hydrolytic activity of cellulases and are essential for cost-efficient conversion of lignocellulosic biomass. Here, using structural and spectroscopic analyses, we found that native HjLPMO9A contains a catalytic domain and a family-1 carbohydrate-binding module (CBM1) connected via a linker sequence. A C terminally truncated variant of HjLPMO9A containing 21 residues of the predicted linker was expressed at levels sufficient for analysis. Here, using structural, spectroscopic, and biochemical analyses, we found that this truncated variant exhibited reduced binding to and activity on cellulose compared with the full-length enzyme. Importantly, a 0.95-Å resolution X-ray structure of truncated HjLPMO9A revealed that the linker forms an integral part of the catalytic domain structure, covering a hydrophobic patch on the catalytic AA9 module. We noted that the oxidized catalytic center contains a Cu(II) coordinated by two His ligands, one of which has a His-brace in which the His-1 terminal amine group also coordinates to a copper. The final equatorial position of the Cu(II) is occupied by a water-derived ligand. The spectroscopic characteristics of the truncated variant were not measurably different from those of full-length HjLPMO9A, indicating that the presence of the CBM1 module increases the affinity of HjLPMO9A for cellulose binding, but does not affect the active site.

  View details for PubMedID 28900033

  View details for PubMedCentralID PMC5704490

• Sulfur K-Edge XAS Studies of the Effect of DNA Binding on the [Fe4S4] Site in EndoIII and MutY JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ha, Y., Arnold, A. R., Nunez, N. N., Bartels, P. L., Zhou, A., David, S. S., Barton, J. K., Hedman, B., Hodgson, K., Solomon, E. I. 2017; 139 (33): 11434–42

  Abstract

  S K-edge X-ray absorption spectroscopy (XAS) was used to study the [Fe4S4] clusters in the DNA repair glycosylases EndoIII and MutY to evaluate the effects of DNA binding and solvation on Fe-S bond covalencies (i.e., the amount of S 3p character mixed into the Fe 3d valence orbitals). Increased covalencies in both iron-thiolate and iron-sulfide bonds would stabilize the oxidized state of the [Fe4S4] clusters. The results are compared to those on previously studied [Fe4S4] model complexes, ferredoxin (Fd), and to new data on high-potential iron-sulfur protein (HiPIP). A limited decrease in covalency is observed upon removal of solvent water from EndoIII and MutY, opposite to the significant increase observed for Fd, where the [Fe4S4] cluster is solvent exposed. Importantly, in EndoIII and MutY, a large increase in covalency is observed upon DNA binding, which is due to the effect of its negative charge on the iron-sulfur bonds. In EndoIII, this change in covalency can be quantified and makes a significant contribution to the observed decrease in reduction potential found experimentally in DNA repair proteins, enabling their HiPIP-like redox behavior.

  View details for PubMedID 28715891

• Investigation of the 4 H+/4 e- reduction of oxygen performed by heme-copper oxidases Schaefer, A., Adam, S., Kieber-Emmons, M., Karlin, K., Solomon, E. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000429556700839

• Oxygen activation by Cu sites Solomon, E. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000429556700511

• New insight into the reaction mechanism of the formylglycine generating enzyme: A spectroscopic perspective Meier, K., Appel, M., Solomon, E. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000429556700914

• Insight into the electronic structure of transition metal ion complexes from resonant inelastic X-ray scattering Kroll, T., Hadt, R., Wilson, S., Baker, M., Lundberg, M., Yan, J., Weng, T., Sokaras, D., Alonso-Mori, R., Casa, D., Upton, M., Hedman, B., Hodgson, K., Solomon, E. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000429556700220

• K- and L-edge X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) determination of differential orbital covalency (DOC) of transition metal sites COORDINATION CHEMISTRY REVIEWS Baker, M. L., Mara, M. W., Yan, J. J., Hodgson, K. O., Hedman, B., Solomon, E. I. 2017; 345: 182–208

  Abstract

  Continual advancements in the development of synchrotron radiation sources have resulted in X-ray based spectroscopic techniques capable of probing the electronic and structural properties of numerous systems. This review gives an overview of the application of metal K-edge and L-edge X-ray absorption spectroscopy (XAS), as well as K resonant inelastic X-ray scattering (RIXS), to the study of electronic structure in transition metal sites with emphasis on experimentally quantifying 3d orbital covalency. The specific sensitivities of K-edge XAS, L-edge XAS, and RIXS are discussed emphasizing the complementary nature of the methods. L-edge XAS and RIXS are sensitive to mixing between 3d orbitals and ligand valence orbitals, and to the differential orbital covalency (DOC), that is, the difference in the covalencies for different symmetry sets of the d orbitals. Both L-edge XAS and RIXS are highly sensitive to and enable separation of and donor bonding and back bonding contributions to bonding. Applying ligand field multiplet simulations, including charge transfer via valence bond configuration interactions, DOC can be obtained for direct comparison with density functional theory calculations and to understand chemical trends. The application of RIXS as a probe of frontier molecular orbitals in a heme enzyme demonstrates the potential of this method for the study of metal sites in highly covalent coordination sites in bioinorganic chemistry.

  View details for PubMedID 28970624

  View details for PubMedCentralID PMC5621773

• A Mononuclear Nonheme Iron(V)-Imido Complex JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hong, S., Sutherlin, K. D., Vardhaman, A., Yan, J. J., Park, S., Lee, Y., Jang, S., Lu, X., Ohta, T., Ogura, T., Solomon, E. I., Nam, W. 2017; 139 (26): 8800–8803

  Abstract

  Mononuclear nonheme iron(V)-oxo complexes have been reported previously. Herein, we report the first example of a mononuclear nonheme iron(V)-imido complex bearing a tetraamido macrocyclic ligand (TAML), [(TAML)FeV(NTs)]- (1). The spectroscopic characterization of 1 revealed an S = 1/2 Fe(V) oxidation state, an Fe-N bond length of 1.65(4) Å, and an Fe-N vibration at 817 cm-1. The reactivity of 1 was demonstrated in C-H bond functionalization and nitrene transfer reactions.

  View details for PubMedID 28628312

• RIBONUCLEOTIDE REDUCTASE, AND A COMPARISON OF THE DIMANGANESE ACTIVE SITES OF MANGANESE CATALASE Lofstad, M., Bottger, L. H., Hammerstad, M., Rohr, A., Hersleth, H., Zaltariov, M. F., Arion, V. B., Solomon, E. I., Andersson, K. SPRINGER. 2017: S157

  View details for Web of Science ID 000419606600126

• Geometric and Electronic Structural Contributions to Fe/O-2 Reactivity Solomon, E. I. SPRINGER. 2017: S163

  View details for Web of Science ID 000419606600132

• Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2 '- bipyridine)(2)(CN)(2)] STRUCTURAL DYNAMICS Kjaer, K. S., Zhang, W., Alonso-Mori, R., Bergmann, U., Chollet, M., Hadt, R. G., Hartsock, R. W., Harlang, T., Kroll, T., Kubicek, K., Lemke, H. T., Liang, H. W., Liu, Y., Nielsen, M. M., Robinson, J. S., Solomon, E. I., Sokaras, D., van Driel, T. B., Weng, T., Zhu, D., Persson, P., Warnmark, K., Sundstrom, V., Gaffney, K. J. 2017; 4 (4): 044030

  Abstract

  We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)2(CN)2], where bpy=2,2'-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2'-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy)2(CN)2] undergoes ultrafast spin crossover to a metal-centered quintet excited state through a short lived metal-centered triplet transient species. These measurements of [Fe(bpy)2(CN)2] complement prior measurement performed on [Fe(bpy)3]2+ and [Fe(bpy)(CN)4]2- in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy)N(CN)6-2N]2N-4, where N = 1-3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.

  View details for PubMedID 28653021

• Metalloprotein entatic control of ligand-metal bonds quantified by ultrafast x-ray spectroscopy SCIENCE Mara, M. W., Hadt, R. G., Reinhard, M., Kroll, T., Lim, H., Hartsock, R. W., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Kunnus, K., Hodgson, K. O., Hedman, B., Bergmann, U., Gaffney, K. J., Solomon, E. I. 2017; 356 (6344): 1276-+

  Abstract

  The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe-S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe-S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.

  View details for PubMedID 28642436

  View details for PubMedCentralID PMC5706643

• Reduction in Heme-Copper Oxidases. Journal of the American Chemical Society Schaefer, A. W., Kieber-Emmons, M. T., Adam, S. M., Karlin, K. D., Solomon, E. I. 2017

  Abstract

  This study evaluates the reaction of a biomimetic heme-peroxo-copper complex, {[(DCHIm)(F8)Fe(III)]-(O2(2-))-[Cu(II)(AN)]}(+) (1), with a phenolic substrate, involving a net H-atom abstraction to cleave the bridging peroxo O-O bond that produces Fe(IV)═O, Cu(II)-OH, and phenoxyl radical moieties, analogous to the chemistry carried out in heme-copper oxidases (HCOs). A 3D potential energy surface generated for this reaction reveals two possible reaction pathways: one involves nearly complete proton transfer (PT) from the phenol to the peroxo ligand before the barrier; the other involves O-O homolysis, where the phenol remains H-bonding to the peroxo OCu in the transition state (TS) and transfers the H(+) after the barrier. In both mechanisms, electron transfer (ET) from phenol occurs after the PT (and after the barrier); therefore, only the interaction with the H(+) is involved in lowering the O-O cleavage barrier. The relative barriers depend on covalency (which governs ET from Fe), and therefore vary with DFT functional. However, as these mechanisms differ by the amount of PT at the TS, kinetic isotope experiments were conducted to determine which mechanism is active. It is found that the phenolic proton exhibits a secondary kinetic isotope effect, consistent with the calculations for the H-bonded O-O homolysis mechanism. The consequences of these findings are discussed in relation to O-O cleavage in HCOs, supporting a model in which a peroxo intermediate serves as the active H(+) acceptor, and both the H(+) and e(-) required for O-O cleavage derive from the cross-linked Tyr residue present at the active site.

  View details for DOI 10.1021/jacs.7b03292

  View details for PubMedID 28521498

• Peroxide Activation for Electrophilic Reactivity by the Binuclear Non-heme Iron Enzyme AurF. Journal of the American Chemical Society Park, K., Li, N., Kwak, Y., Srnec, M., Bell, C. B., Liu, L. V., Wong, S. D., Yoda, Y., Kitao, S., Seto, M., Hu, M., Zhao, J., Krebs, C., Bollinger, J. M., Solomon, E. I. 2017; 139 (20): 7062-7070

  Abstract

  Binuclear non-heme iron enzymes activate O2 for diverse chemistries that include oxygenation of organic substrates and hydrogen atom abstraction. This process often involves the formation of peroxo-bridged biferric intermediates, only some of which can perform electrophilic reactions. To elucidate the geometric and electronic structural requirements to activate peroxo reactivity, the active peroxo intermediate in 4-aminobenzoate N-oxygenase (AurF) has been characterized spectroscopically and computationally. A magnetic circular dichroism study of reduced AurF shows that its electronic and geometric structures are poised to react rapidly with O2. Nuclear resonance vibrational spectroscopic definition of the peroxo intermediate formed in this reaction shows that the active intermediate has a protonated peroxo bridge. Density functional theory computations on the structure established here show that the protonation activates peroxide for electrophilic/single-electron-transfer reactivity. This activation of peroxide by protonation is likely also relevant to the reactive peroxo intermediates in other binuclear non-heme iron enzymes.

  View details for DOI 10.1021/jacs.7b02997

  View details for PubMedID 28457126

• Determination of differential orbital covalency of heme active sites by L-edge spectroscopy Baker, M., Alpert, B., Cho, H., Denison, E., Doriese, W., Fowler, J., Gaffney, K., Gard, J., Gao, B., Hilton, G., Irwin, K., Joe, Y., Kenney, C., Knight, J., Kroll, T., Lee, S., Li, D., Marks, R., Minitti, M., Morgan, K., Mori, R., Ogasawara, H., O'Neil, G., Schmidt, D., Sokaras, D., Swetz, D., Song, Y., Titus, C., Ullom, J., Weng, T., Williams, C., Yan, J., Young, B., Nordlund, D., Solomon, E. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000430569103813

• Defining the active sites of low-temperature methane hydroxylation in iron and copper zeolites Snyder, B., Vanelderen, P., Woertink, J., Sels, B., Schoonheydt, R., Solomon, E. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000430569100012

• Structure-function relationships in G4DFsc variants containing a 4-His/3-carboxylate active site Oshea, K., Dorsheimer, J., Biernat, K., Jacobs, A., Solomon, E., Wu, Y., Degrado, W., Reig, A. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000430569102619

• Bioinorganic spectroscopy: Activating metal sites for biological electron transfer Solomon, E. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000430569103032

• Biophysical characterization and catalytic reactivity of rubrerythrin and symerythrin model proteins Pellegrino, J., Bell, K., Polinski, R., Cimerol, S., Jacobs, A., Solomon, E., Reig, A. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000430569102617

• Biophysical Characterization and Catalytic Reactivity of Rubrerythrin and Symerythrin Model Proteins Pellegrino, J., Bell, K. A., Polinski, R. Z., Cimerol, S. N., Jacobs, A., Solomon, E. I., Reig, A. FEDERATION AMER SOC EXP BIOL. 2017

  View details for Web of Science ID 000405461403094

• ° Intermediate in Turnover of Nitrous Oxide Reductase and Molecular Insight into the Catalytic Mechanism. Journal of the American Chemical Society Johnston, E. M., Carreira, C., Dell'Acqua, S., Dey, S. G., Pauleta, S. R., Moura, I., Solomon, E. I. 2017; 139 (12): 4462-4476

  Abstract

  Spectroscopic methods and density functional theory (DFT) calculations are used to determine the geometric and electronic structure of CuZ°, an intermediate form of the Cu4S active site of nitrous oxide reductase (N2OR) that is observed in single turnover of fully reduced N2OR with N2O. Electron paramagnetic resonance (EPR), absorption, and magnetic circular dichroism (MCD) spectroscopies show that CuZ° is a 1-hole (i.e., 3Cu(I)Cu(II)) state with spin density delocalized evenly over CuI and CuIV. Resonance Raman spectroscopy shows two Cu-S vibrations at 425 and 413 cm(-1), the latter with a -3 cm(-1) O(18) solvent isotope shift. DFT calculations correlated to these spectral features show that CuZ° has a terminal hydroxide ligand coordinated to CuIV, stabilized by a hydrogen bond to a nearby lysine residue. CuZ° can be reduced via electron transfer from CuA using a physiologically relevant reductant. We obtain a lower limit on the rate of this intramolecular electron transfer (IET) that is >10(4) faster than the unobserved IET in the resting state, showing that CuZ° is the catalytically relevant oxidized form of N2OR. Terminal hydroxide coordination to CuIV in the CuZ° intermediate yields insight into the nature of N2O binding and reduction, specifying a molecular mechanism in which N2O coordinates in a μ-1,3 fashion to the fully reduced state, with hydrogen bonding from Lys397, and two electrons are transferred from the fully reduced μ4S(2-) bridged tetranuclear copper cluster to N2O via a single Cu atom to accomplish N-O bond cleavage.

  View details for DOI 10.1021/jacs.6b13225

  View details for PubMedID 28228011

• Spectroscopic Definition of the Cu-Z degrees Intermediate in Turnover of Nitrous Oxide Reductase and Molecular Insight into the Catalytic Mechanism JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Johnston, E. M., Carreira, C., Dell'Acqua, S., Dey, S. G., Pauleta, S. R., Moura, I., Solomon, E. I. 2017; 139 (12): 4462-4476

  Abstract

  Spectroscopic methods and density functional theory (DFT) calculations are used to determine the geometric and electronic structure of CuZ°, an intermediate form of the Cu4S active site of nitrous oxide reductase (N2OR) that is observed in single turnover of fully reduced N2OR with N2O. Electron paramagnetic resonance (EPR), absorption, and magnetic circular dichroism (MCD) spectroscopies show that CuZ° is a 1-hole (i.e., 3Cu(I)Cu(II)) state with spin density delocalized evenly over CuI and CuIV. Resonance Raman spectroscopy shows two Cu-S vibrations at 425 and 413 cm(-1), the latter with a -3 cm(-1) O(18) solvent isotope shift. DFT calculations correlated to these spectral features show that CuZ° has a terminal hydroxide ligand coordinated to CuIV, stabilized by a hydrogen bond to a nearby lysine residue. CuZ° can be reduced via electron transfer from CuA using a physiologically relevant reductant. We obtain a lower limit on the rate of this intramolecular electron transfer (IET) that is >10(4) faster than the unobserved IET in the resting state, showing that CuZ° is the catalytically relevant oxidized form of N2OR. Terminal hydroxide coordination to CuIV in the CuZ° intermediate yields insight into the nature of N2O binding and reduction, specifying a molecular mechanism in which N2O coordinates in a μ-1,3 fashion to the fully reduced state, with hydrogen bonding from Lys397, and two electrons are transferred from the fully reduced μ4S(2-) bridged tetranuclear copper cluster to N2O via a single Cu atom to accomplish N-O bond cleavage.

  View details for DOI 10.1021/jacs.6b13225

  View details for Web of Science ID 000398247100043

• Cores with Enhanced Oxidative Reactivity. Journal of the American Chemical Society Garcia-Bosch, I., Cowley, R. E., Díaz, D. E., Peterson, R. L., Solomon, E. I., Karlin, K. D. 2017; 139 (8): 3186-3195

  Abstract

  Copper-dependent metalloenzymes are widespread throughout metabolic pathways, coupling the reduction of O2 with the oxidation of organic substrates. Small-molecule synthetic analogs are useful platforms to generate L/Cu/O2 species that reproduce the structural, spectroscopic, and reactive properties of some copper-/O2-dependent enzymes. Landmark studies have shown that the conversion between dicopper(II)-peroxo species (L2Cu(II)2(O2(2-)) either side-on peroxo, (S)P, or end-on trans-peroxo, (T)P) and dicopper(III)-bis(μ-oxo) (L2Cu(III)2(O(2-))2: O) can be controlled through ligand design, reaction conditions (temperature, solvent, and counteranion), or substrate coordination. We recently published ( J. Am. Chem. Soc. 2012 , 134 , 8513 , DOI: 10.1021/ja300674m ) the crystal structure of an unusual (S)P species [(MeAN)2Cu(II)2(O2(2-))](2+) ((S)P(MeAN), MeAN: N-methyl-N,N-bis[3-(dimethylamino)propyl]amine) that featured an elongated O-O bond but did not lead to O-O cleavage or reactivity toward external substrates. Herein, we report that (S)P(MeAN) can be activated to generate O(MeAN) and perform the oxidation of external substrates by two complementary strategies: (i) coordination of substituted sodium phenolates to form the substrate-bound O(MeAN)-RPhO(-) species that leads to ortho-hydroxylation in a tyrosinase-like fashion and (ii) addition of stoichiometric amounts (1 or 2 equiv) of Lewis acids (LA's) to form an unprecedented series of O-type species (O(MeAN)-LA) able to oxidize C-H and O-H bonds. Spectroscopic, computational, and mechanistic studies emphasize the unique plasticity of the (S)P(MeAN) core, which combines the assembly of exogenous reagents in the primary (phenolates) and secondary (Lewis acids association to the MeAN ligand) coordination spheres with O-O cleavage. These findings are reminiscent of the strategy followed by several metalloproteins and highlight the possible implication of O-type species in copper-/dioxygen-dependent enzymes such as tyrosinase (Ty) and particulate methane monooxygenase (pMMO).

  View details for DOI 10.1021/jacs.6b12990

  View details for PubMedID 28195739

• Substrate and Lewis Acid Coordination Promote O-O Bond Cleavage of an Unreactive L(2)Cu(II)2(O-2(2-)) Species to Form L2Cu2III(O)(2) Cores with Enhanced Oxidative Reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Garcia-Bosch, I., Cowley, R. E., Diaz, D. E., Peterson, R. L., Solomon, E. I., Karlin, K. D. 2017; 139 (8): 3186-3195

  Abstract

  Copper-dependent metalloenzymes are widespread throughout metabolic pathways, coupling the reduction of O2 with the oxidation of organic substrates. Small-molecule synthetic analogs are useful platforms to generate L/Cu/O2 species that reproduce the structural, spectroscopic, and reactive properties of some copper-/O2-dependent enzymes. Landmark studies have shown that the conversion between dicopper(II)-peroxo species (L2Cu(II)2(O2(2-)) either side-on peroxo, (S)P, or end-on trans-peroxo, (T)P) and dicopper(III)-bis(μ-oxo) (L2Cu(III)2(O(2-))2: O) can be controlled through ligand design, reaction conditions (temperature, solvent, and counteranion), or substrate coordination. We recently published ( J. Am. Chem. Soc. 2012 , 134 , 8513 , DOI: 10.1021/ja300674m ) the crystal structure of an unusual (S)P species [(MeAN)2Cu(II)2(O2(2-))](2+) ((S)P(MeAN), MeAN: N-methyl-N,N-bis[3-(dimethylamino)propyl]amine) that featured an elongated O-O bond but did not lead to O-O cleavage or reactivity toward external substrates. Herein, we report that (S)P(MeAN) can be activated to generate O(MeAN) and perform the oxidation of external substrates by two complementary strategies: (i) coordination of substituted sodium phenolates to form the substrate-bound O(MeAN)-RPhO(-) species that leads to ortho-hydroxylation in a tyrosinase-like fashion and (ii) addition of stoichiometric amounts (1 or 2 equiv) of Lewis acids (LA's) to form an unprecedented series of O-type species (O(MeAN)-LA) able to oxidize C-H and O-H bonds. Spectroscopic, computational, and mechanistic studies emphasize the unique plasticity of the (S)P(MeAN) core, which combines the assembly of exogenous reagents in the primary (phenolates) and secondary (Lewis acids association to the MeAN ligand) coordination spheres with O-O cleavage. These findings are reminiscent of the strategy followed by several metalloproteins and highlight the possible implication of O-type species in copper-/dioxygen-dependent enzymes such as tyrosinase (Ty) and particulate methane monooxygenase (pMMO).

  View details for DOI 10.1021/jacs.6b12990

  View details for Web of Science ID 000395493400053

• Frontier Molecular Orbital Contributions to Chlorination versus Hydroxylation Selectivity in the Non-Heme Iron Halogenase SyrB2. Journal of the American Chemical Society Srnec, M., Solomon, E. I. 2017; 139 (6): 2396-2407

  Abstract

  The ability of an Fe(IV)═O intermediate in SyrB2 to perform chlorination versus hydroxylation was computationally evaluated for different substrates that had been studied experimentally. The π-trajectory for H atom abstraction (Fe(IV)═O oriented perpendicular to the C-H bond of substrate) was found to lead to the S = 2 five-coordinate HO-Fe(III)-Cl complex with the C(•) of the substrate, π-oriented relative to both the Cl(-) and the OH(-) ligands. From this ferric intermediate, hydroxylation is thermodynamically favored, but chlorination is intrinsically more reactive due to the energy splitting between two key redox-active dπ* frontier molecular orbitals (FMOs). The splitting is determined by the differential ligand field effect of Cl(-) versus OH(-) on the Fe center. This makes chlorination effectively competitive with hydroxylation. Chlorination versus hydroxylation selectivity is then determined by the orientation of the substrate with respect to the HO-Fe-Cl plane that controls either the Cl(-) or the OH(-) to rebound depending on the relative π-overlap with the substrate C radical. The differential contribution of the two FMOs to chlorination versus hydroxylation selectivity in SyrB2 is related to a reaction mechanism that involves two asynchronous transfers: electron transfer from the substrate radical to the iron center followed by late ligand (Cl(-) or OH(-)) transfer to the substrate.

  View details for DOI 10.1021/jacs.6b11995

  View details for PubMedID 28095695

  View details for PubMedCentralID PMC5310988

• L-Edge X-ray Absorption Spectroscopic Investigation of {FeNO}(6): Delocalization vs Antiferromagnetic Coupling JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yan, J. J., Gonzales, M. A., Mascharak, P. K., Hedman, B., Hodgson, K. O., Solomon, E. I. 2017; 139 (3): 1215-1225

  Abstract

  NO is a classic non-innocent ligand, and iron nitrosyls can have different electronic structure descriptions depending on their spin state and coordination environment. These highly covalent ligands are found in metalloproteins and are also used as models for Fe-O2 systems. This study utilizes iron L-edge X-ray absorption spectroscopy (XAS), interpreted using a valence bond configuration interaction multiplet model, to directly experimentally probe the electronic structure of the S = 0 {FeNO}(6) compound [Fe(PaPy3)NO](2+) (PaPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide) and the S = 0 [Fe(PaPy3)CO](+) reference compound. This method allows separation of the σ-donation and π-acceptor interactions of the ligand through ligand-to-metal and metal-to-ligand charge-transfer mixing pathways. The analysis shows that the {FeNO}(6) electronic structure is best described as Fe(III)-NO(neutral), with no localized electron in an NO π* orbital or electron hole in an Fe dπ orbital. This delocalization comes from the large energy gap between the Fe-NO π-bonding and antibonding molecular orbitals relative to the exchange interactions between electrons in these orbitals. This study demonstrates the utility of L-edge XAS in experimentally defining highly delocalized electronic structures.

  View details for DOI 10.1021/jacs.6b11260

  View details for Web of Science ID 000393541000029

  View details for PubMedCentralID PMC5322818

• : Delocalization vs Antiferromagnetic Coupling. Journal of the American Chemical Society Yan, J. J., Gonzales, M. A., Mascharak, P. K., Hedman, B., Hodgson, K. O., Solomon, E. I. 2017; 139 (3): 1215-1225

  Abstract

  NO is a classic non-innocent ligand, and iron nitrosyls can have different electronic structure descriptions depending on their spin state and coordination environment. These highly covalent ligands are found in metalloproteins and are also used as models for Fe-O2 systems. This study utilizes iron L-edge X-ray absorption spectroscopy (XAS), interpreted using a valence bond configuration interaction multiplet model, to directly experimentally probe the electronic structure of the S = 0 {FeNO}(6) compound [Fe(PaPy3)NO](2+) (PaPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide) and the S = 0 [Fe(PaPy3)CO](+) reference compound. This method allows separation of the σ-donation and π-acceptor interactions of the ligand through ligand-to-metal and metal-to-ligand charge-transfer mixing pathways. The analysis shows that the {FeNO}(6) electronic structure is best described as Fe(III)-NO(neutral), with no localized electron in an NO π* orbital or electron hole in an Fe dπ orbital. This delocalization comes from the large energy gap between the Fe-NO π-bonding and antibonding molecular orbitals relative to the exchange interactions between electrons in these orbitals. This study demonstrates the utility of L-edge XAS in experimentally defining highly delocalized electronic structures.

  View details for DOI 10.1021/jacs.6b11260

  View details for PubMedID 28006897

  View details for PubMedCentralID PMC5322818

• Critical Aspects of Heme-Peroxo-Cu Complex Structure and Nature of Proton Source Dictate Metal-O-peroxo Breakage versus Reductive O-O Cleavage Chemistry JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Adam, S. M., Garcia-Bosch, I., Schaefer, A. W., Sharma, S. K., Siegler, M. A., Solomon, E. I., Karlin, K. D. 2017; 139 (1): 472-481

  Abstract

  The 4H(+)/4e(-) reduction of O2 to water, a key fuel-cell reaction also carried out in biology by oxidase enzymes, includes the critical O-O bond reductive cleavage step. Mechanistic investigations on active-site model compounds, which are synthesized by rational design to incorporate systematic variations, can focus on and resolve answers to fundamental questions, including protonation and/or H-bonding aspects, which accompany electron transfer. Here, we describe the nature and comparative reactivity of two low-spin heme-peroxo-Cu complexes, LS-4DCHIm, [(DCHIm)F8Fe(III)-(O2(2-))-Cu(II)(DCHIm)4](+), and LS-3DCHIm, [(DCHIm)F8Fe(III)-(O2(2-))-Cu(II)(DCHIm)3](+) (F8 = tetrakis(2,6-difluorophenyl)-porphyrinate; DCHIm = 1,5-dicyclohexylimidazole), toward different proton (4-nitrophenol and [DMF·H(+)](CF3SO3(-))) (DMF = dimethyl-formamide) or electron (decamethylferrocene (Fc*)) sources. Spectroscopic reactivity studies show that differences in structure and electronic properties of LS-3DCHIm and LS-4DCHIm lead to significant differences in behavior. LS-3DCHIm is resistant to reduction, is unreactive toward weakly acidic 4-NO2-phenol, and stronger acids cleave the metal-O bonds, releasing H2O2. By contrast, LS-4DCHIm forms an adduct with 4-NO2-phenol, which includes an H-bond to the peroxo O-atom distal to Fe (resonance Raman (rR) spectroscopy and DFT). With addition of Fc* (2 equiv overall required), O-O reductive cleavage occurs, giving water, Fe(III), and Cu(II) products; however, a kinetic study reveals a one-electron rate-determining process, ket = 1.6 M(-1) s(-1) (-90 °C). The intermediacy of a high-valent [(DCHIm)F8Fe(IV)═O] species is thus implied, and separate experiments show that one-electron reduction-protonation of [(DCHIm)F8Fe(IV)═O] occurs faster (ket2 = 5.0 M(-1) s(-1)), consistent with the overall postulated mechanism. The importance of the H-bonding interaction as a prerequisite for reductive cleavage is highlighted.

  View details for DOI 10.1021/jacs.6b11322

  View details for Web of Science ID 000392036900066

  View details for PubMedCentralID PMC5274545

• Breakage versus Reductive O-O Cleavage Chemistry. Journal of the American Chemical Society Adam, S. M., Garcia-Bosch, I., Schaefer, A. W., Sharma, S. K., Siegler, M. A., Solomon, E. I., Karlin, K. D. 2017; 139 (1): 472-481

  Abstract

  The 4H(+)/4e(-) reduction of O2 to water, a key fuel-cell reaction also carried out in biology by oxidase enzymes, includes the critical O-O bond reductive cleavage step. Mechanistic investigations on active-site model compounds, which are synthesized by rational design to incorporate systematic variations, can focus on and resolve answers to fundamental questions, including protonation and/or H-bonding aspects, which accompany electron transfer. Here, we describe the nature and comparative reactivity of two low-spin heme-peroxo-Cu complexes, LS-4DCHIm, [(DCHIm)F8Fe(III)-(O2(2-))-Cu(II)(DCHIm)4](+), and LS-3DCHIm, [(DCHIm)F8Fe(III)-(O2(2-))-Cu(II)(DCHIm)3](+) (F8 = tetrakis(2,6-difluorophenyl)-porphyrinate; DCHIm = 1,5-dicyclohexylimidazole), toward different proton (4-nitrophenol and [DMF·H(+)](CF3SO3(-))) (DMF = dimethyl-formamide) or electron (decamethylferrocene (Fc*)) sources. Spectroscopic reactivity studies show that differences in structure and electronic properties of LS-3DCHIm and LS-4DCHIm lead to significant differences in behavior. LS-3DCHIm is resistant to reduction, is unreactive toward weakly acidic 4-NO2-phenol, and stronger acids cleave the metal-O bonds, releasing H2O2. By contrast, LS-4DCHIm forms an adduct with 4-NO2-phenol, which includes an H-bond to the peroxo O-atom distal to Fe (resonance Raman (rR) spectroscopy and DFT). With addition of Fc* (2 equiv overall required), O-O reductive cleavage occurs, giving water, Fe(III), and Cu(II) products; however, a kinetic study reveals a one-electron rate-determining process, ket = 1.6 M(-1) s(-1) (-90 °C). The intermediacy of a high-valent [(DCHIm)F8Fe(IV)═O] species is thus implied, and separate experiments show that one-electron reduction-protonation of [(DCHIm)F8Fe(IV)═O] occurs faster (ket2 = 5.0 M(-1) s(-1)), consistent with the overall postulated mechanism. The importance of the H-bonding interaction as a prerequisite for reductive cleavage is highlighted.

  View details for DOI 10.1021/jacs.6b11322

  View details for PubMedID 28029788

• Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution CHEMICAL SCIENCE Zhang, W., Kjaer, K. S., Alonso-Mori, R., Bergmann, U., Chollet, M., Fredin, L. A., Hadt, R. G., Hartsock, R. W., Harlang, T., Kroll, T., Kubicek, K., Lemke, H. T., Liang, H. W., Liu, Y., Nielsen, M. M., Persson, P., Robinson, J. S., Solomon, E. I., Sun, Z., Sokaras, D., van Driel, T. B., Weng, T., Zhu, D., Warnmark, K., Sundstromb, V., Gaffney, K. J. 2017; 8 (1): 515-523

  Abstract

  Developing light-harvesting and photocatalytic molecules made with iron could provide a cost effective, scalable, and environmentally benign path for solar energy conversion. To date these developments have been limited by the sub-picosecond metal-to-ligand charge transfer (MLCT) electronic excited state lifetime of iron based complexes due to spin crossover - the extremely fast intersystem crossing and internal conversion to high spin metal-centered excited states. We revitalize a 30 year old synthetic strategy for extending the MLCT excited state lifetimes of iron complexes by making mixed ligand iron complexes with four cyanide (CN-) ligands and one 2,2'-bipyridine (bpy) ligand. This enables MLCT excited state and metal-centered excited state energies to be manipulated with partial independence and provides a path to suppressing spin crossover. We have combined X-ray Free-Electron Laser (XFEL) Kβ hard X-ray fluorescence spectroscopy with femtosecond time-resolved UV-visible absorption spectroscopy to characterize the electronic excited state dynamics initiated by MLCT excitation of [Fe(CN)4(bpy)]2-. The two experimental techniques are highly complementary; the time-resolved UV-visible measurement probes allowed electronic transitions between valence states making it sensitive to ligand-centered electronic states such as MLCT states, whereas the Kβ fluorescence spectroscopy provides a sensitive measure of changes in the Fe spin state characteristic of metal-centered excited states. We conclude that the MLCT excited state of [Fe(CN)4(bpy)]2- decays with roughly a 20 ps lifetime without undergoing spin crossover, exceeding the MLCT excited state lifetime of [Fe(2,2'-bipyridine)3]2+ by more than two orders of magnitude.

  View details for DOI 10.1039/c6sc03070j

  View details for Web of Science ID 000391454500060

  View details for PubMedCentralID PMC5341207

• Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution. Chemical science Zhang, W., Kjær, K. S., Alonso-Mori, R., Bergmann, U., Chollet, M., Fredin, L. A., Hadt, R. G., Hartsock, R. W., Harlang, T., Kroll, T., Kubiček, K., Lemke, H. T., Liang, H. W., Liu, Y., Nielsen, M. M., Persson, P., Robinson, J. S., Solomon, E. I., Sun, Z., Sokaras, D., van Driel, T. B., Weng, T. C., Zhu, D., Wärnmark, K., Sundström, V., Gaffney, K. J. 2017; 8 (1): 515-523

  Abstract

  Developing light-harvesting and photocatalytic molecules made with iron could provide a cost effective, scalable, and environmentally benign path for solar energy conversion. To date these developments have been limited by the sub-picosecond metal-to-ligand charge transfer (MLCT) electronic excited state lifetime of iron based complexes due to spin crossover - the extremely fast intersystem crossing and internal conversion to high spin metal-centered excited states. We revitalize a 30 year old synthetic strategy for extending the MLCT excited state lifetimes of iron complexes by making mixed ligand iron complexes with four cyanide (CN-) ligands and one 2,2'-bipyridine (bpy) ligand. This enables MLCT excited state and metal-centered excited state energies to be manipulated with partial independence and provides a path to suppressing spin crossover. We have combined X-ray Free-Electron Laser (XFEL) Kβ hard X-ray fluorescence spectroscopy with femtosecond time-resolved UV-visible absorption spectroscopy to characterize the electronic excited state dynamics initiated by MLCT excitation of [Fe(CN)4(bpy)]2-. The two experimental techniques are highly complementary; the time-resolved UV-visible measurement probes allowed electronic transitions between valence states making it sensitive to ligand-centered electronic states such as MLCT states, whereas the Kβ fluorescence spectroscopy provides a sensitive measure of changes in the Fe spin state characteristic of metal-centered excited states. We conclude that the MLCT excited state of [Fe(CN)4(bpy)]2- decays with roughly a 20 ps lifetime without undergoing spin crossover, exceeding the MLCT excited state lifetime of [Fe(2,2'-bipyridine)3]2+ by more than two orders of magnitude.

  View details for DOI 10.1039/c6sc03070j

  View details for PubMedID 28451198

  View details for PubMedCentralID PMC5341207

• Mechanism of chloride inhibition of bilirubin oxidases and its dependence on potential and pH. ACS catalysis de Poulpiquet, A. n., Kjaergaard, C. H., Rouhana, J. n., Mazurenko, I. n., Infossi, P. n., Gounel, S. n., Gadiou, R. n., Giudici-Orticoni, M. T., Solomon, E. I., Mano, N. n., Lojou, E. n. 2017; 7 (6): 3916–23

  Abstract

  Bilirubin oxidases (BODs) belong to the multi-copper oxidase (MCO) family and efficiently reduce O2 at neutral pH and in physiological conditions where chloride concentrations are over 100 mM. BODs were consequently considered to be Cl- resistant contrary to laccases. However, there has not been a detailed study on the related effect of chloride and pH on the redox state of immobilized BODs. Here, we investigate by electrochemistry the catalytic mechanism of O2 reduction by the thermostable Bacillus pumilus BOD immobilized on carbon nanofibers in the presence of NaCl. The addition of chloride results in the formation of a redox state of the enzyme, previously observed for different BODs and laccases, which is only active after a reductive step. This behavior has not been previously investigated. We show for the first time that the kinetics of formation of this state is strongly dependent on pH, temperature, Cl- concentration and on the applied redox potential. UV-visible spectroscopy allows us to correlate the inhibition process by chloride with the formation of the alternative resting form of the enzyme. We demonstrate that O2 is not required for its formation and show that the application of an oxidative potential is sufficient. In addition, our results suggest that the reactivation may proceed thought the T3 β.

  View details for PubMedID 29930880

• Reactivity of a Cobalt(III)-Hydroperoxo Complex in Electrophilic Reactions INORGANIC CHEMISTRY Shin, B., Sutherlin, K. D., Ohta, T., Ogura, T., Solomon, E. I., Cho, J. 2016; 55 (23): 12391-12399

  Abstract

  The reactivity of mononuclear metal-hydroperoxo adducts has fascinated researchers in many areas due to their diverse biological and catalytic processes. In this study, a mononuclear cobalt(III)-peroxo complex bearing a tetradentate macrocyclic ligand, [Co(III)(Me3-TPADP)(O2)](+) (Me3-TPADP = 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane), was prepared by reacting [Co(II)(Me3-TPADP)(CH3CN)2](2+) with H2O2 in the presence of triethylamine. Upon protonation, the cobalt(III)-peroxo intermediate was converted into a cobalt(III)-hydroperoxo complex, [Co(III)(Me3-TPADP)(O2H)(CH3CN)](2+). The mononuclear cobalt(III)-peroxo and -hydroperoxo intermediates were characterized by a variety of physicochemical methods. Results of electrospray ionization mass spectrometry clearly show the transformation of the intermediates: the peak at m/z 339.2 assignable to the cobalt(III)-peroxo species disappears with concomitant growth of the peak at m/z 190.7 corresponding to the cobalt(III)-hydroperoxo complex (with bound CH3CN). Isotope labeling experiments further support the existence of the cobalt(III)-peroxo and -hydroperoxo complexes. In particular, the O-O bond stretching frequency of the cobalt(III)-hydroperoxo complex was determined to be 851 cm(-1) for (16)O2H samples (803 cm(-1) for (18)O2H samples), and its Co-O vibrational energy was observed at 571 cm(-1) for (16)O2H samples (551 cm(-1) for (18)O2H samples; 568 cm(-1) for (16)O2(2)H samples) by resonance Raman spectroscopy. Reactivity studies performed with the cobalt(III)-peroxo and -hydroperoxo complexes in organic functionalizations reveal that the latter is capable of conducting oxygen atom transfer with an electrophilic character, whereas the former exhibits no oxygen atom transfer reactivity under the same reaction conditions. Alternatively, the cobalt(III)-hydroperoxo complex does not perform hydrogen atom transfer reactions, while analogous low-spin Fe(III)-hydroperoxo complexes are capable of this reactivity. Density functional theory calculations indicate that this lack of reactivity is due to the high free energy cost of O-O bond homolysis that would be required to produce the hypothetical Co(IV)-oxo product.

  View details for DOI 10.1021/acs.inargchem.6b02288

  View details for Web of Science ID 000389497600035

  View details for PubMedID 27934432

  View details for PubMedCentralID PMC5363059

• O-2 Activation by Non-Heme Iron Enzymes BIOCHEMISTRY Solomon, E. I., Goudarzi, S., Sutherlin, K. D. 2016; 55 (46): 6363-6374

  Abstract

  The non-heme Fe enzymes are ubiquitous in nature and perform a wide range of functions involving O2 activation. These had been difficult to study relative to heme enzymes; however, spectroscopic methods that provide significant insight into the correlation of structure with function have now been developed. This Current Topics article summarizes both the molecular mechanism these enzymes use to control O2 activation in the presence of cosubstrates and the oxygen intermediates these reactions generate. Three types of O2 activation are observed. First, non-heme reactivity is shown to be different from heme chemistry where a low-spin Fe(III)-OOH non-heme intermediate directly reacts with substrate. Also, two subclasses of non-heme Fe enzymes generate high-spin Fe(IV)═O intermediates that provide both σ and π frontier molecular orbitals that can control selectivity. Finally, for several subclasses of non-heme Fe enzymes, binding of the substrate to the Fe(II) site leads to the one-electron reductive activation of O2 to an Fe(III)-superoxide capable of H atom abstraction and electrophilic attack.

  View details for DOI 10.1021/acs.biochem.6b00635

  View details for Web of Science ID 000388913700003

  View details for PubMedID 27792301

  View details for PubMedCentralID PMC5345855

• Nuclear Resonance Vibrational Spectroscopic Definition of Peroxy Intermediates in Nonheme Iron Sites. Journal of the American Chemical Society Sutherlin, K. D., Liu, L. V., Lee, Y., Kwak, Y., Yoda, Y., Saito, M., Kurokuzu, M., Kobayashi, Y., Seto, M., Que, L., Nam, W., Solomon, E. I. 2016; 138 (43): 14294-14302

  Abstract

  Fe(III)-(hydro)peroxy intermediates have been isolated in two classes of mononuclear nonheme Fe enzymes that are important in bioremediation: the Rieske dioxygenases and the extradiol dioxygenases. The binding mode and protonation state of the peroxide moieties in these intermediates are not well-defined, due to a lack of vibrational structural data. Nuclear resonance vibrational spectroscopy (NRVS) is an important technique for obtaining vibrational information on these and other intermediates, as it is sensitive to all normal modes with Fe displacement. Here, we present the NRVS spectra of side-on Fe(III)-peroxy and end-on Fe(III)-hydroperoxy model complexes and assign these spectra using calibrated DFT calculations. We then use DFT calculations to define and understand the changes in the NRVS spectra that arise from protonation and from opening the Fe-O-O angle. This study identifies four spectroscopic handles that will enable definition of the binding mode and protonation state of Fe(III)-peroxy intermediates in mononuclear nonheme Fe enzymes. These structural differences are important in determining the frontier molecular orbitals available for reactivity.

  View details for PubMedID 27726349

• Mechanism of O2 activation and substrate hydroxylation in noncoupled binuclear copper monooxygenases. Proceedings of the National Academy of Sciences of the United States of America Cowley, R. E., Tian, L., Solomon, E. I. 2016; 113 (43): 12035-12040

  Abstract

  Peptidylglycine α-hydroxylating monooxygenase (PHM) and dopamine β-monooxygenase (DβM) are copper-dependent enzymes that are vital for neurotransmitter regulation and hormone biosynthesis. These enzymes feature a unique active site consisting of two spatially separated (by 11 Å in PHM) and magnetically noncoupled copper centers that enables 1e(-) activation of O2 for hydrogen atom abstraction (HAA) of substrate C-H bonds and subsequent hydroxylation. Although the structures of the resting enzymes are known, details of the hydroxylation mechanism and timing of long-range electron transfer (ET) are not clear. This study presents density-functional calculations of the full reaction coordinate, which demonstrate: (i) the importance of the end-on coordination of superoxide to Cu for HAA along the triplet spin surface; (ii) substrate radical rebound to a Cu(II) hydroperoxide favors the proximal, nonprotonated oxygen; and (iii) long-range ET can only occur at a late step with a large driving force, which serves to inhibit deleterious Fenton chemistry. The large inner-sphere reorganization energy at the ET site is used as a control mechanism to arrest premature ET and dictate the correct timing of ET.

  View details for PubMedID 27790986

• Activation in Cofactor Biogenesis. Journal of the American Chemical Society Cowley, R. E., Cirera, J., Qayyum, M. F., Rokhsana, D., Hedman, B., Hodgson, K. O., Dooley, D. M., Solomon, E. I. 2016; 138 (40): 13219-13229

  Abstract

  Galactose oxidase (GO) is a copper-dependent enzyme that accomplishes 2e(-) substrate oxidation by pairing a single copper with an unusual cysteinylated tyrosine (Cys-Tyr) redox cofactor. Previous studies have demonstrated that the post-translational biogenesis of Cys-Tyr is copper- and O2-dependent, resulting in a self-processing enzyme system. To investigate the mechanism of cofactor biogenesis in GO, the active-site structure of Cu(I)-loaded GO was determined using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, and density-functional theory (DFT) calculations were performed on this model. Our results show that the active-site tyrosine lowers the Cu potential to enable the thermodynamically unfavorable 1e(-) reduction of O2, and the resulting Cu(II)-O2(•-) is activated toward H atom abstraction from cysteine. The final step of biogenesis is a concerted reaction involving coordinated Tyr ring deprotonation where Cu(II) coordination enables formation of the Cys-Tyr cross-link. These spectroscopic and computational results highlight the role of the Cu(I) in enabling O2 activation by 1e(-) and the role of the resulting Cu(II) in enabling substrate activation for biogenesis.

  View details for PubMedID 27626829

• Structure of the Reduced Copper Active Site in Preprocessed Galactose Oxidase: Ligand Tuning for One-Electron O-2 Activation in Cofactor Biogenesis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Cowley, R. E., Cirera, J., Qayyum, M. F., Rokhsana, D., Hedman, B., Hodgson, K. O., Dooley, D. M., Solomon, E. I. 2016; 138 (40): 13219-13229

  Abstract

  Galactose oxidase (GO) is a copper-dependent enzyme that accomplishes 2e(-) substrate oxidation by pairing a single copper with an unusual cysteinylated tyrosine (Cys-Tyr) redox cofactor. Previous studies have demonstrated that the post-translational biogenesis of Cys-Tyr is copper- and O2-dependent, resulting in a self-processing enzyme system. To investigate the mechanism of cofactor biogenesis in GO, the active-site structure of Cu(I)-loaded GO was determined using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, and density-functional theory (DFT) calculations were performed on this model. Our results show that the active-site tyrosine lowers the Cu potential to enable the thermodynamically unfavorable 1e(-) reduction of O2, and the resulting Cu(II)-O2(•-) is activated toward H atom abstraction from cysteine. The final step of biogenesis is a concerted reaction involving coordinated Tyr ring deprotonation where Cu(II) coordination enables formation of the Cys-Tyr cross-link. These spectroscopic and computational results highlight the role of the Cu(I) in enabling O2 activation by 1e(-) and the role of the resulting Cu(II) in enabling substrate activation for biogenesis.

  View details for DOI 10.1021/jacs.6b05792

  View details for Web of Science ID 000385469600026

  View details for PubMedID 27626829

  View details for PubMedCentralID PMC5061629

• Activating Metal Sites for Biological Electron Transfer ISRAEL JOURNAL OF CHEMISTRY Solomon, E. I., Hadt, R. G., Snyder, B. E. 2016; 56 (9-10): 649-659

  Abstract

  This review focuses on the unique spectroscopic features of the blue copper active sites. These reflect a novel electronic structure that activates the site for rapid long-range electron transfer in its biological function. The role of the protein in determining the geometric and electronic structure of this site is defined, as is its contribution to function. This has been referred to as the entatic/rack-induced state. These concepts are then extended to cytochrome c, which is also determined to be in an entatic state.

  View details for DOI 10.1002/ijch.201600016

  View details for Web of Science ID 000385666100003

  View details for PubMedCentralID PMC5517049

• Activating Metal Sites for Biological Electron Transfer. Israel journal of chemistry Solomon, E. I., Hadt, R. G., Snyder, B. E. 2016; 56 (9-10): 649-659

  Abstract

  This review focuses on the unique spectroscopic features of the blue copper active sites. These reflect a novel electronic structure that activates the site for rapid long-range electron transfer in its biological function. The role of the protein in determining the geometric and electronic structure of this site is defined, as is its contribution to function. This has been referred to as the entatic/rack-induced state. These concepts are then extended to cytochrome c, which is also determined to be in an entatic state.

  View details for DOI 10.1002/ijch.201600016

  View details for PubMedID 28736456

  View details for PubMedCentralID PMC5517049

• Structure/function correlations over binuclear non-heme iron active sites. Journal of biological inorganic chemistry Solomon, E. I., Park, K. 2016; 21 (5-6): 575-588

  Abstract

  Binuclear non-heme iron enzymes activate O2 to perform diverse chemistries. Three different structural mechanisms of O2 binding to a coupled binuclear iron site have been identified utilizing variable-temperature, variable-field magnetic circular dichroism spectroscopy (VTVH MCD). For the μ-OH-bridged Fe(II)2 site in hemerythrin, O2 binds terminally to a five-coordinate Fe(II) center as hydroperoxide with the proton deriving from the μ-OH bridge and the second electron transferring through the resulting μ-oxo superexchange pathway from the second coordinatively saturated Fe(II) center in a proton-coupled electron transfer process. For carboxylate-only-bridged Fe(II)2 sites, O2 binding as a bridged peroxide requires both Fe(II) centers to be coordinatively unsaturated and has good frontier orbital overlap with the two orthogonal O2 π* orbitals to form peroxo-bridged Fe(III)2 intermediates. Alternatively, carboxylate-only-bridged Fe(II)2 sites with only a single open coordination position on an Fe(II) enable the one-electron formation of Fe(III)-O2 (-) or Fe(III)-NO(-) species. Finally, for the peroxo-bridged Fe(III)2 intermediates, further activation is necessary for their reactivities in one-electron reduction and electrophilic aromatic substitution, and a strategy consistent with existing spectral data is discussed.

  View details for DOI 10.1007/s00775-016-1372-9

  View details for PubMedID 27369780

  View details for PubMedCentralID PMC5010389

• Biophysical characterization and catalytic reactivity of rubrerythrin and symerythrin model proteins Pellegrino, J., Bell, K., Polinski, R., Cimerol, S., Jacobs, A., Solomon, E., Reig, A. AMER CHEMICAL SOC. 2016

  View details for Web of Science ID 000431460202219

• The active site of low-temperature methane hydroxylation in iron-containing zeolites. Nature Snyder, B. E., Vanelderen, P., Bols, M. L., Hallaert, S. D., Böttger, L. H., Ungur, L., Pierloot, K., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2016; 536 (7616): 317-321

  Abstract

  An efficient catalytic process for converting methane into methanol could have far-reaching economic implications. Iron-containing zeolites (microporous aluminosilicate minerals) are noteworthy in this regard, having an outstanding ability to hydroxylate methane rapidly at room temperature to form methanol. Reactivity occurs at an extra-lattice active site called α-Fe(ii), which is activated by nitrous oxide to form the reactive intermediate α-O; however, despite nearly three decades of research, the nature of the active site and the factors determining its exceptional reactivity are unclear. The main difficulty is that the reactive species-α-Fe(ii) and α-O-are challenging to probe spectroscopically: data from bulk techniques such as X-ray absorption spectroscopy and magnetic susceptibility are complicated by contributions from inactive 'spectator' iron. Here we show that a site-selective spectroscopic method regularly used in bioinorganic chemistry can overcome this problem. Magnetic circular dichroism reveals α-Fe(ii) to be a mononuclear, high-spin, square planar Fe(ii) site, while the reactive intermediate, α-O, is a mononuclear, high-spin Fe(iv)=O species, whose exceptional reactivity derives from a constrained coordination geometry enforced by the zeolite lattice. These findings illustrate the value of our approach to exploring active sites in heterogeneous systems. The results also suggest that using matrix constraints to activate metal sites for function-producing what is known in the context of metalloenzymes as an 'entatic' state-might be a useful way to tune the activity of heterogeneous catalysts.

  View details for DOI 10.1038/nature19059

  View details for PubMedID 27535535

• Reversible S-nitrosylation in an engineered azurin NATURE CHEMISTRY Tian, S., Liu, J., Cowley, R. E., Hosseinzadeh, P., Marshall, N. M., Yu, Y., Robinson, H., Nilges, M. J., Blackburn, N. J., Solomon, E. I., Lu, Y. 2016; 8 (7): 670-677

  Abstract

  S-Nitrosothiols are known as reagents for NO storage and transportation and as regulators in many physiological processes. Although the S-nitrosylation catalysed by haem proteins is well known, no direct evidence of S-nitrosylation in copper proteins has been reported. Here, we report reversible insertion of NO into a copper-thiolate bond in an engineered copper centre in Pseudomonas aeruginosa azurin by rational design of the primary coordination sphere and tuning its reduction potential by deleting a hydrogen bond in the secondary coordination sphere. The results not only provide the first direct evidence of S-nitrosylation of Cu(II)-bound cysteine in metalloproteins, but also shed light on the reaction mechanism and structural features responsible for stabilizing the elusive Cu(I)-S(Cys)NO species. The fast, efficient and reversible S-nitrosylation reaction is used to demonstrate its ability to prevent NO inhibition of cytochrome bo3 oxidase activity by competing for NO binding with the native enzyme under physiologically relevant conditions.

  View details for DOI 10.1038/nchem.2489

  View details for PubMedID 27325093

• Peroxo and Superoxo Moieties Bound to Copper Ion: Electron-Transfer Equilibrium with a Small Reorganization Energy JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Cao, R., Saracini, C., Ginsbach, J. W., Kieber-Emmons, M. T., Siegler, M. A., Solomon, E. I., Fukuzumi, S., Karlin, K. D. 2016; 138 (22): 7055-7066

  Abstract

  Oxygenation of [Cu2(UN-O(-))(DMF)](2+) (1), a structurally characterized dicopper Robin-Day class I mixed-valent Cu(II)Cu(I) complex, with UN-O(-) as a binucleating ligand and where dimethylformamide (DMF) binds to the Cu(II) ion, leads to a superoxo-dicopper(II) species [Cu(II)2(UN-O(-))(O2(•-))](2+) (2). The formation kinetics provide that kon = 9 × 10(-2) M(-1) s(-1) (-80 °C), ΔH(‡) = 31.1 kJ mol(-1) and ΔS(‡) = -99.4 J K(-1) mol(-1) (from -60 to -90 °C data). Complex 2 can be reversibly reduced to the peroxide species [Cu(II)2(UN-O(-))(O2(2-))](+) (3), using varying outer-sphere ferrocene or ferrocenium redox reagents. A Nernstian analysis could be performed by utilizing a monodiphenylamine substituted ferrocenium salt to oxidize 3, leading to an equilibrium mixture with Ket = 5.3 (-80 °C); a standard reduction potential for the superoxo-peroxo pair is calculated to be E° = +130 mV vs SCE. A literature survey shows that this value falls into the range of biologically relevant redox reagents, e.g., cytochrome c and an organic solvent solubilized ascorbate anion. Using mixed-isotope resonance Raman (rRaman) spectroscopic characterization, accompanied by DFT calculations, it is shown that the superoxo complex consists of a mixture of μ-1,2- (2(1,2)) and μ-1,1- (2(1,1)) isomers, which are in rapid equilibrium. The electron transfer process involves only the μ-1,2-superoxo complex [Cu(II)2(UN-O(-))(μ-1,2-O2(•-))](2+) (2(1,2)) and μ-1,2-peroxo structures [Cu(II)2(UN-O(-))(O2(2-))](+) (3) having a small bond reorganization energy of 0.4 eV (λin). A stopped-flow kinetic study results reveal an outer-sphere electron transfer process with a total reorganization energy (λ) of 1.1 eV between 2(1,2) and 3 calculated in the context of Marcus theory.

  View details for DOI 10.1021/jacs.6b02404

  View details for Web of Science ID 000377643300027

  View details for PubMedID 27228314

  View details for PubMedCentralID PMC4950875

• Electronic Structure of the Ferryl Intermediate in the alpha-Ketoglutarate Dependent Non-Heme Iron Halogenase SyrB2: Contributions to H Atom Abstraction Reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Srnec, M., Wong, S. D., Matthews, M. L., Krebs, C., Bollinger, J. M., Solomon, E. I. 2016; 138 (15): 5110-5122

  Abstract

  Low temperature magnetic circular dichroism (LT MCD) spectroscopy in combination with quantum-chemical calculations are used to define the electronic structure associated with the geometric structure of the Fe(IV)═O intermediate in SyrB2 that was previously determined by nuclear resonance vibrational spectroscopy. These studies elucidate key frontier molecular orbitals (FMOs) and their contribution to H atom abstraction reactivity. The VT MCD spectra of the enzymatic S = 2 Fe(IV)═O intermediate with Br(-) ligation contain information-rich features that largely parallel the corresponding spectra of the S = 2 model complex (TMG3tren)Fe(IV)═O (Srnec, M.; Wong, S. D.; England, J; Que, L; Solomon, E. I. Proc. Natl. Acad. Sci. USA 2012, 109, 14326-14331). However, quantitative differences are observed that correlate with π-anisotropy and oxo donor strength that perturb FMOs and affect reactivity. Due to π-anisotropy, the Fe(IV)═O active site exhibits enhanced reactivity in the direction of the substrate cavity that proceeds through a π-channel that is controlled by perpendicular orientation of the substrate C-H bond relative to the halide-Fe(IV)═O plane. Also, the increased intrinsic reactivity of the SyrB2 intermediate relative to the ferryl model complex is correlated to a higher oxyl character of the Fe(IV)═O at the transition states resulting from the weaker ligand field of the halogenase.

  View details for DOI 10.1021/jacs.6b01151

  View details for Web of Science ID 000374812100020

  View details for PubMedID 27021969

  View details for PubMedCentralID PMC4927264

• Dioxygen Activation by a Macrocyclic Copper Complex Leads to a Cu2O2 Core with Unexpected Structure and Reactivity CHEMISTRY-A EUROPEAN JOURNAL Garcia-Bosch, I., Cowley, R. E., Diaz, D. E., Siegler, M. A., Nam, W., Solomon, E. I., Karlin, K. D. 2016; 22 (15): 5133-5137

  Abstract

  We report the Cu(I) /O2 chemistry of complexes derived from the macrocylic ligands 14-TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and 12-TMC (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). While [(14-TMC)Cu(I) ](+) is unreactive towards dioxygen, the smaller analog [(12-TMC)Cu(I) (CH3 CN)](+) reacts with O2 to give a side-on bound peroxo-dicopper(II) species ((S) P), confirmed by spectroscopic and computational methods. Intriguingly, 12-TMC as a N4 donor ligand generates (S) P species, thus in contrast with the previous observation that such species are generated by N2 and N3 ligands. In addition, the reactivity of this macrocyclic side-on peroxo-dicopper(II) differs from typical (S) P species, because it reacts only with acid to release H2 O2 , in contrast with the classic reactivity of Cu2 O2 cores. Kinetics and computations are consistent with a protonation mechanism whereby the TMC acts as a hemilabile ligand and shuttles H(+) to an isomerized peroxo core.

  View details for DOI 10.1002/chem.201600551

  View details for Web of Science ID 000373483600012

  View details for PubMedID 26919169

  View details for PubMedCentralID PMC4852750

• Spectroscopic and Theoretical Study of Cu-I Binding to His111 in the Human Prion Protein Fragment 106-115 INORGANIC CHEMISTRY Arcos-Lopez, T., Qayyum, M., Rivillas-Acevedo, L., Miotto, M. C., Grande-Aztatzi, R., Fernandez, C. O., Hedman, B., Hodgson, K. O., Vela, A., Solomon, E. I., Quintanar, L. 2016; 55 (6): 2909-2922

  Abstract

  The ability of the cellular prion protein (PrP(C)) to bind copper in vivo points to a physiological role for PrP(C) in copper transport. Six copper binding sites have been identified in the nonstructured N-terminal region of human PrP(C). Among these sites, the His111 site is unique in that it contains a MKHM motif that would confer interesting Cu(I) and Cu(II) binding properties. We have evaluated Cu(I) coordination to the PrP(106-115) fragment of the human PrP protein, using NMR and X-ray absorption spectroscopies and electronic structure calculations. We find that Met109 and Met112 play an important role in anchoring this metal ion. Cu(I) coordination to His111 is pH-dependent: at pH >8, 2N1O1S species are formed with one Met ligand; in the range of pH 5-8, both methionine (Met) residues bind to Cu(I), forming a 1N1O2S species, where N is from His111 and O is from a backbone carbonyl or a water molecule; at pH <5, only the two Met residues remain coordinated. Thus, even upon drastic changes in the chemical environment, such as those occurring during endocytosis of PrP(C) (decreased pH and a reducing potential), the two Met residues in the MKHM motif enable PrP(C) to maintain the bound Cu(I) ions, consistent with a copper transport function for this protein. We also find that the physiologically relevant Cu(I)-1N1O2S species activates dioxygen via an inner-sphere mechanism, likely involving the formation of a copper(II) superoxide complex. In this process, the Met residues are partially oxidized to sulfoxide; this ability to scavenge superoxide may play a role in the proposed antioxidant properties of PrP(C). This study provides further insight into the Cu(I) coordination properties of His111 in human PrP(C) and the molecular mechanism of oxygen activation by this site.

  View details for DOI 10.1021/acs.inorgchem.5b02794

  View details for Web of Science ID 000372677800028

  View details for PubMedCentralID PMC4804749

• Spectroscopic and Theoretical Study of Cu(I) Binding to His111 in the Human Prion Protein Fragment 106-115. Inorganic chemistry Arcos-López, T., Qayyum, M., Rivillas-Acevedo, L., Miotto, M. C., Grande-Aztatzi, R., Fernández, C. O., Hedman, B., Hodgson, K. O., Vela, A., Solomon, E. I., Quintanar, L. 2016; 55 (6): 2909-2922

  Abstract

  The ability of the cellular prion protein (PrP(C)) to bind copper in vivo points to a physiological role for PrP(C) in copper transport. Six copper binding sites have been identified in the nonstructured N-terminal region of human PrP(C). Among these sites, the His111 site is unique in that it contains a MKHM motif that would confer interesting Cu(I) and Cu(II) binding properties. We have evaluated Cu(I) coordination to the PrP(106-115) fragment of the human PrP protein, using NMR and X-ray absorption spectroscopies and electronic structure calculations. We find that Met109 and Met112 play an important role in anchoring this metal ion. Cu(I) coordination to His111 is pH-dependent: at pH >8, 2N1O1S species are formed with one Met ligand; in the range of pH 5-8, both methionine (Met) residues bind to Cu(I), forming a 1N1O2S species, where N is from His111 and O is from a backbone carbonyl or a water molecule; at pH <5, only the two Met residues remain coordinated. Thus, even upon drastic changes in the chemical environment, such as those occurring during endocytosis of PrP(C) (decreased pH and a reducing potential), the two Met residues in the MKHM motif enable PrP(C) to maintain the bound Cu(I) ions, consistent with a copper transport function for this protein. We also find that the physiologically relevant Cu(I)-1N1O2S species activates dioxygen via an inner-sphere mechanism, likely involving the formation of a copper(II) superoxide complex. In this process, the Met residues are partially oxidized to sulfoxide; this ability to scavenge superoxide may play a role in the proposed antioxidant properties of PrP(C). This study provides further insight into the Cu(I) coordination properties of His111 in human PrP(C) and the molecular mechanism of oxygen activation by this site.

  View details for DOI 10.1021/acs.inorgchem.5b02794

  View details for PubMedID 26930130

  View details for PubMedCentralID PMC4804749

• Nuclear resonance vibrational spectroscopic elucidation of binuclear non-heme iron enzyme intermediates Park, K., Solomon, E. AMER CHEMICAL SOC. 2016

  View details for Web of Science ID 000431905702488

• Structural and functional characterization of G4DFsc variants containing a 4-His/3-carboxylate active site O'Shea, K., Dorsheimer, J., Biernat, K., Jacobs, A., Solomon, E., Wu, Y., Degrado, W., Reig, A. AMER CHEMICAL SOC. 2016

  View details for Web of Science ID 000431905702246

• Creation and characterization of rubrerythrin and symerythrin model proteins Pellegrino, J., Bell, K., Polinski, R., Cimerol, S., Jacobs, A., Solomon, E., Reig, A. AMER CHEMICAL SOC. 2016

  View details for Web of Science ID 000431905702245

• Award Address (Alfred Bader Award in Bioinorganic or Bioorganic Chemistry sponsored by the Alfred R. Bader Fund). Dioxygen binding, activation, and reduction to H2O by Cu enzymes Solomon, E. AMER CHEMICAL SOC. 2016

  View details for Web of Science ID 000431905702347

• From electronic properties of non-heme iron active sites to biocatalysis Srnec, M., Solomon, E. AMER CHEMICAL SOC. 2016

  View details for Web of Science ID 000431905702489

• Catalytic cycle of multi-copper oxidases studied by theoretical methods Rulisek, L., Solomon, E., Ryde, U. AMER CHEMICAL SOC. 2016

  View details for Web of Science ID 000431905702491

• Differential oxidase and oxygenase reactivities in de novo Due Ferri proteins Reig, A., Snyder, R., Butch, S., Degrado, W., Solomon, E. AMER CHEMICAL SOC. 2016

  View details for Web of Science ID 000431905702572

• Dioxygen Binding, Activation, and Reduction to H2O by Cu Enzymes. Inorganic chemistry Solomon, E. I. 2016; 55 (13): 6364–75

  Abstract

  Oxygen intermediates in copper enzymes exhibit unique spectroscopic features that reflect novel geometric and electronic structures that are key to reactivity. This perspective will describe: (1) the bonding origin of the unique spectroscopic features of the coupled binuclear copper enzymes and how this overcomes the spin forbiddenness of O2 binding and activates monooxygenase activity, (2) how the difference in exchange coupling in the non-coupled binuclear Cu enzymes controls the reaction mechanism, and (3) how the trinuclear Cu cluster present in the multicopper oxidases leads to a major structure/function difference in enabling the irreversible reductive cleavage of the O-O bond with little overpotential and generating a fully oxidized intermediate, different from the resting enzyme studied by crystallography, that is key in enabling fast PCET in the reductive half of the catalytic cycle.

  View details for PubMedID 27299802

• The active site of low-temperature methane hydroxylation in iron-containing zeolites Nature Snyder, B. E., Vanelderen, P., Bols, M. L., Hallaert, S. D., Boettger, L. H., Ungur, L., Pierloot, K., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2016; 536 (7616): 317-321

  Abstract

  An efficient catalytic process for converting methane into methanol could have far-reaching economic implications. Iron-containing zeolites (microporous aluminosilicate minerals) are noteworthy in this regard, having an outstanding ability to hydroxylate methane rapidly at room temperature to form methanol. Reactivity occurs at an extra-lattice active site called α-Fe(ii), which is activated by nitrous oxide to form the reactive intermediate α-O; however, despite nearly three decades of research, the nature of the active site and the factors determining its exceptional reactivity are unclear. The main difficulty is that the reactive species-α-Fe(ii) and α-O-are challenging to probe spectroscopically: data from bulk techniques such as X-ray absorption spectroscopy and magnetic susceptibility are complicated by contributions from inactive 'spectator' iron. Here we show that a site-selective spectroscopic method regularly used in bioinorganic chemistry can overcome this problem. Magnetic circular dichroism reveals α-Fe(ii) to be a mononuclear, high-spin, square planar Fe(ii) site, while the reactive intermediate, α-O, is a mononuclear, high-spin Fe(iv)=O species, whose exceptional reactivity derives from a constrained coordination geometry enforced by the zeolite lattice. These findings illustrate the value of our approach to exploring active sites in heterogeneous systems. The results also suggest that using matrix constraints to activate metal sites for function-producing what is known in the context of metalloenzymes as an 'entatic' state-might be a useful way to tune the activity of heterogeneous catalysts.

  View details for DOI 10.1038/nature19059

• High-Spin and Low-Spin States in {FeNO}(7), Fe-IV=O, and Fe-III-OOH Complexes and Their Correlations to Reactivity SPIN STATES IN BIOCHEMISTRY AND INORGANIC CHEMISTRY: INFLUENCE ON STRUCTURE AND REACTIVITY Solomon, E. I., Sutherlin, K. D., Srnec, M., Swart, M., Costas, M. 2016: 369–407

  View details for DOI 10.1002/9781118898277.ch15

  View details for Web of Science ID 000384712200016

• CD/MCD/VTVH-MCD Studies of Escherichia coli Bacterioferritin Support a Binuclear Iron Cofactor Site BIOCHEMISTRY Kwak, Y., Schwartz, J. K., Huang, V. W., Boice, E., Kurtz, D. M., Solomon, E. I. 2015; 54 (47): 7010-7018

  Abstract

  Ferritins and bacterioferritins (Bfrs) utilize a binuclear non-heme iron binding site to catalyze oxidation of Fe(II), leading to formation of an iron mineral core within a protein shell. Unlike ferritins, in which the diiron site binds Fe(II) as a substrate, which then autoxidizes and migrates to the mineral core, the diiron site in Bfr has a 2-His/4-carboxylate ligand set that is commonly found in diiron cofactor enzymes. Bfrs could, therefore, utilize the diiron site as a cofactor rather than for substrate iron binding. In this study, we applied circular dichroism (CD), magnetic CD (MCD), and variable-temperature, variable-field MCD (VTVH-MCD) spectroscopies to define the geometric and electronic structures of the biferrous active site in Escherichia coli Bfr. For these studies, we used an engineered M52L variant, which is known to eliminate binding of a heme cofactor but to have very minor effects on either iron oxidation or mineral core formation. We also examined an H46A/D50A/M52L Bfr variant, which additionally disrupts a previously observed mononuclear non-heme iron binding site inside the protein shell. The spectral analyses define a binuclear and an additional mononuclear ferrous site. The biferrous site shows two different five-coordinate centers. After O2 oxidation and re-reduction, only the mononuclear ferrous signal is eliminated. The retention of the biferrous but not the mononuclear ferrous site upon O2 cycling supports a mechanism in which the binuclear site acts as a cofactor for the O2 reaction, while the mononuclear site binds the substrate Fe(II) that, after its oxidation to Fe(III), migrates to the mineral core.

  View details for DOI 10.1021/acs.biochem.5b01033

  View details for Web of Science ID 000365930700005

  View details for PubMedID 26551523

• Spectroscopic and Computational Studies of Nitrile Hydratase: Insights into Geometric and Electronic Structure and the Mechanism of Amide Synthesis. Chemical science Light, K. M., Yamanaka, Y., Odaka, M., Solomon, E. I. 2015; 6 (11): 6280-6294

  Abstract

  Nitrile hydratases (NHases) are mononuclear nonheme enzymes that catalyze the hydration of nitriles to amides. NHase is unusual in that it utilizes a low-spin (LS) FeIII center and a unique ligand set comprised of two deprotonated backbone amides, cysteine-based sulfenic acid (RSO(H)) and sulfinic acid (RSO2-), and an unmodified cysteine trans to an exogenous ligand site. Electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD) and low-temperature absorption (LT-Abs) spectroscopies are used to determine the geometric and electronic structures of butyrate-bound (NHaseBA) and active (NHaseAq) NHase. These data calibrate DFT models, which are then extended to explore the mechanism of nitrile hydration by NHase. In particular, the nitrile is activated by coordination to the LS FeIII and the sulfenate group is found to be deprotonated and a significantly better nucleophile than water that can attack the coordinated nitrile to form a cyclic species. Attack at the sulfenate S atom of the cyclic species is favorable and leads to a lower kinetic barrier than attack by water on coordinated, uncyclized nitrile, while attack at the C of the cyclic species is unfavorable. The roles of the unique ligand set and low-spin nature of the NHase active site in function are also explored. It is found that the oxidized thiolate ligands are crucial to maintaining the LS state, which is important in the binding and activation of nitrile susbtrates. The dominant role of the backbone amidate ligands appears to be as a chelate in keeping the sulfenate properly oriented for nucleophilic attack on the coordinated substrate.

  View details for DOI 10.1039/C5SC02012C

  View details for PubMedID 26508996

  View details for PubMedCentralID PMC4618400

• Final-State Projection Method in Charge-Transfer Multiplet Calculations: An Analysis of Ti L-Edge Absorption Spectra JOURNAL OF PHYSICAL CHEMISTRY B Kroll, T., Solomon, E. I., de Groot, F. M. 2015; 119 (43): 13852-13858

  Abstract

  A projection method to determine the final-state configuration character of all peaks in a charge transfer multiplet calculation of a 2p X-ray absorption spectrum is presented using a d(0) system as an example. The projection method is used to identify the most important influences on spectral shape and to map out the configuration weights. The spectral shape of a 2p X-ray absorption or L2,3-edge spectrum is largely determined by the ratio of the 2p core-hole interactions relative to the 2p3d atomic multiplet interaction. This leads to a nontrivial spectral assignment, which makes a detailed theoretical description of experimental spectra valuable for the analysis of bonding.

  View details for DOI 10.1021/acs.jpcb.5b04133

  View details for Web of Science ID 000363994000043

  View details for PubMedID 26226507

  View details for PubMedCentralID PMC4779055

• Protonation state of the Cu4S2 CuZ site in nitrous oxide reductase: redox dependence and insight into reactivity. Chemical science Johnston, E. M., Dell'Acqua, S., Pauleta, S. R., Moura, I., Solomon, E. I. 2015; 6 (10): 5670-5679

  Abstract

  Spectroscopic and computational methods have been used to determine the protonation state of the edge sulfur ligand in the Cu4S2 CuZ form of the active site of nitrous oxide reductase (N2OR) in its 3CuICuII (1-hole) and 2CuI2CuII (2-hole) redox states. The EPR, absorption, and MCD spectra of 1-hole CuZ indicate that the unpaired spin in this site is evenly delocalized over CuI, CuII, and CuIV. 1-hole CuZ is shown to have a μ2-thiolate edge ligand from the observation of S-H bending modes in the resonance Raman spectrum at 450 and 492 cm-1 that have significant deuterium isotope shifts (-137 cm-1) and are not perturbed up to pH 10. 2-hole CuZ is characterized with absorption and resonance Raman spectroscopies as having two Cu-S stretching vibrations that profile differently. DFT models of the 1-hole and 2-hole CuZ sites are correlated to these spectroscopic features to determine that 2-hole CuZ has a μ2-sulfide edge ligand at neutral pH. The slow two electron (+1 proton) reduction of N2O by 1-hole CuZ is discussed and the possibility of a reaction between 2-hole CuZ and O2 is considered.

  View details for DOI 10.1039/C5SC02102B

  View details for PubMedID 26417423

  View details for PubMedCentralID PMC4583207

• Systematic Perturbations of Binuclear Non-heme Iron Sites: Structure and Dioxygen Reactivity of de Novo Due Ferri Proteins BIOCHEMISTRY Snyder, R. A., Betzu, J., Butch, S. E., Reig, A. J., DeGrado, W. F., Solomon, E. I. 2015; 54 (30): 4637-4651

  Abstract

  DFsc (single-chain due ferri) proteins allow for modeling binuclear non-heme iron enzymes with a similar fold. Three 4A → 4G variants of DFsc were studied to investigate the effects of (1) increasing the size of the substrate/solvent access channel (G4DFsc), (2) including an additional His residue in the first coordination sphere along with three additional helix-stabilizing mutations [3His-G4DFsc(Mut3)], and (3) the three helix-stabilizing mutations alone [G4DFsc(Mut3)] on the biferrous structures and their O2 reactivities. Near-infrared circular dichroism and magnetic circular dichroism (MCD) spectroscopy show that the 4A → 4G mutations increase coordination of the diiron site from 4-coordinate/5-coordinate to 5-coordinate/5-coordinate, likely reflecting increased solvent accessibility. While the three helix-stabilizing mutations [G4DFsc(Mut3)] do not affect the coordination number, addition of the third active site His residue [3His-G4DFsc(Mut3)] results in a 5-coordinate/6-coordinate site. Although all 4A→ 4G variants have significantly slower pseudo-first-order rates when reacting with excess O2 than DFsc (∼2 s(-1)), G4DFsc and 3His-G4DFsc(Mut3) have rates (∼0.02 and ∼0.04 s(-1)) faster than that of G4DFsc(Mut3) (∼0.002 s(-1)). These trends in the rate of O2 reactivity correlate with exchange coupling between the Fe(II) sites and suggest that the two-electron reduction of O2 occurs through end-on binding at one Fe(II) rather than through a peroxy-bridged intermediate. UV-vis absorption and MCD spectroscopies indicate that an Fe(III)Fe(III)-OH species first forms in all three variants but converts into an Fe(III)-μ-OH-Fe(III) species only in the 2-His forms, a process inhibited by the additional active site His ligand that coordinatively saturates one of the iron centers in 3His-G4DFsc(Mut3).

  View details for DOI 10.1021/acs.biochem.5b00324

  View details for Web of Science ID 000359277800007

  View details for PubMedID 26154739

• Molecular-Level Insight into the Differential Oxidase and Oxygenase Reactivities of de Novo Due Ferri Proteins. Journal of the American Chemical Society Snyder, R. A., Butch, S. E., Reig, A. J., DeGrado, W. F., Solomon, E. I. 2015; 137 (29): 9302-9314

  Abstract

  Using the single-chain due ferri (DFsc) peptide scaffold, the differential oxidase and oxygenase reactivities of two 4A→4G variants, one with two histidines at the diiron center (G4DFsc) and the other with three histidines (3His-G4DFsc(Mut3)), are explored. By controlling the reaction conditions, the active form responsible for 4-aminophenol (4-AP) oxidase activity in both G4DFsc and 3His-G4DFsc(Mut3) is determined to be the substrate-bound biferrous site. Using circular dichroism (CD), magnetic CD (MCD), and variable-temperature, variable-field (VTVH) MCD spectroscopies, 4-AP is found to bind directly to the biferrous sites of the DF proteins. In G4DFsc, 4-AP increases the coordination of the biferrous site, while in 3His-G4DFsc(Mut3), the coordination number remains the same and the substrate likely replaces the additional bound histidine. This substrate binding enables a two-electron process where 4-AP is oxidized to benzoquinone imine and O2 is reduced to H2O2. In contrast, only the biferrous 3His variant is found to be active in the oxygenation of p-anisidine to 4-nitroso-methoxybenzene. From CD, MCD, and VTVH MCD, p-anisidine addition is found to minimally perturb the biferrous centers of both G4DFsc and 3His-G4DFsc(Mut3), indicating that this substrate binds near the biferrous site. In 3His-G4DFsc(Mut3), the coordinative saturation of one iron leads to the two-electron reduction of O2 at the second iron to generate an end-on hydroperoxo-Fe(III) active oxygenating species.

  View details for DOI 10.1021/jacs.5b03524

  View details for PubMedID 26090726

• Two-Electron Reduction versus One-Electron Oxidation of the Type 3 Pair in the Multicopper Oxidases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Kjaergaard, C. H., Jones, S. M., Gounel, S., Mano, N., Solomon, E. I. 2015; 137 (27): 8783-8794

  Abstract

  Multicopper oxidases (MCOs) utilize an electron shuttling Type 1 Cu (T1) site in conjunction with a mononuclear Type 2 (T2) and a binuclear Type 3 (T3) site, arranged in a trinuclear copper cluster (TNC), to reduce O2 to H2O. Reduction of O2 occurs with limited overpotential indicating that all the coppers in the active site can be reduced via high-potential electron donors. Two forms of the resting enzyme have been observed in MCOs: the alternative resting form (AR), where only one of the three TNC Cu's is oxidized, and the resting oxidized form (RO), where all three TNC Cu's are oxidized. In contrast to the AR form, we show that in the RO form of a high-potential MCO, the binuclear T3 Cu(II) site can be reduced via the 700 mV T1 Cu. Systematic spectroscopic evaluation reveals that this proceeds by a two-electron process, where delivery of the first electron, forming a high energy, metastable half reduced T3 state, is followed by the rapid delivery of a second energetically favorable electron to fully reduce the T3 site. Alternatively, when this fully reduced binuclear T3 site is oxidized via the T1 Cu, a different thermodynamically favored half oxidized T3 form, i.e., the AR site, is generated. This behavior is evaluated by DFT calculations, which reveal that the protein backbone plays a significant role in controlling the environment of the active site coppers. This allows for the formation of the metastable, half reduced state and thus the complete reductive activation of the enzyme for catalysis.

  View details for DOI 10.1021/jacs.5b04136

  View details for Web of Science ID 000358186700024

  View details for PubMedCentralID PMC4504817

• Two-Electron Reduction versus One-Electron Oxidation of the Type 3 Pair in the Multicopper Oxidases. Journal of the American Chemical Society Kjaergaard, C. H., Jones, S. M., Gounel, S., Mano, N., Solomon, E. I. 2015; 137 (27): 8783-94

  Abstract

  Multicopper oxidases (MCOs) utilize an electron shuttling Type 1 Cu (T1) site in conjunction with a mononuclear Type 2 (T2) and a binuclear Type 3 (T3) site, arranged in a trinuclear copper cluster (TNC), to reduce O2 to H2O. Reduction of O2 occurs with limited overpotential indicating that all the coppers in the active site can be reduced via high-potential electron donors. Two forms of the resting enzyme have been observed in MCOs: the alternative resting form (AR), where only one of the three TNC Cu's is oxidized, and the resting oxidized form (RO), where all three TNC Cu's are oxidized. In contrast to the AR form, we show that in the RO form of a high-potential MCO, the binuclear T3 Cu(II) site can be reduced via the 700 mV T1 Cu. Systematic spectroscopic evaluation reveals that this proceeds by a two-electron process, where delivery of the first electron, forming a high energy, metastable half reduced T3 state, is followed by the rapid delivery of a second energetically favorable electron to fully reduce the T3 site. Alternatively, when this fully reduced binuclear T3 site is oxidized via the T1 Cu, a different thermodynamically favored half oxidized T3 form, i.e., the AR site, is generated. This behavior is evaluated by DFT calculations, which reveal that the protein backbone plays a significant role in controlling the environment of the active site coppers. This allows for the formation of the metastable, half reduced state and thus the complete reductive activation of the enzyme for catalysis.

  View details for DOI 10.1021/jacs.5b04136

  View details for PubMedID 26075678

  View details for PubMedCentralID PMC4504817

• Spectroscopic Definition of the Copper Active Sites in Mordenite: Selective Methane Oxidation JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Vanelderen, P., Snyder, B. E., Tsai, M., Hadt, R. G., Vancauwenbergh, J., Coussens, O., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2015; 137 (19): 6383-6392

  Abstract

  Two distinct [Cu-O-Cu](2+) sites with methane monooxygenase activity are identified in the zeolite Cu-MOR, emphasizing that this Cu-O-Cu active site geometry, having a ∠Cu-O-Cu ∼140°, is particularly formed and stabilized in zeolite topologies. Whereas in ZSM-5 a similar [Cu-O-Cu](2+) active site is located in the intersection of the two 10 membered rings, Cu-MOR provides two distinct local structures, situated in the 8 membered ring windows of the side pockets. Despite their structural similarity, as ascertained by electronic absorption and resonance Raman spectroscopy, the two Cu-O-Cu active sites in Cu-MOR clearly show different kinetic behaviors in selective methane oxidation. This difference in reactivity is too large to be ascribed to subtle differences in the ground states of the Cu-O-Cu sites, indicating the zeolite lattice tunes their reactivity through second-sphere effects. The MOR lattice is therefore functionally analogous to the active site pocket of a metalloenzyme, demonstrating that both the active site and its framework environment contribute to and direct reactivity in transition metal ion-zeolites.

  View details for DOI 10.1021/jacs.5b02817

  View details for Web of Science ID 000355053100041

  View details for PubMedID 25914019

• Evolution of thioether S-ligated primary CuI/O2 adducts: The 1st example of CuII-superoxo species with enhanced reactivity Lee, J., Kim, S., Cowley, R., Ginsbach, J., Siegler, M., Solomon, E., Karlin, K. AMER CHEMICAL SOC. 2015

  View details for Web of Science ID 000411186501490

• Structure/function correlations over non-heme iron enzymes Solomon, E. AMER CHEMICAL SOC. 2015

  View details for Web of Science ID 000411186501176

• Creation and characterization of rubrerythrin and symerythrin model proteins Pellegrino, J., Polinski, R., Cimerol, S., Jacobs, A., Solomon, E., Reig, A. AMER CHEMICAL SOC. 2015

  View details for Web of Science ID 000411186501255

• Amine Oxidative N-Dealkylation via Cupric Hydroperoxide Cu-OOH Homolytic Cleavage Followed by Site-Specific Fenton Chemistry JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Kim, S., Ginsbach, J. W., Lee, J. Y., Peterson, R. L., Liu, J. J., Siegler, M. A., Sarjeant, A. A., Solomon, E. I., Karlin, K. D. 2015; 137 (8): 2867-2874

  Abstract

  Copper(II) hydroperoxide species are significant intermediates in processes such as fuel cells and (bio)chemical oxidations, all involving stepwise reduction of molecular oxygen. We previously reported a Cu(II)-OOH species that performs oxidative N-dealkylation on a dibenzylamino group that is appended to the 6-position of a pyridyl donor of a tripodal tetradentate ligand. To obtain insights into the mechanism of this process, reaction kinetics and products were determined employing ligand substrates with various para-substituent dibenzyl pairs (-H,-H; -H,-Cl; -H,-OMe, and -Cl,-OMe), or with partially or fully deuterated dibenzyl N-(CH2Ph)2 moieties. A series of ligand-copper(II) bis-perchlorate complexes were synthesized, characterized, and the X-ray structures of the -H,-OMe analogue were determined. The corresponding metastable Cu(II)-OOH species were generated by addition of H2O2/base in acetone at -90 °C. These convert (t1/2 ≈ 53 s) to oxidatively N-dealkylated products, producing para-substituted benzaldehydes. Based on the experimental observations and supporting DFT calculations, a reaction mechanism involving dibenzylamine H-atom abstraction or electron-transfer oxidation by the Cu(II)-OOH entity could be ruled out. It is concluded that the chemistry proceeds by rate limiting Cu-O homolytic cleavage of the Cu(II)-(OOH) species, followed by site-specific copper Fenton chemistry. As a process of broad interest in copper as well as iron oxidative (bio)chemistries, a detailed computational analysis was performed, indicating that a Cu(I)OOH species undergoes O-O homolytic cleavage to yield a hydroxyl radical and Cu(II)OH rather than heterolytic cleavage to yield water and a Cu(II)-O(•-) species.

  View details for DOI 10.1021/ja508371q

  View details for Web of Science ID 000350614700019

  View details for PubMedID 25706825

• A N3S(thioether)-Ligated Cull-Superoxo with Enhanced Reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Kim, S., Lee, J. Y., Cowley, R. E., Ginsbach, J. W., Siegler, M. A., Soomon, E. I., Karlin, K. D. 2015; 137 (8): 2796-2799

  Abstract

  Previous efforts to synthesize a cupric superoxide complex possessing a thioether donor have resulted in the formation of an end-on trans-peroxo-dicopper(II) species, [{(Ligand)Cu(II)}2(μ-1,2-O2(2-))](2+). Redesign/modification of previous N3S tetradentate ligands has now allowed for the stabilization of the monomeric, superoxide product possessing a S(thioether) ligation, [((DMA)N3S)Cu(II)(O2(•-))](+) (2(S)), as characterized by UV-vis and resonance Raman spectroscopies. This complex mimics the putative Cu(II)(O2(•-)) active species of the copper monooxygenase PHM and exhibits enhanced reactivity toward both O-H and C-H substrates in comparison to close analogues [(L)Cu(II)(O2(•-))](+), where L contains only nitrogen donor atoms. Also, comparisons of [(L)Cu(II/I)](n+) compound reduction potentials (L = various N4 vs (DMA)N3S ligands) provide evidence that (DMA)N3S is a weaker donor to copper ion than is found for any N4 ligand-complex.

  View details for DOI 10.1021/ja511504n

  View details for Web of Science ID 000350614700003

  View details for PubMedCentralID PMC4482613

• New Insights into Structure and Luminescence of Eu-III and Sm-III Complexes of the 3,4,3-L1(1,2-HOPO) Ligand JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Daumann, L. J., Tatum, D. S., Snyder, B. E., Ni, C., Law, G., Solomon, E. I., Raymond, K. N. 2015; 137 (8): 2816-2819

  Abstract

  We report the preparation and new insight into photophysical properties of luminescent hydroxypyridonate complexes [M(III)L](-) (M = Eu or Sm) of the versatile 3,4,3-LI(1,2-HOPO) ligand (L). We report the crystal structure of this ligand with Eu(III) as well as insights into the coordination behavior and geometry in solution by using magnetic circular dichroism. In addition TD-DFT calculations were used to examine the excited states of the two different chromophores present in the 3,4,3-LI(1,2-HOPO) ligand. We find that the Eu(III) and Sm(III) complexes of this ligand undergo a transformation after in situ preparation to yield complexes with higher quantum yield (QY) over time. It is proposed that the lower QY in the in situ complexes is not only due to water quenching but could also be due to a lower degree of f-orbital overlap (in a kinetic isomer) as indicated by magnetic circular dichroism measurements.

  View details for DOI 10.1021/ja5116524

  View details for Web of Science ID 000350614700008

  View details for PubMedID 25607882

• A N3S(thioether)-ligated Cu(II)-superoxo with enhanced reactivity. Journal of the American Chemical Society Kim, S., Lee, J. Y., Cowley, R. E., Ginsbach, J. W., Siegler, M. A., Solomon, E. I., Karlin, K. D. 2015; 137 (8): 2796-2799

  Abstract

  Previous efforts to synthesize a cupric superoxide complex possessing a thioether donor have resulted in the formation of an end-on trans-peroxo-dicopper(II) species, [{(Ligand)Cu(II)}2(μ-1,2-O2(2-))](2+). Redesign/modification of previous N3S tetradentate ligands has now allowed for the stabilization of the monomeric, superoxide product possessing a S(thioether) ligation, [((DMA)N3S)Cu(II)(O2(•-))](+) (2(S)), as characterized by UV-vis and resonance Raman spectroscopies. This complex mimics the putative Cu(II)(O2(•-)) active species of the copper monooxygenase PHM and exhibits enhanced reactivity toward both O-H and C-H substrates in comparison to close analogues [(L)Cu(II)(O2(•-))](+), where L contains only nitrogen donor atoms. Also, comparisons of [(L)Cu(II/I)](n+) compound reduction potentials (L = various N4 vs (DMA)N3S ligands) provide evidence that (DMA)N3S is a weaker donor to copper ion than is found for any N4 ligand-complex.

  View details for DOI 10.1021/ja511504n

  View details for PubMedID 25697226

• Electron transfer and reaction mechanism of laccases. Cellular and molecular life sciences Jones, S. M., Solomon, E. I. 2015; 72 (5): 869-883

  Abstract

  Laccases are part of the family of multicopper oxidases (MCOs), which couple the oxidation of substrates to the four electron reduction of O2 to H2O. MCOs contain a minimum of four Cu's divided into Type 1 (T1), Type 2 (T2), and binuclear Type 3 (T3) Cu sites that are distinguished based on unique spectroscopic features. Substrate oxidation occurs near the T1, and electrons are transferred approximately 13 Å through the protein via the Cys-His pathway to the T2/T3 trinuclear copper cluster (TNC), where dioxygen reduction occurs. This review outlines the electron transfer (ET) process in laccases, and the mechanism of O2 reduction as elucidated through spectroscopic, kinetic, and computational data. Marcus theory is used to describe the relevant factors which impact ET rates including the driving force, reorganization energy, and electronic coupling matrix element. Then, the mechanism of O2 reaction is detailed with particular focus on the intermediates formed during the two 2e(-) reduction steps. The first 2e(-) step forms the peroxide intermediate, followed by the second 2e(-) step to form the native intermediate, which has been shown to be the catalytically relevant fully oxidized form of the enzyme.

  View details for DOI 10.1007/s00018-014-1826-6

  View details for PubMedID 25572295

  View details for PubMedCentralID PMC4323859

• A "Naked" Fe-III-(O-2(2-))-Cu-II Species Allows for Structural and Spectroscopic Tuning of Low-Spin Heme-Peroxo-Cu Complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Garcia-Bosch, I., Adam, S. M., Schaefer, A. W., Sharma, S. K., Peterson, R. L., Solomon, E. I., Karlin, K. D. 2015; 137 (3): 1032-1035

  Abstract

  Here we describe a new approach for the generation of heme-peroxo-Cu compounds, using a "naked" complex synthon, [(F8)Fe(III)-(O2(2-))-Cu(II)(MeTHF)3](+) (MeTHF = 2-methyltetrahydrofuran; F8 = tetrakis(2,6-difluorophenyl)porphyrinate). Addition of varying ligands (L) for Cu allows the generation and spectroscopic characterization of a family of high- and low-spin Fe(III)-(O2(2-))-Cu(II)(L) complexes. These possess markedly varying Cu(II) coordination geometries, leading to tunable Fe-O, O-O, and Cu-O bond strengths. DFT calculations accompanied by vibrational data correlations give detailed structural insights.

  View details for DOI 10.1021/ja5115198

  View details for Web of Science ID 000348690100007

  View details for PubMedID 25594533

  View details for PubMedCentralID PMC4311974

• Spectroscopic and computational studies of nitrile hydratase: insights into geometric and electronic structure and the mechanism of amide synthesis CHEMICAL SCIENCE Light, K. M., Yamanaka, Y., Odaka, M., Solomon, E. I. 2015; 6 (11): 6280-6294

  Abstract

  Nitrile hydratases (NHases) are mononuclear nonheme enzymes that catalyze the hydration of nitriles to amides. NHase is unusual in that it utilizes a low-spin (LS) FeIII center and a unique ligand set comprised of two deprotonated backbone amides, cysteine-based sulfenic acid (RSO(H)) and sulfinic acid (RSO2-), and an unmodified cysteine trans to an exogenous ligand site. Electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD) and low-temperature absorption (LT-Abs) spectroscopies are used to determine the geometric and electronic structures of butyrate-bound (NHaseBA) and active (NHaseAq) NHase. These data calibrate DFT models, which are then extended to explore the mechanism of nitrile hydration by NHase. In particular, the nitrile is activated by coordination to the LS FeIII and the sulfenate group is found to be deprotonated and a significantly better nucleophile than water that can attack the coordinated nitrile to form a cyclic species. Attack at the sulfenate S atom of the cyclic species is favorable and leads to a lower kinetic barrier than attack by water on coordinated, uncyclized nitrile, while attack at the C of the cyclic species is unfavorable. The roles of the unique ligand set and low-spin nature of the NHase active site in function are also explored. It is found that the oxidized thiolate ligands are crucial to maintaining the LS state, which is important in the binding and activation of nitrile susbtrates. The dominant role of the backbone amidate ligands appears to be as a chelate in keeping the sulfenate properly oriented for nucleophilic attack on the coordinated substrate.

  View details for DOI 10.1039/c5sc02012c

  View details for Web of Science ID 000362977000031

  View details for PubMedCentralID PMC4618400

• MOLECULAR PROPERTIES AND REACTION MECHANISM OF MULTICOPPER OXIDASES RELATED TO THEIR USE IN BIOFUEL CELLS ELECTROCHEMICAL PROCESSES IN BIOLOGICAL SYSTEMS Solomon, E. I., Kjaergaard, C. H., Heppner, D. E., Lewenstam, A., Gorton, L. 2015: 169–212

  View details for Web of Science ID 000374415900009

• Protonation state of the Cu4S2 Cu-Z site in nitrous oxide reductase: redox dependence and insight into reactivity CHEMICAL SCIENCE Johnston, E. M., Dell'Acqua, S., Pauleta, S. R., Moura, I., Solomon, E. I. 2015; 6 (10): 5670-5679

  Abstract

  Spectroscopic and computational methods have been used to determine the protonation state of the edge sulfur ligand in the Cu4S2 CuZ form of the active site of nitrous oxide reductase (N2OR) in its 3CuICuII (1-hole) and 2CuI2CuII (2-hole) redox states. The EPR, absorption, and MCD spectra of 1-hole CuZ indicate that the unpaired spin in this site is evenly delocalized over CuI, CuII, and CuIV. 1-hole CuZ is shown to have a μ2-thiolate edge ligand from the observation of S-H bending modes in the resonance Raman spectrum at 450 and 492 cm-1 that have significant deuterium isotope shifts (-137 cm-1) and are not perturbed up to pH 10. 2-hole CuZ is characterized with absorption and resonance Raman spectroscopies as having two Cu-S stretching vibrations that profile differently. DFT models of the 1-hole and 2-hole CuZ sites are correlated to these spectroscopic features to determine that 2-hole CuZ has a μ2-sulfide edge ligand at neutral pH. The slow two electron (+1 proton) reduction of N2O by 1-hole CuZ is discussed and the possibility of a reaction between 2-hole CuZ and O2 is considered.

  View details for DOI 10.1039/c5sc02102b

  View details for Web of Science ID 000361212000038

  View details for PubMedCentralID PMC4583207

• Resonant Inelastic X-ray Scattering on Ferrous and Ferric Bis-imidazole Porphyrin and Cytochrome c: Nature and Role of the Axial Methionine-Fe Bond. Journal of the American Chemical Society Kroll, T., Hadt, R. G., Wilson, S. A., Lundberg, M., Yan, J. J., Weng, T., Sokaras, D., Alonso-Mori, R., Casa, D., Upton, M. H., Hedman, B., Hodgson, K. O., Solomon, E. I. 2014; 136 (52): 18087-18099

  Abstract

  Axial Cu-S(Met) bonds in electron transfer (ET) active sites are generally found to lower their reduction potentials. An axial S(Met) bond is also present in cytochrome c (cyt c) and is generally thought to increase the reduction potential. The highly covalent nature of the porphyrin environment in heme proteins precludes using many spectroscopic approaches to directly study the Fe site to experimentally quantify this bond. Alternatively, L-edge X-ray absorption spectroscopy (XAS) enables one to directly focus on the 3d-orbitals in a highly covalent environment and has previously been successfully applied to porphyrin model complexes. However, this technique cannot be extended to metalloproteins in solution. Here, we use metal K-edge XAS to obtain L-edge like data through 1s2p resonance inelastic X-ray scattering (RIXS). It has been applied here to a bis-imidazole porphyrin model complex and cyt c. The RIXS data on the model complex are directly correlated to L-edge XAS data to develop the complementary nature of these two spectroscopic methods. Comparison between the bis-imidazole model complex and cyt c in ferrous and ferric oxidation states show quantitative differences that reflect differences in axial ligand covalency. The data reveal an increased covalency for the S(Met) relative to N(His) axial ligand and a higher degree of covalency for the ferric states relative to the ferrous states. These results are reproduced by DFT calculations, which are used to evaluate the thermodynamics of the Fe-S(Met) bond and its dependence on redox state. These results provide insight into a number of previous chemical and physical results on cyt c.

  View details for DOI 10.1021/ja5100367

  View details for PubMedID 25475739

  View details for PubMedCentralID PMC4291809

• Mechanism of the Reduction of the Native Intermediate in the Multicopper Oxidases: Insights into Rapid Intramolecular Electron Transfer in Turnover JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Heppner, D. E., Kjaergaard, C. H., Solomon, E. I. 2014; 136 (51): 17788-17801

  Abstract

  The multicopper oxidases (MCOs) are the family of enzymes that catalyze the 4-electron reduction of O2 to H2O coupled to the four 1-electron oxidations of substrate. In the catalytic cycle electrons are transferred intramolecularly over ∼13 Å from a Type 1 (T1) Cu site that accepts electrons from substrate to a trinuclear Cu cluster (TNC) where O2 is reduced to H2O at rapid rates consistent with turnover (560 s(-1)). The oxygen reduction mechanism for the MCOs is well-characterized, whereas the rereduction is less understood. Our initial study of Rhus vernicifera Laccase (Heppner et al. J. Am. Chem. Soc. 2013, 135, 12212) experimentally established that the native intermediate (NI), the species formed upon O-O bond cleavage, is reduced with an IET rate >700 s(-1) and is the catalytically relevant fully oxidized form of the enzyme, rather than the resting state. In this report, we present kinetic and spectroscopic results coupled to DFT calculations that evaluate the mechanism of the 3 e(-)/3 H(+) reduction of NI, where all three catalytically relevant intramolecular electron transfer (IET) steps are rapid and involve three different structural changes. These three rapid IET processes reflect the sophisticated mechanistic control of the TNC to enable rapid turnover. All three IET processes are fast due to the associated protonation of the bridging oxo and hydroxo ligands, generated by O-O cleavage, to form water products that are extruded from the TNC upon full reduction, thereby defining a unifying mechanism for oxygen reduction and rapid IET by the TNC in the catalytic cycle of the MCOs.

  View details for DOI 10.1021/ja509150j

  View details for Web of Science ID 000347139200016

  View details for PubMedID 25490729

  View details for PubMedCentralID PMC4291763

• Reactivity of the binuclear non-heme iron active site of ?? desaturase studied by large-scale multireference ab initio calculations. Journal of the American Chemical Society Chalupský, J., Rokob, T. A., Kurashige, Y., Yanai, T., Solomon, E. I., Rulíšek, L., Srnec, M. 2014; 136 (45): 15977-15991

  Abstract

  The results of density matrix renormalization group complete active space self-consistent field (DMRG-CASSCF) and second-order perturbation theory (DMRG-CASPT2) calculations are presented on various structural alternatives for the O-O and first C-H activating step of the catalytic cycle of the binuclear nonheme iron enzyme Δ(9) desaturase. This enzyme is capable of inserting a double bond into an alkyl chain by double hydrogen (H) atom abstraction using molecular O2. The reaction step studied here is presumably associated with the highest activation barrier along the full pathway; therefore, its quantitative assessment is of key importance to the understanding of the catalysis. The DMRG approach allows unprecedentedly large active spaces for the explicit correlation of electrons in the large part of the chemically important valence space, which is apparently conditio sine qua non for obtaining well-converged reaction energetics. The derived reaction mechanism involves protonation of the previously characterized 1,2-μ peroxy Fe(III)Fe(III) (P) intermediate to a 1,1-μ hydroperoxy species, which abstracts an H atom from the C10 site of the substrate. An Fe(IV)-oxo unit is generated concomitantly, supposedly capable of the second H atom abstraction from C9. In addition, several popular DFT functionals were compared to the computed DMRG-CASPT2 data. Notably, many of these show a preference for heterolytic C-H cleavage, erroneously predicting substrate hydroxylation. This study shows that, despite its limitations, DMRG-CASPT2 is a significant methodological advancement toward the accurate computational treatment of complex bioinorganic systems, such as those with the highly open-shell diiron active sites.

  View details for DOI 10.1021/ja506934k

  View details for PubMedID 25313991

• Anisotropic Covalency Contributions to Superexchange Pathways in Type One Copper Active Sites JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hadt, R. G., Gorelsky, S. I., Solomon, E. I. 2014; 136 (42): 15034-15045

  Abstract

  Type one (T1) Cu sites deliver electrons to catalytic Cu active sites: the mononuclear type two (T2) Cu site in nitrite reductases (NiRs) and the trinuclear Cu cluster in the multicopper oxidases (MCOs). The T1 Cu and the remote catalytic sites are connected via a Cys-His intramolecular electron-transfer (ET) bridge, which contains two potential ET pathways: P1 through the protein backbone and P2 through the H-bond between the Cys and the His. The high covalency of the T1 Cu-S(Cys) bond is shown here to activate the T1 Cu site for hole superexchange via occupied valence orbitals of the bridge. This covalency-activated electronic coupling (H(DA)) facilitates long-range ET through both pathways. These pathways can be selectively activated depending on the geometric and electronic structure of the T1 Cu site and thus the anisotropic covalency of the T1 Cu-S(Cys) bond. In NiRs, blue (π-type) T1 sites utilize P1 and green (σ-type) T1 sites utilize P2, with P2 being more efficient. Comparing the MCOs to NiRs, the second-sphere environment changes the conformation of the Cys-His pathway, which selectively activates HDA for superexchange by blue π sites for efficient turnover in catalysis. These studies show that a given protein bridge, here Cys-His, provides different superexchange pathways and electronic couplings depending on the anisotropic covalencies of the donor and acceptor metal sites.

  View details for DOI 10.1021/ja508361h

  View details for Web of Science ID 000343686500061

  View details for PubMedCentralID PMC4210080

• Anisotropic covalency contributions to superexchange pathways in type one copper active sites. Journal of the American Chemical Society Hadt, R. G., Gorelsky, S. I., Solomon, E. I. 2014; 136 (42): 15034-15045

  Abstract

  Type one (T1) Cu sites deliver electrons to catalytic Cu active sites: the mononuclear type two (T2) Cu site in nitrite reductases (NiRs) and the trinuclear Cu cluster in the multicopper oxidases (MCOs). The T1 Cu and the remote catalytic sites are connected via a Cys-His intramolecular electron-transfer (ET) bridge, which contains two potential ET pathways: P1 through the protein backbone and P2 through the H-bond between the Cys and the His. The high covalency of the T1 Cu-S(Cys) bond is shown here to activate the T1 Cu site for hole superexchange via occupied valence orbitals of the bridge. This covalency-activated electronic coupling (H(DA)) facilitates long-range ET through both pathways. These pathways can be selectively activated depending on the geometric and electronic structure of the T1 Cu site and thus the anisotropic covalency of the T1 Cu-S(Cys) bond. In NiRs, blue (π-type) T1 sites utilize P1 and green (σ-type) T1 sites utilize P2, with P2 being more efficient. Comparing the MCOs to NiRs, the second-sphere environment changes the conformation of the Cys-His pathway, which selectively activates HDA for superexchange by blue π sites for efficient turnover in catalysis. These studies show that a given protein bridge, here Cys-His, provides different superexchange pathways and electronic couplings depending on the anisotropic covalencies of the donor and acceptor metal sites.

  View details for DOI 10.1021/ja508361h

  View details for PubMedID 25310460

  View details for PubMedCentralID PMC4210080

• Modeling nuclear resonance vibrational spectroscopic data of binuclear nonheme iron enzymes using density functional theory CANADIAN JOURNAL OF CHEMISTRY Park, K., Solomon, E. I. 2014; 92 (10): 975-978

  Abstract

  Nuclear resonance vibrational spectroscopy (NRVS) is a powerful technique that can provide geometric structural information on key reaction intermediates of Fe-containing systems when utilized in combination with density functional theory (DFT). However, in the case of binuclear non-heme iron enzymes, DFT-predicted NRVS spectra have been found to be sensitive to truncation method used to model the active sites of the enzymes. Therefore, in this study various-level truncation schemes have been tested to predict the NRVS spectrum of a binuclear non-heme iron enzyme, and a reasonably sized DFT model that is suitable for employing the NRVS/DFT combined methodology to characterize binuclear non-heme iron enzymes has been developed.

  View details for DOI 10.1139/cjc-2014-0067

  View details for Web of Science ID 000343118800011

  View details for PubMedCentralID PMC5607781

• Modeling nuclear resonance vibrational spectroscopic data of binuclear non-heme iron enzymes using density functional theory. Canadian journal of chemistry Park, K., Solomon, E. I. 2014; 92 (10): 975-978

  Abstract

  Nuclear resonance vibrational spectroscopy (NRVS) is a powerful technique that can provide geometric structural information on key reaction intermediates of Fe-containing systems when utilized in combination with density functional theory (DFT). However, in the case of binuclear non-heme iron enzymes, DFT-predicted NRVS spectra have been found to be sensitive to truncation method used to model the active sites of the enzymes. Therefore, in this study various-level truncation schemes have been tested to predict the NRVS spectrum of a binuclear non-heme iron enzyme, and a reasonably sized DFT model that is suitable for employing the NRVS/DFT combined methodology to characterize binuclear non-heme iron enzymes has been developed.

  View details for DOI 10.1139/cjc-2014-0067

  View details for PubMedID 28943644

  View details for PubMedCentralID PMC5607781

• Spectroscopic and computational insight into the activation of O-2 by the mononuclear Cu center in Polysaccharide monooxygenases Kjaergaard, C. H., Qayyum, M. F., Wong, S. D., Xu, F., Hemsworth, G. R., Davies, G. J., Walton, P. H., Johansen, K. S., Hodgson, K. O., Hedman, B., Solomon, E. I. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349167401519

• Structure/function correlations over non-heme iron enzymes Solomon, E. I. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349165102029

• Copper/dioxygen (bio)inorganic chemistry Solomon, E. I. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349167401215

• Reactive intermediates in Cu MOR zeolites for alkane oxidation Vanelderen, P., Hadt, R. G., Kirschhock, C., Schoonheydt, R. A., Solomon, E. I., Sels, B. F. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349167401228

• Spectroscopy and redox chemistry of copper in mordenite Vanelderen, P., Vancauwenbergh, J., Hadt, R. G., Tsai, M., Snyder, B. R., Solomon, E. I., Schoonheydt, R. A., Sels, B. F. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349167401428

• Molecular insights into the rates of intramolecular electron transfer in the multicopper oxidases Heppner, D. E., Kjaergaard, C. H., Solomon, E. I. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349167401515

• Electronic structure and reactivities of resting and intermediate forms of the tetranuclear copper cluster in nitrous oxide reductase Johnston, E. M., Dell'Acqua, S., Gorelsky, S., Pauleta, S. R., Moura, I., Solomon, E. I. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349167401517

• Structure and function studies of systematic perturbations on de novo Due Ferri proteins: Insights into oxygen-dependent reactivity Snyder, R., Reig, A. J., Butch, S. E., Betzu, J., DeGrado, W. F., Solomon, E. I. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349167401584

• O-2 and N2O activation in transition metal containing zeolites: Comparing heterogeneous and enzymatic catalysis Hadt, R. G., Tsai, M., Vanelderen, P., Snyder, B. R., Sels, B. F., Schoonheydt, R. A., Solomon, E. I. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349167401589

• Structure/function correlations in the coupled binuclear copper enzyme family Ginsbach, J. W., Solomon, E. I. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349167401587

• Geometric and electronic structural contributions to Fe/O-2 reactivity Solomon, E. I. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349167401161

• Structure/function relations in binuclear non-heme iron enzymes Boettger, L. H., Light, K. M., Knoot, C., Farrugia, M., Park, K., Sutherlin, K. D., Libscomb, J. D., Shanklin, J., Hausinger, R. P., Solomon, E. I. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349167401433

• Electronic structure and oxo transfer reactivity in Mo enzymes Ha, Y., Tenderholt, A. L., Holm, R. H., Hedman, B., Hodgson, K. O., Solomon, E. I. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000349167401513

• Mechanistic Insights into the Oxidation of Substituted Phenols via Hydrogen Atom Abstraction by a Cupric-Superoxo Complex JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lee, J. Y., Peterson, R. L., Ohkubo, K., Garcia-Bosch, I., Himes, R. A., Woertink, J., Moore, C. D., Solomon, E. I., Fukuzumi, S., Karlin, K. D. 2014; 136 (28): 9925-9937

  Abstract

  To obtain mechanistic insights into the inherent reactivity patterns for copper(I)-O2 adducts, a new cupric-superoxo complex [(DMM-tmpa)Cu(II)(O2(•-))](+) (2) [DMM-tmpa = tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine] has been synthesized and studied in phenol oxidation-oxygenation reactions. Compound 2 is characterized by UV-vis, resonance Raman, and EPR spectroscopies. Its reactions with a series of para-substituted 2,6-di-tert-butylphenols (p-X-DTBPs) afford 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) in up to 50% yields. Significant deuterium kinetic isotope effects and a positive correlation of second-order rate constants (k2) compared to rate constants for p-X-DTBPs plus cumylperoxyl radical reactions indicate a mechanism that involves rate-limiting hydrogen atom transfer (HAT). A weak correlation of (k(B)T/e) ln k2 versus E(ox) of p-X-DTBP indicates that the HAT reactions proceed via a partial transfer of charge rather than a complete transfer of charge in the electron transfer/proton transfer pathway. Product analyses, (18)O-labeling experiments, and separate reactivity employing the 2,4,6-tri-tert-butylphenoxyl radical provide further mechanistic insights. After initial HAT, a second molar equiv of 2 couples to the phenoxyl radical initially formed, giving a Cu(II)-OO-(ArO') intermediate, which proceeds in the case of p-OR-DTBP substrates via a two-electron oxidation reaction involving hydrolysis steps which liberate H2O2 and the corresponding alcohol. By contrast, four-electron oxygenation (O-O cleavage) mainly occurs for p-R-DTBP which gives (18)O-labeled DTBQ and elimination of the R group.

  View details for DOI 10.1021/ja503105b

  View details for Web of Science ID 000339228200033

  View details for PubMedID 24953129

  View details for PubMedCentralID PMC4102632

• Preface for the forum on insights into spectroscopy and reactivity from electronic structure theory. Inorganic chemistry Gagliardi, L., Solomon, E. I. 2014; 53 (13): 6357-6360

  View details for DOI 10.1021/ic5013654

  View details for PubMedID 24999856

• The Role of Chloride in the Mechanism of O-2 Activation at the Mononuclear Nonheme Fe(II) Center of the Halogenase HctB JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pratter, S. M., Light, K. M., Solomon, E. I., Straganz, G. D. 2014; 136 (26): 9385-9395

  Abstract

  Mononuclear nonheme Fe(II) (MNH) and α-ketoglutarate (α-KG) dependent halogenases activate O2 to perform oxidative halogenations of activated and nonactivated carbon centers. While the mechanism of halide incorporation into a substrate has been investigated, the mechanism by which halogenases prevent oxidations in the absence of chloride is still obscure. Here, we characterize the impact of chloride on the metal center coordination and reactivity of the fatty acyl-halogenase HctB. Stopped-flow kinetic studies show that the oxidative transformation of the Fe(II)-α-KG-enzyme complex is >200-fold accelerated by saturating concentrations of chloride in both the absence and presence of a covalently bound substrate. By contrast, the presence of substrate, which generally brings about O2 activation at enzymatic MNH centers, only has an ∼10-fold effect in the absence of chloride. Circular dichroism (CD) and magnetic CD (MCD) studies demonstrate that chloride binding triggers changes in the metal center ligation: chloride binding induces the proper binding of the substrate as shown by variable-temperature, variable-field (VTVH) MCD studies of non-α-KG-containing forms and the conversion from six-coordinate (6C) to 5C/6C mixtures when α-KG is bound. In the presence of substrate, a site with square pyramidal five-coordinate (5C) geometry is observed, which is required for O2 activation at enzymatic MNH centers. In the absence of substrate an unusual trigonal bipyramidal site is formed, which accounts for the observed slow, uncoupled reactivity. Molecular dynamics simulations suggest that the binding of chloride to the metal center of HctB leads to a conformational change in the enzyme that makes the active site more accessible to the substrate and thus facilitates the formation of the catalytically competent enzyme-substrate complex. Results are discussed in relation to other MNH dependent halogenases.

  View details for DOI 10.1021/ja503179m

  View details for Web of Science ID 000338692700025

  View details for PubMedID 24847780

  View details for PubMedCentralID PMC4091267

• Efficient C-H Bond Activations via O2 Cleavage by a Dianionic Cobalt(II) Complex. Chemical science Nguyen, A. I., Hadt, R. G., Solomon, E. I., Tilley, T. D. 2014; 5 (7): 2874-2878

  Abstract

  A dianionic, square planar cobalt(II) complex reacts with O2 in the presence of acetonitrile to give a cyanomethylcobalt(III) complex formed by C-H bond cleavage. Interestingly, PhIO and p-tolylazide react similarly to give the same cyanomethylcobalt(III) complex. Competition studies with various hydrocarbon substrates indicate that the rate of C-H bond cleavage greatly depends on the p Ka of the C-H bond, rather than on the C-H bond dissociation energy. Kinetic isotope experiments reveal a moderate KIE value of ca. 3.5 using either O2 or PhIO. The possible involvement of a cobalt(IV) oxo species in this chemistry is discussed.

  View details for DOI 10.1039/C4SC00108G

  View details for PubMedID 25071930

  View details for PubMedCentralID PMC4111274

• Sulfur K-Edge X-ray Absorption Spectroscopy and Density Functional Theory Calculations on Monooxo Mo-IV and Bisoxo Mo-VI Bis-dithiolenes: Insights into the Mechanism of Oxo Transfer in Sulfite Oxidase and Its Relation to the Mechanism of DMSO Reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ha, Y., Tenderholt, A. L., Holm, R. H., Hedman, B., Hodgson, K. O., Solomon, E. I. 2014; 136 (25): 9094-9105

  Abstract

  Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations have been used to determine the electronic structures of two complexes [Mo(IV)O(bdt)2](2-) and [Mo(VI)O2(bdt)2](2-) (bdt = benzene-1,2-dithiolate(2-)) that relate to the reduced and oxidized forms of sulfite oxidase (SO). These are compared with those of previously studied dimethyl sulfoxide reductase (DMSOr) models. DFT calculations supported by the data are extended to evaluate the reaction coordinate for oxo transfer to a phosphite ester substrate. Three possible transition states are found with the one at lowest energy, stabilized by a P-S interaction, in good agreement with experimental kinetics data. Comparison of both oxo transfer reactions shows that in DMSOr, where the oxo is transferred from the substrate to the metal ion, the oxo transfer induces electron transfer, while in SO, where the oxo transfer is from the metal site to the substrate, the electron transfer initiates oxo transfer. This difference in reactivity is related to the difference in frontier molecular orbitals (FMO) of the metal-oxo and substrate-oxo bonds. Finally, these experimentally related calculations are extended to oxo transfer by sulfite oxidase. The presence of only one dithiolene at the enzyme active site selectively activates the equatorial oxo for transfer, and allows facile structural reorganization during turnover.

  View details for DOI 10.1021/ja503316p

  View details for Web of Science ID 000338184200041

  View details for PubMedCentralID PMC4073832

• Sulfur K-edge X-ray absorption spectroscopy and density functional theory calculations on monooxo Mo(IV) and bisoxo Mo(VI) bis-dithiolenes: insights into the mechanism of oxo transfer in sulfite oxidase and its relation to the mechanism of DMSO reductase. Journal of the American Chemical Society Ha, Y., Tenderholt, A. L., Holm, R. H., Hedman, B., Hodgson, K. O., Solomon, E. I. 2014; 136 (25): 9094-9105

  Abstract

  Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations have been used to determine the electronic structures of two complexes [Mo(IV)O(bdt)2](2-) and [Mo(VI)O2(bdt)2](2-) (bdt = benzene-1,2-dithiolate(2-)) that relate to the reduced and oxidized forms of sulfite oxidase (SO). These are compared with those of previously studied dimethyl sulfoxide reductase (DMSOr) models. DFT calculations supported by the data are extended to evaluate the reaction coordinate for oxo transfer to a phosphite ester substrate. Three possible transition states are found with the one at lowest energy, stabilized by a P-S interaction, in good agreement with experimental kinetics data. Comparison of both oxo transfer reactions shows that in DMSOr, where the oxo is transferred from the substrate to the metal ion, the oxo transfer induces electron transfer, while in SO, where the oxo transfer is from the metal site to the substrate, the electron transfer initiates oxo transfer. This difference in reactivity is related to the difference in frontier molecular orbitals (FMO) of the metal-oxo and substrate-oxo bonds. Finally, these experimentally related calculations are extended to oxo transfer by sulfite oxidase. The presence of only one dithiolene at the enzyme active site selectively activates the equatorial oxo for transfer, and allows facile structural reorganization during turnover.

  View details for DOI 10.1021/ja503316p

  View details for PubMedID 24884723

• Spectroscopic and computational insight into the activation of O-2 by the mononuclear Cu center in polysaccharide monooxygenases PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Kjaergaard, C. H., Qayyum, M. F., Wong, S. D., Xu, F., Hemsworth, G. R., Walton, D. J., Young, N. A., Davies, G. J., Walton, P. H., Johansen, K. S., Hodgson, K. O., Hedman, B., Solomon, E. I. 2014; 111 (24): 8797-8802

  Abstract

  Strategies for O2 activation by copper enzymes were recently expanded to include mononuclear Cu sites, with the discovery of the copper-dependent polysaccharide monooxygenases, also classified as auxiliary-activity enzymes 9-11 (AA9-11). These enzymes are finding considerable use in industrial biofuel production. Crystal structures of polysaccharide monooxygenases have emerged, but experimental studies are yet to determine the solution structure of the Cu site and how this relates to reactivity. From X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies, we observed a change from four-coordinate Cu(II) to three-coordinate Cu(I) of the active site in solution, where three protein-derived nitrogen ligands coordinate the Cu in both redox states, and a labile hydroxide ligand is lost upon reduction. The spectroscopic data allowed for density functional theory calculations of an enzyme active site model, where the optimized Cu(I) and (II) structures were consistent with the experimental data. The O2 reactivity of the Cu(I) site was probed by EPR and stopped-flow absorption spectroscopies, and a rapid one-electron reduction of O2 and regeneration of the resting Cu(II) enzyme were observed. This reactivity was evaluated computationally, and by calibration to Cu-superoxide model complexes, formation of an end-on Cu-AA9-superoxide species was found to be thermodynamically favored. We discuss how this thermodynamically difficult one-electron reduction of O2 is enabled by the unique protein structure where two nitrogen ligands from His1 dictate formation of a T-shaped Cu(I) site, which provides an open coordination position for strong O2 binding with very little reorganization energy.

  View details for DOI 10.1073/pnas.1408115111

  View details for Web of Science ID 000337300100032

  View details for PubMedCentralID PMC4066490

• Spectroscopic and computational insight into the activation of O2 by the mononuclear Cu center in polysaccharide monooxygenases. Proceedings of the National Academy of Sciences of the United States of America Kjaergaard, C. H., Qayyum, M. F., Wong, S. D., Xu, F., Hemsworth, G. R., Walton, D. J., Young, N. A., Davies, G. J., Walton, P. H., Johansen, K. S., Hodgson, K. O., Hedman, B., Solomon, E. I. 2014; 111 (24): 8797-8802

  Abstract

  Strategies for O2 activation by copper enzymes were recently expanded to include mononuclear Cu sites, with the discovery of the copper-dependent polysaccharide monooxygenases, also classified as auxiliary-activity enzymes 9-11 (AA9-11). These enzymes are finding considerable use in industrial biofuel production. Crystal structures of polysaccharide monooxygenases have emerged, but experimental studies are yet to determine the solution structure of the Cu site and how this relates to reactivity. From X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies, we observed a change from four-coordinate Cu(II) to three-coordinate Cu(I) of the active site in solution, where three protein-derived nitrogen ligands coordinate the Cu in both redox states, and a labile hydroxide ligand is lost upon reduction. The spectroscopic data allowed for density functional theory calculations of an enzyme active site model, where the optimized Cu(I) and (II) structures were consistent with the experimental data. The O2 reactivity of the Cu(I) site was probed by EPR and stopped-flow absorption spectroscopies, and a rapid one-electron reduction of O2 and regeneration of the resting Cu(II) enzyme were observed. This reactivity was evaluated computationally, and by calibration to Cu-superoxide model complexes, formation of an end-on Cu-AA9-superoxide species was found to be thermodynamically favored. We discuss how this thermodynamically difficult one-electron reduction of O2 is enabled by the unique protein structure where two nitrogen ligands from His1 dictate formation of a T-shaped Cu(I) site, which provides an open coordination position for strong O2 binding with very little reorganization energy.

  View details for DOI 10.1073/pnas.1408115111

  View details for PubMedID 24889637

• A Zinc Linchpin Motif in the MUTYH Glycosylase Interdomain Connector Is Required for Efficient Repair of DNA Damage JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Engstrom, L. M., Brinkmeyer, M. K., Ha, Y., Raetz, A. G., Hedman, B., Hodgson, K. O., Solomon, E. I., David, S. S. 2014; 136 (22): 7829-7832

  Abstract

  Mammalian MutY glycosylases have a unique architecture that features an interdomain connector (IDC) that joins the catalytic N-terminal domain and 8-oxoguanine (OG) recognition C-terminal domain. The IDC has been shown to be a hub for interactions with protein partners involved in coordinating downstream repair events and signaling apoptosis. Herein, a previously unidentified zinc ion and its coordination by three Cys residues of the IDC region of eukaryotic MutY organisms were characterized by mutagenesis, ICP-MS, and EXAFS. In vitro kinetics and cellular assays on WT and Cys to Ser mutants have revealed an important function for zinc coordination on overall protein stability, iron-sulfur cluster insertion, and ability to mediate DNA damage repair. We propose that this "zinc linchpin" motif serves to structurally organize the IDC and coordinate the damage recognition and base excision functions of the C- and N-terminal domains.

  View details for DOI 10.1021/ja502942d

  View details for Web of Science ID 000337014400012

  View details for PubMedCentralID PMC4063174

• Tuning of the copper-thioether bond in tetradentate N3S(thioether) ligands; O-O bond reductive cleavage via a [Cu(II)2(µ-1,2-peroxo)]²?/[Cu(III)2(µ-oxo)2]²? equilibrium. Journal of the American Chemical Society Kim, S., Ginsbach, J. W., Billah, A. I., Siegler, M. A., Moore, C. D., Solomon, E. I., Karlin, K. D. 2014; 136 (22): 8063-8071

  Abstract

  Current interest in copper/dioxygen reactivity includes the influence of thioether sulfur ligation, as it concerns the formation, structures, and properties of derived copper-dioxygen complexes. Here, we report on the chemistry of {L-Cu(I)}2-(O2) species L = (DMM)ESE, (DMM)ESP, and (DMM)ESDP, which are N3S(thioether)-based ligands varied in the nature of a substituent on the S atom, along with a related N3O(ether) (EOE) ligand. Cu(I) and Cu(II) complexes have been synthesized and crystallographically characterized. Copper(I) complexes are dimeric in the solid state, [{L-Cu(I)}2](B(C6F5)4)2, however are shown by diffusion-ordered NMR spectroscopy to be mononuclear in solution. Copper(II) complexes with a general formulation [L-Cu(II)(X)](n+) {X = ClO4(-), n = 1, or X = H2O, n = 2} exhibit distorted square pyramidal coordination geometries and progressively weaker axial thioether ligation across the series. Oxygenation (-130 °C) of {((DMM)ESE)Cu(I)}(+) results in the formation of a trans-μ-1,2-peroxodicopper(II) species [{((DMM)ESE)Cu(II)}2(μ-1,2-O2(2-))](2+) (1(P)). Weakening the Cu-S bond via a change to the thioether donor found in (DMM)ESP leads to the initial formation of [{((DMM)ESP)Cu(II)}2(μ-1,2-O2(2-))](2+) (2(P)) that subsequently isomerizes to a bis-μ-oxodicopper(III) complex, [{((DMM)ESP)Cu(III)}2(μ-O(2-))2](2+) (2(O)), with 2(P) and 2(O) in equilibrium (K(eq) = [2(O)]/[2(P)] = 2.6 at -130 °C). Formulations for these Cu/O2 adducts were confirmed by resonance Raman (rR) spectroscopy. This solution mixture is sensitive to the addition of methylsulfonate, which shifts the equilibrium toward the bis-μ-oxo isomer. Further weakening of the Cu-S bond in (DMM)ESDP or substitution with an ether donor in (DMM)EOE leads to only a bis-μ-oxo species (3(O) and 4(O), respectively). Reactivity studies indicate that the bis-μ-oxodicopper(III) species (2(O), 3(O)) and not the trans-peroxo isomers (1(P) and 2(P)) are responsible for the observed ligand sulfoxidation. Our findings concerning the existence of the 2(P)/2(O) equilibrium contrast with previously established ligand-Cu(I)/O2 reactivity and possible implications are discussed.

  View details for DOI 10.1021/ja502974c

  View details for PubMedID 24854766

• A zinc linchpin motif in the MUTYH glycosylase interdomain connector is required for efficient repair of DNA damage. Journal of the American Chemical Society Engstrom, L. M., Brinkmeyer, M. K., Ha, Y., Raetz, A. G., Hedman, B., Hodgson, K. O., Solomon, E. I., David, S. S. 2014; 136 (22): 7829-7832

  Abstract

  Mammalian MutY glycosylases have a unique architecture that features an interdomain connector (IDC) that joins the catalytic N-terminal domain and 8-oxoguanine (OG) recognition C-terminal domain. The IDC has been shown to be a hub for interactions with protein partners involved in coordinating downstream repair events and signaling apoptosis. Herein, a previously unidentified zinc ion and its coordination by three Cys residues of the IDC region of eukaryotic MutY organisms were characterized by mutagenesis, ICP-MS, and EXAFS. In vitro kinetics and cellular assays on WT and Cys to Ser mutants have revealed an important function for zinc coordination on overall protein stability, iron-sulfur cluster insertion, and ability to mediate DNA damage repair. We propose that this "zinc linchpin" motif serves to structurally organize the IDC and coordinate the damage recognition and base excision functions of the C- and N-terminal domains.

  View details for DOI 10.1021/ja502942d

  View details for PubMedID 24841533

• Tuning of the Copper-Thioether Bond in Tetradentate N3S(thioether) Ligands; O-O Bond Reductive Cleavage via a [Cu-2(II)(mu-1,2-peroxo)](2+)/[Cu-2(III)(mu-oxo)(2)](2+) Equilibrium JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Kim, S., Ginsbach, J. W., Billah, A. I., Siegler, M. A., Moore, C. D., Solomon, E. I., Karlin, K. D. 2014; 136 (22): 8063-8071

  Abstract

  Current interest in copper/dioxygen reactivity includes the influence of thioether sulfur ligation, as it concerns the formation, structures, and properties of derived copper-dioxygen complexes. Here, we report on the chemistry of {L-Cu(I)}2-(O2) species L = (DMM)ESE, (DMM)ESP, and (DMM)ESDP, which are N3S(thioether)-based ligands varied in the nature of a substituent on the S atom, along with a related N3O(ether) (EOE) ligand. Cu(I) and Cu(II) complexes have been synthesized and crystallographically characterized. Copper(I) complexes are dimeric in the solid state, [{L-Cu(I)}2](B(C6F5)4)2, however are shown by diffusion-ordered NMR spectroscopy to be mononuclear in solution. Copper(II) complexes with a general formulation [L-Cu(II)(X)](n+) {X = ClO4(-), n = 1, or X = H2O, n = 2} exhibit distorted square pyramidal coordination geometries and progressively weaker axial thioether ligation across the series. Oxygenation (-130 °C) of {((DMM)ESE)Cu(I)}(+) results in the formation of a trans-μ-1,2-peroxodicopper(II) species [{((DMM)ESE)Cu(II)}2(μ-1,2-O2(2-))](2+) (1(P)). Weakening the Cu-S bond via a change to the thioether donor found in (DMM)ESP leads to the initial formation of [{((DMM)ESP)Cu(II)}2(μ-1,2-O2(2-))](2+) (2(P)) that subsequently isomerizes to a bis-μ-oxodicopper(III) complex, [{((DMM)ESP)Cu(III)}2(μ-O(2-))2](2+) (2(O)), with 2(P) and 2(O) in equilibrium (K(eq) = [2(O)]/[2(P)] = 2.6 at -130 °C). Formulations for these Cu/O2 adducts were confirmed by resonance Raman (rR) spectroscopy. This solution mixture is sensitive to the addition of methylsulfonate, which shifts the equilibrium toward the bis-μ-oxo isomer. Further weakening of the Cu-S bond in (DMM)ESDP or substitution with an ether donor in (DMM)EOE leads to only a bis-μ-oxo species (3(O) and 4(O), respectively). Reactivity studies indicate that the bis-μ-oxodicopper(III) species (2(O), 3(O)) and not the trans-peroxo isomers (1(P) and 2(P)) are responsible for the observed ligand sulfoxidation. Our findings concerning the existence of the 2(P)/2(O) equilibrium contrast with previously established ligand-Cu(I)/O2 reactivity and possible implications are discussed.

  View details for DOI 10.1021/ja502974c

  View details for Web of Science ID 000337014400044

  View details for PubMedCentralID PMC4063178

• Hydroxo-Bridged Dicopper(II,III) and -(III,III) Complexes: Models for Putative Intermediates in Oxidation Catalysis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Halvagar, M. R., Solntsev, P. V., Lim, H., Hedman, B., Hodgson, K. O., Solomon, E. I., Cramer, C. J., Tolman, W. B. 2014; 136 (20): 7269-7272

  Abstract

  A macrocyclic ligand (L(4-)) comprising two pyridine(dicarboxamide) donors was used to target reactive copper species relevant to proposed intermediates in catalytic hydrocarbon oxidations by particulate methane monooxygenase and heterogeneous zeolite systems. Treatment of LH4 with base and Cu(OAc)2·H2O yielded (Me4N)2[L2Cu4(μ4-O)] (1) or (Me4N)[LCu2(μ-OH)] (2), depending on conditions. Complex 2 was found to undergo two reversible 1-electron oxidations via cyclic voltammetry and low-temperature chemical reactions. On the basis of spectroscopy and theory, the oxidation products were identified as novel hydroxo-bridged mixed-valent Cu(II)Cu(III) and symmetric Cu(III)2 species, respectively, that provide the first precedence for such moieties as oxidation catalysis intermediates.

  View details for DOI 10.1021/ja503629r

  View details for Web of Science ID 000336416600021

  View details for PubMedID 24821432

  View details for PubMedCentralID PMC4046753

• Tracking excited-state charge and spin dynamics in iron coordination complexes. Nature Zhang, W., Alonso-Mori, R., Bergmann, U., Bressler, C., Chollet, M., Galler, A., Gawelda, W., Hadt, R. G., Hartsock, R. W., Kroll, T., Kjær, K. S., Kubicek, K., Lemke, H. T., Liang, H. W., Meyer, D. A., Nielsen, M. M., Purser, C., Robinson, J. S., Solomon, E. I., Sun, Z., Sokaras, D., van Driel, T. B., Vankó, G., Weng, T., Zhu, D., Gaffney, K. J. 2014; 509 (7500): 345-348

  Abstract

  Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2'-bipyridine)3](2+), where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2'-bipyridine)3](2+) on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

  View details for DOI 10.1038/nature13252

  View details for PubMedID 24805234

• Tracking excited-state charge and spin dynamics in iron coordination complexes. Nature Zhang, W., Alonso-Mori, R., Bergmann, U., Bressler, C., Chollet, M., Galler, A., Gawelda, W., Hadt, R. G., Hartsock, R. W., Kroll, T., Kjær, K. S., Kubicek, K., Lemke, H. T., Liang, H. W., Meyer, D. A., Nielsen, M. M., Purser, C., Robinson, J. S., Solomon, E. I., Sun, Z., Sokaras, D., van Driel, T. B., Vankó, G., Weng, T., Zhu, D., Gaffney, K. J. 2014; 509 (7500): 345-348

  Abstract

  Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2'-bipyridine)3](2+), where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2'-bipyridine)3](2+) on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

  View details for DOI 10.1038/nature13252

  View details for PubMedID 24805234

• Observation of a Cu-2(II)(mu-1,2-peroxo)/Cu-2(III)(mu-oxo)(2) Equilibrium and its Implications for Copper-Dioxygen Reactivity ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Kieber-Emmons, M. T., Ginsbach, J. W., Wick, P. K., Lucas, H. R., Helton, M. E., Lucchese, B., Suzuki, M., Zuberbuehler, A. D., Karlin, K. D., Solomon, E. I. 2014; 53 (19): 4935-4939

  Abstract

  Synthesis of small-molecule Cu2 O2 adducts has provided insight into the related biological systems and their reactivity patterns including the interconversion of the Cu(II) 2 (μ-η(2) :η(2) -peroxo) and Cu(III) 2 (μ-oxo)2 isomers. In this study, absorption spectroscopy, kinetics, and resonance Raman data show that the oxygenated product of [(BQPA)Cu(I) ](+) initially yields an "end-on peroxo" species, that subsequently converts to the thermodynamically more stable "bis-μ-oxo" isomer (Keq =3.2 at -90 °C). Calibration of density functional theory calculations to these experimental data suggest that the electrophilic reactivity previously ascribed to end-on peroxo species is in fact a result of an accessible bis-μ-oxo isomer, an electrophilic Cu2 O2 isomer in contrast to the nucleophilic reactivity of binuclear Cu(II) end-on peroxo species. This study is the first report of the interconversion of an end-on peroxo to bis-μ-oxo species in transition metal-dioxygen chemistry.

  View details for DOI 10.1002/anie.201402166

  View details for Web of Science ID 000335202700032

  View details for PubMedID 24700427

  View details for PubMedCentralID PMC4041702

• Introduction: Bioinorganic Enzymology II CHEMICAL REVIEWS Holm, R. H., Solomon, E. I. 2014; 114 (8): 4039–40

  View details for DOI 10.1021/cr5001332

  View details for Web of Science ID 000335086300001

  View details for PubMedID 24758378

• Evolution of Iron(II)-Finger Peptides by Using a Bipyridyl Amino Acid CHEMBIOCHEM Kang, M., Light, K., Ai, H., Shen, W., Kim, C. H., Chen, P. R., Lee, H. S., Solomon, E. I., Schultz, P. G. 2014; 15 (6): 822-825

  Abstract

  We report the engineering of zinc-finger-like motifs containing the unnatural amino acid (2,2'-bipyridin-5-yl)alanine (Bpy-Ala). A phage-display library was constructed in which five residues in the N-terminal finger of zif268 were randomized to include both canonical amino acids and Bpy-Ala. Panning of this library against a nine-base-pair DNA binding site identified several Bpy-Ala-containing functional Zif268 mutants. These mutants bind the Zif268 recognition site with affinities comparable to that of the wild-type protein. Further characterization indicated that the mutant fingers bind low-spin Fe(II) rather than Zn(II) . This work demonstrates that an expanded genetic code can lead to new metal ion binding motifs that can serve as structural, catalytic, or regulatory elements in proteins.

  View details for DOI 10.1002/cbic.201300727

  View details for Web of Science ID 000333966100008

  View details for PubMedID 24591102

  View details for PubMedCentralID PMC4010245

• Copper active sites in biology. Chemical reviews Solomon, E. I., Heppner, D. E., Johnston, E. M., Ginsbach, J. W., Cirera, J., Qayyum, M., Kieber-Emmons, M. T., Kjaergaard, C. H., Hadt, R. G., Tian, L. 2014; 114 (7): 3659-3853

  View details for DOI 10.1021/cr400327t

  View details for PubMedID 24588098

• Introduction: Bioinorganic Enzymology II CHEMICAL REVIEWS Holm, R. H., Solomon, E. I. 2014; 114 (7): 3367–68

  View details for DOI 10.1021/cr500118g

  View details for Web of Science ID 000334572600001

  View details for PubMedID 24712923

• [Cu2O](2+) Active Site Formation in Cu-ZSM-5: Geometric and Electronic Structure Requirements for N2O Activation. Journal of the American Chemical Society Tsai, M., Hadt, R. G., Vanelderen, P., Sels, B. F., Schoonheydt, R. A., Solomon, E. I. 2014; 136 (9): 3522-3529

  Abstract

  Understanding the formation mechanism of the [Cu2O](2+) active site in Cu-ZSM-5 is important for the design of efficient catalysts to selectively convert methane to methanol and related value-added chemicals and for N2O decomposition. Spectroscopically validated DFT calculations are used here to evaluate the thermodynamic and kinetic requirements for formation of [Cu2O](2+) active sites from the reaction between binuclear Cu(I) sites and N2O in the 10-membered rings Cu-ZSM-5. Thermodynamically, the most stable Cu(I) center prefers bidentate coordination with a close to linear bite angle. This binuclear Cu(I) site reacts with N2O to generate the experimentally observed [Cu2O](2+) site. Kinetically, the reaction coordinate was evaluated for two representative binuclear Cu(I) sites. When the Cu-Cu distance is sufficiently short (<4.2 Å), N2O can bind in a "bridged" μ-1,1-O fashion and the oxo-transfer reaction is calculated to proceed with a low activation energy barrier (2 kcal/mol). This is in good agreement with the experimental Ea for N2O activation (2.5 ± 0.5 kcal/mol). However, when the Cu-Cu distance is long (>5.0 Å), N2O binds in a "terminal" η(1)-O fashion to a single Cu(I) site of the dimer and the resulting Ea for N2O activation is significantly higher (16 kcal/mol). Therefore, bridging N2O between two Cu(I) centers is necessary for its efficient two-electron activation in [Cu2O](2+) active site formation. In nature, this N2O reduction reaction is catalyzed by a tetranuclear CuZ cluster that has a higher Ea. The lower Ea for Cu-ZSM-5 is attributed to the larger thermodynamic driving force resulting from formation of strong Cu(II)-oxo bonds in the ZSM-5 framework.

  View details for DOI 10.1021/ja4113808

  View details for PubMedID 24524659

• Spectroscopic Studies of Single and Double Variants of M Ferritin: Lack of Conversion of a Biferrous Substrate Site into a Cofactor Site for O-2 Activation BIOCHEMISTRY Kwak, Y., Schwartz, J. K., Haldar, S., Behera, R. K., Tosha, T., Theil, E. C., Solomon, E. I. 2014; 53 (3): 473-482

  Abstract

  Ferritin has a binuclear non-heme iron active site that functions to oxidize iron as a substrate for formation of an iron mineral core. Other enzymes of this class have tightly bound diiron cofactor sites that activate O2 to react with substrate. Ferritin has an active site ligand set with 1-His/4-carboxylate/1-Gln rather than the 2-His/4-carboxylate set of the cofactor site. This ligand variation has been thought to make a major contribution to this biferrous substrate rather than cofactor site reactivity. However, the Q137E/D140H double variant of M ferritin, has a ligand set that is equivalent to most of the diiron cofactor sites, yet did not rapidly react with O2 or generate the peroxy intermediate observed in the cofactor sites. Therefore, in this study, a combined spectroscopic methodology of circular dichroism (CD)/magnetic CD (MCD)/variable temperature, variable field (VTVH) MCD has been applied to evaluate the factors required for the rapid O2 activation observed in cofactor sites. This methodology defines the coordination environment of each iron and the bridging ligation of the biferrous active sites in the double and corresponding single variants of frog M ferritin. Based on spectral changes, the D140H single variant has the new His ligand binding, and the Q137E variant has the new carboxylate forming a μ-1,3 bridge. The spectra for the Q137E/D140H double variant, which has the cofactor ligand set, however, reflects a site that is more coordinately saturated than the cofactor sites in other enzymes including ribonucleotide reductase, indicating the presence of additional water ligation. Correlation of this double variant and the cofactor sites to their O2 reactivities indicates that electrostatic and steric changes in the active site and, in particular, the hydrophobic nature of a cofactor site associated with its second sphere protein environment, make important contributions to the activation of O2 by the binuclear non-heme iron enzymes.

  View details for DOI 10.1021/bi4013726

  View details for Web of Science ID 000330543100004

  View details for PubMedID 24397299

  View details for PubMedCentralID PMC3985457

• First- and second-sphere contributions to Fe(II) site activation by cosubstrate binding in non-heme Fe enzymes. Dalton transactions Light, K. M., Hangasky, J. A., Knapp, M. J., Solomon, E. I. 2014; 43 (4): 1505-1508

  Abstract

  Non-heme Fe(II) enzymes exhibit a general mechanistic strategy where binding all cosubstrates opens a coordination site on the Fe(II) for O2 activation. This study shows that strong-donor ligands, steric interactions with the substrate and second-sphere H-bonding to the facial triad carboxylate allow for five-coordinate site formation in this enzyme superfamily.

  View details for DOI 10.1039/c3dt53201a

  View details for PubMedID 24292428

  View details for PubMedCentralID PMC3976902

• Copper-sulfenate complex from oxidation of a cavity mutant of Pseudomonas aeruginosa azurin PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Sieracki, N. A., Tian, S., Hadt, R. G., Zhang, J., Woertink, J. S., Nilges, M. J., Sun, F., Solomon, E. I., Lu, Y. 2014; 111 (3): 924-929

  Abstract

  Metal-sulfenate centers are known to play important roles in biology and yet only limited examples are known due to their instability and high reactivity. Herein we report a copper-sulfenate complex characterized in a protein environment, formed at the active site of a cavity mutant of an electron transfer protein, type 1 blue copper azurin. Reaction of hydrogen peroxide with Cu(I)-M121G azurin resulted in a species with strong visible absorptions at 350 and 452 nm and a relatively low electron paramagnetic resonance gz value of 2.169 in comparison with other normal type 2 copper centers. The presence of a side-on copper-sulfenate species is supported by resonance Raman spectroscopy, electrospray mass spectrometry using isotopically enriched hydrogen peroxide, and density functional theory calculations correlated to the experimental data. In contrast, the reaction with Cu(II)-M121G or Zn(II)-M121G azurin under the same conditions did not result in Cys oxidation or copper-sulfenate formation. Structural and computational studies strongly suggest that the secondary coordination sphere noncovalent interactions are critical in stabilizing this highly reactive species, which can further react with oxygen to form a sulfinate and then a sulfonate species, as demonstrated by mass spectrometry. Engineering the electron transfer protein azurin into an active copper enzyme that forms a copper-sulfenate center and demonstrating the importance of noncovalent secondary sphere interactions in stabilizing it constitute important contributions toward the understanding of metal-sulfenate species in biological systems.

  View details for DOI 10.1073/pnas.1316483111

  View details for Web of Science ID 000329928400027

  View details for PubMedID 24390543

  View details for PubMedCentralID PMC3903256

• Determination of the Active Form of the Tetranuclear Copper Sulfur Cluster in Nitrous Oxide Reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Johnston, E. M., Dell'Acqua, S., Ramos, S., Pauleta, S. R., Moura, I., Solomon, E. I. 2014; 136 (2): 614-617

  Abstract

  N2OR has been found to have two structural forms of its tetranuclear copper active site, the 4CuS Cu(Z)* form and the 4Cu2S Cu(Z) form. EPR, resonance Raman, and MCD spectroscopies have been used to determine the redox states of these sites under different reductant conditions, showing that the Cu(Z)* site accesses the 1-hole and fully reduced redox states, while the Cu(Z) site accesses the 2-hole and 1-hole redox states. Single-turnover reactions of N2OR for Cu(Z) and Cu(Z)* poised in these redox states and steady-state turnover assays with different proportions of Cu(Z) and Cu(Z)* show that only fully reduced Cu(Z)* is catalytically competent in rapid turnover with N2O.

  View details for DOI 10.1021/ja411500p

  View details for Web of Science ID 000330018600019

  View details for PubMedID 24364717

  View details for PubMedCentralID PMC3927536

• Spectroscopy and Redox Chemistry of Copper in Mordenite CHEMPHYSCHEM Vanelderen, P., Vancauwenbergh, J., Tsai, M., Hadt, R. G., Solomon, E. I., Schoonheydt, R. A., Sels, B. F. 2014; 15 (1): 91-99

  Abstract

  Copper-containing zeolites, such as mordenite (MOR), have recently gained increased attention as a consequence of their catalytic potential. While the preferred copper loadings in these catalytic studies are generally high, the literature lacks appropriate spectroscopic and structural information on such Cu-rich zeolite samples. Higher copper loadings increase the complexity of the copper identity and their location in the zeolite host, but they also provide the opportunity to create novel Cu sites, which are perhaps energetically less favorable, but possibly more reactive and more suitable for catalysis. In order to address the different role of each Cu site in catalysis, we here report a combined electron paramagnetic resonance (EPR), UV/Vis-NIR and temperature-programmed reduction (TPR) study on highly copper-loaded MOR. Highly resolved diffuse reflectance (DR) spectra of the CuMOR samples were obtained due to the increased copper loading, allowing the differentiation of two isolated mononuclear Cu(2+) sites and the unambiguous correlation with extensively reported features in the EPR spectrum. Ligand field theory is applied together with earlier suggested theoretical calculations to determine their coordination chemistry and location within the zeolite matrix, and the theoretical analysis further allowed us to define factors governing their redox behavior. In addition to monomeric species, an EPR-silent, possibly dimeric, copper site is present in accordance with its charge transfer absorption feature at 22200 cm(-1), and quantified with TPR. Its full description and true location in MOR is currently being investigated.

  View details for DOI 10.1002/cphc.201300730

  View details for Web of Science ID 000329510500003

  View details for PubMedID 24399800

• Excited state potential energy surfaces and their interactions in Fe-IV=O active sites DALTON TRANSACTIONS Srnec, M., Wong, S. D., Solomon, E. I. 2014; 43 (47): 17567-17577

  Abstract

  The non-heme ferryl active sites are of significant interest for their application in biomedical and green catalysis. These sites have been shown to have an S = 1 or S = 2 ground spin state; the latter is functional in biology. Low-temperature magnetic circular dichroism (LT MCD) spectroscopy probes the nature of the excited states in these species including ligand-field (LF) states that are otherwise difficult to study by other spectroscopies. In particular, the temperature dependences of MCD features enable their unambiguous assignment and thus determination of the low-lying excited states in two prototypical S = 1 and S = 2 NHFe(IV)[double bond, length as m-dash]O complexes. Furthermore, some MCD bands exhibit vibronic structures that allow mapping of excited-state interactions and their effects on the potential energy surfaces (PESs). For the S = 2 species, there is also an unusual spectral feature in both near-infrared absorption and MCD spectra - Fano antiresonance (dip in Abs) and Fano resonance (sharp peak in MCD) that indicates the weak spin-orbit coupling of an S = 1 state with the S = 2 LF state. These experimental data are correlated with quantum-chemical calculations that are further extended to analyze the low-lying electronic states and the evolution of their multiconfigurational characters along the Fe-O PESs. These investigations show that the lowest-energy states develop oxyl Fe(III) character at distances that are relevant to the transition state (TS) for H-atom abstraction and define the frontier molecular orbitals that participate in the reactivity of S = 1 vs. S = 2 non-heme Fe(IV)[double bond, length as m-dash]O active sites. The S = 1 species has only one available channel that requires the C-H bond of a substrate to approach perpendicular to the Fe-oxo bond (the π channel). In contrast, there are three channels (one σ and two π) available for the S = 2 non-heme Fe(IV)[double bond, length as m-dash]O system allowing C-H substrate approach both along and perpendicular to the Fe-oxo bond that have important implications for enzymatic selectivity.

  View details for DOI 10.1039/c4dt01366b

  View details for Web of Science ID 000345065600002

  View details for PubMedID 24916844

  View details for PubMedCentralID PMC4229428

• First- and second-sphere contributions to Fe(II) site activation by cosubstrate binding in non-heme Fe enzymes DALTON TRANSACTIONS Light, K. M., Hangasky, J. A., Knapp, M. J., Solomon, E. I. 2014; 43 (4): 1505-1508

  Abstract

  Non-heme Fe(II) enzymes exhibit a general mechanistic strategy where binding all cosubstrates opens a coordination site on the Fe(II) for O2 activation. This study shows that strong-donor ligands, steric interactions with the substrate and second-sphere H-bonding to the facial triad carboxylate allow for five-coordinate site formation in this enzyme superfamily.

  View details for DOI 10.1039/c3dt53201a

  View details for Web of Science ID 000328885300005

  View details for PubMedCentralID PMC3976902

• Crystallographic and spectroscopic characterization and reactivities of a mononuclear non-haem iron(III)-superoxo complex. Nature communications Hong, S., Sutherlin, K. D., Park, J., Kwon, E., Siegler, M. A., Solomon, E. I., Nam, W. 2014; 5: 5440-?

  Abstract

  Mononuclear non-haem iron(III)-superoxo species (Fe(III)-O2(-·)) have been implicated as key intermediates in the catalytic cycles of dioxygen activation by non-haem iron enzymes. Although non-haem iron(III)-superoxo species have been trapped and characterized spectroscopically in enzymatic and biomimetic reactions, no structural information has yet been obtained. Here we report the isolation, spectroscopic characterization and crystal structure of a mononuclear side-on (η(2)) iron(III)-superoxo complex with a tetraamido macrocyclic ligand. The non-haem iron(III)-superoxo species undergoes both electrophilic and nucleophilic oxidation reactions, as well as O2-transfer between metal complexes. In the O2-transfer reaction, the iron(III)-superoxo complex transfers the bound O2 unit to a manganese(III) analogue, resulting in the formation of a manganese(IV)-peroxo complex, which is characterized structurally and spectroscopically as a mononuclear side-on (η(2)) manganese(IV)-peroxo complex. The difference in the redox distribution between the metal ions and O2 in iron(III)-superoxo and manganese(IV)-peroxo complexes is rationalized using density functional theory calculations.

  View details for DOI 10.1038/ncomms6440

  View details for PubMedID 25510711

• Efficient C-H bond activations via O-2 cleavage by a dianionic cobalt(II) complex CHEMICAL SCIENCE Nguyen, A. I., Hadt, R. G., Solomon, E. I., Tilley, T. D. 2014; 5 (7): 2874-2878

  Abstract

  A dianionic, square planar cobalt(II) complex reacts with O2 in the presence of acetonitrile to give a cyanomethylcobalt(III) complex formed by C-H bond cleavage. Interestingly, PhIO and p-tolylazide react similarly to give the same cyanomethylcobalt(III) complex. Competition studies with various hydrocarbon substrates indicate that the rate of C-H bond cleavage greatly depends on the p Ka of the C-H bond, rather than on the C-H bond dissociation energy. Kinetic isotope experiments reveal a moderate KIE value of ca. 3.5 using either O2 or PhIO. The possible involvement of a cobalt(IV) oxo species in this chemistry is discussed.

  View details for DOI 10.1039/c4sc00108g

  View details for Web of Science ID 000337108200037

  View details for PubMedCentralID PMC4111274

• Geometric and Electronic Structure of the Mn(IV)Fe(III) Cofactor in Class Ic Ribonucleotide Reductase: Correlation to the Class Ia Binuclear Non-Heme Iron Enzyme. Journal of the American Chemical Society Kwak, Y., Jiang, W., Dassama, L. M., Park, K., Bell, C. B., Liu, L. V., Wong, S. D., Saito, M., Kobayashi, Y., Kitao, S., Seto, M., Yoda, Y., Alp, E. E., Zhao, J., Bollinger, J. M., Krebs, C., Solomon, E. I. 2013; 135 (46): 17573-17584

  Abstract

  The class Ic ribonucleotide reductase (RNR) from Chlamydia trachomatis (Ct) utilizes a Mn/Fe heterobinuclear cofactor, rather than the Fe/Fe cofactor found in the β (R2) subunit of the class Ia enzymes, to react with O2. This reaction produces a stable Mn(IV)Fe(III) cofactor that initiates a radical, which transfers to the adjacent α (R1) subunit and reacts with the substrate. We have studied the Mn(IV)Fe(III) cofactor using nuclear resonance vibrational spectroscopy (NRVS) and absorption (Abs)/circular dichroism (CD)/magnetic CD (MCD)/variable temperature, variable field (VTVH) MCD spectroscopies to obtain detailed insight into its geometric/electronic structure and to correlate structure with reactivity; NRVS focuses on the Fe(III), whereas MCD reflects the spin-allowed transitions mostly on the Mn(IV). We have evaluated 18 systematically varied structures. Comparison of the simulated NRVS spectra to the experimental data shows that the cofactor has one carboxylate bridge, with Mn(IV) at the site proximal to Phe127. Abs/CD/MCD/VTVH MCD data exhibit 12 transitions that are assigned as d-d and oxo and OH(-) to metal charge-transfer (CT) transitions. Assignments are based on MCD/Abs intensity ratios, transition energies, polarizations, and derivative-shaped pseudo-A term CT transitions. Correlating these results with TD-DFT calculations defines the Mn(IV)Fe(III) cofactor as having a μ-oxo, μ-hydroxo core and a terminal hydroxo ligand on the Mn(IV). From DFT calculations, the Mn(IV) at site 1 is necessary to tune the redox potential to a value similar to that of the tyrosine radical in class Ia RNR, and the OH(-) terminal ligand on this Mn(IV) provides a high proton affinity that could gate radical translocation to the α (R1) subunit.

  View details for DOI 10.1021/ja409510d

  View details for PubMedID 24131208

• Geometric and electronic structure of the Mn(IV)Fe(III) cofactor in class Ic ribonucleotide reductase: correlation to the class Ia binuclear non-heme iron enzyme. Journal of the American Chemical Society Kwak, Y., Jiang, W., Dassama, L. M., Park, K., Bell, C. B., Liu, L. V., Wong, S. D., Saito, M., Kobayashi, Y., Kitao, S., Seto, M., Yoda, Y., Alp, E. E., Zhao, J., Bollinger, J. M., Krebs, C., Solomon, E. I. 2013; 135 (46): 17573-17584

  Abstract

  The class Ic ribonucleotide reductase (RNR) from Chlamydia trachomatis (Ct) utilizes a Mn/Fe heterobinuclear cofactor, rather than the Fe/Fe cofactor found in the β (R2) subunit of the class Ia enzymes, to react with O2. This reaction produces a stable Mn(IV)Fe(III) cofactor that initiates a radical, which transfers to the adjacent α (R1) subunit and reacts with the substrate. We have studied the Mn(IV)Fe(III) cofactor using nuclear resonance vibrational spectroscopy (NRVS) and absorption (Abs)/circular dichroism (CD)/magnetic CD (MCD)/variable temperature, variable field (VTVH) MCD spectroscopies to obtain detailed insight into its geometric/electronic structure and to correlate structure with reactivity; NRVS focuses on the Fe(III), whereas MCD reflects the spin-allowed transitions mostly on the Mn(IV). We have evaluated 18 systematically varied structures. Comparison of the simulated NRVS spectra to the experimental data shows that the cofactor has one carboxylate bridge, with Mn(IV) at the site proximal to Phe127. Abs/CD/MCD/VTVH MCD data exhibit 12 transitions that are assigned as d-d and oxo and OH(-) to metal charge-transfer (CT) transitions. Assignments are based on MCD/Abs intensity ratios, transition energies, polarizations, and derivative-shaped pseudo-A term CT transitions. Correlating these results with TD-DFT calculations defines the Mn(IV)Fe(III) cofactor as having a μ-oxo, μ-hydroxo core and a terminal hydroxo ligand on the Mn(IV). From DFT calculations, the Mn(IV) at site 1 is necessary to tune the redox potential to a value similar to that of the tyrosine radical in class Ia RNR, and the OH(-) terminal ligand on this Mn(IV) provides a high proton affinity that could gate radical translocation to the α (R1) subunit.

  View details for DOI 10.1021/ja409510d

  View details for PubMedID 24131208

• L-Edge X-ray Absorption Spectroscopy and DFT Calculations on Cu2O2 Species: Direct Electrophilic Aromatic Attack by Side-on Peroxo Bridged Dicopper(II) Complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Qayyum, M. F., Sarangi, R., Fujisawa, K., Stack, T. D., Karlin, K. D., Hodgson, K. O., Hedman, B., Solomon, E. I. 2013; 135 (46): 17417-17431

  Abstract

  The hydroxylation of aromatic substrates catalyzed by coupled binuclear copper enzymes has been observed with side-on-peroxo-dicopper(II) (P) and bis-μ-oxo-dicopper(III) (O) model complexes. The substrate-bound-O intermediate in [Cu(II)2(DBED)2(O)2](2+) (DBED = N,N'-di-tert-butyl-ethylenediamine) was shown to perform aromatic hydroxylation. For the [Cu(II)2(NO2-XYL)(O2)](2+) complex, only a P species was spectroscopically observed. However, it was not clear whether this O-O bond cleaves to proceed through an O-type structure along the reaction coordinate for hydroxylation of the aromatic xylyl linker. Accurate evaluation of these reaction coordinates requires reasonable quantitative descriptions of the electronic structures of the P and O species. We have performed Cu L-edge XAS on two well-characterized P and O species to experimentally quantify the Cu 3d character in their ground state wave functions. The lower per-hole Cu character (40 ± 6%) corresponding to higher covalency in the O species compared to the P species (52 ± 4%) reflects a stronger bonding interaction of the bis-μ-oxo core with the Cu(III) centers. DFT calculations show that 10-20% Hartree-Fock (HF) mixing for P and ~38% for O species are required to reproduce the Cu-O bonding; for the P species this HF mixing is also required for an antiferromagnetically coupled description of the two Cu(II) centers. B3LYP (with 20% HF) was, therefore, used to calculate the hydroxylation reaction coordinate of P in [Cu(II)2(NO2-XYL)(O2)](2+). These experimentally calibrated calculations indicate that the electrophilic attack on the aromatic ring does not involve formation of a Cu(III)2(O(2-))2 species. Rather, there is direct electron donation from the aromatic ring into the peroxo σ* orbital of the Cu(II)2(O2(2-)) species, leading to concerted C-O bond formation with O-O bond cleavage. Thus, species P is capable of direct hydroxylation of aromatic substrates without the intermediacy of an O-type species.

  View details for DOI 10.1021/ja4078717

  View details for Web of Science ID 000327413300032

  View details for PubMedID 24102191

  View details for PubMedCentralID PMC3891796

• Geometric and Electronic Structure Contributions to Function in Non-heme Iron Enzymes ACCOUNTS OF CHEMICAL RESEARCH Solomon, E. I., Light, K. M., Liu, L. V., Srnec, M., Wong, S. D. 2013; 46 (11): 2725-2739

  Abstract

  Mononuclear non-heme Fe (NHFe) enzymes play key roles in DNA repair, the biosynthesis of antibiotics, the response to hypoxia, cancer therapy, and many other biological processes. These enzymes catalyze a diverse range of oxidation reactions, including hydroxylation, halogenation, ring closure, desaturation, and electrophilic aromatic substitution (EAS). Most of these enzymes use an Fe(II) site to activate dioxygen, but traditional spectroscopic methods have not allowed researchers to insightfully probe these ferrous active sites. We have developed a methodology that provides detailed geometric and electronic structure insights into these NHFe(II) active sites. Using these data, we have defined a general mechanistic strategy that many of these enzymes use: they control O2 activation (and limit autoxidation and self-hydroxylation) by allowing Fe(II) coordination unsaturation only in the presence of cosubstrates. Depending on the type of enzyme, O2 activation either involves a 2e(-) reduced Fe(III)-OOH intermediate or a 4e(-) reduced Fe(IV)═O intermediate. Nuclear resonance vibrational spectroscopy (NRVS) has provided the geometric structure of these intermediates, and magnetic circular dichroism (MCD) has defined the frontier molecular orbitals (FMOs), the electronic structure that controls reactivity. This Account emphasizes that experimental spectroscopy is critical in evaluating the results of electronic structure calculations. Therefore these data are a key mechanistic bridge between structure and reactivity. For the Fe(III)-OOH intermediates, the anticancer drug activated bleomycin (BLM) acts as the non-heme Fe analog of compound 0 in heme (e.g., P450) chemistry. However BLM shows different reactivity: the low-spin (LS) Fe(III)-OOH can directly abstract a H atom from DNA. The LS and high-spin (HS) Fe(III)-OOHs have fundamentally different transition states. The LS transition state goes through a hydroxyl radical, but the HS transition state is activated for EAS without O-O cleavage. This activation is important in one class of NHFe enzymes that utilizes a HS Fe(III)-OOH intermediate in dioxygenation. For Fe(IV)═O intermediates, the LS form has a π-type FMO activated for attack perpendicular to the Fe-O bond. However, the HS form (present in the NHFe enzymes) has a π FMO activated perpendicular to the Fe-O bond and a σ FMO positioned along the Fe-O bond. For the NHFe enzymes, the presence of π and σ FMOs enables enzymatic control in determining the type of reactivity: EAS or H-atom extraction for one substrate with different enzymes and halogenation or hydroxylation for one enzyme with different substrates.

  View details for DOI 10.1021/ar400149m

  View details for Web of Science ID 000327360800037

  View details for PubMedID 24070107

  View details for PubMedCentralID PMC3905672

• Correlation of the Electronic and Geometric Structures in Mononuclear Copper(II) Superoxide Complexes. Inorganic chemistry Ginsbach, J. W., Peterson, R. L., Cowley, R. E., Karlin, K. D., Solomon, E. I. 2013; 52 (22): 12872-12874

  Abstract

  The geometry of mononuclear copper(II) superoxide complexes has been shown to determine their ground state where side-on bonding leads to a singlet ground state and end-on complexes have triplet ground states. In an apparent contrast to this trend, the recently synthesized (HIPT3tren)Cu(II)O2(•-) (1) was proposed to have an end-on geometry and a singlet ground state. However, reexamination of 1 with resonance Raman, magnetic circular dichroism, and (2)H NMR spectroscopies indicate that 1 is, in fact, an end-on superoxide species with a triplet ground state that results from the single Cu(II)O2(•-) bonding interaction being weaker than the spin-pairing energy.

  View details for DOI 10.1021/ic402357u

  View details for PubMedID 24164429

• Metal-Ligand Covalency of Iron Complexes from High-Resolution Resonant Inelastic X-ray Scattering. Journal of the American Chemical Society Lundberg, M., Kroll, T., DeBeer, S., Bergmann, U., Wilson, S. A., Glatzel, P., Nordlund, D., Hedman, B., Hodgson, K. O., Solomon, E. I. 2013; 135 (45): 17121-17134

  Abstract

  Data from Kα resonant inelastic X-ray scattering (RIXS) have been used to extract electronic structure information, i.e., the covalency of metal-ligand bonds, for four iron complexes using an experimentally based theoretical model. Kα RIXS involves resonant 1s→3d excitation and detection of the 2p→1s (Kα) emission. This two-photon process reaches similar final states as single-photon L-edge (2p→3d) X-ray absorption spectroscopy (XAS), but involves only hard X-rays and can therefore be used to get high-resolution L-edge-like spectra for metal proteins, solution catalysts and their intermediates. To analyze the information content of Kα RIXS spectra, data have been collected for four characteristic σ-donor and π-back-donation complexes: ferrous tacn [Fe(II)(tacn)2]Br2, ferrocyanide [Fe(II)(CN)6]K4, ferric tacn [Fe(III)(tacn)2]Br3 and ferricyanide [Fe(III)(CN)6]K3. From these spectra metal-ligand covalencies can be extracted using a charge-transfer multiplet model, without previous information from the L-edge XAS experiment. A direct comparison of L-edge XAS and Kα RIXS spectra show that the latter reaches additional final states, e.g., when exciting into the eg (σ*) orbitals, and the splitting between final states of different symmetry provides an extra dimension that makes Kα RIXS a more sensitive probe of σ-bonding. Another key difference between L-edge XAS and Kα RIXS is the π-back-bonding features in ferro- and ferricyanide that are significantly more intense in L-edge XAS compared to Kα RIXS. This shows that two methods are complementary in assigning electronic structure. The Kα RIXS approach can thus be used as a stand-alone method, in combination with L-edge XAS for strongly covalent systems that are difficult to probe by UV/vis spectroscopy, or as an extension to conventional absorption spectroscopy for a wide range of transition metal enzymes and catalysts.

  View details for DOI 10.1021/ja408072q

  View details for PubMedID 24131028

• Stepwise Protonation and Electron-Transfer Reduction of a Primary Copper-Dioxygen Adduct JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Peterson, R. L., Ginsbach, J. W., Cowley, R. E., Qayyum, M. F., Himes, R. A., Siegler, M. A., Moore, C. D., Hedman, B., Hodgson, K. O., Fukuzumi, S., Solomon, E. I., Karlin, K. D. 2013; 135 (44): 16454-16467

  Abstract

  The protonation–reduction of a dioxygen adduct with [LCu(I)][B(C6F5)4], cupric superoxo complex [LCu(II)(O2(•–))]+ (1) (L = TMG3tren (1,1,1-tris[2-[N(2)-(1,1,3,3-tetramethylguanidino)]ethyl]amine)) has been investigated. Trifluoroacetic acid (HOAcF) reversibly associates with the superoxo ligand in ([LCu(II)(O2(•–))]+) in a 1:1 adduct [LCu(II)(O2(•–))(HOAcF)](+) (2), as characterized by UV–visible, resonance Raman (rR), nuclear magnetic resonance (NMR), and X-ray absorption (XAS) spectroscopies, along with density functional theory (DFT) calculations. Chemical studies reveal that for the binding of HOAcF with 1 to give 2, Keq = 1.2 × 10(5) M(–1) (−130 °C) and ΔH° = −6.9(7) kcal/mol, ΔS° = −26(4) cal mol(–1) K(–1)). Vibrational (rR) data reveal a significant increase (29 cm(–1)) in vO–O (= 1149 cm(–1)) compared to that known for [LCu(II)(O2(•–))](+) (1). Along with results obtained from XAS and DFT calculations, hydrogen bonding of HOAcF to a superoxo O-atom in 2 is established. Results from NMR spectroscopy of 2 at −120 °C in 2-methyltetrahydrofuran are also consistent with 1/HOAcF = 1:1 formulation of 2 and with this complex possessing a triplet (S = 1) ground state electronic configuration, as previously determined for 1. The pre-equilibrium acid association to 1 is followed by outer-sphere electron-transfer reduction of 2 by decamethylferrocene (Me10Fc) or octamethylferrocene (Me8Fc), leading to the products H2O2, the corresponding ferrocenium salt, and [LCu(II)(OAcF)](+). Second-order rate constants for electron transfer (ket) were determined to be 1365 M(–1) s(–1) (Me10Fc) and 225 M(–1) s(–1) (Me8Fc) at −80 °C. The (bio)chemical relevance of the proton-triggered reduction of the metal-bound dioxygen-derived fragment is discussed.

  View details for DOI 10.1021/ja4065377

  View details for Web of Science ID 000326774300044

  View details for PubMedCentralID PMC3874213

• Circular Dichroism, Magnetic Circular Dichroism, and Variable Temperature Variable Field Magnetic Circular Dichroism Studies of Biferrous and Mixed-Valent myo-Inositol Oxygenase: Insights into Substrate Activation of O2 Reactivity. Journal of the American Chemical Society Snyder, R. A., Bell, C. B., Diao, Y., Krebs, C., Bollinger, J. M., Solomon, E. I. 2013; 135 (42): 15851-15863

  Abstract

  myo-Inositol oxygenase (MIOX) catalyzes the 4e(-) oxidation of myo-inositol (MI) to d-glucuronate using a substrate activated Fe(II)Fe(III) site. The biferrous and Fe(II)Fe(III) forms of MIOX were studied with circular dichroism (CD), magnetic circular dichroism (MCD), and variable temperature variable field (VTVH) MCD spectroscopies. The MCD spectrum of biferrous MIOX shows two ligand field (LF) transitions near 10000 cm(-1), split by ∼2000 cm(-1), characteristic of six coordinate (6C) Fe(II) sites, indicating that the modest reactivity of the biferrous form toward O2 can be attributed to the saturated coordination of both irons. Upon oxidation to the Fe(II)Fe(III) state, MIOX shows two LF transitions in the ∼10000 cm(-1) region, again implying a coordinatively saturated Fe(II) site. Upon MI binding, these split in energy to 5200 and 11200 cm(-1), showing that MI binding causes the Fe(II) to become coordinatively unsaturated. VTVH MCD magnetization curves of unbound and MI-bound Fe(II)Fe(III) forms show that upon substrate binding, the isotherms become more nested, requiring that the exchange coupling and ferrous zero-field splitting (ZFS) both decrease in magnitude. These results imply that MI binds to the ferric site, weakening the Fe(III)-μ-OH bond and strengthening the Fe(II)-μ-OH bond. This perturbation results in the release of a coordinated water from the Fe(II) that enables its O2 activation.

  View details for DOI 10.1021/ja406635k

  View details for PubMedID 24066857

• Preparation of Non-heme {FeNO}(7) Models of Cysteine Dioxygenase: Sulfur versus Nitrogen Ligation and Photorelease of Nitric Oxide JOURNAL OF THE AMERICAN CHEMICAL SOCIETY McQuilken, A. C., Ha, Y., Sutherlin, K. D., Siegler, M. A., Hodgson, K. O., Hedman, B., Solomon, E. I., Jameson, G. N., Goldberg, D. P. 2013; 135 (38): 14024-14027

  Abstract

  We present the synthesis and spectroscopic characterization of [Fe(NO)(N3PyS)]BF4 (3), the first structural and electronic model of NO-bound cysteine dioxygenase. The nearly isostructural all-N-donor analogue [Fe(NO)(N4Py)](BF4)2 (4) was also prepared, and comparisons of 3 and 4 provide insight regarding the influence of S vs N ligation in {FeNO}(7) species. One key difference occurs upon photoirradiation, which causes the fully reversible release of NO from 3, but not from 4.

  View details for DOI 10.1021/ja4064487

  View details for Web of Science ID 000330162900007

  View details for PubMedID 24040838

  View details for PubMedCentralID PMC3831609

• VTVH MCD studies of substrate activation of the binuclear nonheme iron active site of myo-inositol oxygenase Snyder, R., Diao, Y., Bell, C. B., Krebs, C., Bollinger, J., Solomon, E. I. AMER CHEMICAL SOC. 2013

  View details for Web of Science ID 000329618404521

• Spectroscopic and computational investigation of FIH: Second-sphere contributions to reactivity in nonheme iron enzymes Light, K. M., Hangasky, J. A., Knapp, M. J., Solomon, E. I. AMER CHEMICAL SOC. 2013

  View details for Web of Science ID 000329618404360

• Axial interactions in the mixed-valent CuA active site and role of the axial methionine in electron transfer. Proceedings of the National Academy of Sciences of the United States of America Tsai, M., Hadt, R. G., Marshall, N. M., Wilson, T. D., Lu, Y., Solomon, E. I. 2013; 110 (36): 14658-14663

  Abstract

  Within Cu-containing electron transfer active sites, the role of the axial ligand in type 1 sites is well defined, yet its role in the binuclear mixed-valent CuA sites is less clear. Recently, the mutation of the axial Met to Leu in a CuA site engineered into azurin (CuA Az) was found to have a limited effect on E(0) relative to this mutation in blue copper (BC). Detailed low-temperature absorption and magnetic circular dichroism, resonance Raman, and electron paramagnetic resonance studies on CuA Az (WT) and its M123X (X = Q, L, H) axial ligand variants indicated stronger axial ligation in M123L/H. Spectroscopically validated density functional theory calculations show that the smaller ΔE(0) is attributed to H2O coordination to the Cu center in the M123L mutant in CuA but not in the equivalent BC variant. The comparable stabilization energy of the oxidized over the reduced state in CuA and BC (CuA ∼ 180 mV; BC ∼ 250 mV) indicates that the S(Met) influences E(0) similarly in both. Electron delocalization over two Cu centers in CuA was found to minimize the Jahn-Teller distortion induced by the axial Met ligand and lower the inner-sphere reorganization energy. The Cu-S(Met) bond in oxidized CuA is weak (5.2 kcal/mol) but energetically similar to that of BC, which demonstrates that the protein matrix also serves an entatic role in keeping the Met bound to the active site to tune down E(0) while maintaining a low reorganization energy required for rapid electron transfer under physiological conditions.

  View details for DOI 10.1073/pnas.1314242110

  View details for PubMedID 23964128

  View details for PubMedCentralID PMC3767567

• Molecular Origin of Rapid versus Slow Intramolecular Electron Transfer in the Catalytic Cycle of the Multicopper Oxidases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Heppner, D. E., Kjaergaard, C. H., Solomon, E. I. 2013; 135 (33): 12212-12215

  Abstract

  Kinetic measurements on single-turnover processes in laccase established fast type-1 Cu to trinuclear Cu cluster (TNC) intramolecular electron transfer (IET) in the reduction of the native intermediate (NI), the fully oxidized form of the enzyme formed immediately after O-O bond cleavage in the mechanism of O2 reduction. Alternatively, slow IET kinetics was observed in the reduction of the resting enzyme, which involves proton-coupled electron transfer on the basis of isotope measurements. The >10(3) difference between the IET rates for these two processes confirms that the NI, rather than the resting enzyme that has been defined by crystallography, is the fully oxidized form of the TNC in catalytic turnover. Computational modeling showed that reduction of NI is fast because of the larger driving force associated with a more favorable proton affinity of its μ3-oxo moiety generated by reductive cleavage of the O-O bond. This defines a unifying mechanism in which reductive cleavage of the O-O bond is coupled to rapid IET in the multicopper oxidases.

  View details for DOI 10.1021/ja4064525

  View details for Web of Science ID 000323536100016

  View details for PubMedID 23902255

  View details for PubMedCentralID PMC3807568

• Elucidation of the Fe(IV)=O intermediate in the catalytic cycle of the halogenase SyrB2. Nature Wong, S. D., Srnec, M., Matthews, M. L., Liu, L. V., Kwak, Y., Park, K., Bell, C. B., Alp, E. E., Zhao, J., Yoda, Y., Kitao, S., Seto, M., Krebs, C., Bollinger, J. M., Solomon, E. I. 2013; 499 (7458): 320-323

  Abstract

  Mononuclear non-haem iron (NHFe) enzymes catalyse a broad range of oxidative reactions, including halogenation, hydroxylation, ring closure, desaturation and aromatic ring cleavage reactions. They are involved in a number of biological processes, including phenylalanine metabolism, the production of neurotransmitters, the hypoxic response and the biosynthesis of secondary metabolites. The reactive intermediate in the catalytic cycles of these enzymes is a high-spin S = 2 Fe(IV)=O species, which has been trapped for a number of NHFe enzymes, including the halogenase SyrB2 (syringomycin biosynthesis enzyme 2). Computational studies aimed at understanding the reactivity of this Fe(IV)=O intermediate are limited in applicability owing to the paucity of experimental knowledge about its geometric and electronic structure. Synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) is a sensitive and effective method that defines the dependence of the vibrational modes involving Fe on the nature of the Fe(IV)=O active site. Here we present NRVS structural characterization of the reactive Fe(IV)=O intermediate of a NHFe enzyme, namely the halogenase SyrB2 from the bacterium Pseudomonas syringae pv. syringae. This intermediate reacts via an initial hydrogen-atom abstraction step, performing subsequent halogenation of the native substrate or hydroxylation of non-native substrates. A correlation of the experimental NRVS data to electronic structure calculations indicates that the substrate directs the orientation of the Fe(IV)=O intermediate, presenting specific frontier molecular orbitals that can activate either selective halogenation or hydroxylation.

  View details for DOI 10.1038/nature12304

  View details for PubMedID 23868262

• Elucidation of the Fe(IV)=O intermediate in the catalytic cycle of the halogenase SyrB2 NATURE Wong, S. D., Srnec, M., Matthews, M. L., Liu, L. V., Kwak, Y., Park, K., Bell, C. B., Alp, E. E., Zhao, J., Yoda, Y., Kitao, S., Seto, M., Krebs, C., Bollinger, J. M., Solomon, E. I. 2013; 499 (7458): 320-?

  Abstract

  Mononuclear non-haem iron (NHFe) enzymes catalyse a broad range of oxidative reactions, including halogenation, hydroxylation, ring closure, desaturation and aromatic ring cleavage reactions. They are involved in a number of biological processes, including phenylalanine metabolism, the production of neurotransmitters, the hypoxic response and the biosynthesis of secondary metabolites. The reactive intermediate in the catalytic cycles of these enzymes is a high-spin S = 2 Fe(IV)=O species, which has been trapped for a number of NHFe enzymes, including the halogenase SyrB2 (syringomycin biosynthesis enzyme 2). Computational studies aimed at understanding the reactivity of this Fe(IV)=O intermediate are limited in applicability owing to the paucity of experimental knowledge about its geometric and electronic structure. Synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) is a sensitive and effective method that defines the dependence of the vibrational modes involving Fe on the nature of the Fe(IV)=O active site. Here we present NRVS structural characterization of the reactive Fe(IV)=O intermediate of a NHFe enzyme, namely the halogenase SyrB2 from the bacterium Pseudomonas syringae pv. syringae. This intermediate reacts via an initial hydrogen-atom abstraction step, performing subsequent halogenation of the native substrate or hydroxylation of non-native substrates. A correlation of the experimental NRVS data to electronic structure calculations indicates that the substrate directs the orientation of the Fe(IV)=O intermediate, presenting specific frontier molecular orbitals that can activate either selective halogenation or hydroxylation.

  View details for DOI 10.1038/nature12304

  View details for Web of Science ID 000321910700032

  View details for PubMedID 23868262

• Spectroscopic Studies of the Mononuclear Non-Heme Fe-II Enzyme FIH: Second-Sphere Contributions to Reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Light, K. M., Hangasky, J. A., Knapp, M. J., Solomon, E. I. 2013; 135 (26): 9665-9674

  Abstract

  Factor inhibiting hypoxia-inducible factor (FIH) is an α-ketoglutarate (αKG)-dependent enzyme which catalyzes hydroxylation of residue Asn803 in the C-terminal transactivation domain (CAD) of hypoxia-inducible factor 1α (HIF-1α) and plays an important role in cellular oxygen sensing and hypoxic response. Circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature, variable-field (VTVH) MCD spectroscopies are used to determine the geometric and electronic structures of FIH in its (Fe(II)), (Fe(II)/αKG), and (Fe(II)/αKG/CAD) forms. (Fe(II))FIH and (Fe(II)/αKG)FIH are found to be six-coordinate (6C), whereas (Fe(II)/αKG/CAD)FIH is found to be a 5C/6C mixture. Thus, FIH follows the general mechanistic strategy of non-heme Fe(II) enzymes. Modeling shows that, when Arg238 of FIH is removed, the facial triad carboxylate binds to Fe(II) in a bidentate mode with concomitant lengthening of the Fe(II)/αKG carbonyl bond, which would inhibit the O2 reaction. Correlations over α-keto acid-dependent enzymes and with the extradiol dioxygenases show that members of these families (where both the electron source and O2 bind to Fe(II)) have a second-sphere residue H-bonding to the terminal oxygen of the carboxylate, which stays monodentate. Alternatively, structures of the pterin-dependent and Rieske dioxygenases, which do not have substrate binding to Fe(II), lack H-bonds to the carboxylate and thus allow its bidentate coordination which would direct O2 reactivity. Finally, vis-UV MCD spectra show an unusually high-energy Fe(II) → αKG π* metal-to-ligand charge transfer transition in (Fe(II)/αKG)FIH which is red-shifted upon CAD binding. This red shift indicates formation of H-bonds to the αKG that lower the energy of its carbonyl LUMO, activating it for nucleophilic attack by the Fe-O2 intermediate formed along the reaction coordinate.

  View details for DOI 10.1021/0312571m

  View details for Web of Science ID 000321541800027

  View details for PubMedID 23742069

  View details for PubMedCentralID PMC3712650

• Modified Reactivity toward O-2 in First Shell Variants of Fet3p: Geometric and Electronic Structure Requirements for a Functioning Trinuclear Copper Cluster BIOCHEMISTRY Kjaergaard, C. H., Qayyum, M. F., Augustine, A. J., Ziegler, L., Kosman, D. J., Hodgson, K. O., Hedman, B., Solomon, E. I. 2013; 52 (21): 3702-3711

  Abstract

  Multicopper oxidases (MCOs) carry out the most energy efficient reduction of O2 to H2O known, i.e., with the lowest overpotential. This four-electron process requires an electron mediating type 1 (T1) Cu site and an oxygen reducing trinuclear Cu cluster (TNC), consisting of a binuclear type 3 (T3)- and a mononuclear type 2 (T2) Cu center. The rate-determining step in O2 reduction is the first two-electron transfer from one of the T3 Cu's (T3β) and the T2 Cu, forming a bridged peroxide intermediate (PI). This reaction has been investigated in T3β Cu variants of the Fet3p, where a first shell His ligand is mutated to Glu or Gln. This converts the fast two-electron reaction of the wild-type (WT) enzyme to a slow one-electron oxidation of the TNC. Both variants initially react to form a common T3β Cu(II) intermediate that converts to the Glu or Gln bound resting state. From spectroscopic evaluation, the nonmutated His ligands coordinate linearly to the T3β Cu in the reduced TNCs in the two variants, in contrast to the trigonal arrangement observed in the WT enzyme. This structural perturbation is found to significantly alter the electronic structure of the reduced TNC, which is no longer capable of rapidly transferring two electrons to the two perpendicular half occupied π*-orbitals of O2, in contrast to the WT enzyme. This study provides new insight into the geometric and electronic structure requirements of a fully functional TNC for the rate determining two-electron reduction of O2 in the MCOs.

  View details for DOI 10.1021/bi4002826

  View details for Web of Science ID 000319795500012

  View details for PubMedCentralID PMC3809158

• Modified reactivity toward O2 in first shell variants of Fet3p: geometric and electronic structure requirements for a functioning trinuclear copper cluster. Biochemistry Kjaergaard, C. H., Qayyum, M. F., Augustine, A. J., Ziegler, L., Kosman, D. J., Hodgson, K. O., Hedman, B., Solomon, E. I. 2013; 52 (21): 3702-3711

  Abstract

  Multicopper oxidases (MCOs) carry out the most energy efficient reduction of O2 to H2O known, i.e., with the lowest overpotential. This four-electron process requires an electron mediating type 1 (T1) Cu site and an oxygen reducing trinuclear Cu cluster (TNC), consisting of a binuclear type 3 (T3)- and a mononuclear type 2 (T2) Cu center. The rate-determining step in O2 reduction is the first two-electron transfer from one of the T3 Cu's (T3β) and the T2 Cu, forming a bridged peroxide intermediate (PI). This reaction has been investigated in T3β Cu variants of the Fet3p, where a first shell His ligand is mutated to Glu or Gln. This converts the fast two-electron reaction of the wild-type (WT) enzyme to a slow one-electron oxidation of the TNC. Both variants initially react to form a common T3β Cu(II) intermediate that converts to the Glu or Gln bound resting state. From spectroscopic evaluation, the nonmutated His ligands coordinate linearly to the T3β Cu in the reduced TNCs in the two variants, in contrast to the trigonal arrangement observed in the WT enzyme. This structural perturbation is found to significantly alter the electronic structure of the reduced TNC, which is no longer capable of rapidly transferring two electrons to the two perpendicular half occupied π*-orbitals of O2, in contrast to the WT enzyme. This study provides new insight into the geometric and electronic structure requirements of a fully functional TNC for the rate determining two-electron reduction of O2 in the MCOs.

  View details for DOI 10.1021/bi4002826

  View details for PubMedID 23631422

• Characterization of Metastable Intermediates Formed in the Reaction between a Mn(II) Complex and Dioxygen, Including a Crystallographic Structure of a Binuclear Mn(III)-Peroxo Species JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Coggins, M. K., Sun, X., Kwak, Y., Solomon, E. I., Rybak-Akimova, E., Kovacs, J. A. 2013; 135 (15): 5631-5640

  Abstract

  Transition-metal peroxos have been implicated as key intermediates in a variety of critical biological processes involving O2. Because of their highly reactive nature, very few metal-peroxos have been characterized. The dioxygen chemistry of manganese remains largely unexplored despite the proposed involvement of a Mn-peroxo, either as a precursor to, or derived from, O2, in both photosynthetic H2O oxidation and DNA biosynthesis. These are arguably two of the most fundamental processes of life. Neither of these biological intermediates has been observed. Herein we describe the dioxygen chemistry of coordinatively unsaturated [Mn(II)(S(Me2)N4(6-Me-DPEN))] (+) (1), and the characterization of intermediates formed en route to a binuclear mono-oxo-bridged Mn(III) product {[Mn(III)(S(Me2)N4(6-Me-DPEN)]2(μ-O)}(2+) (2), the oxo atom of which is derived from (18)O2. At low-temperatures, a dioxygen intermediate, [Mn(S(Me2)N4(6-Me-DPEN))(O2)](+) (4), is observed (by stopped-flow) to rapidly and irreversibly form in this reaction (k1(-10 °C) = 3780 ± 180 M(-1) s(-1), ΔH1(++) = 26.4 ± 1.7 kJ mol(-1), ΔS1(++) = -75.6 ± 6.8 J mol(-1) K(-1)) and then convert more slowly (k2(-10 °C) = 417 ± 3.2 M(-1) s(-1), ΔH2(++) = 47.1 ± 1.4 kJ mol(-1), ΔS2(++) = -15.0 ± 5.7 J mol(-1) K(-1)) to a species 3 with isotopically sensitive stretches at νO-O(Δ(18)O) = 819(47) cm(-1), kO-O = 3.02 mdyn/Å, and νMn-O(Δ(18)O) = 611(25) cm(-1) consistent with a peroxo. Intermediate 3 releases approximately 0.5 equiv of H2O2 per Mn ion upon protonation, and the rate of conversion of 4 to 3 is dependent on [Mn(II)] concentration, consistent with a binuclear Mn(O2(2-)) Mn peroxo. This was verified by X-ray crystallography, where the peroxo of {[Mn(III)(S(Me2)N4(6-Me-DPEN)]2(trans-μ-1,2-O2)}(2+) (3) is shown to be bridging between two Mn(III) ions in an end-on trans-μ-1,2-fashion. This represents the first characterized example of a binuclear Mn(III)-peroxo, and a rare case in which more than one intermediate is observed en route to a binuclear μ-oxo-bridged product derived from O2. Vibrational and metrical parameters for binuclear Mn-peroxo 3 are compared with those of related binuclear Fe- and Cu-peroxo compounds.

  View details for DOI 10.1021/ja311166u

  View details for Web of Science ID 000317872800027

  View details for PubMedID 23470101

  View details for PubMedCentralID PMC3709604

• Nuclear resonance vibrational spectroscopic and computational study of high-valent diiron complexes relevant to enzyme intermediates. Proceedings of the National Academy of Sciences of the United States of America Park, K., Bell, C. B., Liu, L. V., Wang, D., Xue, G., Kwak, Y., Wong, S. D., Light, K. M., Zhao, J., Alp, E. E., Yoda, Y., Saito, M., Kobayashi, Y., Ohta, T., Seto, M., Que, L., Solomon, E. I. 2013; 110 (16): 6275-6280

  Abstract

  High-valent intermediates of binuclear nonheme iron enzymes are structurally unknown despite their importance for understanding enzyme reactivity. Nuclear resonance vibrational spectroscopy combined with density functional theory calculations has been applied to structurally well-characterized high-valent mono- and di-oxo bridged binuclear Fe model complexes. Low-frequency vibrational modes of these high-valent diiron complexes involving Fe motion have been observed and assigned. These are independent of Fe oxidation state and show a strong dependence on spin state. It is important to note that they are sensitive to the nature of the Fe2 core bridges and provide the basis for interpreting parallel nuclear resonance vibrational spectroscopy data on the high-valent oxo intermediates in the binuclear nonheme iron enzymes.

  View details for DOI 10.1073/pnas.1304238110

  View details for PubMedID 23576760

• Nuclear resonance vibrational spectroscopic and computational study of high-valent diiron complexes relevant to enzyme intermediates PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Park, K., Bell, C. B., Liu, L. V., Wang, D., Xue, G., Kwak, Y., Wong, S. D., Light, K. M., Zhao, J., Alp, E. E., Yoda, Y., Saito, M., Kobayashi, Y., Ohta, T., Seto, M., Que, L., Solomon, E. I. 2013; 110 (16): 6269-6280

  Abstract

  High-valent intermediates of binuclear nonheme iron enzymes are structurally unknown despite their importance for understanding enzyme reactivity. Nuclear resonance vibrational spectroscopy combined with density functional theory calculations has been applied to structurally well-characterized high-valent mono- and di-oxo bridged binuclear Fe model complexes. Low-frequency vibrational modes of these high-valent diiron complexes involving Fe motion have been observed and assigned. These are independent of Fe oxidation state and show a strong dependence on spin state. It is important to note that they are sensitive to the nature of the Fe2 core bridges and provide the basis for interpreting parallel nuclear resonance vibrational spectroscopy data on the high-valent oxo intermediates in the binuclear nonheme iron enzymes.

  View details for DOI 10.1073/pnas.1304238110

  View details for Web of Science ID 000318041500021

  View details for PubMedCentralID PMC3631696

• Comparison of High-Spin and Low-Spin Nonheme Fe-III-OOH Complexes in O-O Bond Homolysis and H-Atom Abstraction Reactivities JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Liu, L. V., Hong, S., Cho, J., Nam, W., Solomon, E. I. 2013; 135 (8): 3286-3299

  Abstract

  The geometric and electronic structures and reactivity of an S = 5/2 (HS) mononuclear nonheme (TMC)Fe(III)-OOH complex are studied by spectroscopies, calculations, and kinetics and compared with the results of previous studies of S = 1/2 (LS) Fe(III)-OOH complexes to understand parallels and differences in mechanisms of O-O bond homolysis and electrophilic H-atom abstraction reactions. The homolysis reaction of the HS [(TMC)Fe(III)-OOH](2+) complex is found to involve axial ligand coordination and a crossing to the LS surface for O-O bond homolysis. Both HS and LS Fe(III)-OOH complexes are found to perform direct H-atom abstraction reactions but with very different reaction coordinates. For the LS Fe(III)-OOH, the transition state is late in O-O and early in C-H coordinates. However, for the HS Fe(III)-OOH, the transition state is early in O-O and further along in the C-H coordinate. In addition, there is a significant amount of electron transfer from the substrate to the HS Fe(III)-OOH at transition state, but that does not occur in the LS transition state. Thus, in contrast to the behavior of LS Fe(III)-OOH, the H-atom abstraction reactivity of HS Fe(III)-OOH is found to be highly dependent on both the ionization potential and the C-H bond strength of the substrate. LS Fe(III)-OOH is found to be more effective in H-atom abstraction for strong C-H bonds, while the higher reduction potential of HS Fe(III)-OOH allows it to be active in electrophilic reactions without the requirement of O-O bond cleavage. This is relevant to the Rieske dioxygenases, which are proposed to use a HS Fe(III)-OOH to catalyze cis-dihydroxylation of a wide range of aromatic compounds.

  View details for DOI 10.1021/ja400183g

  View details for Web of Science ID 000315618900064

  View details for PubMedID 23368958

  View details for PubMedCentralID PMC3614352

• Iron L-Edge X-ray Absorption Spectroscopy of Oxy-Picket Fence Porphyrin: Experimental Insight into Fe-O-2 Bonding JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Wilson, S. A., Kroll, T., Decreau, R. A., Hocking, R. K., Lundberg, M., Hedman, B., Hodgson, K. O., Solomon, E. I. 2013; 135 (3): 1124-1136

  Abstract

  The electronic structure of the Fe-O(2) center in oxy-hemoglobin and oxy-myoglobin is a long-standing issue in the field of bioinorganic chemistry. Spectroscopic studies have been complicated by the highly delocalized nature of the porphyrin, and calculations require interpretation of multideterminant wave functions for a highly covalent metal site. Here, iron L-edge X-ray absorption spectroscopy, interpreted using a valence bond configuration interaction multiplet model, is applied to directly probe the electronic structure of the iron in the biomimetic Fe-O(2) heme complex [Fe(pfp)(1-MeIm)O(2)] (pfp ("picket fence porphyrin") = meso-tetra(α,α,α,α-o-pivalamidophenyl)porphyrin or TpivPP). This method allows separate estimates of σ-donor, π-donor, and π-acceptor interactions through ligand-to-metal charge transfer and metal-to-ligand charge transfer mixing pathways. The L-edge spectrum of [Fe(pfp)(1-MeIm)O(2)] is further compared to those of [Fe(II)(pfp)(1-MeIm)(2)], [Fe(II)(pfp)], and [Fe(III)(tpp)(ImH)(2)]Cl (tpp = meso-tetraphenylporphyrin) which have Fe(II)S = 0, Fe(II)S = 1, and Fe(III)S = 1/2 ground states, respectively. These serve as references for the three possible contributions to the ground state of oxy-pfp. The Fe-O(2) pfp site is experimentally determined to have both significant σ-donation and a strong π-interaction of the O(2) with the iron, with the latter having implications with respect to the spin polarization of the ground state.

  View details for DOI 10.1021/ja3103583

  View details for Web of Science ID 000314141200029

  View details for PubMedID 23259487

  View details for PubMedCentralID PMC3614349

• Nuclear Resonance Vibrational Spectroscopy and DFT study of Peroxo-Bridged Biferric Complexes: Structural Insight into Peroxo Intermediates of Binuclear Non-heme Iron Enzymes ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Park, K., Tsugawa, T., Furutachi, H., Kwak, Y., Liu, L. V., Wong, S. D., Yoda, Y., Kobayashi, Y., Saito, M., Kurokuzu, M., Seto, M., Suzuki, M., Solomon, E. I. 2013; 52 (4): 1294-1298

  View details for DOI 10.1002/anie.201208240

  View details for Web of Science ID 000313719300042

  View details for PubMedID 23225363

• Mononuclear nickel(II)-superoxo and nickel(III)-peroxo complexes bearing a common macrocyclic TMC ligand CHEMICAL SCIENCE Cho, J., Kang, H. Y., Liu, L. V., Sarangi, R., Solomon, E. I., Nam, W. 2013; 4 (4): 1502-1508

  Abstract

  Mononuclear metal-dioxygen adducts, such as metal-superoxo and -peroxo species, are generated as key intermediates in the catalytic cycles of dioxygen activation by heme and non-heme metalloenzymes. We have shown recently that the geometric and electronic structure of the Ni-O2 core in [Ni(n-TMC)(O2)]+ (n = 12 and 14) varies depending on the ring size of the supporting TMC ligand. In this study, mononuclear Ni(II)-superoxo and Ni(III)-peroxo complexes bearing a common macrocylic 13-TMC ligand, such as [NiII(13-TMC)(O2)]+ and [NiIII(13-TMC)(O2)]+, were synthesized in the reaction of [NiII(13-TMC)(CH3CN)]2+ and H2O2 in the presence of tetramethylammonium hydroxide (TMAH) and triethylamine (TEA), respectively. The Ni(II)-superoxo and Ni(III)-peroxo complexes bearing the common 13-TMC ligand were successfully characterized by various spectroscopic methods, X-ray crystallography, and DFT calculations. Based on the combined experimental and theoretical studies, we conclude that the superoxo ligand in [NiII(13-TMC)(O2)]+ is bound in an end-on fashion to the nickel(II) center, whereas the peroxo ligand in [NiIII(13-TMC)(O2)]+ is bound in a side-on fashion to the nickel(III) center. Reactivity studies performed with the Ni(II)-superoxo and Ni(III)-peroxo complexes toward organic substrates reveal that the former possesses an electrophilic character, whereas the latter is an active oxidant in nucleophilic reaction.

  View details for DOI 10.1039/c3sc22173c

  View details for Web of Science ID 000315597900013

  View details for PubMedCentralID PMC3646059

• Ribonucleotide reductase class I with different radical generating clusters COORDINATION CHEMISTRY REVIEWS Tomter, A. B., Zoppellaro, G., Andersen, N. H., Hersleth, H., Hammerstad, M., Rohr, A. K., Sandvik, G. K., Strand, K. R., Nilsson, G. E., Bell, C. B., Barra, A., Blasco, E., Le Pape, L., Solomon, E. I., Andersson, K. K. 2013; 257 (1): 3-26

  View details for DOI 10.1016/j.ccr.2012.05.021

  View details for Web of Science ID 000312972700002

• Bilirubin oxidase from Magnaporthe oryzae: an attractive new enzyme for biotechnological applications APPLIED MICROBIOLOGY AND BIOTECHNOLOGY Durand, F., Gounel, S., Kjaergaard, C. H., Solomon, E. I., Mano, N. 2012; 96 (6): 1489-1498

  Abstract

  A novel bilirubin oxidase (BOD), from the rice blast fungus Magnaporthe oryzae, has been identified and isolated. The 64-kDa protein containing four coppers was successfully overexpressed in Pichia pastoris and purified to homogeneity in one step. Protein yield is more than 100 mg for 2 L culture, twice that of Myrothecium verrucaria. The k(cat)/K(m) ratio for conjugated bilirubin (1,513 mM⁻¹ s⁻¹) is higher than that obtained for the BOD from M. verrucaria expressed in native fungus (980 mM⁻¹ s⁻¹), with the lowest K(m) measured for any BOD highly desirable for detection of bilirubin in medical samples. In addition, this protein exhibits a half-life for deactivation >300 min at 37 °C, high stability at pH 7, and high tolerance towards urea, making it an ideal candidate for the elaboration of biofuel cells, powering implantable medical devices. Finally, this new BOD is efficient in decolorizing textile dyes such as Remazol brilliant Blue R, making it useful for environmentally friendly industrial applications.

  View details for DOI 10.1007/s00253-012-3926-2

  View details for Web of Science ID 000311244800009

  View details for PubMedID 22350257

• Alteration of the oxygen-dependent reactivity of de novo Due Ferri proteins NATURE CHEMISTRY Reig, A. J., Pires, M. M., Snyder, R. A., Wu, Y., Jo, H., Kulp, D. W., Butch, S. E., Calhoun, J. R., Szyperski, T. G., Solomon, E. I., DeGrado, W. F. 2012; 4 (11): 900-906

  Abstract

  De novo proteins provide a unique opportunity to investigate the structure-function relationships of metalloproteins in a minimal, well-defined and controlled scaffold. Here, we describe the rational programming of function in a de novo designed di-iron carboxylate protein from the Due Ferri family. Originally created to catalyse the O(2)-dependent, two-electron oxidation of hydroquinones, the protein was reprogrammed to catalyse the selective N-hydroxylation of arylamines by remodelling the substrate access cavity and introducing a critical third His ligand to the metal-binding cavity. Additional second- and third-shell modifications were required to stabilize the His ligand in the core of the protein. These structural changes resulted in at least a 10(6)-fold increase in the relative rate between the arylamine N-hydroxylation and hydroquinone oxidation reactions. This result highlights the potential for using de novo proteins as scaffolds for future investigations of the geometric and electronic factors that influence the catalytic tuning of di-iron active sites.

  View details for DOI 10.1038/NCHEM.1454

  View details for Web of Science ID 000310436600008

  View details for PubMedID 23089864

  View details for PubMedCentralID PMC3568993

• Spectroscopic and DFT Studies of Second-Sphere Variants of the Type 1 Copper Site in Azurin: Covalent and Nonlocal Electrostatic Contributions to Reduction Potentials JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hadt, R. G., Sun, N., Marshall, N. M., Hodgson, K. O., Hedman, B., Lu, Y., Solomon, E. I. 2012; 134 (40): 16701-16716

  Abstract

  The reduction potentials (E(0)) of type 1 (T1) or blue copper (BC) sites in proteins and enzymes with identical first coordination spheres around the redox active copper ion can vary by ~400 mV. Here, we use a combination of low-temperature electronic absorption and magnetic circular dichroism, electron paramagnetic resonance, resonance Raman, and S K-edge X-ray absorption spectroscopies to investigate a series of second-sphere variants--F114P, N47S, and F114N in Pseudomonas aeruginosa azurin--which modulate hydrogen bonding to and protein-derived dipoles nearby the Cu-S(Cys) bond. Density functional theory calculations correlated to the experimental data allow for the fractionation of the contributions to tuning E(0) into covalent and nonlocal electrostatic components. These are found to be significant, comparable in magnitude, and additive for active H-bonds, while passive H-bonds are mostly nonlocal electrostatic in nature. For dipoles, these terms can be additive to or oppose one another. This study provides a methodology for uncoupling covalency from nonlocal electrostatics, which, when coupled to X-ray crystallographic data, distinguishes specific local interactions from more long-range protein/active interactions, while affording further insight into the second-sphere mechanisms available to the protein to tune the E(0) of electron-transfer sites in biology.

  View details for DOI 10.1021/ja306438n

  View details for Web of Science ID 000309566400044

  View details for PubMedID 22985400

  View details for PubMedCentralID PMC3506006

• Analysis of resonance Raman data on the blue copper site in pseudoazurin: Excited state pi and sigma charge transfer distortions and their relation to ground state reorganization energy JOURNAL OF INORGANIC BIOCHEMISTRY Hadt, R. G., Xie, X., Pauleta, S. R., Moura, I., Solomon, E. I. 2012; 115: 155-162

  Abstract

  The short Cu(2+)-S(Met) bond in pseudoazurin (PAz) results in the presence of two relatively intense S(p)(π) and S(p)(σ) charge transfer (CT) transitions. This has enabled resonance Raman (rR) data to be obtained for each excited state. The rR data show very different intensity distribution patterns for the vibrations in the 300-500 cm(-1) region. Time-dependent density functional theory (TDDFT) calculations have been used to determine that the change in intensity distribution between the S(p)(π) and S(p)(σ) excited states reflects the differential enhancement of S(Cys) backbone modes with Cu-S(Cys)-C(β) out-of-plane (oop) and in-plane (ip) bend character in their respective potential energy distributions (PEDs). The rR excited state distortions have been related to ground state reorganization energies (λ s) and predict that, in addition to M-L stretches, the Cu-S(Cys)-C(β) oop bend needs to be considered. DFT calculations predict a large distortion in the Cu-S(Cys)-C(β) oop bending coordinate upon reduction of a blue copper (BC) site; however, this distortion is not present in the X-ray crystal structures of reduced BC sites. The lack of Cu-S(Cys)-C(β) oop distortion upon reduction corresponds to a previously unconsidered constraint on the thiolate ligand orientation in the reduced state of BC proteins and can be considered as a contribution to the entatic/rack nature of BC sites.

  View details for DOI 10.1016/j.jinorgbio.2012.03.006

  View details for Web of Science ID 000309990500021

  View details for PubMedID 22560510

• pi-Frontier molecular orbitals in S=2 ferryl species and elucidation of their contributions to reactivity PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Srnec, M., Wong, S. D., England, J., Que, L., Solomon, E. I. 2012; 109 (36): 14326-14331

  Abstract

  S = 2 Fe(IV) ═ O species are key intermediates in the catalysis of most nonheme iron enzymes. This article presents detailed spectroscopic and high-level computational studies on a structurally-defined S = 2 Fe(IV) ═ O species that define its frontier molecular orbitals, which allow its high reactivity. Importantly, there are both π- and σ-channels for reaction, and both are highly reactive because they develop dominant oxyl character at the transition state. These π- and σ-channels have different orientation dependences defining how the same substrate can undergo different reactions (H-atom abstraction vs. electrophilic aromatic attack) with Fe(IV) ═ O sites in different enzymes, and how different substrates can undergo different reactions (hydroxylation vs. halogenation) with an Fe(IV) ═ O species in the same enzyme.

  View details for DOI 10.1073/pnas.1212693109

  View details for Web of Science ID 000308912600016

  View details for PubMedID 22908238

  View details for PubMedCentralID PMC3437891

• (Fe-IV=O(TBC)(CH3CN)](2+): Comparative Reactivity of Iron(IV)-Oxo Species with Constrained Equatorial Cyclam Ligation JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Wilson, S. A., Chen, J., Hong, S., Lee, Y., Clemancey, M., Garcia-Serres, R., Nomura, T., Ogura, T., Latour, J., Hedman, B., Hodgson, K. O., Nam, W., Solomon, E. I. 2012; 134 (28): 11791-11806

  Abstract

  [Fe(IV)═O(TBC)(CH(3)CN)](2+) (TBC = 1,4,8,11-tetrabenzyl-1,4,8,11-tetraazacyclotetradecane) is characterized, and its reactivity differences relative to [Fe(IV)═O(TMC)(CH(3)CN)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) are evaluated in hydrogen atom (H-atom) abstraction and oxo-transfer reactions. Structural differences are defined using X-ray absorption spectroscopy and correlated to reactivities using density functional theory. The S = 1 ground states are highly similar and result in large activation barriers (~25 kcal/mol) due to steric interactions between the cyclam chelate and the substrate (e.g., ethylbenzene) associated with the equatorial π-attack required by this spin state. Conversely, H-atom abstraction reactivity on an S = 2 surface allows for a σ-attack with an axial substrate approach. This results in decreased steric interactions with the cyclam and a lower barrier (~9 kcal/mol). For [Fe(IV)═O(TBC)(CH(3)CN)](2+), the S = 2 excited state in the reactant is lower in energy and therefore more accessible at the transition state due to a weaker ligand field associated with the steric interactions of the benzyl substituents with the trans-axial ligand. This study is further extended to the oxo-transfer reaction, which is a two-electron process requiring both σ- and π-electron transfer and thus a nonlinear transition state. In oxo-transfer, the S = 2 has a lower barrier due to sequential vs concerted (S = 1) two electron transfer which gives a high-spin ferric intermediate at the transition state. The [Fe(IV)═O(TBC)(CH(3)CN)](2+) complex is more distorted at the transition state, with the iron farther out of the equatorial plane due to the steric interaction of the benzyl groups with the trans-axial ligand. This allows for better orbital overlap with the substrate, a lower barrier, and an increased rate of oxo-transfer.

  View details for DOI 10.1021/ja03046298

  View details for PubMedID 22708532

• Structure/function correlations among coupled binuclear copper proteins through spectroscopic and reactivity studies of NspF PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Ginsbach, J. W., Kieber-Emmons, M. T., Nomoto, R., Noguchi, A., Ohnishi, Y., Solomon, E. I. 2012; 109 (27): 10793-10797

  Abstract

  The terminal step of 4-hydroxy-3-nitrosobenzamide biosynthesis in Streptomyces murayamaensis is performed by NspF, a mono-oxygenase that converts o-aminophenols to the corresponding nitroso product (hydroxyanilinase activity). Previous biochemical characterization of the resting form of NspF suggested that this enzyme belonged to the coupled binuclear copper enzyme (CBC) family. Another member of this enzyme family, tyrosinase, is able to mono-oxygenate monophenols (monophenolase activity) but not o-aminophenols. To gain insight into the unique reactivity of NspF, we have generated and characterized the oxy form of its active site. The observation of spectral features identical to those of oxy-tyrosinase indicates that oxy-NspF contains a Cu(2)O(2) core where peroxide is coordinated in a μ-η(2):η(2) mode, confirming that NspF is a CBC enzyme. This oxy form is found to react with monophenols, indicating that, like tyrosinase, NspF also possesses monophenolase activity. A comparison of the two electrophilic mechanisms for the monophenolase and hydroxyanilinase activity indicates a large geometric change between their respective transition states. The potential for specific interactions between the protein pocket and the substrate in each transition state is discussed within the context of the differential reactivity of this family of enzymes with equivalent μ-η(2):η(2) peroxy bridged coupled binuclear copper active sites.

  View details for DOI 10.1073/pnas.1208718109

  View details for Web of Science ID 000306641100022

  View details for PubMedID 22711806

  View details for PubMedCentralID PMC3390868

• Geometric and Electronic Structure of [{Cu(MeAN)}(2)(mu-eta(2):eta(2)(O-2(2-)))](2+) with an Unusually Long O-O Bond: O-O Bond Weakening vs Activation for Reductive Cleavage JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Park, G. Y., Qayyum, M. F., Woertink, J., Hodgson, K. O., Hedman, B., Sarjeant, A. A., Solomon, E. I., Karlin, K. D. 2012; 134 (20): 8513-8524

  Abstract

  Certain side-on peroxo-dicopper(II) species with particularly low ν(O-O) (710-730 cm(-1)) have been found in equilibrium with their bis-μ-oxo-dicopper(III) isomer. An issue is whether such side-on peroxo bridges are further activated for O-O cleavage. In a previous study (Liang, H.-C., et al. J. Am. Chem. Soc.2002, 124, 4170), we showed that oxygenation of the three-coordinate complex [Cu(I)(MeAN)](+) (MeAN = N-methyl-N,N-bis[3-(dimethylamino)propyl]amine) leads to a low-temperature stable [{Cu(II)(MeAN)}(2)(μ-η(2):η(2)-O(2)(2-))](2+) peroxo species with low ν(O-O) (721 cm(-1)), as characterized by UV-vis absorption and resonance Raman (rR) spectroscopies. Here, this complex has been crystallized as its SbF(6)(-) salt, and an X-ray structure indicates the presence of an unusually long O-O bond (1.540(5) Å) consistent with the low ν(O-O). Extended X-ray absorption fine structure and rR spectroscopic and reactivity studies indicate the exclusive formation of [{Cu(II)(MeAN)}(2)(μ-η(2):η(2)-O(2)(2-))](2+) without any bis-μ-oxo-dicopper(III) isomer present. This is the first structure of a side-on peroxo-dicopper(II) species with a significantly long and weak O-O bond. DFT calculations show that the weak O-O bond results from strong σ donation from the MeAN ligand to Cu that is compensated by a decrease in the extent of peroxo to Cu charge transfer. Importantly, the weak O-O bond does not reflect an increase in backbonding into the σ* orbital of the peroxide. Thus, although the O-O bond is unusually weak, this structure is not further activated for reductive cleavage to form a reactive bis-μ-oxo dicopper(III) species. These results highlight the necessity of understanding electronic structure changes associated with spectral changes for correlations to reactivity.

  View details for DOI 10.1021/ja300674m

  View details for Web of Science ID 000304285700048

  View details for PubMedCentralID PMC3437010

• Geometric and electronic structure of [{Cu(MeAN)}2(µ-?2:?2(O2(2-)))]2+ with an unusually long O-O bond: O-O bond weakening vs activation for reductive cleavage. Journal of the American Chemical Society Park, G. Y., Qayyum, M. F., Woertink, J., Hodgson, K. O., Hedman, B., Narducci Sarjeant, A. A., Solomon, E. I., Karlin, K. D. 2012; 134 (20): 8513-8524

  Abstract

  Certain side-on peroxo-dicopper(II) species with particularly low ν(O-O) (710-730 cm(-1)) have been found in equilibrium with their bis-μ-oxo-dicopper(III) isomer. An issue is whether such side-on peroxo bridges are further activated for O-O cleavage. In a previous study (Liang, H.-C., et al. J. Am. Chem. Soc.2002, 124, 4170), we showed that oxygenation of the three-coordinate complex [Cu(I)(MeAN)](+) (MeAN = N-methyl-N,N-bis[3-(dimethylamino)propyl]amine) leads to a low-temperature stable [{Cu(II)(MeAN)}(2)(μ-η(2):η(2)-O(2)(2-))](2+) peroxo species with low ν(O-O) (721 cm(-1)), as characterized by UV-vis absorption and resonance Raman (rR) spectroscopies. Here, this complex has been crystallized as its SbF(6)(-) salt, and an X-ray structure indicates the presence of an unusually long O-O bond (1.540(5) Å) consistent with the low ν(O-O). Extended X-ray absorption fine structure and rR spectroscopic and reactivity studies indicate the exclusive formation of [{Cu(II)(MeAN)}(2)(μ-η(2):η(2)-O(2)(2-))](2+) without any bis-μ-oxo-dicopper(III) isomer present. This is the first structure of a side-on peroxo-dicopper(II) species with a significantly long and weak O-O bond. DFT calculations show that the weak O-O bond results from strong σ donation from the MeAN ligand to Cu that is compensated by a decrease in the extent of peroxo to Cu charge transfer. Importantly, the weak O-O bond does not reflect an increase in backbonding into the σ* orbital of the peroxide. Thus, although the O-O bond is unusually weak, this structure is not further activated for reductive cleavage to form a reactive bis-μ-oxo dicopper(III) species. These results highlight the necessity of understanding electronic structure changes associated with spectral changes for correlations to reactivity.

  View details for DOI 10.1021/ja300674m

  View details for PubMedID 22571744

• Bilirubin oxidase from Bacillus pumilus: A promising enzyme for the elaboration of efficient cathodes in biofuel cells BIOSENSORS & BIOELECTRONICS Durand, F., Kjaergaard, C. H., Suraniti, E., Gounel, S., Hadt, R. G., Solomon, E. I., Mano, N. 2012; 35 (1): 140-146

  Abstract

  A CotA multicopper oxidase (MCO) from Bacillus pumilus, previously identified as a laccase, has been studied and characterized as a new bacterial bilirubin oxidase (BOD). The 59 kDa protein containing four coppers, was successfully over-expressed in Escherichia coli and purified to homogeneity in one step. This 509 amino-acid enzyme, having 67% and 26% sequence identity with CotA from Bacillus subtilis and BOD from Myrothecium verrucaria, respectively, shows higher turnover activity towards bilirubin compared to other bacterial MCOs. The current density for O(2) reduction, when immobilized in a redox hydrogel, is only 12% smaller than the current obtained with Trachyderma tsunodae BOD. Under continuous electrocatalysis, an electrode modified with the new BOD is more stable, and has a higher tolerance towards NaCl, than a T. tsunodae BOD modified electrode. This makes BOD from B. pumilus an attractive new candidate for application in biofuel cells (BFCs) and biosensors.

  View details for DOI 10.1016/j.bios.2012.02.033

  View details for Web of Science ID 000305036000021

  View details for PubMedID 22410485

• Spectroscopic and Crystallographic Characterization of "Alternative Resting" and "Resting Oxidized" Enzyme Forms of Bilirubin Oxidase: Implications for Activity and Electrochemical Behavior of Multicopper Oxidases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Kjaergaard, C. H., Durand, F., Tasca, F., Qayyum, M. F., Kauffmann, B., Gounel, S., Suraniti, E., Hodgson, K. O., Hedman, B., Mano, N., Solomon, E. I. 2012; 134 (12): 5548-5551

  Abstract

  While there is broad agreement on the catalytic mechanism of multicopper oxidases (MCOs), the geometric and electronic structures of the resting trinuclear Cu cluster have been variable, and their relevance to catalysis has been debated. Here, we present a spectroscopic characterization, complemented by crystallographic data, of two resting forms occurring in the same enzyme and define their interconversion. The resting oxidized form shows similar features to the resting form in Rhus vernicifera and Trametes versicolor laccase, characterized by "normal" type 2 Cu electron paramagnetic resonance (EPR) features, 330 nm absorption shoulder, and a short type 3 (T3) Cu-Cu distance, while the alternative resting form shows unusually small A(||) and high g(||) EPR features, lack of 330 nm absorption intensity, and a long T3 Cu-Cu distance. These different forms are evaluated with respect to activation for catalysis, and it is shown that the alternative resting form can only be activated by low-potential reduction, in contrast to the resting oxidized form which is activated via type 1 Cu at high potential. This difference in activity is correlated to differences in redox states of the two forms and highlights the requirement for efficient sequential reduction of resting MCOs for their involvement in catalysis.

  View details for DOI 10.1021/ja211872j

  View details for Web of Science ID 000302489500031

  View details for PubMedID 22413777

  View details for PubMedCentralID PMC3339634

• Reactive intermediates in methane to methanol conversion over Cu containing zeolites Vanelderen, P., Smeets, P. J., Hadt, R. G., Woertink, J. S., Schoonheydt, R. A., Solomon, E. I., Sels, B. F. AMER CHEMICAL SOC. 2012

  View details for Web of Science ID 000324503201326

• Substrate and Metal Control of Barrier Heights for Oxo Transfer to Mo and W Bis-dithiolene Sites INORGANIC CHEMISTRY Tenderholt, A. L., Hodgson, K. O., Hedman, B., Holm, R. H., Solomon, E. I. 2012; 51 (6): 3436-3442

  Abstract

  Reaction coordinates for oxo transfer from the substrates Me(3)NO, Me(2)SO, and Me(3)PO to the biologically relevant Mo(IV) bis-dithiolene complex [Mo(OMe)(mdt)(2)](-) where mdt = 1,2-dimethyl-ethene-1,2-dithiolate(2-), and from Me(2)SO to the analogous W(IV) complex, have been calculated using density functional theory. In each case, the reaction first proceeds through a transition state (TS1) to an intermediate with substrate weakly bound, followed by a second transition state (TS2) around which breaking of the substrate X-O bond begins. By analyzing the energetic contributions to each barrier, it is shown that the nature of the substrate and metal determines which transition state controls the rate-determining step of the reaction.

  View details for DOI 10.1021/ic2020397

  View details for Web of Science ID 000301624500013

  View details for PubMedID 22372518

  View details for PubMedCentralID PMC3319056

• Spectroscopic Studies of the Iron and Manganese Reconstituted Tyrosyl Radical in Bacillus Cereus Ribonucleotide Reductase R2 Protein PLOS ONE Tomter, A. B., Zoppellaro, G., Bell, C. B., Barra, A., Andersen, N. H., Solomon, E. I., Andersson, K. K. 2012; 7 (3)

  Abstract

  Ribonucleotide reductase (RNR) catalyzes the rate limiting step in DNA synthesis where ribonucleotides are reduced to the corresponding deoxyribonucleotides. Class Ib RNRs consist of two homodimeric subunits: R1E, which houses the active site; and R2F, which contains a metallo cofactor and a tyrosyl radical that initiates the ribonucleotide reduction reaction. We studied the R2F subunit of B. cereus reconstituted with iron or alternatively with manganese ions, then subsequently reacted with molecular oxygen to generate two tyrosyl-radicals. The two similar X-band EPR spectra did not change significantly over 4 to 50 K. From the 285 GHz EPR spectrum of the iron form, a g(1)-value of 2.0090 for the tyrosyl radical was extracted. This g(1)-value is similar to that observed in class Ia E. coli R2 and class Ib R2Fs with iron-oxygen cluster, suggesting the absence of hydrogen bond to the phenoxyl group. This was confirmed by resonance Raman spectroscopy, where the stretching vibration associated to the radical (C-O, ν(7a) = 1500 cm(-1)) was found to be insensitive to deuterium-oxide exchange. Additionally, the (18)O-sensitive Fe-O-Fe symmetric stretching (483 cm(-1)) of the metallo-cofactor was also insensitive to deuterium-oxide exchange indicating no hydrogen bonding to the di-iron-oxygen cluster, and thus, different from mouse R2 with a hydrogen bonded cluster. The HF-EPR spectrum of the manganese reconstituted RNR R2F gave a g(1)-value of ∼2.0094. The tyrosyl radical microwave power saturation behavior of the iron-oxygen cluster form was as observed in class Ia R2, with diamagnetic di-ferric cluster ground state, while the properties of the manganese reconstituted form indicated a magnetic ground state of the manganese-cluster. The recent activity measurements (Crona et al., (2011) J Biol Chem 286: 33053-33060) indicates that both the manganese and iron reconstituted RNR R2F could be functional. The manganese form might be very important, as it has 8 times higher activity.

  View details for DOI 10.1371/journal.pone.0033436

  View details for Web of Science ID 000303198600065

  View details for PubMedID 22432022

  View details for PubMedCentralID PMC3303829

• Structural and Spectroscopic Properties of the Peroxodiferric Intermediate of Ricinus communis Soluble Delta(9) Desaturase INORGANIC CHEMISTRY Srnec, M., Rokob, T. A., Schwartz, J. K., Kwak, Y., Rulisek, L., Solomon, E. I. 2012; 51 (5): 2806-2820

  Abstract

  Large-scale quantum and molecular mechanical methods (QM/MM) and QM calculations were carried out on the soluble Δ(9) desaturase (Δ(9)D) to investigate various structural models of the spectroscopically defined peroxodiferric (P) intermediate. This allowed us to formulate a consistent mechanistic picture for the initial stages of the reaction mechanism of Δ(9)D, an important diferrous nonheme iron enzyme that cleaves the C-H bonds in alkane chains resulting in the highly specific insertion of double bonds. The methods (density functional theory (DFT), time-dependent DFT (TD-DFT), QM(DFT)/MM, and TD-DFT with electrostatic embedding) were benchmarked by demonstrating that the known spectroscopic effects and structural perturbation caused by substrate binding to diferrous Δ(9)D can be qualitatively reproduced. We show that structural models whose spectroscopic (absorption, circular dichroism (CD), vibrational and Mössbauer) characteristics correlate best with experimental data for the P intermediate correspond to the μ-1,2-O(2)(2-) binding mode. Coordination of Glu196 to one of the iron centers (Fe(B)) is demonstrated to be flexible, with the monodentate binding providing better agreement with spectroscopic data, and the bidentate structure being slightly favored energetically (1-10 kJ mol(-1)). Further possible structures, containing an additional proton or water molecule are also evaluated in connection with the possible activation of the P intermediate. Specifically, we suggest that protonation of the peroxide moiety, possibly preceded by water binding in the Fe(A) coordination sphere, could be responsible for the conversion of the P intermediate in Δ(9)D into a form capable of hydrogen abstraction. Finally, results are compared with recent findings on the related ribonucleotide reductase and toluene/methane monooxygenase enzymes.

  View details for DOI 10.1021/ic2018067

  View details for Web of Science ID 000301007100014

  View details for PubMedID 22332845

• Spectroscopic Elucidation of a New Heme/Copper Dioxygen Structure Type: Implications for O center dot center dot center dot O Bond Rupture in Cytochrome c Oxidase ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Kieber-Emmons, M. T., Qayyum, M. F., Li, Y., Halime, Z., Hodgson, K. O., Hedman, B., Karlin, K. D., Solomon, E. I. 2012; 51 (1): 168-172

  View details for DOI 10.1002/anie.201104080

  View details for Web of Science ID 000298598500025

  View details for PubMedID 22095556

  View details for PubMedCentralID PMC3517061

• Ligand Field and Molecular Orbital Theories of Transition Metal X-ray Absorption Edge Transitions MOLECULAR ELECTRONIC STRUCTURES OF TRANSITION METAL COMPLEXES I Hocking, R. K., Solomon, E. I. 2012; 142: 155-184

  View details for DOI 10.1007/430_2011_60

  View details for Web of Science ID 000321627600008

• Cu-ZSM-5: A biomimetic inorganic model for methane oxidation JOURNAL OF CATALYSIS Vanelderen, P., Hadt, R. G., Smeets, P. J., Solomon, E. I., Schoonheydt, R. A., Sels, B. F. 2011; 284 (2): 157-164

  Abstract

  The present work highlights recent advances in elucidating the methane oxidation mechanism of inorganic Cu-ZSM-5 biomimic and in identifying the reactive intermediates that are involved. Such molecular understanding is important in view of upgrading abundantly available methane, but also to comprehend the working mechanism of genuine Cu-containing oxidation enzymes.

  View details for DOI 10.1016/j.jcat.2011.10.009

  View details for Web of Science ID 000298527000005

  View details for PubMedCentralID PMC3593946

• S K-edge XAS and DFT Calculations on SAM Dependent Pyruvate Formate-Lyase Activating Enzyme: Nature of Interaction between the Fe4S4 Cluster and SAM and its Role in Reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Peng, Y., Broderick, W. E., Hedman, B., Hodgson, K. O., Broderick, J. B., Solomon, E. I. 2011; 133 (46): 18656-18662

  Abstract

  S K-edge X-ray absorption spectroscopy on the resting oxidized and the S-adenosyl-l-methionine (SAM) bound forms of pyruvate formate-lyase activating enzyme are reported. The data show an increase in pre-edge intensity, which is due to additional contributions from sulfide and thiolate of the Fe(4)S(4) cluster into the C-S σ* orbital. This experimentally demonstrates that there is a backbonding interaction between the Fe(4)S(4) cluster and C-S σ* orbitals of SAM in this inner sphere complex. DFT calculations that reproduce the data indicate that this backbonding is enhanced in the reduced form and that this configurational interaction between the donor and acceptor orbitals facilitates the electron transfer from the cluster to the SAM, which otherwise has a large outer sphere electron transfer barrier. The energy of the reductive cleavage of the C-S bond is sensitive to the dielectric of the protein in the immediate vicinity of the site as a high dielectric stabilizes the more charge separated reactant increasing the reaction barrier. This may provide a mechanism for generation of the 5'-deoxyadenosyl radical upon substrate binding.

  View details for DOI 10.1021/ja203780t

  View details for Web of Science ID 000297398900036

  View details for PubMedID 21992686

  View details for PubMedCentralID PMC3235791

• Electronic Structure of a Low-Spin Heme/Cu Peroxide Complex: Spin-State and Spin-Topology Contributions to Reactivity INORGANIC CHEMISTRY Kieber-Emmons, M. T., Li, Y., Halime, Z., Karlin, K. D., Solomon, E. I. 2011; 50 (22): 11777-11786

  Abstract

  This study details the electronic structure of the heme–peroxo–copper adduct {[(F8)Fe(DCHIm)]-O2-[Cu(AN)]}+ (LS(AN)) in which O2(2–) bridges the metals in a μ-1,2 or “end-on” configuration. LS(AN) is generated by addition of coordinating base to the parent complex {[(F8)Fe]-O2-[Cu(AN)]}+ (HS(AN)) in which the O2(2–) bridges the metals in an μ-η2:η2 or “side-on” mode. In addition to the structural change of the O2(2–) bridging geometry, coordination of the base changes the spin state of the heme fragment (from S = 5/2 in HS(AN) to S = 1/2 in LS(AN)) that results in an antiferromagnetically coupled diamagnetic ground state in LS(AN). The strong ligand field of the porphyrin modulates the high-spin to low-spin effect on Fe–peroxo bonding relative to nonheme complexes, which is important in the O–O bond cleavage process. On the basis of DFT calculations, the ground state of LS(AN) is dependent on the Fe–O–O–Cu dihedral angle, wherein acute angles (<~150°) yield an antiferromagnetically coupled electronic structure while more obtuse angles yield a ferromagnetic ground state. LS(AN) is diamagnetic and thus has an antiferromagnetically coupled ground state with a calculated Fe–O–O–Cu dihedral angle of 137°. The nature of the bonding in LS(AN) and the frontier molecular orbitals which lead to this magneto-structural correlation provide insight into possible spin topology contributions to O–O bond cleavage by cytochrome c oxidase.

  View details for DOI 10.1021/ic2018727

  View details for Web of Science ID 000296830400061

  View details for PubMedID 22007669

  View details for PubMedCentralID PMC3226806

• Activation of alpha-Keto Acid-Dependent Dioxygenases: Application of an {FeNO}(7)/{FeO2}(8) Methodology for Characterizing the Initial Steps of O-2 Activation JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Diebold, A. R., Brown-Marshall, C. D., Neidig, M. L., Brownlee, J. M., Moran, G. R., Solomon, E. I. 2011; 133 (45): 18148-18160

  Abstract

  The α-keto acid-dependent dioxygenases are a major subgroup within the O(2)-activating mononuclear nonheme iron enzymes. For these enzymes, the resting ferrous, the substrate plus cofactor-bound ferrous, and the Fe(IV)═O states of the reaction have been well studied. The initial O(2)-binding and activation steps are experimentally inaccessible and thus are not well understood. In this study, NO is used as an O(2) analogue to probe the effects of α-keto acid binding in 4-hydroxyphenylpyruvate dioxygenase (HPPD). A combination of EPR, UV-vis absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field (VTVH) MCD spectroscopies in conjunction with computational models is used to explore the HPPD-NO and HPPD-HPP-NO complexes. New spectroscopic features are present in the α-keto acid bound {FeNO}(7) site that reflect the strong donor interaction of the α-keto acid with the Fe. This promotes the transfer of charge from the Fe to NO. The calculations are extended to the O(2) reaction coordinate where the strong donation associated with the bound α-keto acid promotes formation of a new, S = 1 bridged Fe(IV)-peroxy species. These studies provide insight into the effects of a strong donor ligand on O(2) binding and activation by Fe(II) in the α-keto acid-dependent dioxygenases and are likely relevant to other subgroups of the O(2) activating nonheme ferrous enzymes.

  View details for DOI 10.1021/ja202549q

  View details for Web of Science ID 000297381200038

  View details for PubMedID 21981763

  View details for PubMedCentralID PMC3212634

• Rapid C-H Bond Activation by a Monocopper(III)-Hydroxide Complex JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Donoghue, P. J., Tehranchi, J., Cramer, C. J., Sarangi, R., Solomon, E. I., Tolman, W. B. 2011; 133 (44): 17602-17605

  Abstract

  One-electron oxidation of the tetragonal Cu(II) complex [Bu(4)N][LCuOH] at -80 °C generated the reactive intermediate LCuOH, which was shown to be a Cu(III) complex on the basis of spectroscopy and theory (L = N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide). The complex LCuOH reacts with dihydroanthracene to yield anthracene and the Cu(II) complex LCu(OH(2)). Kinetic studies showed that the reaction occurs via H-atom abstraction via a second-order rate law at high rates (cf. k = 1.1(1) M(-1) s(-1) at -80 °C, ΔH(‡) = 5.4(2) kcal mol(-1), ΔS(‡) = -30(2) eu) and with very large kinetic isotope effects (cf. k(H)/k(D) = 44 at -70 °C). The findings suggest that a Cu(III)-OH moiety is a viable reactant in oxidation catalysis.

  View details for DOI 10.1021/ja207882h

  View details for Web of Science ID 000296312200022

  View details for PubMedID 22004091

  View details for PubMedCentralID PMC3213683

• X-ray Absorption Spectroscopic and Computational Investigation of a Possible S center dot center dot center dot S Interaction in the [Cu3S2](3+) Core JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sarangi, R., Yang, L., Winikoff, S. G., Gagliardi, L., Cramer, C. J., Tolman, W. B., Solomon, E. I. 2011; 133 (43): 17180-17191

  Abstract

  The electronic structure of the [Cu(3)S(2)](3+) core of [(LCu)(3)(S)(2)](3+) (L = N,N,N',N'-tetramethyl-2R,3R-cyclohexanediamine) is investigated using a combination of Cu and S K-edge X-ray absorption spectroscopy and calculations at the density functional and multireference second-order perturbation levels of theory. The results show that the [Cu(3)S(2)](3+) core is best described as having all copper centers close to but more oxidized than Cu(2+), while the charge on the S(2) fragment is between that of a sulfide (S(2-)) and a subsulfide (S(2)(3-)) species. The [Cu(3)S(2)](3+) core thus is different from a previously described, analogous [Cu(3)O(2)](3+) core, which has a localized [(Cu(3+)Cu(2+)Cu(2+))(O(2-))(2)](3+) electronic structure. The difference in electronic structure between the two analogues is attributed to increased covalent overlap between the Cu 3d and S 3p orbitals and the increased radial distribution function of the S 3p orbital (relative to O 2p). These features result in donation of electron density from the S-S σ* to the Cu and result in some bonding interaction between the two S atoms at ~2.69 Å in [Cu(3)S(2)](3+), stabilizing a delocalized S = 1 ground state.

  View details for DOI 10.1021/ja111323m

  View details for Web of Science ID 000297380900021

  View details for PubMedID 21923178

  View details for PubMedCentralID PMC3202975

• Structure and reactivity of a mononuclear non-haem iron(III)-peroxo complex NATURE Cho, J., Jeon, S., Wilson, S. A., Liu, L. V., Kang, E. A., Braymer, J. J., Lim, M. H., Hedman, B., Hodgson, K. O., Valentine, J. S., Solomon, E. I., Nam, W. 2011; 478 (7370): 502-505

  Abstract

  Oxygen-containing mononuclear iron species--iron(III)-peroxo, iron(III)-hydroperoxo and iron(IV)-oxo--are key intermediates in the catalytic activation of dioxygen by iron-containing metalloenzymes. It has been difficult to generate synthetic analogues of these three active iron-oxygen species in identical host complexes, which is necessary to elucidate changes to the structure of the iron centre during catalysis and the factors that control their chemical reactivities with substrates. Here we report the high-resolution crystal structure of a mononuclear non-haem side-on iron(III)-peroxo complex, [Fe(III)(TMC)(OO)](+). We also report a series of chemical reactions in which this iron(III)-peroxo complex is cleanly converted to the iron(III)-hydroperoxo complex, [Fe(III)(TMC)(OOH)](2+), via a short-lived intermediate on protonation. This iron(III)-hydroperoxo complex then cleanly converts to the ferryl complex, [Fe(IV)(TMC)(O)](2+), via homolytic O-O bond cleavage of the iron(III)-hydroperoxo species. All three of these iron species--the three most biologically relevant iron-oxygen intermediates--have been spectroscopically characterized; we note that they have been obtained using a simple macrocyclic ligand. We have performed relative reactivity studies on these three iron species which reveal that the iron(III)-hydroperoxo complex is the most reactive of the three in the deformylation of aldehydes and that it has a similar reactivity to the iron(IV)-oxo complex in C-H bond activation of alkylaromatics. These reactivity results demonstrate that iron(III)-hydroperoxo species are viable oxidants in both nucleophilic and electrophilic reactions by iron-containing enzymes.

  View details for DOI 10.1038/nature10535

  View details for Web of Science ID 000296194200040

  View details for PubMedID 22031443

  View details for PubMedCentralID PMC3306242

• Spectroscopic and Computational Studies of alpha-Keto Acid Binding to Dke1: Understanding the Role of the Facial Triad and the Reactivity of beta-Diketones JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Diebold, A. R., Straganz, G. D., Solomon, E. I. 2011; 133 (40): 15979-15991

  Abstract

  The O(2) activating mononuclear nonheme iron enzymes generally have a common facial triad (two histidine and one carboxylate (Asp or Glu) residue) ligating Fe(II) at the active site. Exceptions to this motif have recently been identified in nonheme enzymes, including a 3His triad in the diketone cleaving dioxygenase Dke1. This enzyme is used to explore the role of the facial triad in directing reactivity. A combination of spectroscopic studies (UV-vis absorption, MCD, and resonance Raman) and DFT calculations is used to define the nature of the binding of the α-keto acid, 4-hydroxyphenlpyruvate (HPP), to the active site in Dke1 and the origin of the atypical cleavage (C2-C3 instead of C1-C2) pattern exhibited by this enzyme in the reaction of α-keto acids with dioxygen. The reduced charge of the 3His triad induces α-keto acid binding as the enolate dianion, rather than the keto monoanion, found for α-keto acid binding to the 2His/1 carboxylate facial triad enzymes. The mechanistic insight from the reactivity of Dke1 with the α-keto acid substrate is then extended to understand the reaction mechanism of this enzyme with its native substrate, acac. This study defines a key role for the 2His/1 carboxylate facial triad in α-keto acid-dependent mononuclear nonheme iron enzymes in stabilizing the bound α-keto acid as a monoanion for its decarboxylation to provide the two additional electrons required for O(2) activation.

  View details for DOI 10.1021/ja203005j

  View details for Web of Science ID 000296036700043

  View details for PubMedID 21870808

  View details for PubMedCentralID PMC3191879

• Hybrid Genetic Algorithm with an Adaptive Penalty Function for Fitting Multimodal Experimental Data: Application to Exchange-Coupled Non-Kramers Binuclear Iron Active Sites JOURNAL OF CHEMICAL INFORMATION AND MODELING Beaser, E., Schwartz, J. K., Bell, C. B., Solomon, E. I. 2011; 51 (9): 2164-2173

  Abstract

  A Genetic Algorithm (GA) is a stochastic optimization technique based on the mechanisms of biological evolution. These algorithms have been successfully applied in many fields to solve a variety of complex nonlinear problems. While they have been used with some success in chemical problems such as fitting spectroscopic and kinetic data, many have avoided their use due to the unconstrained nature of the fitting process. In engineering, this problem is now being addressed through incorporation of adaptive penalty functions, but their transfer to other fields has been slow. This study updates the Nanakorrn Adaptive Penalty function theory, expanding its validity beyond maximization problems to minimization as well. The expanded theory, using a hybrid genetic algorithm with an adaptive penalty function, was applied to analyze variable temperature variable field magnetic circular dichroism (VTVH MCD) spectroscopic data collected on exchange coupled Fe(II)Fe(II) enzyme active sites. The data obtained are described by a complex nonlinear multimodal solution space with at least 6 to 13 interdependent variables and are costly to search efficiently. The use of the hybrid GA is shown to improve the probability of detecting the global optimum. It also provides large gains in computational and user efficiency. This method allows a full search of a multimodal solution space, greatly improving the quality and confidence in the final solution obtained, and can be applied to other complex systems such as fitting of other spectroscopic or kinetics data.

  View details for DOI 10.1021/ci2001296

  View details for Web of Science ID 000295114700016

  View details for PubMedID 21819138

• Geometric and electronic structure contributions to Cu/O-2 reactivity Solomon, E. I. AMER CHEMICAL SOC. 2011

  View details for Web of Science ID 000299378304180

• Oxygen precursor to the reactive intermediate in methanol synthesis by Cu-ZSM-5 Vanelderen, P., Smeets, P. J., Hadt, R. G., Woertink, J. S., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. AMER CHEMICAL SOC. 2011

  View details for Web of Science ID 000299378303418

• Comparative molecular chemistry of molybdenum and tungsten and its relation to hydroxylase and oxotransferase enzymes COORDINATION CHEMISTRY REVIEWS Holm, R. H., Solomon, E. I., Majumdar, A., Tenderholt, A. 2011; 255 (9-10): 993-1015

  View details for DOI 10.1016/j.ccr.2010.10.017

  View details for Web of Science ID 000289822400002

• Recent advances in understanding blue copper proteins COORDINATION CHEMISTRY REVIEWS Solomon, E. I., Hadt, R. G. 2011; 255 (7-8): 774-789

  View details for DOI 10.1016/j.ccr.2010.12.008

  View details for Web of Science ID 000289663500012

• Covalent and electrostatic tuning of the reduction potential of a type 1 blue copper site through second coordination sphere interactions 241st National Meeting and Exposition of the American-Chemical-Society (ACS) Hadt, R. G., Sun, N., Marshall, N. M., Lu, Y., Hodgson, K. O., Hedman, B., Solomon, E. I. AMER CHEMICAL SOC. 2011

  View details for Web of Science ID 000291982802606

• Variable temperature variable field magnetic circular dichroism (VTVH MCD) spectroscopy in bioinorganic chemistry Solomon, E. I. AMER CHEMICAL SOC. 2011

  View details for Web of Science ID 000291982802597

• Variable-temperature variable-field magnetic circular dichroism (VTVH MCD) and nuclear resonance vibrational spectroscopy (NRVS) studies on Fe-IV=O intermediates: Electronic and geometric structural insight into reactivity 241st National Meeting and Exposition of the American-Chemical-Society (ACS) Wong, S. D., Bell, C. B., Liu, L. V., Kwak, Y., England, J., Zhao, J., Que, L., Solomon, E. I. AMER CHEMICAL SOC. 2011

  View details for Web of Science ID 000291982802611

• A Codeposition Route to CuI-Pyridine Coordination Complexes for Organic Light-Emitting Diodes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Liu, Z., Qayyum, M. F., Wu, C., Whited, M. T., Djurovich, P. I., Hodgson, K. O., Hedman, B., Solomon, E. I., Thompson, M. E. 2011; 133 (11): 3700-3703

  Abstract

  We demonstrate a new approach for utilizing CuI coordination complexes as emissive layers in organic light-emitting diodes that involves in situ codeposition of CuI and 3,5-bis(carbazol-9-yl)pyridine (mCPy). With a simple three-layer device structure, pure green electroluminescence at 530 nm from a Cu(I) complex was observed. A maximum luminance and external quantum efficiency (EQE) of 9700 cd/m(2) and 4.4%, respectively, were achieved. The luminescent species was identified as [CuI(mCPy)(2)](2) on the basis of photophysical studies of model complexes and X-ray absorption spectroscopy.

  View details for DOI 10.1021/ja1065653

  View details for Web of Science ID 000288889900005

  View details for PubMedID 21366248

  View details for PubMedCentralID PMC3066052

• Cupric Superoxo-Mediated Intermolecular C-H Activation Chemistry JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Peterson, R. L., Himes, R. A., Kotani, H., Suenobu, T., Tian, L., Siegler, M. A., Solomon, E. I., Fukuzumi, S., Karlin, K. D. 2011; 133 (6): 1702-1705

  Abstract

  The new cupric superoxo complex [LCu(II)(O(2)(•-))](+), which possesses particularly strong O-O and Cu-O bonding, is capable of intermolecular C-H activation of the NADH analogue 1-benzyl-1,4-dihydronicotinamide (BNAH). Kinetic studies indicated a first-order dependence on both the Cu complex and BNAH with a deuterium kinetic isotope effect (KIE) of 12.1, similar to that observed for certain copper monooxygenases.

  View details for DOI 10.1021/ja110466q

  View details for Web of Science ID 000287831800024

  View details for PubMedID 21265534

  View details for PubMedCentralID PMC3091961

• XAS and DFT Investigation of Mononuclear Cobalt(III) Peroxo Complexes: Electronic Control of the Geometric Structure in CoO2 versus NiO2 Systems INORGANIC CHEMISTRY Sarangi, R., Cho, J., Nam, W., Solomon, E. I. 2011; 50 (2): 614-620

  Abstract

  The geometric and electronic structures of two mononuclear [(L)CoO(2)](+) complexes, [(12-TMC)CoO(2)](ClO(4)) (1) and [(14-TMC)CoO(2)](ClO(4)) (2), have been evaluated using Co K-edge X-ray absorption spectroscopy (XAS) and extended X-ray absorption fine structure (EXAFS) and correlated with density functional theory (DFT) calculations to evaluate the differences in the geometric and electronic structures due to changes in the TMC chelate ring size. Co K-edge XAS shows that both 1 and 2 are Co(III) species. Co K-edge EXAFS data show that both 1 and 2 are side-on O(2)-bound cobalt(III) peroxide complexes. A combination of EXAFS and DFT calculations reveals that while the constrained 12-TMC ring in 1 allows for side-on O(2) binding to the Co center with ease, the 14-TMC chelate in 2 has to undergo significant distortion of the ring to overcome steric hindrance posed by the four cis-methyl groups of the chelate to allow side-on O(2) binding to the Co center. The Ni analogue of 2, [(14-TMC)NiO(2)](+), has been shown to form an end-on-bound nickel(II) superoxide species. The electronic and geometric factors that determine the different electronic structures of 2 and [(14-TMC)NiO(2)](+) are evaluated using DFT calculations. The results show that while the sterics of the cis-14-TMC chelate contribute to the geometry of O(2) binding and result in an end-on-bound Ni(II)O(2)(-) complex in [(14-TMC)NiO(2)](+), the higher thermodynamic driving force for oxidation of Co(II) overcomes this steric constraint, resulting in stabilization of a side-on-bound Co(III)O(2)(2-) electronic structure in 2.

  View details for DOI 10.1021/ic101730r

  View details for Web of Science ID 000285956600030

  View details for PubMedID 21142119

  View details for PubMedCentralID PMC3130071

• S K-Edge X-Ray Absorption Spectroscopy and Density Functional Theory Studies of High and Low Spin {FeNO}(7) Thiolate Complexes: Exchange Stabilization of Electron Delocalization in {FeNO}(7) and {FeO2}(8) INORGANIC CHEMISTRY Sun, N., Liu, L. V., Dey, A., Villar-Acevedo, G., Kovacs, J. A., Darensbourg, M. Y., Hodgson, K. O., Hedman, B., Solomon, E. I. 2011; 50 (2): 427-436

  Abstract

  S K-edge X-ray absorption spectroscopy (XAS) is a direct experimental probe of metal ion electronic structure as the pre-edge energy reflects its oxidation state, and the energy splitting pattern of the pre-edge transitions reflects its spin state. The combination of sulfur K-edge XAS and density functional theory (DFT) calculations indicates that the electronic structures of {FeNO}(7) (S = 3/2) (S(Me2)N(4)(tren)Fe(NO), complex I) and {FeNO}(7) (S = 1/2) ((bme-daco)Fe(NO), complex II) are Fe(III)(S = 5/2)-NO(-)(S = 1) and Fe(III)(S = 3/2)-NO(-)(S = 1), respectively. When an axial ligand is computationally added to complex II, the electronic structure becomes Fe(II)(S = 0)-NO•(S = 1/2). These studies demonstrate how the ligand field of the Fe center defines its spin state and thus changes the electron exchange, an important factor in determining the electron distribution over {FeNO}(7) and {FeO(2)}(8) sites.

  View details for DOI 10.1021/ic1006378

  View details for Web of Science ID 000285956600011

  View details for PubMedCentralID PMC3130116

• Copper dioxygen (bio) inorganic chemistry FARADAY DISCUSSIONS Solomon, E. I., Ginsbach, J. W., Heppner, D. E., Kieber-Emmons, M. T., Kjaergaard, C. H., Smeets, P. J., Tian, L., Woertink, J. S. 2011; 148: 11-39

  Abstract

  Cu/O2 intermediates in biological, homogeneous, and heterogeneous catalysts exhibit unique spectral features that reflect novel geometric and electronic structures that make significant contributions to reactivity. This review considers how the respective intermediate electronic structures overcome the spin-forbidden nature of O2 binding, activate O2 for electrophilic aromatic attack and H-atom abstraction, catalyze the 4 e- reduction of O2 to H2O, and discusses the role of exchange coupling between Cu ions in determining reactivity.

  View details for DOI 10.1039/c005500j

  View details for Web of Science ID 000285361500002

  View details for PubMedID 21322475

  View details for PubMedCentralID PMC3062954

• Nuclear Resonance Vibrational Spectroscopy on the Fe-IV=O S=2 Non-Heme Site in TMG(3)tren: Experimentally Calibrated Insights into Reactivity ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Wong, S. D., Bell, C. B., Liu, L. V., Kwak, Y., England, J., Alp, E. E., Zhao, J., Que, L., Solomon, E. I. 2011; 50 (14): 3215-3218

  View details for DOI 10.1002/anie.201007692

  View details for Web of Science ID 000288796600017

  View details for PubMedID 21370371

  View details for PubMedCentralID PMC3085250

• Definition of the intermediates and mechanism of the anticancer drug bleomycin using nuclear resonance vibrational spectroscopy and related methods PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Liu, L. V., Bell, C. B., Wong, S. D., Wilson, S. A., Kwak, Y., Chow, M. S., Zhao, J., Hodgson, K. O., Hedman, B., Solomon, E. I. 2010; 107 (52): 22419-22424

  Abstract

  Bleomycin (BLM) is a glycopeptide anticancer drug capable of effecting single- and double-strand DNA cleavage. The last detectable intermediate prior to DNA cleavage is a low spin Fe(III) peroxy level species, termed activated bleomycin (ABLM). DNA strand scission is initiated through the abstraction of the C-4' hydrogen atom of the deoxyribose sugar unit. Nuclear resonance vibrational spectroscopy (NRVS) aided by extended X-ray absorption fine structure spectroscopy and density functional theory (DFT) calculations are applied to define the natures of Fe(III)BLM and ABLM as (BLM)Fe(III)─OH and (BLM)Fe(III)(η(1)─OOH) species, respectively. The NRVS spectra of Fe(III)BLM and ABLM are strikingly different because in ABLM the δFe─O─O bending mode mixes with, and energetically splits, the doubly degenerate, intense O─Fe─N(ax) transaxial bends. DFT calculations of the reaction of ABLM with DNA, based on the species defined by the NRVS data, show that the direct H-atom abstraction by ABLM is thermodynamically favored over other proposed reaction pathways.

  View details for DOI 10.1073/pnas.1016323107

  View details for Web of Science ID 000285684200017

  View details for PubMedID 21149675

  View details for PubMedCentralID PMC3012509

• S K-Edge X-Ray Absorption Spectroscopy and Density Functional Theory Studies of High and Low Spin {FeNO}(7) Thiolate Complexes: Exchange Stabilization of Electron Delocalization in {FeNO}(7) and {FeO(2)}(8). Inorganic chemistry Sun, N., Liu, L. V., Dey, A., Villar-Acevedo, G., Kovacs, J. A., Darensbourg, M. Y., Hodgson, K. O., Hedman, B., Solomon, E. I. 2010

  Abstract

  S K-edge X-ray absorption spectroscopy (XAS) is a direct experimental probe of metal ion electronic structure as the pre-edge energy reflects its oxidation state, and the energy splitting pattern of the pre-edge transitions reflects its spin state. The combination of sulfur K-edge XAS and density functional theory (DFT) calculations indicates that the electronic structures of {FeNO}(7) (S = 3/2) (S(Me2)N(4)(tren)Fe(NO), complex I) and {FeNO}(7) (S = 1/2) ((bme-daco)Fe(NO), complex II) are Fe(III)(S = 5/2)-NO(-)(S = 1) and Fe(III)(S = 3/2)-NO(-)(S = 1), respectively. When an axial ligand is computationally added to complex II, the electronic structure becomes Fe(II)(S = 0)-NO•(S = 1/2). These studies demonstrate how the ligand field of the Fe center defines its spin state and thus changes the electron exchange, an important factor in determining the electron distribution over {FeNO}(7) and {FeO(2)}(8) sites.

  View details for DOI 10.1021/ic1006378

  View details for PubMedID 21158471

  View details for PubMedCentralID PMC3130116

• CD and MCD Spectroscopic Studies of the Two Dps Miniferritin Proteins from Bacillus anthracis: Role of O-2 and H2O2 Substrates in Reactivity of the Diiron Catalytic Centers BIOCHEMISTRY Schwartz, J. K., Liu, X. S., Tosha, T., Diebold, A., Theil, E. C., Solomon, E. I. 2010; 49 (49): 10516-10525

  Abstract

  DNA protection during starvation (Dps) proteins are miniferritins found in bacteria and archaea that provide protection from uncontrolled Fe(II)/O radical chemistry; thus the catalytic sites are targets for antibiotics against pathogens, such as anthrax. Ferritin protein cages synthesize ferric oxymineral from Fe(II) and O(2)/H(2)O(2), which accumulates in the large central cavity; for Dps, H(2)O(2) is the more common Fe(II) oxidant contrasting with eukaryotic maxiferritins that often prefer dioxygen. To better understand the differences in the catalytic sites of maxi- versus miniferritins, we used a combination of NIR circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD (VTVH MCD) to study Fe(II) binding to the catalytic sites of the two Bacillus anthracis miniferritins: one in which two Fe(II) react with O(2) exclusively (Dps1) and a second in which both O(2) or H(2)O(2) can react with two Fe(II) (Dps2). Both result in the formation of iron oxybiomineral. The data show a single 5- or 6-coordinate Fe(II) in the absence of oxidant; Fe(II) binding to Dps2 is 30× more stable than Dps1; and the lower limit of K(D) for binding a second Fe(II), in the absence of oxidant, is 2-3 orders of magnitude weaker than for the binding of the single Fe(II). The data fit an equilibrium model where binding of oxidant facilitates formation of the catalytic site, in sharp contrast to eukaryotic M-ferritins where the binuclear Fe(II) centers are preformed before binding of O(2). The two different binding sequences illustrate the mechanistic range possible for catalytic sites of the family of ferritins.

  View details for DOI 10.1021/bi101346c

  View details for Web of Science ID 000284975000017

  View details for PubMedID 21028901

  View details for PubMedCentralID PMC3075618

• Bis(mu-oxo) Dicopper(III) Species of the Simplest Peralkylated Diamine: Enhanced Reactivity toward Exogenous Substrates INORGANIC CHEMISTRY Kang, P., Bobyr, E., Dustman, J., Hodgson, K. O., Hedman, B., Solomon, E. I., Stack, T. D. 2010; 49 (23): 11030-11038

  Abstract

  N,N,N',N'-tetramethylethylenediamine (TMED), the simplest and most extensively used peralkylated diamine ligand, is conspicuously absent from those known to form a bis(μ-oxo)dicopper(III) (O) species, [(TMED)(2)Cu(III)(2)(μ(2)-O)(2)](2+), upon oxygenation of its Cu(I) complex. Presented here is the characterization of this O species and its reactivity toward exogenous substrates. Its formation is complicated both by the instability of the [(TMED)Cu(I)](1+) precursor and by competitive formation of a presumed mixed-valent trinuclear [(TMED)(3)Cu(III)Cu(II)(2)(μ(3)-O)(2)](3+) (T) species. Under most reaction conditions, the T species dominates, yet, the O species can be formed preferentially (>80%) upon oxygenation of acetone solutions, if the copper concentration is low (<2 mM) and [(TMED)Cu(I)](1+) is prepared immediately before use. The experimental data of this simplest O species provide a benchmark by which to evaluate density functional theory (DFT) computational methods for geometry optimization and spectroscopic predictions. The enhanced thermal stability of [(TMED)(2)Cu(III)(2)(μ(2)-O)(2)](2+) and its limited steric demands compared to other O species allows more efficient oxidation of exogenous substrates, including benzyl alcohol to benzaldehyde (80% yield), highlighting the importance of ligand structure to not only enhance the oxidant stability but also maintain accessibility to the nascent metal/O(2) oxidant.

  View details for DOI 10.1021/ic101515g

  View details for Web of Science ID 000284518800037

  View details for PubMedID 21028910

  View details for PubMedCentralID PMC2993838

• Synthesis, Structural, and Spectroscopic Characterization and Reactivities of Mononuclear Cobalt(III)-Peroxo Complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Cho, J., Sarangi, R., Kang, H. Y., Lee, J. Y., Kubo, M., Ogura, T., Solomon, E. I., Nam, W. 2010; 132 (47): 16977-16986

  Abstract

  Metal-dioxygen adducts are key intermediates detected in the catalytic cycles of dioxygen activation by metalloenzymes and biomimetic compounds. In this study, mononuclear cobalt(III)-peroxo complexes bearing tetraazamacrocyclic ligands, [Co(12-TMC)(O(2))](+) and [Co(13-TMC)(O(2))](+), were synthesized by reacting [Co(12-TMC)(CH(3)CN)](2+) and [Co(13-TMC)(CH(3)CN)](2+), respectively, with H(2)O(2) in the presence of triethylamine. The mononuclear cobalt(III)-peroxo intermediates were isolated and characterized by various spectroscopic techniques and X-ray crystallography, and the structural and spectroscopic characterization demonstrated unambiguously that the peroxo ligand is bound in a side-on η(2) fashion. The O-O bond stretching frequency of [Co(12-TMC)(O(2))](+) and [Co(13-TMC)(O(2))](+) was determined to be 902 cm(-1) by resonance Raman spectroscopy. The structural properties of the CoO(2) core in both complexes are nearly identical; the O-O bond distances of [Co(12-TMC)(O(2))](+) and [Co(13-TMC)(O(2))](+) were 1.4389(17) Å and 1.438(6) Å, respectively. The cobalt(III)-peroxo complexes showed reactivities in the oxidation of aldehydes and O(2)-transfer reactions. In the aldehyde oxidation reactions, the nucleophilic reactivity of the cobalt-peroxo complexes was significantly dependent on the ring size of the macrocyclic ligands, with the reactivity of [Co(13-TMC)(O(2))](+) > [Co(12-TMC)(O(2))](+). In the O(2)-transfer reactions, the cobalt(III)-peroxo complexes transferred the bound peroxo group to a manganese(II) complex, affording the corresponding cobalt(II) and manganese(III)-peroxo complexes. The reactivity of the cobalt-peroxo complexes in O(2)-transfer was also significantly dependent on the ring size of tetraazamacrocycles, and the reactivity order in the O(2)-transfer reactions was the same as that observed in the aldehyde oxidation reactions.

  View details for DOI 10.1021/ja107177m

  View details for Web of Science ID 000284972400041

  View details for PubMedID 21062059

  View details for PubMedCentralID PMC2995300

• Oxygen Precursor to the Reactive Intermediate in Methanol Synthesis by Cu-ZSM-5 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Smeets, P. J., Hadt, R. G., Woertink, J. S., Vanelderen, P., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2010; 132 (42): 14736-14738

  Abstract

  The reactive oxidizing species in the selective oxidation of methane to methanol in oxygen activated Cu-ZSM-5 was recently defined to be a bent mono(μ-oxo)dicopper(II) species, [Cu(2)O](2+). In this communication we report the formation of an O(2)-precursor of this reactive site with an associated absorption band at 29,000 cm(-1). Laser excitation into this absorption feature yields a resonance Raman (rR) spectrum characterized by (18)O(2) isotope sensitive and insensitive vibrations, νO-O and νCu-Cu, at 736 (Δ(18)O(2) = 41 cm(-1)) and 269 cm(-1), respectively. These define the precursor to be a μ-(η(2):η(2)) peroxo dicopper(II) species, [Cu(2)(O(2))](2+). rR experiments in combination with UV-vis absorption data show that this [Cu(2)(O(2))](2+) species transforms directly into the [Cu(2)O](2+) reactive site. Spectator Cu(+) sites in the zeolite ion-exchange sites provide the two electrons required to break the peroxo bond in the precursor. O(2)-TPD experiments with (18)O(2) show the incorporation of the second (18)O atom into the zeolite lattice in the transformation of [Cu(2)(O(2))](2+) into [Cu(2)O](2+). This study defines the mechanism of oxo-active site formation in Cu-ZSM-5.

  View details for DOI 10.1021/ja106283u

  View details for Web of Science ID 000283403200016

  View details for PubMedID 20923156

  View details for PubMedCentralID PMC2974621

• Spectroscopic and Computational Studies of an End-on Bound Superoxo-Cu(II) Complex: Geometric and Electronic Factors That Determine the Ground State INORGANIC CHEMISTRY Woertink, J. S., Tian, L., Maiti, D., Lucas, H. R., Himes, R. A., Karlin, K. D., Neese, F., Wuertele, C., Holthausen, M. C., Bill, E., Sundermeyer, J., Schindler, S., Solomon, E. I. 2010; 49 (20): 9450-9459

  Abstract

  A variety of techniques including absorption, magnetic circular dichroism (MCD), variable-temperature, variable-field MCD (VTVH-MCD), and resonance Raman (rR) spectroscopies are combined with density functional theory (DFT) calculations to elucidate the electronic structure of the end-on (η(1)) bound superoxo-Cu(II) complex [TMG(3)trenCuO(2)](+) (where TMG(3)tren is 1,1,1-tris[2-[N(2)-(1,1,3,3-tetramethylguanidino)]ethyl]amine). The spectral features of [TMG(3)trenCuO(2)](+) are assigned, including the first definitive assignment of a superoxo intraligand transition in a metal-superoxo complex, and a detailed description of end-on superoxo-Cu(II) bonding is developed. The lack of overlap between the two magnetic orbitals of [TMG(3)trenCuO(2)](+) eliminates antiferromagnetic coupling between the copper(II) and the superoxide, while the significant superoxo π*(σ) character of the copper dz(2) orbital leads to its ferromagnetically coupled, triplet, ground state.

  View details for DOI 10.1021/ic101138u

  View details for Web of Science ID 000282783400045

  View details for PubMedID 20857998

  View details for PubMedCentralID PMC2963092

• Density functional theory calculations on Fe-O and O-O cleavage of ferric hydroperoxide species: Role of axial ligand and spin state INORGANICA CHIMICA ACTA Dey, A., Solomon, E. I. 2010; 363 (12): 2762-2767

  Abstract

  Density Functional Theory (DFT) calculations are performed on thiolate bound hydroperoxide complexes. O-O and Fe-O cleavage reaction coordinates, relevant to the active sites of Cytocrome P450 and Superoxide Reductase enzymes, were investigated for both high and low spin states and for cis and trans orientations of the thiolate ligand with respect to the hydroperoxide ligand. The results indicate that the presence of a thiolate ligand produces significant elongation of the Fe-O bond and reduction of Fe-O vibrational frequency. While the fate of the O-O cleavage reaction is not significantly altered, the presence of a thiolate induces a heterolytic Fe-O cleavage irrespective of the spin state and orientation which is very different from results obtained with a trans ammine ligand.

  View details for DOI 10.1016/j.ica.2010.03.059

  View details for Web of Science ID 000282360200010

  View details for PubMedCentralID PMC2967774

• Sulfur Donor Atom Effects on Copper(I)/O-2 Chemistry with Thioanisole Containing Tetradentate N3S Ligand Leading to mu-1,2-Peroxo-Dicopper(II) Species INORGANIC CHEMISTRY Lee, Y., Lee, D., Park, G. Y., Lucas, H. R., Sarjeant, A. A., Kieber-Emmons, M. T., Vance, M. A., Milligan, A. E., Solomon, E. I., Karlin, K. D. 2010; 49 (19): 8873-8885

  Abstract

  To better understand the effect of thioether coordination in copper-O(2) chemistry, the tetradentate N(3)S ligand L(ASM) (2-(methylthio)-N,N-bis((pyridin-2-yl)methyl)benzenamine) and related alkylether ligand L(EOE) (2-ethoxy-N,N-bis((pyridin-2-yl)methyl)ethanamine) have been studied. The corresponding copper(I) complexes, [(L(ASM))Cu(I)](+) (1a) and [(L(EOE))Cu(I)](+) (3a), were studied as were the related compound [(L(ESE))Cu(I)](+) (2a, L(ESE) = (2-ethylthio-N,N-bis((pyridin-2-yl)methyl)ethanamine). The X-ray structure of 1a and its solution conductivity reveal a monomeric molecular structure possessing thioether coordination which persists in solution. In contrast, the C-O stretching frequencies of the derivative Cu(I)-CO complexes reveal that for these complexes, the modulated ligand arms, whether arylthioether, alkylthioether, or ether, are not coordinated to the cuprous ion. Electrochemical data for 1a and 2a in CH(3)CN and N,N-dimethylformamide (DMF) show the thioanisole moiety to be a poor electron donor compared to alkylthioether (1a is ∼200 mV more positive than 2a). The structures of [(L(ASM))Cu(II)(CH(3)OH)](2+) (1c) and [(L(ESE))Cu(II)(CH(3)OH)](2+) (2c) have also been obtained and indicate nearly identical copper coordination environments. Oxygenation of 1a at reduced temperature gives a characteristic deep blue intermediate [{(L(ASM))Cu(II)}(2)(O(2)(2-))](2+) (1b(P)) with absorption features at 442 (1,500 M(-1) cm(-1)), 530 (8,600 M(-1) cm(-1)), and 605 nm (10,400 M(-1) cm(-1)); these values compare well to the ligand-to-metal charge-transfer (LMCT) transitions previously reported for [{(L(ESE))Cu(II)}(2)(O(2)(2-))](2+) (2b(P)). Resonance Raman data for [{(L(ASM))Cu(II)}(2)(O(2)(2-))](2+) (1b(P)) support the formation of μ-1,2-peroxo species ν(O-O) = 828 cm(-1)(Δ((18)O(2)) = 48), ν(sym)(Cu-O) = 547 cm(-1) (Δ((18)O(2)) = 23), and ν(asym)(Cu-O) = 497 cm(-1) (Δ((18)O(2)) = 22) and suggest the L(ASM) ligand is a poorer electron donor to copper than is L(ESE). In contrast, the oxygenation of [(L(EOE))Cu(I)](+) (3a), possessing an ether donor as an analogue of the thioether in L(ESE), led to the formation of a bis(μ-oxo) species [{(L(EOE))Cu(III)}(2)(O(2-))(2)](2+) (3b(O); 380 nm, ε ∼ 10,000 M(-1) cm(-1)). This result provides further support for the sulfur influence in 1b(P) and 2b(P), in particular coordination of the sulfur to the Cu. Thermal decomposition of 1b(P) is accompanied by ligand sulfoxidation. The structure of [{(L(EOE))Cu(II)(Cl)}(2)](+) (3c) generated from the reductive dehalogenation of organic chlorides suggests that the ether moiety is weakly bound to the cupric ion. A detailed discussion of the spectroscopic and structural characteristics of 1b(P), 2b(P), and 3b(O) is presented.

  View details for DOI 10.1021/ic101041m

  View details for Web of Science ID 000282084600029

  View details for PubMedID 20822156

  View details for PubMedCentralID PMC2949281

• Solvation Effects on S K-Edge XAS Spectra of Fe-S Proteins: Normal and Inverse Effects on WT and Mutant Rubredoxin JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sun, N., Dey, A., Xiao, Z., Wedd, A. G., Hodgson, K. O., Hedman, B., Solomon, E. I. 2010; 132 (36): 12639-12647

  Abstract

  S K-edge X-ray absorption spectroscopy (XAS) was performed on wild type Cp rubredoxin and its Cys --> Ser mutants in both solution and lyophilized forms. For wild type rubredoxin and for the mutants where an interior cysteine residue (C6 or C39) is substituted by serine, a normal solvent effect is observed, that is, the S covalency increases upon lyophilization. For the mutants where a solvent accessible surface cysteine residue is substituted by serine, the S covalency decreases upon lyophilization which is an inverse solvent effect. Density functional theory (DFT) calculations reproduce these experimental results and show that the normal solvent effect reflects the covalency decrease due to solvent H-bonding to the surface thiolates and that the inverse solvent effect results from the covalency compensation from the interior thiolates. With respect to the Cys --> Ser substitution, the S covalency decreases. Calculations indicate that the stronger bonding interaction of the alkoxide with the Fe relative to that of thiolate increases the energy of the Fe d orbitals and reduces their bonding interaction with the remaining cysteines. The solvent effects support a surface solvent tuning contribution to electron transfer, and the Cys --> Ser result provides an explanation for the change in properties of related iron-sulfur sites with this mutation.

  View details for DOI 10.1021/ja102807x

  View details for Web of Science ID 000282074200026

  View details for PubMedID 20726554

  View details for PubMedCentralID PMC2946794

• Geometric and electronic structure contributions to Cu/02 reactivity Solomon, E. I. AMER CHEMICAL SOC. 2010

  View details for Web of Science ID 000208164705775

• The Three-His Triad in Dke1: Comparisons to the Classical Facial Triad BIOCHEMISTRY Diebold, A. R., Neidig, M. L., Moran, G. R., Straganz, G. D., Solomon, E. I. 2010; 49 (32): 6945-6952

  Abstract

  The oxygen activating mononuclear non-heme ferrous enzymes catalyze a diverse range of chemistry yet typically maintain a common structural motif: two histidines and a carboxylate coordinating the iron center in a facial triad. A new Fe(II) coordinating triad has been observed in two enzymes, diketone-cleaving dioxygenase, Dke1, and cysteine dioxygenase (CDO), and is composed of three histidine residues. The effect of this three-His motif in Dke1 on the geometric and electronic structure of the Fe(II) center is explored via a combination of absorption, CD, MCD, and VTVH MCD spectroscopies and DFT calculations. This geometric and electronic structure of the three-His triad is compared to that of the classical (2-His-1-carboxylate) facial triad in the alpha-ketoglutarate (alphaKG)-dependent dioxygenases clavaminate synthase 2 (CS2) and hydroxyphenylpyruvate dioxygenase (HPPD). Comparison of the ligand fields at the Fe(II) shows little difference between the three-His and 2-His-1-carboxylate facial triad sites. Acetylacetone, the substrate for Dke1, will also bind to HPPD and is identified as a strong donor, similar to alphaKG. The major difference between the three-His and 2-His-1-carboxylate facial triad sites is in MLCT transitions observed for both types of triads and reflects their difference in charge. These studies provide insight into the effects of perturbation of the facial triad ligation of the non-heme ferrous enzymes on their geometric and electronic structure and their possible contributions to reactivity.

  View details for DOI 10.1021/bi100892w

  View details for Web of Science ID 000280668000014

  View details for PubMedID 20695531

  View details for PubMedCentralID PMC2924660

• Sulfur K-Edge X-ray Absorption Spectroscopy and Density Functional Calculations on Mo(IV) and Mo(VI)=O Bis-dithiolenes: Insights into the Mechanism of Oxo Transfer in DMSO Reductase and Related Functional Analogues JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Tenderholt, A. L., Wang, J., Szilagyi, R. K., Holm, R. H., Hodgson, K. O., Hedman, B., Solomon, E. I. 2010; 132 (24): 8359-8371

  Abstract

  Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations have been used to determine the electronic structures of two Mo bis-dithiolene complexes, [Mo(OSi)(bdt)(2)](1-) and [MoO(OSi)(bdt)(2)](1-), where OSi = [OSiPh(2)(t)Bu](1-) and bdt = benzene-1,2-dithiolate(2-), that model the Mo(IV) and Mo(VI)=O states of the DMSO reductase family of molybdenum enzymes. These results show that the Mo(IV) complex undergoes metal-based oxidation unlike Mo tris-dithiolene complexes, indicating that the dithiolene ligands are behaving innocently. Experimentally validated calculations have been extended to model the oxo transfer reaction coordinate using dimethylsulfoxide (DMSO) as a substrate. The reaction proceeds through a transition state (TS1) to an intermediate with DMSO weakly bound, followed by a subsequent transition state (TS2) which is the largest barrier of the reaction. The factors that control the energies of these transition states, the nature of the oxo transfer process, and the role of the dithiolene ligand are discussed.

  View details for DOI 10.1021/ja910369c

  View details for Web of Science ID 000278905700034

  View details for PubMedID 20499905

  View details for PubMedCentralID PMC2907113

• Multireference Ab Initio Calculations of g tensors for Trinuclear Copper Clusters in Multicopper Oxidases JOURNAL OF PHYSICAL CHEMISTRY B Vancoillie, S., Chalupsky, J., Ryde, U., Solomon, E. I., Pierloot, K., Neese, F., Rulisek, L. 2010; 114 (22): 7692-7702

  Abstract

  EPR spectroscopy has proven to be an indispensable tool in elucidating the structure of metal sites in proteins. In recent years, experimental EPR data have been complemented by theoretical calculations, which have become a standard tool of many quantum chemical packages. However, there have only been a few attempts to calculate EPR g tensors for exchange-coupled systems with more than two spins. In this work, we present a quantum chemical study of structural, electronic, and magnetic properties of intermediates in the reaction cycle of multicopper oxidases and of their inorganic models. All these systems contain three copper(II) ions bridged by hydroxide or O(2-) anions and their ground states are antiferromagnetically coupled doublets. We demonstrate that only multireference methods, such as CASSCF/CASPT2 or MRCI can yield qualitatively correct results (compared to the experimental values) and consider the accuracy of the calculated EPR g tensors as the current benchmark of quantum chemical methods. By decomposing the calculated g tensors into terms arising from interactions of the ground state with the various excited states, the origin of the zero-field splitting is explained. The results of the study demonstrate that a truly quantitative prediction of the g tensors of exchange-coupled systems is a great challenge to contemporary theory. The predictions strongly depend on small energy differences that are difficult to predict with sufficient accuracy by any quantum chemical method that is applicable to systems of the size of our target systems.

  View details for DOI 10.1021/jp103098r

  View details for Web of Science ID 000278301000033

  View details for PubMedID 20469875

  View details for PubMedCentralID PMC2885356

• Systematic Perturbation of the Trinuclear Copper Cluster in the Multicopper Oxidases: The Role of Active Site Asymmetry in Its Reduction of O-2 to H2O JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Augustine, A. J., Kjaergaard, C., Qayyum, M., Ziegler, L., Kosman, D. J., Hodgson, K. O., Hedman, B., Solomon, E. I. 2010; 132 (17): 6057-6067

  Abstract

  The multicopper oxidase Fet3p catalyzes the four-electron reduction of dioxygen to water, coupled to the one-electron oxidation of four equivalents of substrate. To carry out this process, the enzyme utilizes four Cu atoms: a type 1, a type 2, and a coupled binuclear, type 3 site. Substrates are oxidized at the T1 Cu, which rapidly transfers electrons, 13 A away, to a trinuclear copper cluster composed of the T2 and T3 sites, where dioxygen is reduced to water in two sequential 2e(-) steps. This study focuses on two variants of Fet3p, H126Q and H483Q, that perturb the two T3 Cu's, T3alpha and T3beta, respectively. The variants have been isolated in both holo and type 1 depleted (T1D) forms, T1DT3alphaQ and T1DT3betaQ, and their trinuclear copper clusters have been characterized in their oxidized and reduced states. While the variants are only mildly perturbed relative to T1D in the resting oxidized state, in contrast to T1D they are both found to have lost a ligand in their reduced states. Importantly, T1DT3alphaQ reacts with O(2), but T1DT3betaQ does not. Thus loss of a ligand at T3beta, but not at T3alpha, turns off O(2) reactivity, indicating that T3beta and T2 are required for the 2e(-) reduction of O(2) to form the peroxide intermediate (PI), whereas T3alpha remains reduced. This is supported by the spectroscopic features of PI in T1DT3alphaQ, which are identical to T1D PI. This selective redox activity of one edge of the trinuclear cluster demonstrates its asymmetry in O(2) reactivity. The structural origin of this asymmetry between the T3alpha and T3beta is discussed, as is its contribution to reactivity.

  View details for DOI 10.1021/ja909143d

  View details for Web of Science ID 000277158500040

  View details for PubMedID 20377263

  View details for PubMedCentralID PMC2886579

• Transition-Metal Ions in Zeolites: Coordination and Activation of Oxygen INORGANIC CHEMISTRY Smeets, P. J., Woertink, J. S., Sels, B. F., Solomon, E. I., Schoonheydt, R. A. 2010; 49 (8): 3573-3583

  Abstract

  Zeolites containing transition-metal ions (TMIs) often show promising activity as heterogeneous catalysts in pollution abatement and selective oxidation reactions. In this paper, two aspects of research on the TMIs Cu, Co, and Fe in zeolites are discussed: (i) coordination to the lattice and (ii) activated oxygen species. At low loading, TMIs preferably occupy exchange sites in six-membered oxygen rings (6MR), where the TMIs preferentially coordinate with the O atoms of Al tetrahedra. High TMI loadings result in a variety of TMI species formed at the zeolite surface. Removal of the extralattice O atoms during high-temperature pretreatments can result in autoreduction. Oxidation of reduced TMI sites often results in the formation of highly reactive oxygen species. In Cu-ZSM-5, calcination with O(2) results in the formation of a species, which was found to be a crucial intermediate in both the direct decomposition of NO and N(2)O and the selective oxidation of methane into methanol. An activated oxygen species, called alpha-O, is formed in Fe-ZSM5 and reported to be the active site in the partial oxidation of methane and benzene into methanol and phenol, respectively. However, this reactive alpha-O can only be formed with N(2)O, not with O(2). O(2)-activated Co intermediates in faujasite (FAU) zeolites can selectively oxidize alpha-pinene and epoxidize styrene. In Co-FAU, Co(III) superoxo and peroxo complexes are suggested to be the active cores, whereas in Cu and Fe-ZSM-5, various monomeric and dimeric sites have been proposed, but no consensus has been obtained. Very recently, the active site in Cu-ZSM-5 was identified as a bent [Cu-O-Cu](2+) core (Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 18908-18913). Overall, O(2) activation depends on the interplay of structural factors such as the type of zeolite and sizes of the channels and cages and chemical factors such as the Si/Al ratio and the nature, charge, and distribution of the charge-balancing cations. The presence of several different TMI sites hinders the direct study of the spectroscopic features of the active site. Spectroscopic techniques capable of selectively probing these sites, even if they only constitute a minor fraction of the total amount of TMI sites, are thus required. Fundamental knowledge of the geometric and electronic structures of the reactive active site can help in the design of novel selective oxidation catalysts.

  View details for DOI 10.1021/ic901814f

  View details for Web of Science ID 000276556900004

  View details for PubMedID 20380459

  View details for PubMedCentralID PMC2881549

• Preface: Forum on Dioxygen Activation and Reduction INORGANIC CHEMISTRY Tolman, W. B., Solomon, E. I. 2010; 49 (8): 3555–56

  View details for DOI 10.1021/ic100161a

  View details for Web of Science ID 000276556900001

  View details for PubMedID 20380456

• Heme-Copper-Dioxygen Complexes: Toward Understanding Ligand-Environmental Effects on the Coordination Geometry, Electronic Structure, and Reactivity INORGANIC CHEMISTRY Halime, Z., Kieber-Emmons, M. T., Qayyum, M. F., Mondal, B., Gandhi, T., Puiu, S. C., Chufan, E. E., Sarjeant, A. A., Hodgson, K. O., Hedman, B., Solomon, E. I., Karlin, K. D. 2010; 49 (8): 3629-3645

  Abstract

  The nature of the ligand is an important aspect of controlling the structure and reactivity in coordination chemistry. In connection with our study of heme-copper-oxygen reactivity relevant to cytochrome c oxidase dioxygen-reduction chemistry, we compare the molecular and electronic structures of two high-spin heme-peroxo-copper [Fe(III)O(2)(2-)Cu(II)](+) complexes containing N(4) tetradentate (1) or N(3) tridentate (2) copper ligands. Combining previously reported and new resonance Raman and EXAFS data coupled to density functional theory calculations, we report a geometric structure and more complete electronic description of the high-spin heme-peroxo-copper complexes 1 and 2, which establish mu-(O(2)(2-)) side-on to the Fe(III) and end-on to Cu(II) (mu-eta(2):eta(1)) binding for the complex 1 but side-on/side-on (mu-eta(2):eta(2)) mu-peroxo coordination for the complex 2. We also compare and summarize the differences and similarities of these two complexes in their reactivity toward CO, PPh(3), acid, and phenols. The comparison of a new X-ray structure of mu-oxo complex 2a with the previously reported 1a X-ray structure, two thermal decomposition products respectively of 2 and 1, reveals a considerable difference in the Fe-O-Cu angle between the two mu-oxo complexes ( angleFe-O-Cu = 178.2 degrees in 1a and angleFe-O-Cu = 149.5 degrees in 2a). The reaction of 2 with 1 equiv of an exogenous nitrogen-donor axial base leads to the formation of a distinctive low-temperature-stable, low-spin heme-dioxygen-copper complex (2b), but under the same conditions, the addition of an axial base to 1 leads to the dissociation of the heme-peroxo-copper assembly and the release of O(2). 2b reacts with phenols performing H-atom (e(-) + H(+)) abstraction resulting in O-O bond cleavage and the formation of high-valent ferryl [Fe(IV)=O] complex (2c). The nature of 2c was confirmed by a comparison of its spectroscopic features and reactivity with those of an independently prepared ferryl complex. The phenoxyl radical generated by the H-atom abstraction was either (1) directly detected by electron paramagnetic resonance spectroscopy using phenols that produce stable radicals or (2) indirectly detected by the coupling product of two phenoxyl radicals.

  View details for DOI 10.1021/ic9020993

  View details for Web of Science ID 000276556900010

  View details for PubMedID 20380465

  View details for PubMedCentralID PMC2893725

• Fe L-Edge X-ray Absorption Spectroscopy Determination of Differential Orbital Covalency of Siderophore Model Compounds: Electronic Structure Contributions to High Stability Constants JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hocking, R. K., George, S. D., Raymond, K. N., Hodgson, K. O., Hedman, B., Solomon, E. I. 2010; 132 (11): 4006-4015

  Abstract

  Most bacteria and fungi produce low-molecular-weight iron chelators called siderophores. Although different siderophore structures have been characterized, the iron-binding moieties often contain catecholate or hydroxamate groups. Siderophores function because of their extraordinarily high stability constants (K(STAB) = 10(30)-10(49)) and selectivity for Fe(III), yet the origin of these high stability constants has been difficult to quantify experimentally. Herein, we utilize Fe L-edge X-ray absorption spectroscopy to determine the differential orbital covalency (i.e., the differences in the mixing of the metal d-orbitals with ligand valence orbitals) of a series of siderophore model compounds. The results enable evaluation of the electronic structure contributions to their high stability constants in terms of sigma- and pi-donor covalent bonding, ionic bonding, and solvent effects. The results indicate substantial differences in the covalent contributions to stability constants of hydroxamate and catecholate complexes and show that increased sigma as well as pi bonding contributes to the high stability constants of catecholate complexes.

  View details for DOI 10.1021/ja9090098

  View details for Web of Science ID 000275868700061

  View details for PubMedID 20187651

  View details for PubMedCentralID PMC2890247

• Spectroscopic and DFT studies of activated bleomycin and its reactivity Liu, L. V., Chow, M. S., Bell, C. B., Wong, S. D., Zhao, J., Solomon, E. I. AMER CHEMICAL SOC. 2010

  View details for Web of Science ID 000208189303311

• Spectroscopic and electronic structure studies of phenolate-Cu(II) complexes: Phenolate activation related to cofactor biogenesis in amine oxidase Cirera, J., Ghosh, S., Vance, M. A., Ono, T., Fujisawa, K., Solomon, E. I. AMER CHEMICAL SOC. 2010

  View details for Web of Science ID 000208189302775

• Geometric and electronic structure of LMO2 (M=Ni, Co) complexes: The effect of ring size on the nature of M-O-2 bonding Sarangi, R., Solomon, E. I., Wonwoo Nam, Cho, J. AMER CHEMICAL SOC. 2010

  View details for Web of Science ID 000208189302812

• Active site in the oxidation of methane to methanol in Cu-ZSM-5: A [Cu2O](2+) core Woertink, J. S., Smeets, P. J., Sels, B. F., Schoonheydt, R. A., Solomon, E. I. AMER CHEMICAL SOC. 2010

  View details for Web of Science ID 000208189302871

• Spectroscopic and DFT study of oxygen-activated Cu-ZSM-5, the active site of selective CH4 oxidation Smeets, P. J., Woertink, J. S., Sels, B. F., Schoonheydt, R. A., Solomon, E. I. AMER CHEMICAL SOC. 2010

  View details for Web of Science ID 000208189303124

• Spectroscopic and computational studies of a mononuclear copper superoxo complex: A model of the non-coupled binuclear copper enzyme intermediate Tian, L., Woertink, J. S., Maiti, D., Karlin, K. D., Neese, F., Schindler, S., Sundermeyer, J., Solomon, E. I. AMER CHEMICAL SOC. 2010

  View details for Web of Science ID 000208189303312

• Spectroscopic and computational studies on cytochrome oxidase model complexes: Role of the copper ligand denticity on geometric and electronic structure Kieber-Emmons, M. T., Halime, Z., Qayyum, M. F., Hodgson, K. O., Hedman, B., Karlin, K. D., Solomon, E. I. AMER CHEMICAL SOC. 2010

  View details for Web of Science ID 000208189302776

• Phenolate stabilized bis(mu-oxo)dicopper(III) species: An intermediate prior to the phenolate hydroxylation Kang, P., Woertink, J., Wasinger, E., Solomon, E. I., Stack, T. D. AMER CHEMICAL SOC. 2010

  View details for Web of Science ID 000208189302777

• Reaction Coordinate of Isopenicillin N Synthase: Oxidase versus Oxygenase Activity BIOCHEMISTRY Brown-Marshall, C. D., Diebold, A. R., Solomon, E. I. 2010; 49 (6): 1176-1182

  Abstract

  Isopenicillin N synthase (IPNS) can have both oxidase and oxygenase activity depending on the substrate. For the native substrate, ACV, oxidase activity exists; however, for the substrate analogue ACOV, which lacks an amide nitrogen, IPNS exhibits oxygenase activity. The potential energy surfaces for the O-O bond elongation and cleavage were calculated for three different reactions: homolytic cleavage via traditional Fenton chemistry, heterolytic cleavage, and nucleophilic attack. These surfaces show that the hydroperoxide-ferrous intermediate, formed by O(2)-activated H atom abstraction from the substrate, can exploit different reaction pathways and that interactions with the substrate govern the pathway. The hydrogen bonds from hydroperoxide to the amide nitrogen of ACV polarize the sigma* orbital of the peroxide toward the proximal oxygen, facilitating heterolytic cleavage. For the substrate analogue ACOV, this hydrogen bond is no longer present, leading to nucleophilic attack on the substrate intermediate C-S bond. After cleavage of the hydroperoxide, the two reaction pathways proceed with minimal barriers, resulting in the closure of the beta-lactam ring for the oxidase activity (ACV) or formation of the thiocarboxylate for oxygenase activity (ACOV).

  View details for DOI 10.1021/bi901772w

  View details for Web of Science ID 000274342000014

  View details for PubMedID 20078029

  View details for PubMedCentralID PMC2838496

• Kinetic and CD/MCD Spectroscopic Studies of the Atypical, Three-His-Ligated, Non-Heme Fe2+ Center in Diketone Dioxygenase: The Role of Hydrophilic Outer Shell Residues in Catalysis BIOCHEMISTRY Straganz, G. D., Diebold, A. R., Egger, S., Nidetzky, B., Solomon, E. I. 2010; 49 (5): 996-1004

  Abstract

  Diketone cleaving enzyme (Dke1) is a dioxygenase with an atypical, three-histidine-ligated, mononuclear non-heme Fe(2+) center. To assess the role in enzyme catalysis of the hydrophilic residues in the active site pocket, residues Glu98, Arg80, Tyr70, and Thr107 were subjected to mutational analysis. Steady state and pre-steady state kinetics indicated a role for Glu98 in promoting both substrate binding and O(2) reduction. Additionally, the Glu98 substitution eliminated the pH dependence of substrate binding (k(cat)(app)/K(M)(app)-pH profile) present in wild-type Dke1 (pK(a) = 6.3 +/- 0.4 and 8.4 +/- 0.4). MCD spectroscopy revealed that the Glu98 --> Gln mutation leads to the conversion of the six-coordinate (6C) resting Fe(2+) center present in the wild-type enzyme at pH 7.0 to a mixture of five-coordinate (5C) and 6C sites. The 6C geometry was restored with a pH shift to 9.5 which also resulted in ligand field (LF) energy splittings identical to that found for wild-type (WT) Dke1 at pH 9.5. In WT Dke1, these LF transitions are shifted up in energy by approximately 300 cm(-1) at pH 9.5 relative to pH 7.0. These data, combined with CD pH titrations which reveal a pK(a) of approximately 8.2 for resting WT Dke1 and the Glu98 --> Gln variant, indicate the deprotonation of a metal-ligated water. Together, the kinetic and spectroscopic data reveal a stabilizing effect of Glu98 on the 6C geometry of the metal center, priming it for substrate ligation. Arg80 and Tyr70 are shown to promote O(2) reduction, while Thr107 stabilizes the Fe(II) cofactor.

  View details for DOI 10.1021/bi901339n

  View details for Web of Science ID 000274094300020

  View details for PubMedID 20050606

  View details for PubMedCentralID PMC2882036

• Thioether S-ligation in a side-on mu-eta(2):eta(2)-peroxodicopper(II) complex CHEMICAL COMMUNICATIONS Park, G. Y., Lee, Y., Lee, D., Woertink, J. S., Sarjeant, A. A., Solomon, E. I., Karlin, K. D. 2010; 46 (1): 91-93

  Abstract

  [(ANS)Cu(I)(CH(3)CN)](+) reacts with O(2) giving [{(ANS)Cu(II)}(2)(micro-eta(2):eta(2)-O(2)(2-))](2+), nu(O-O) = 731 cm(-1), shown to possess S-thioether ligation, based on comparisons with analogues having all N-ligands or a -S(Ph) group. The finding is a rare occurrence and new for side-on O(2)(2-) binding.

  View details for DOI 10.1039/b918616f

  View details for Web of Science ID 000272679200014

  View details for PubMedID 20024303

  View details for PubMedCentralID PMC2908150

• Copper(I)/O(2)Chemistry with Imidazole Containing Tripodal Tetradentate Ligands Leading to mu-1,2-Peroxo-Dicopper(II) Species INORGANIC CHEMISTRY Lee, Y., Park, G. Y., Lucas, H. R., Vajda, P. L., Kamaraj, K., Vance, M. A., Milligan, A. E., Woertink, J. S., Siegler, M. A., Sarjeant, A. A., Zakharov, L. N., Rheingold, A. L., Solomon, E. I., Karlin, K. D. 2009; 48 (23): 11297-11309

  Abstract

  Cuprous and cupric complexes with the new imidazolyl containing tripodal tetradentate ligands {L(MIm), (1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine, and L(EIm), 2-(1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)ethanamine}, have been investigated to probe differences in their chemistry, especially in copper(I)-dioxygen chemistry, compared to that already known for the pyridyl analogue TMPA, tris(2-pyridyl)methyl)amine. Infrared (IR) stretching frequencies obtained from carbon monoxide adducts of [(L(MIm))Cu(I)](+) (1a) and [(L(EIm))Cu(I)](+) (2a) show that the imidazolyl donor is stronger than its pyridyl analogue. Electrochemical data suggest differences in the binding constant of Cu(II) to L(EIm) compared to TMPA and L(MIm), reflecting geometric changes. Oxygenation of [(L(MIm))Cu(I)](+) (1a) in 2-methyltetrahydrofuran (MeTHF) solvent at -128 degrees C leads to an intensely purple colored species with a UV-vis spectrum characteristic of an end-on bound peroxodicopper(II) complex [{(L(MIm))Cu(II)}(2)(mu-1,2-O(2)(2-))](2+) (1b(P)) {lambda(max) = 528 nm}, very similar to the previously well characterized complex [{(TMPA)Cu(II)}(2)(mu-1,2-O(2)(2-))](2+) {lambda(max) = 520 nm (epsilon = 12 000 M(-1) cm(-1)), in MeTHF; resonance Raman (rR) spectroscopy: nu(O-O) = 832 (Delta((18)O(2)) = -44) cm(-1)}. In the low-temperature oxygenation of 2a, benchtop (-128 degrees C) and stopped-flow (-90 degrees C) experiments reveal the formation of an initial superoxo-Cu(II) species [(L(EIm))Cu(II)(O(2)(*-))](+) (2b(S)), lambda(max) = 431 nm in THF) . This converts to the low-temperature stable peroxo complex [{(L(EIm))Cu(II)}(2)(mu-1,2-O(2)(2-))](2+) (2b(P)) {rR spectroscopy: nu(O-O) = 822 (Delta((18)O(2)) = -46) cm(-1)}. Complex 2b(P) possess distinctly reduced Cu-O and O-O stretching frequencies and a red-shifted UV-vis feature {to lambda(max) = 535 nm (epsilon = 11 000 M(-1) cm(-1))} compared to the TMPA analogue due to a distortion from trigonal bipyramidal (TBP) to a square pyramidal ligand field. This distortion is supported by the structural characterization of related ligand-copper(II) complexes: A stable tetramer cluster complex [(mu(2)-L(EIm-))(4)(Cu(II))(4)](4+), obtained from thermal decomposition of 2b(P) (with formation of H(2)O(2)), also exhibits a distorted square pyramidal Cu(II) ion geometry as does the copper(II) complex [(L(EIm))Cu(II)(CH(3)CN)](2+) (2c), characterized by X-ray crystallography and solution electron paramagnetic resonance (EPR) spectroscopy.

  View details for DOI 10.1021/ic9017695

  View details for Web of Science ID 000272037500061

  View details for PubMedID 19886646

  View details for PubMedCentralID PMC2787896

• A [Cu2O](2+) core in Cu-ZSM-5, the active site in the oxidation of methane to methanol PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Woertink, J. S., Smeets, P. J., Groothaert, M. H., Vance, M. A., Sels, B. F., Schoonheydt, R. A., Solomon, E. I. 2009; 106 (45): 18908-18913

  Abstract

  Driven by the depletion of crude oil, the direct oxidation of methane to methanol has been of considerable interest. Promising low-temperature activity of an oxygen-activated zeolite, Cu-ZSM-5, has recently been reported in this selective oxidation and the active site in this reaction correlates with an absorption feature at 22,700 cm(-1). In the present study, this absorption band is used to selectively resonance enhance Raman vibrations of this active site. (18)O(2) labeling experiments allow definitive assignment of the observed vibrations and exclude all previously characterized copper-oxygen species for the active site. In combination with DFT and normal coordinate analysis calculations, the oxygen activated Cu core is uniquely defined as a bent mono-(mu-oxo)dicupric site. Spectroscopically validated electronic structure calculations show polarization of the low-lying singly-occupied molecular orbital of the [Cu(2)O](2+) core, which is directed into the zeolite channel, upon approach of CH(4). This induces significant oxyl character into the bridging O atom leading to a low transition state energy consistent with experiment and explains why the bent mono-(mu-oxo)dicupric core is highly activated for H atom abstraction from CH(4). The oxygen intermediate of Cu-ZSM-5 is now the most well defined species active in the methane monooxygenase reaction.

  View details for DOI 10.1073/pnas.0910461106

  View details for Web of Science ID 000271637500009

  View details for PubMedID 19864626

  View details for PubMedCentralID PMC2776445

• A peroxynitrite complex of copper: formation from a copper-nitrosyl complex, transformation to nitrite and exogenous phenol oxidative coupling or nitration JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY Park, G. Y., Deepalatha, S., Puiu, S. C., Lee, D., Mondal, B., Sarjeant, A. A., del Rio, D., Pau, M. Y., Solomon, E. I., Karlin, K. D. 2009; 14 (8): 1301-1311

  Abstract

  Reaction of nitrogen monoxide with a copper(I) complex possessing a tridentate alkylamine ligand gives a Cu(I)-(*NO) adduct, which when exposed to dioxygen generates a peroxynitrite (O=NOO(-))-Cu(II) species. This undergoes thermal transformation to produce a copper(II) nitrito (NO(2) (-)) complex and 0.5 mol equiv O(2). In the presence of a substituted phenol, the peroxynitrite complex effects oxidative coupling, whereas addition of chloride ion to dissociate the peroxynitrite moiety instead leads to phenol ortho nitration. Discussions include the structures (including electronic description) of the copper-nitrosyl and copper-peroxynitrite complexes and the formation of the latter, based on density functional theory calculations and accompanying spectroscopic data.

  View details for DOI 10.1007/s00775-009-0575-8

  View details for Web of Science ID 000271422100014

  View details for PubMedID 19662443

  View details for PubMedCentralID PMC2908284

• A variable temperature spectroscopic study on Paracoccus pantotrophus pseudoazurin: Protein constraints on the blue Cu site 1st Latin American Meeting on Biological Inorganic Chemistry (LABIC2008) Xie, X., Hadt, R. G., Pauleta, S. R., Gonzalez, P. J., Un, S., Moura, I., Solomon, E. I. ELSEVIER SCIENCE INC. 2009: 1307–13

  Abstract

  The blue or Type 1 (T1) copper site of Paracoccuspantotrophus pseudoazurin exhibits significant absorption intensity in both the 450 and 600 nm regions. These are sigma and pi S(Cys) to Cu(2+) charge transfer (CT) transitions. The temperature dependent absorption, EPR, and resonance Raman (rR) vibrations enhanced by these bands indicate that a single species is present at all temperatures. This contrasts the temperature dependent behavior of the T1 center in nitrite reductase [S. Ghosh, X. Xie, A. Dey, Y. Sun, C. Scholes, E. Solomon, Proc. Natl. Acad. Sci. 106 (2009) 4969-4974] which has a thioether ligand that is unconstrained by the protein. The lack of temperature dependence in the T1 site in pseudoazurin indicates the presence of a protein constraint similar to the blue Cu site in plastocyanin where the thioether ligand is constrained at 2.8 A. However, plastocyanin exhibits only pi CT. This spectral difference between pseudoazurin and plastocyanin reflects a coupled distortion of the site where the axial thioether in pseudoazurin is also constrained, but at a shorter Cu-S(Met) bond length. This leads to an increase in the Cu(2+)-S(Cys) bond length, and the site undergoes a partial tetragonal distortion in pseudoazurin. Thus, its ground state wavefunction has both sigma and pi character in the Cu(2+)-S(Cys) bond.

  View details for DOI 10.1016/j.jinorgbio.2009.04.012

  View details for Web of Science ID 000270795900003

  View details for PubMedID 19481814

• Geometric and electronic structure and reactivity of a mononuclear 'side-on' nickel(III)-peroxo complex NATURE CHEMISTRY Cho, J., Sarangi, R., Annaraj, J., Kim, S. Y., Kubo, M., Ogura, T., Solomon, E. I., Nam, W. 2009; 1 (7): 568-572

  Abstract

  Metal-dioxygen adducts, such as metal-superoxo and -peroxo species, are key intermediates often detected in the catalytic cycles of dioxygen activation by metalloenzymes and biomimetic compounds. The synthesis and spectroscopic characterization of an end-on nickel(II)-superoxo complex with a 14-membered macrocyclic ligand was reported previously. Here we report the isolation, spectroscopic characterization, and high-resolution crystal structure of a mononuclear side-on nickel(III)-peroxo complex with a 12-membered macrocyclic ligand, [Ni(12-TMC)(O(2))](+) (1) (12-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). Different from the end-on Ni(II)-superoxo complex, the Ni(III)-peroxo complex is not reactive in electrophilic reactions, but is capable of conducting nucleophilic reactions. The Ni(III)-peroxo complex transfers the bound dioxygen to manganese(II) complexes, thus affording the corresponding nickel(II) and manganese(III)-peroxo complexes. The present results demonstrate the significance of supporting ligands in tuning the geometric and electronic structures and reactivities of metal-O(2) intermediates that have been shown to have biological as well as synthetic usefulness in biomimetic reactions.

  View details for DOI 10.1038/NCHEM.366

  View details for Web of Science ID 000270077200017

  View details for PubMedID 20711413

  View details for PubMedCentralID PMC2920495

• Molecular Oxygen and Sulfur Reactivity of a Cyclotriveratrylene Derived Trinuclear Copper(I) Complex INORGANIC CHEMISTRY Maiti, D., Woertink, J. S., Ghiladi, R. A., Solomon, E. I., Karlin, K. D. 2009; 48 (17): 8342-8356

  Abstract

  Our continuing efforts into developing copper coordination chemistry relevant to dioxygen-processing copper proteins has led us to design and synthesize a cyclotriveratrylene (CTV)-based trinucleating ligand, CTV-TMPA, which employs tetradentate tris(2-pyridylmethyl)-amine chelates (TMPA) for their copper ion binding sites. Binding of three copper ions per CTV-TMPA unit was established by various chemical and spectroscopic methods such as UV-vis and resonance Raman (rR) spectroscopies. The following complexes were observed: A tricopper(I) complex [(CTV-TMPA)Cu(I)(3)](3+) (1), a CO adduct [(CTV-TMPA)Cu(I)(3)(CO)(3)](3+) (1-CO; nu(C=O) = 2094 cm(-1)), a triphenylphosphine adduct [(CTV-TMPA)Cu(I)(3)(PPh(3))(3)](3+) (1-PPh(3)), a tricopper(II) complex [(CTV-TMPA)Cu(II)(3)](3+) (1-Ox), and its tris-monochloride or tris-monobromide adducts. Also, introduction of dioxygen to the -80 degrees C solutions of 1 leads to O(2)-adducts, the first example of a synthetic copper complex which can stabilize a mononuclear Cu(II)-superoxo and dinuclear peroxo species simultaneously within one complex {[Cu] = 1.53 mM in THF: (mu-1,2-peroxo complex, lambda(max) = 543 (epsilon 9650) nm): nu(O-O) = 825 ((Delta(18)O(2)) = -47) cm(-1); nu(Cu-O) = 506 ((Delta(18)O(2)) = -26) cm(-1): (superoxo complex, lambda(max) = 427 (epsilon 3150) nm): nu(O-O) = 1129 ((Delta(18)O(2)) = -60) cm(-1); nu(Cu-O) = 463 ((Delta(18)O(2)) = -27) cm(-1)}. Elemental sulfur reacts reversibly with 1 leading to a (proposed) hexanuclear species [{(CTV-TMPA)Cu(II)(3)}(2)(mu-1,2-S(2)(2-))(3)](6+) (1-S) {lambda(max) = 544 (epsilon 7270) nm}, possessing one dicopper(II)-disulfide structural type: {THF solvent) nu(S-S) = 489 ((Delta(34)S) = -10) cm(-1); nu(Cu-S) = 307 ((Delta(34)S) = -5) cm(-1)}. Derivation of spectroscopic, structural, and chemical conclusions were aided by the study of a close mononuclear analogue with one pyridyl group of the TMPA parent possessing a 6-CH(2)OCH(3) substituent, this being part of the CTV-TMPA architecture.

  View details for DOI 10.1021/ic900975y

  View details for Web of Science ID 000269313500042

  View details for PubMedID 19663454

  View details for PubMedCentralID PMC2917907

• Peroxo-Type Intermediates in Class I Ribonucleotide Reductase and Related Binuclear Non-Heme Iron Enzymes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Jensen, K. P., Bell, C. B., Clay, M. D., Solomon, E. I. 2009; 131 (34): 12155-12171

  Abstract

  We have performed a systematic study of chemically possible peroxo-type intermediates occurring in the non-heme di-iron enzyme class Ia ribonucleotide reductase, using spectroscopically calibrated computational chemistry. Density functional computations of equilibrium structures, Fe-O and O-O stretch frequencies, Mossbauer isomer shifts, absorption spectra, J-coupling constants, electron affinities, and free energies of O(2) and proton or water binding are presented for a series of possible intermediates. The results enable structure-property correlations and a new rationale for the changes in carboxylate conformations occurring during the O(2) reaction of this class of non-heme iron enzymes. Our procedure identifies and characterizes various possible candidates for peroxo intermediates experimentally observed along the ribonucleotide reductase dioxygen activation reaction. The study explores how water or a proton can bind to the di-iron site of ribonucleotide reductase and facilitate changes that affect the electronic structure of the iron sites and activate the site for further reaction. Two potential reaction pathways are presented: one where water adds to Fe1 of the cis-mu-1,2 peroxo intermediate P causing opening of a bridging carboxylate to form intermediate P' that has an increased electron affinity and is activated for proton-coupled electron transfer to form the Fe(III)Fe(IV) intermediate X; and one that is more energetically favorable where the P to P' conversion involves addition of a proton to a terminal carboxylate ligand in the site which increases the electron affinity and triggers electron transfer to form X. Both pathways provide a mechanism for the activation of peroxy intermediates in binuclear non-heme iron enzymes for reactivity. The studies further show that water coordination can induce the conformational changes observed in crystal structures of the met state.

  View details for DOI 10.1021/ja809983g

  View details for Web of Science ID 000269379600046

  View details for PubMedID 19663382

• The formation and reaction of a copper-peroxynitrite complex Park, G., Subramanian, D., Puiu, S. C., Lee, D., Mondal, B., Sarjeantl, A., del Rio, D., Pau, M. M., Solomon, E. I., Karlin, K. D. AMER CHEMICAL SOC. 2009

  View details for Web of Science ID 000207862000565

• Spectroscopy and Kinetics of Wild-Type and Mutant Tyrosine Hydroxylase: Mechanistic Insight into O-2 Activation JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chow, M. S., Eser, B. E., Wilson, S. A., Hodgson, K. O., Hedman, B., Fitzpatrick, P. F., Solomon, E. I. 2009; 131 (22): 7685-7698

  Abstract

  Tyrosine hydroxylase (TH) is a pterin-dependent nonheme iron enzyme that catalyzes the hydroxylation of L-tyr to L-DOPA in the rate-limiting step of catecholamine neurotransmitter biosynthesis. We have previously shown that the Fe(II) site in phenylalanine hydroxylase (PAH) converts from six-coordinate (6C) to five-coordinate (5C) only when both substrate + cofactor are bound. However, steady-state kinetics indicate that TH has a different co-substrate binding sequence (pterin + O(2) + L-tyr) than PAH (L-phe + pterin + O(2)). Using X-ray absorption spectroscopy (XAS), and variable-temperature-variable-field magnetic circular dichroism (VTVH MCD) spectroscopy, we have investigated the geometric and electronic structure of the wild-type (WT) TH and two mutants, S395A and E332A, and their interactions with substrates. All three forms of TH undergo 6C --> 5C conversion with tyr + pterin, consistent with the general mechanistic strategy established for O(2)-activating nonheme iron enzymes. We have also applied single-turnover kinetic experiments with spectroscopic data to evaluate the mechanism of the O(2) and pterin reactions in TH. When the Fe(II) site is 6C, the two-electron reduction of O(2) to peroxide by Fe(II) and pterin is favored over individual one-electron reactions, demonstrating that both a 5C Fe(II) and a redox-active pterin are required for coupled O(2) reaction. When the Fe(II) is 5C, the O(2) reaction is accelerated by at least 2 orders of magnitude. Comparison of the kinetics of WT TH, which produces Fe(IV)=O + 4a-OH-pterin, and E332A TH, which does not, shows that the E332 residue plays an important role in directing the protonation of the bridged Fe(II)-OO-pterin intermediate in WT to productively form Fe(IV)=O, which is responsible for hydroxylating L-tyr to L-DOPA.

  View details for DOI 10.1021/ja810080c

  View details for Web of Science ID 000267177900058

  View details for PubMedID 19489646

  View details for PubMedCentralID PMC2698713

• S K-edge XAS and DFT Calculations on Cytochrome P450: Covalent and Ionic Contributions to the Cysteine-Fe Bond and Their Contribution to Reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Jiang, Y., de Montellano, P. O., Hodgson, K. O., Hedman, B., Solomon, E. I. 2009; 131 (22): 7869-7878

  Abstract

  Experimental covalencies of the Fe-S bond for the resting low-spin and substrate-bound high-spin active site of cytochrome P450 are reported. DFT calculations on the active site indicate that one H-bonding interaction from the protein backbone is needed to reproduce the experimental values. The H-bonding to the thiolate from the backbone decreases the anisotropic pi covalency of the Fe-S bond lowering the barrier of free rotation of the exchangeable axial ligand, which is important for reactivity. The anionic axial thiolate ligand is calculated to lower the Fe(III/II) reduction potential of the active site by more than 1 V compared to a neutral imidazole ligand. About half of this derives from its covalent bonding and half from its electrostatic interaction with the oxidized Fe. This axial thiolate ligand increases the pK(a) of compound 0 (Fe(III)-hydroperoxo) favoring its protonation which promotes O-O bond heterolysis forming compound I. The reactivity of compound I is calculated to be relatively insensitive to the nature of the axial ligand due to opposing reduction potential and proton affinity contributions to the H-atom abstraction energy.

  View details for DOI 10.1021/ja901868q

  View details for Web of Science ID 000267177900077

  View details for PubMedID 19438234

  View details for PubMedCentralID PMC2734335

• Reaction Coordinate of a Functional Model of Tyrosinase: Spectroscopic and Computational Characterization JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 't Holt, B. T., Vance, M. A., Mirica, L. M., Heppner, D. E., Stack, T. D., Solomon, E. I. 2009; 131 (18): 6421-6438

  Abstract

  The mu-eta(2):eta(2)-peroxodicopper(II) complex synthesized by reacting the Cu(I) complex of the bis-diamine ligand N,N'-di-tert-butyl-ethylenediamine (DBED) with O(2) is a functional and spectroscopic model of the coupled binuclear copper protein tyrosinase. This complex reacts with 2,4-di-tert-butylphenolate at low temperature to produce a mixture of the catechol and quinone products, which proceeds through three intermediates (A-C) that have been characterized. A, stabilized at 153 K, is characterized as a phenolate-bonded bis-mu-oxo dicopper(III) species, which proceeds at 193 K to B, presumably a catecholate-bridged coupled bis-copper(II) species via an electrophilic aromatic substitution mechanism wherein aromatic ring distortion is the rate-limiting step. Isotopic labeling shows that the oxygen inserted into the aromatic substrate during hydroxylation derives from dioxygen, and a late-stage ortho-H(+) transfer to an exogenous base is associated with C-O bond formation. Addition of a proton to B produces C, determined from resonance Raman spectra to be a Cu(II)-semiquinone complex. The formation of C (the oxidation of catecholate and reduction to Cu(I)) is governed by the protonation state of the distal bridging oxygen ligand of B. Parallels and contrasts are drawn between the spectroscopically and computationally supported mechanism of the DBED system, presented here, and the experimentally derived mechanism of the coupled binuclear copper protein tyrosinase.

  View details for DOI 10.1021/ja807898h

  View details for Web of Science ID 000265939200042

  View details for PubMedID 19368383

  View details for PubMedCentralID PMC2692929

• Geometric and electronic structure differences between the type 3 copper sites of the multicopper oxidases and hemocyanin/tyrosinase PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Yoon, J., Fujii, S., Solomon, E. I. 2009; 106 (16): 6585-6590

  Abstract

  The coupled binuclear "type 3" Cu sites are found in hemocyanin (Hc), tyrosinase (Tyr), and the multicopper oxidases (MCOs), such as laccase (Lc), and play vital roles in O(2) respiration. Although all type 3 Cu sites share the same ground state features, those of Hc/Tyr have very different ligand-binding properties relative to those of the MCOs. In particular, the type 3 Cu site in the MCOs (Lc(T3)) is a part of the trinuclear Cu cluster, and if the third (i.e., type 2) Cu is removed, the Lc(T3) site does not react with O(2). Density functional theory calculations indicate that O(2) binding in Hc is approximately 9 kcal mol(-1) more favorable than for Lc(T3). The difference is mostly found in the total energy difference of the deoxy states (approximately 7 kcal mol(-1)), where the stabilization of deoxy Lc(T3) derives from its long equilibrium Cu-Cu distance of approximately 5.5-6.5 A, relative to approximately 4.2 A in deoxy Hc/Tyr. The O(2) binding in Hc is driven by the electrostatic destabilization of the deoxy Hc site, in which the two Cu(I) centers are kept close together by the protein for facile 2-electron reduction of O(2). Alternatively, the lack of O(2) reactivity in Lc(T3) reflects the flexibility of the active site, capable of minimizing the electrostatic repulsion of the 2 Cu(I)s. Thus, the O(2) reactivity of the MCOs is intrinsic to the trinuclear Cu cluster, leading to different O(2) intermediates as required by its function of irreversible reduction of O(2) to H(2)O.

  View details for DOI 10.1073/pnas.0902127106

  View details for Web of Science ID 000265506800031

  View details for PubMedID 19346471

  View details for PubMedCentralID PMC2672473

• Thermodynamic equilibrium between blue and green copper sites and the role of the protein in controlling function PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Ghosh, S., Xie, X., Dey, A., Sun, Y., Scholes, C. P., Solomon, E. I. 2009; 106 (13): 4969-4974

  Abstract

  A combination of spectroscopies and density functional theory calculations indicate that there are large temperature-dependent absorption spectral changes present in green nitrite reductases (NiRs) due to a thermodynamic equilibrium between a green and a blue type 1 (T1) copper site. The axial methionine (Met) ligand is unconstrained in the oxidized NiRs, which results in an enthalpically favored (DeltaH approximately 4.6 kcal/mol) Met-bound green copper site at low temperatures, and an entropically favored (TDeltaS approximately 4.5 kcal/mol, at room temperature) Met-elongated blue copper site at elevated temperatures. In contrast to the NiRs, the classic blue copper sites in plastocyanin and azurin show no temperature-dependent behavior, indicating that a single species is present at all temperatures. For these blue copper proteins, the polypeptide matrix opposes the gain in entropy that would be associated with the loss of the weak axial Met ligand at physiological temperatures by constraining its coordination to copper. The potential energy surfaces of Met binding indicate that it stabilizes the oxidized state more than the reduced state. This provides a mechanism to tune down the reduction potential of blue copper sites by >200 mV.

  View details for DOI 10.1073/pnas.0900995106

  View details for Web of Science ID 000264790600005

  View details for PubMedID 19282479

  View details for PubMedCentralID PMC2664060

• Copper dioxygen chemistry with diamines at low temperatures Verma, P., Kang, P., Mirica, L. M., Vance, M., Solomon, E. I., Stack, T. D. AMER CHEMICAL SOC. 2009

  View details for Web of Science ID 000207857805389

• Reduction of dioxygen to water by the multicopper oxidases Solomon, E. I. AMER CHEMICAL SOC. 2009

  View details for Web of Science ID 000207857805164

• A functional nitric oxide reductase model Decreau, R., Yang, Y., Dey, A., Ohta, T., Dey, S. G., Solomon, E. I., Collman, J. P. AMER CHEMICAL SOC. 2009

  View details for Web of Science ID 000207857805373

• Toluene and Ethylbenzene Aliphatic C-H Bond Oxidations Initiated by a Dicopper(II)-mu-1,2-Peroxo Complex JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lucas, H. R., Li, L., Sarjeant, A. A., Vance, M. A., Solomon, E. I., Karlin, K. D. 2009; 131 (9): 3230-3245

  Abstract

  With an anisole-containing polypyridylamine potential tetradentate ligand (O)L, a mu-1,2-peroxo-dicopper(II) complex [{(O)LCu(II)}(2)(O(2)(2-))](2+) forms from the reaction of the mononuclear compound [Cu(I)((O)L)(MeCN)]B(C(6)F(5))(4) ((O)LCu(I)) with O(2) in noncoordinating solvents at -80 degrees C. Thermal decay of this peroxo complex in the presence of toluene or ethylbenzene leads to rarely seen C-H activation chemistry; benzaldehyde and acetophenone/1-phenylethanol mixtures, respectively, are formed. Experiments with (18)O(2) confirm that the oxygen source in the products is molecular O(2) and deuterium labeling experiments indicate k(H)/k(D) = 7.5 +/- 1 for the toluene oxygenation. The O(2)-reaction of [Cu(I)((Bz)L)(CH(3)CN)](+) ((Bz)LCu(I)) leads to a dicopper(III)-bis-mu-oxo species [{(Bz)LCu(III)}(2)(mu-O(2-))(2)](2+) at -80 degrees C, and from such solutions, very similar toluene oxygenation chemistry occurs. Ligand (Bz)L is a tridentate chelate, possessing the same moiety found in (O)L, but without the anisole O-atom donor. In these contexts, the nature of the oxidant species in or derived from [{(O)LCu(II)}(2)(O(2)(2-))](2+) is discussed and likely mechanisms of reaction initiated by toluene H-atom abstraction chemistry are detailed. To confirm the structural formulations of the dioxygen-adducts, UV-vis and resonance Raman spectroscopic studies have been carried out and these results are reported and compared to previously described systems including [{Cu(II)((Py)L)}(2)(O(2))](2+) ((Py)L = TMPA = tris(2-methylpyridyl)amine). Using (L)Cu(I), CO-binding properties (i.e., nu(C-O) values) along with electrochemical property comparisons, the relative donor abilities of (O)L, (Bz)L, and (Py)L are assessed.

  View details for DOI 10.1021/ja807081d

  View details for Web of Science ID 000264792400041

  View details for PubMedID 19216527

  View details for PubMedCentralID PMC2765497

• Fe L- and K-edge XAS of Low-Spin Ferric Corrole: Bonding and Reactivity Relative to Low-Spin Ferric Porphyrin INORGANIC CHEMISTRY Hocking, R. K., George, S. D., Gross, Z., Walker, F. A., Hodgson, K. O., Hedman, B., Solomon, E. I. 2009; 48 (4): 1678-1688

  Abstract

  Corrole is a tetrapyrrolic macrocycle that has one carbon atom less than a porphyrin. The ring contraction reduces the symmetry from D(4h) to C(2v), changes the electronic structure of the heterocycle, and leads to a smaller central cavity with three protons rather than the two of a porphyrin. The differences between ferric corroles and porphyrins lead to a number of differences in reactivity including increased axial ligand lability and a tendency to form 5-coordinate complexes. The electronic structure origin of these differences has been difficult to study experimentally as the dominant porphyrin/corrole pi --> pi* transitions obscure the electronic transitions of the metal. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., the differences in mixing of the metal d orbitals with the ligand valence orbitals) using a valence bond configuration interaction model. Herein, we apply this methodology, combined with a ligand field analysis of the Fe K pre-edge to a low-spin ferric corrole, and compare it to a low-spin ferric porphyrin. The experimental results combined with DFT calculations show that the contracted corrole is both a stronger sigma donor and a very anisotropic pi donor. These differences decrease the bonding interactions with axial ligands and contribute to the increased axial ligand lability and reactivity of ferric corroles relative to ferric porphyrins.

  View details for DOI 10.1021/ic802248t

  View details for Web of Science ID 000263227100051

  View details for PubMedID 19149467

  View details for PubMedCentralID PMC2765561

• Peroxo and oxo intermediates in mononuclear nonheme iron enzymes and related active sites CURRENT OPINION IN CHEMICAL BIOLOGY Solomon, E. I., Wong, S. D., Liu, L. V., Decker, A., Chow, M. S. 2009; 13 (1): 99-113

  Abstract

  Fe(III)OOH and Fe(IV)O intermediates have now been documented in a number of nonheme iron active sites. In this Current Opinion we use spectroscopy combined with electronic structure calculations to define the frontier molecular orbitals (FMOs) of these species and their contributions to reactivity. For the low-spin Fe(III)OOH species in activated bleomycin we show that the reactivity of this nonheme iron intermediate is very different from that of the analogous Compound 0 of cytochrome P450. For Fe(IV)O S=1 model species we experimentally define the electronic structure and its contribution to reactivity, and computationally evaluate how this would change for the Fe(IV)O S=2 intermediates found in nonheme iron enzymes.

  View details for DOI 10.1016/j.cbpa.2009.02.011

  View details for Web of Science ID 000266192400013

  View details for PubMedID 19278895

  View details for PubMedCentralID PMC2676221

• Spectroscopic and Computational Studies of Nitrite Reductase: Proton Induced Electron Transfer and Backbonding Contributions to Reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ghosh, S., Dey, A., Sun, Y., Scholes, C. P., Solomon, E. I. 2009; 131 (1): 277-288

  Abstract

  A combination of spectroscopy and DFT calculations has been used to define the geometric and electronic structure of the nitrite bound type 2 (T2) copper site at high and low pH in nitrite reductase from Rhodobacter sphaeroides. At high pH there is no electron transfer from reduced type 1 (T1) to the nitrite bound T2 copper, while protonation triggers T1 --> T2 electron transfer and generation of NO. The DFT calculated reaction coordinate for the N-O bond cleavage in nitrite reduction by the reduced T2 copper suggests that the process is best described as proton transfer triggering electron transfer. Bidentate nitrite binding to copper is calculated to play a major role in activating the reductive cleavage of the nitrite bond through backbonding combined with stabilization of the (-)OH product by coordination to the Cu(2+).

  View details for DOI 10.1021/ja806873e

  View details for Web of Science ID 000262483100059

  View details for PubMedID 19053185

  View details for PubMedCentralID PMC2629382

• Spectroscopic Definition of the Biferrous and Biferric Sites in de Novo Designed Four-Helix Bundle DFsc Peptides: Implications for O-2 Reactivity of Binuclear Non-Heme Iron Enzymes BIOCHEMISTRY Bell, C. B., Calhoun, J. R., Bobyr, E., Wei, P., Hedman, B., Hodgson, K. O., DeGrado, W. F., Solomon, E. T. 2009; 48 (1): 59-73

  Abstract

  DFsc is a single chain de novo designed four-helix bundle peptide that mimics the core protein fold and primary ligand set of various binuclear non-heme iron enzymes. DFsc and the E11D, Y51L, and Y18F single amino acid variants have been studied using a combination of near-IR circular dichroism (CD), magnetic circular dichroism (MCD), variable temperature variable field MCD (VTVH MCD), and X-ray absorption (XAS) spectroscopies. The biferrous sites are all weakly antiferromagnetically coupled with mu-1,3 carboxylate bridges and one 4-coordinate and one 5-coordinate Fe, very similar to the active site of class I ribonucleotide reductase (R2) providing open coordination positions on both irons for dioxygen to bridge. From perturbations of the MCD and VTVH MCD the iron proximal to Y51 can be assigned as the 4-coordinate center, and XAS results show that Y51 is not bound to this iron in the reduced state. The two open coordination positions on one iron in the biferrous state would become occupied by dioxygen and Y51 along the O(2) reaction coordinate. Subsequent binding of Y51 functions as an internal spectral probe of the O(2) reaction and as a proton source that would promote loss of H(2)O(2). Coordination by a ligand that functions as a proton source could be a structural mechanism used by natural binuclear iron enzymes to drive their reactions past peroxo biferric level intermediates.

  View details for DOI 10.1021/bi8016087

  View details for Web of Science ID 000262265900008

  View details for PubMedID 19090676

  View details for PubMedCentralID PMC2660568

• Reactive Intermediates in Oxygenation Reactions with Mononuclear Nonheme Iron Catalysts ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Yoon, J., Wilson, S. A., Jang, Y. K., Seo, M. S., Nehru, K., Hedman, B., Hodgson, K. O., Bill, E., Solomon, E. I., Nam, W. 2009; 48 (7): 1257-1260

  Abstract

  An advanced intermediate: A nonheme iron(IV) oxo complex [Fe(IV)(O)(bqen)(L)](n+) (bqen = N,N'-dimethyl-N,N'-bis(8-quinolyl)ethane-1,2-diamine, L = CH(3)CN or CF(3)SO(3)(-)) activates the C-H bonds of alkanes and alcohols by a hydrogen-atom abstraction mechanism. The catalytic oxidation of these species is proposed to occur through a nonheme iron(V) oxo species, with a high reactivity in oxidation reactions (see picture).

  View details for DOI 10.1002/anie.200802672

  View details for Web of Science ID 000263492400010

  View details for PubMedID 19137521

  View details for PubMedCentralID PMC2863019

• UNIQUE SPECTROSCOPIC FEATURES AND ELECTRONIC STRUCTURES OF COPPER PROTEINS: RELATION TO REACTIVITY HIGH RESOLUTION EPR: APPLICATIONS TO METALLOENZYMES AND METALS IN MEDICINE Yoon, J., Solomon, E. I., Hanson, G., Berliner, L. 2009; 28: 471–504

  View details for DOI 10.1007/978-0-387-84856-3_11

  View details for Web of Science ID 000268457800011

• Geometric Structure Determination of N694C Lipoxygenase: A Comparative Near-Edge X-Ray Absorption Spectroscopy and Extended X-Ray Absorption Fine Structure Study INORGANIC CHEMISTRY Sarangi, R., Hocking, R. K., Neidig, M. L., Benfatto, M., Holman, T. R., Solomon, E. I., Hodgson, K. O., Hedman, B. 2008; 47 (24): 11543-11550

  Abstract

  The mononuclear nonheme iron active site of N694C soybean lipoxygenase (sLO1) has been investigated in the resting ferrous form using a combination of Fe-K-pre-edge, near-edge (using the minuit X-ray absorption near-edge full multiple-scattering approach), and extended X-ray absorption fine structure (EXAFS) methods. The results indicate that the active site is six-coordinate (6C) with a large perturbation in the first-shell bond distances in comparison to the more ordered octahedral site in wild-type sLO1. Upon mutation of the asparagine to cysteine, the short Fe-O interaction with asparagine is replaced by a weak Fe-(H(2)O), which leads to a distorted 6C site with an effective 5C ligand field. In addition, it is shown that near-edge multiple scattering analysis can give important three-dimensional structural information, which usually cannot be accessed using EXAFS analysis. It is further shown that, relative to EXAFS, near-edge analysis is more sensitive to partial coordination numbers and can be potentially used as a tool for structure determination in a mixture of chemical species.

  View details for DOI 10.1021/ic800580f

  View details for Web of Science ID 000261510100016

  View details for PubMedID 18656914

  View details for PubMedCentralID PMC2736335

• Intermediates Involved in the Two Electron Reduction of NO to N2O by a Functional Synthetic Model of Heme Containing Bacterial NO Reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Collman, J. P., Dey, A., Yang, Y., Decreau, R. A., Ohta, T., Solomon, E. I. 2008; 130 (49): 16498-?

  View details for DOI 10.1021/ja807700n

  View details for Web of Science ID 000263320200028

  View details for PubMedID 19049449

  View details for PubMedCentralID PMC3129983

• Spectroscopic and Electronic Structure Studies of Phenolate Cu(II) Complexes: Phenolate Ring Orientation and Activation Related to Cofactor Biogenesis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ghosh, S., Cirera, J., Vance, M. A., Ono, T., Fujisawa, K., Solomon, E. I. 2008; 130 (48): 16262-16273

  Abstract

  A combination of spectroscopies and DFT calculations have been used to define the electronic structures of two crystallographically defined Cu(II)-phenolate complexes. These complexes differ in the orientation of the phenolate ring which results in different bonding interactions of the phenolate donor orbitals with the Cu(II), which are reflected in the very different spectroscopic properties of the two complexes. These differences in electronic structures lead to significant differences in DFT calculated reactivities with oxygen. These calculations suggest that oxygen activation via a Cu(I) phenoxyl ligand-to-metal charge transfer complex is highly endergonic (>50 kcal/mol), hence an unlikely pathway. Rather, the two-electron oxidation of the phenolate forming a bridging Cu(II) peroxoquinone complex is more favorable (11.3 kcal/mol). The role of the oxidized metal in mediating this two-electron oxidation of the coordinated phenolate and its relevance to the biogenesis of the covalently bound topa quinone in amine oxidase are discussed.

  View details for DOI 10.1021/ja8044986

  View details for Web of Science ID 000263319800041

  View details for PubMedID 18998639

  View details for PubMedCentralID PMC2654227

• Circular Dichroism and Magnetic Circular Dichroism Studies of the Biferrous Site of the Class Ib Ribonucleotide Reductase from Bacillus cereus: Comparison to the Class Ia Enzymes BIOCHEMISTRY Tomter, A. B., Bell, C. B., Rohr, A. K., Andersson, K. K., Solomon, E. I. 2008; 47 (43): 11300-11309

  Abstract

  The rate limiting step in DNA biosynthesis is the reduction of ribonucleotides to form the corresponding deoxyribonucleotides. This reaction is catalyzed by ribonucleotide reductases (RNRs) and is an attractive target against rapidly proliferating pathogens. Class I RNRs are binuclear non-heme iron enzymes and can be further divided into subclasses. Class Ia is found in many organisms, including humans, while class Ib has only been found in bacteria, notably some pathogens. Both Bacillus anthracis and Bacillus cereus encode class Ib RNRs with over 98% sequence identity. The geometric and electronic structure of the B. cereus diiron containing subunit (R2F) has been characterized by a combination of circular dichroism, magnetic circular dichroism (MCD) and variable temperature variable field MCD and is compared to class Ia RNRs. While crystallography has given several possible descriptions for the class Ib RNR biferrous site, the spectroscopically defined active site contains a 4-coordinate and a 5-coordinate Fe(II), weakly antiferromagnetically coupled via mu-1,3-carboxylate bridges. Class Ia biferrous sites are also antiferromagnetically coupled 4-coordinate and 5-coordinate Fe(II), however quantitatively differ from class Ib in bridging carboxylate conformation and tyrosine radical positioning relative to the diiron site. Additionally, the iron binding affinity in B. cereus RNR R2F is greater than class Ia RNR and provides the pathogen with a competitive advantage relative to host in physiological, iron-limited environments. These structural differences have potential for the development of selective drugs.

  View details for DOI 10.1021/bi801212f

  View details for Web of Science ID 000260254500016

  View details for PubMedID 18831534

• A functional nitric oxide reductase model PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Collman, J. P., Yang, Y., Dey, A., Decreau, R. A., Ghosh, S., Ohta, T., Solomon, E. I. 2008; 105 (41): 15660-15665

  Abstract

  A functional heme/nonheme nitric oxide reductase (NOR) model is presented. The fully reduced diiron compound reacts with two equivalents of NO leading to the formation of one equivalent of N(2)O and the bis-ferric product. NO binds to both heme Fe and nonheme Fe complexes forming individual ferrous nitrosyl species. The mixed-valence species with an oxidized heme and a reduced nonheme Fe(B) does not show NO reduction activity. These results are consistent with a so-called "trans" mechanism for the reduction of NO by bacterial NOR.

  View details for DOI 10.1073/pnas.0808606105

  View details for Web of Science ID 000260240900007

  View details for PubMedID 18838684

  View details for PubMedCentralID PMC2572950

• Further insights into the mechanism of the reaction of activated bleomycin with DNA PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Chow, M. S., Liu, L. V., Solomon, E. I. 2008; 105 (36): 13241-13245

  Abstract

  Bleomycin (BLM) is a glycopeptide anticancer drug that effectively carries out single- and double-stranded DNA cleavage. Activated BLM (ABLM), a low-spin ferric-hydroperoxide, BLM-Fe(III)-OOH, is the last intermediate detected before DNA cleavage. We have previously shown through experiments and DFT calculations that both ABLM decay and reaction with H atom donors proceed via direct H atom abstraction. However, the rate of ABLM decay had been previously found, based on indirect methods, to be independent of the presence of DNA. In this study, we use a circular dichroism (CD) feature unique to ABLM to directly monitor the kinetics of ABLM reaction with a DNA oligonucleotide. Our results show that the ABLM + DNA reaction is appreciably faster, has a different kinetic isotope effect, and has a lower Arrhenius activation energy than does ABLM decay. In the ABLM reaction with DNA, the small normal k(H)/k(D) ratio is attributed to a secondary solvent effect through DFT vibrational analysis of reactant and transition state (TS) frequencies, and the lower E(a) is attributed to the weaker bond involved in the abstraction reaction (C-H for DNA and N-H for the decay in the absence of DNA). The DNA dependence of the ABLM reaction indicates that DNA is involved in the TS for ABLM decay and thus reacts directly with BLM-Fe(III)-OOH instead of its decay product.

  View details for DOI 10.1073/pnas.0806378105

  View details for Web of Science ID 000259251700014

  View details for PubMedID 18757754

  View details for PubMedCentralID PMC2533175

• INOR 566-Sulfur K-edge X-ray absorption spectroscopic and density functional theory studies of metal bis- and tris-dithiolene complexes 236th National Meeting of the American-Chemical-Society Tenderholt, A. L., Szilagyi, R. K., Holm, R. H., Hodgson, K. O., Hedman, B., Solomon, E. I. AMER CHEMICAL SOC. 2008

  View details for Web of Science ID 000270256306027

• PHYS 174-Reduction of dioxygen to water by the multicopper oxidases Solomon, E. I. AMER CHEMICAL SOC. 2008

  View details for Web of Science ID 000270256307044

• INOR 358-Structure/function correlations over non-heme ferrous enzymes Solomon, E. I. AMER CHEMICAL SOC. 2008

  View details for Web of Science ID 000270256306023

• INOR 37-Role of second coordination sphere carboxylate residues in the reduction of dioxygen by the multicopper oxidases 236th National Meeting of the American-Chemical-Society Augustine, A. J., Yoon, J., Stoj, C. S., Kosman, D., Solomon, E. I. AMER CHEMICAL SOC. 2008

  View details for Web of Science ID 000270256306653

• CD and MCD studies of the effects of component B variant binding on the biferrous active site of methane monooxygenase BIOCHEMISTRY Mitic, N., Schwartz, J. K., Brazeau, B. J., Lipscomb, J. D., Solomon, E. I. 2008; 47 (32): 8386-8397

  Abstract

  The multicomponent soluble form of methane monooxygenase (sMMO) catalyzes the oxidation of methane through the activation of O 2 at a nonheme biferrous center in the hydroxylase component, MMOH. Reactivity is limited without binding of the sMMO effector protein, MMOB. Past studies show that mutations of specific MMOB surface residues cause large changes in the rates of individual steps in the MMOH reaction cycle. To define the structural and mechanistic bases for these observations, CD, MCD, and VTVH MCD spectroscopies coupled with ligand-field (LF) calculations are used to elucidate changes occurring near and at the MMOH biferrous cluster upon binding of MMOB and the MMOB variants. Perturbations to both the CD and MCD are observed upon binding wild-type MMOB and the MMOB variant that similarly increases O 2 reactivity. MMOB variants that do not greatly increase O 2 reactivity fail to cause one or both of these changes. LF calculations indicate that reorientation of the terminal glutamate on Fe2 reproduces the spectral perturbations in MCD. Although this structural change allows O 2 to bridge the diiron site and shifts the redox active orbitals for good overlap, it is not sufficient for enhanced O 2 reactivity of the enzyme. Binding of the T111Y-MMOB variant to MMOH induces the MCD, but not CD changes, and causes only a small increase in reactivity. Thus, both the geometric rearrangement at Fe2 (observed in MCD) coupled with a more global conformational change that may control O 2 access (probed by CD), induced by MMOB binding, are critical factors in the reactivity of sMMO.

  View details for DOI 10.1021/bi800818w

  View details for Web of Science ID 000258225600017

  View details for PubMedID 18627173

  View details for PubMedCentralID PMC2614212

• Oxygen reactivity of the biferrous site in the de novo designed four helix bundle peptide DFsc: Nature of the "intermediate" and reaction mechanism JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Calhoun, J. R., Bell, C. B., Smith, T. J., Thamann, T. J., DeGrado, W. F., Solomon, E. I. 2008; 130 (29): 9188-?

  Abstract

  The DFsc and DFscE11D de novo designed protein scaffolds support biomimetic diiron cofactor sites that react with dioxygen forming a 520 nm "intermediate" species with an apparent pseudo-first-order formation rate constant of 2.2 and 4.8 s-1, respectively. Resonance Raman spectroscopy shows that this absorption feature is due to a phenolate-to-ferric charge transfer transition arising from a single tyrosine residue coordinating terminally to one of the ferric ions in the site. Phenol coordination could provide a proton to promote rapid loss of a putative peroxo species.

  View details for DOI 10.1021/ja801657y

  View details for Web of Science ID 000257796500005

  View details for PubMedID 18572936

• Spectroscopic definition of the ferroxidase site in M ferritin: Comparison of binuclear substrate vs cofactor active sites JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Schwartz, J. K., Liu, X. S., Tosha, T., Theil, E. C., Solomon, E. I. 2008; 130 (29): 9441-9450

  Abstract

  Maxi ferritins, 24 subunit protein nanocages, are essential in humans, plants, bacteria, and other animals for the concentration and storage of iron as hydrated ferric oxide, while minimizing free radical generation or use by pathogens. Formation of the precursors to these ferric oxides is catalyzed at a nonheme biferrous substrate site, which has some parallels with the cofactor sites in other biferrous enzymes. A combination of circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD (VTVH MCD) has been used to probe Fe(II) binding to the substrate active site in frog M ferritin. These data determined that the active site within each subunit consists of two inequivalent five-coordinate (5C) ferrous centers that are weakly antiferromagnetically coupled, consistent with a mu-1,3 carboxylate bridge. The active site ligand set is unusual and likely includes a terminal water bound to each Fe(II) center. The Fe(II) ions bind to the active sites in a concerted manner, and cooperativity among the sites in each subunit is observed, potentially providing a mechanism for the control of ferritin iron loading. Differences in geometric and electronic structure--including a weak ligand field, availability of two water ligands at the biferrous substrate site, and the single carboxylate bridge in ferritin--coincide with the divergent reaction pathways observed between this substrate site and the previously studied cofactor active sites.

  View details for DOI 10.1021/ja801251q

  View details for Web of Science ID 000257796500058

  View details for PubMedID 18576633

  View details for PubMedCentralID PMC2531225

• Interaction of nitric oxide with a functional model of cytochrome c oxidase PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Collman, J. P., Dey, A., Decreau, R. A., Yang, Y., Hosseini, A., Solomon, E. I., Eberspacher, T. A. 2008; 105 (29): 9892-9896

  Abstract

  Cytochrome c oxidase (CcO) is a multimetallic enzyme that carries out the reduction of O2 to H2O and is essential to respiration, providing the energy that powers all aerobic organisms by generating heat and forming ATP. The oxygen-binding heme a(3) should be subject to fatal inhibition by chemicals that could compete with O2 binding. Near the CcO active site is another enzyme, NO synthase, which produces the gaseous hormone NO. NO can strongly bind to heme a(3), thus inhibiting respiration. However, this disaster does not occur. Using functional models for the CcO active site, we show how NO inhibition is avoided; in fact, it is found that NO can protect the respiratory enzyme from other inhibitors such as cyanide, a classic poison.

  View details for DOI 10.1073/pnas.0804257105

  View details for Web of Science ID 000257913200010

  View details for PubMedID 18632561

  View details for PubMedCentralID PMC2481353

• Electronic control of the "Bailar Twist" in formally d(0)-d(2) molybdenum tris(dithiolene) complexes: A sulfur K-edge X-ray absorption spectroscopy and density functional theory study INORGANIC CHEMISTRY Tenderholt, A. L., Szilagyi, R. K., Holm, R. H., Hodgson, K. O., Hedman, B., Solomon, E. I. 2008; 47 (14): 6382-6392

  Abstract

  Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations have been used to determine the electronic structures of a series of Mo tris(dithiolene) complexes, [Mo(mdt)3](z) (where mdt = 1,2-dimethylethene-1,2-dithiolate(2-) and z = 2-, 1-, 0), with near trigonal-prismatic geometries (D3h symmetry). These results show that the formally Mo(IV), Mo(V), and Mo(VI) complexes actually have a (dz(2))(2) configuration, that is, remain effectively Mo(IV) despite oxidation. Comparisons with the XAS data of another set of Mo tris(dithiolene) complexes, [Mo(tbbdt)3](z) (where tbbdt = 3,5-ditert-butylbenzene-1,2-dithiolate(2-) and z = 1-, 0), show that both neutral complexes, [Mo(mdt)3] and [Mo(tbbdt)3], have similar electronic structures while the monoanions do not. Calculations reveal that the "Bailar twist" present in the crystal structure of [Mo(tbbdt)3](1-) (D3 symmetry) but not [Mo(mdt)3](1-) (D3h symmetry) is controlled by electronic factors which arise from bonding differences between the mdt and tbbdt ligands. In the former, configuration interaction between the Mo d(z(2)) and a deeper energy, occupied ligand orbital, which occurs in D3 symmetry, destabilizes the Mo d(z(2)) to above another ligand orbital which is half-occupied in the D3h [Mo(mdt)3](1-) complex. This leads to a metal d(1) configuration with no ligand holes (i.e., d(1)[L3](0h)) for [Mo(tbbdt)3](1-) rather than the metal d(2) configuration with one ligand hole (i.e., d(2)[L3](1h)) for [Mo(mdt)3](1-). Thus, the Bailar twist observed in some metal tris(dithiolene) complexes is the result of configuration interaction between metal and ligand orbitals and can be probed experimentally by S K-edge XAS.

  View details for DOI 10.1021/ic800494h

  View details for Web of Science ID 000257642700037

  View details for PubMedID 18517189

  View details for PubMedCentralID PMC2614217

• Geometric and electronic structure studies of the binuclear nonheme ferrous active site of Toluene-4-monooxygenase: Parallels with methane monooxygenase and insight into the role of the effector proteins in O-2 activation JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Schwartz, J. K., Wei, P., Mitchell, K. H., Fox, B. G., Solomon, E. I. 2008; 130 (22): 7098-7109

  Abstract

  Multicomponent monooxygenases, which carry out a variety of highly specific hydroxylation reactions, are of great interest as potential biocatalysts in a number of applications. These proteins share many similarities in structure and show a marked increase in O2 reactivity upon addition of an effector component. In this study, circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature, variable-field (VTVH) MCD have been used to gain spectroscopic insight into the Fe(II)Fe(II) active site in the hydroxylase component of Toluene-4 monoxygenase (T4moH) and the complex of T4moH bound by its effector protein, T4moD. These results have been correlated to spectroscopic data and density functional theory (DFT) calculations on MmoH and its interaction with MmoB. Together, these data provide further insight into the geometric and electronic structure of these biferrous active sites and, in particular, the perturbation associated with component B/D binding. It is found that binding of the effector protein changes the geometry of one iron center and orientation of its redox active orbital to accommodate the binding of O2 in a bridged structure for efficient 2-electron transfer that can form a peroxo intermediate.

  View details for DOI 10.1021/ja800654d

  View details for Web of Science ID 000256301200046

  View details for PubMedID 18479085

• Reaction of a copper-dioxygen complex with nitrogen Monoxide(center dot NO) leads to a copper(II) - Peroxynitrite species JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Maiti, D., Lee, D., Sarjeant, A. A., Pau, M. Y., Solomon, E. I., Gaoutchenova, K., Sundermeyer, J., Karlin, K. D. 2008; 130 (21): 6700-?

  Abstract

  A discrete peroxynitrite-copper(II) complex, [(TMG3tren)CuII(-OONO)]+ (3), has been generated in solution (ESI-MS, m/z = 565.15; tetragonal EPR) by reacting *NO(g) with superoxo complex [(TMG3tren)CuII(O2*-)]+ (2). Complex 3 undergoes a thermal transformation to give CuII-nitrite complex [(TMG3tren)CuII(-ONO)]+ (4) (X-ray) along with ca. 0.5 molar equiv dioxygen. A DFT calculation derived structure with cyclic bidentate k2-O,O'-OONO bound peroxynitrite moiety and dx2-y2 ground state is proposed. Experiments using 18O2 suggest that the adjacent peroxo oxygen atoms in 3 are derived from molecular oxygen. Further, 18O2 containing 3 undergoes O-O bond cleavage to form singly 18-O-labeled 4. The results suggest the viability of biological CuI/O2/(*NO) peroxynitrite formation and chemistry, that is, not coming from free superoxide plus *NO reaction.

  View details for DOI 10.1021/ja801540e

  View details for Web of Science ID 000256158200024

  View details for PubMedID 18457392

• Copper dioxygen adducts: Formation of bis(mu-oxo)dicopper(III) versus (mu-1,2)peroxodicopper(II) complexes with small changes in one pyridyl-ligand substituent INORGANIC CHEMISTRY Maiti, D., Woertink, J. S., Sarjeant, A. A., Solomon, E. I., Karlin, K. D. 2008; 47 (9): 3787-3800

  Abstract

  The preference for the formation of a particular Cu 2O 2 isomer coming from (ligand)-Cu (I)/O 2 reactivity can be regulated with the steric demands of a TMPA (tris(2-pyridylmethyl)amine) derived ligand possessing 6-pyridyl substituents on one of the three donor groups of the tripodal tetradentate ligand. When this substituent is an -XHR group (X = N or C) the traditional Cu (I)/O 2 adduct forms a (mu-1,2)peroxodicopper(II) species ( A). However, when the substituent is the slightly bulkier XR 2 moiety {aryl or NR 2 (R not equal H)}, a bis(mu-oxo)dicopper(III) structure ( C) is favored. The reactivity of one of the bis(mu-oxo)dicopper(III) species, [{(6tbp)Cu (III)} 2(O (2-)) 2] (2+) ( 7-O 2 ) (6tbp = (6- (t)Bu-phenyl-2-pyridylmethyl)bis(2-pyridylmethyl)amine), was probed, and for the first time, exogenous toluene or ethylbenzene hydrocarbon oxygenation reactions were observed. Typical monooxygenase chemistry occurred: the benzaldehyde product includes an 18-O atom for toluene/ 7- (1) (8)O 2 reactivity, and a H-atom abstraction by 7-O 2 is apparent from study of its reactions with ArOH substrates, as well as the determination of k H/ k D approximately 7 in the toluene oxygenation (i.e., PhCH 3 vs PhCD 3 substrates). Proposed courses of reaction are presented, including the possible involvement of PhCH 2OO (*) and its subsequent reaction with copper(I) complex, the latter derived from dynamic solution behavior of 7-O 2 . External TMPA ligand exchange for copper in 7-O 2 and O-O bond (re)formation chemistry, along with the ability to protonate 7-O 2 and release of H 2O 2 indicate the presence of an equilibrium between [{(6tbp)Cu (III)} 2(O (2-)) 2] (2+) ( 7-O 2 ) and a (mu-1,2)peroxodicopper(II) form.

  View details for DOI 10.1021/ic.702437c

  View details for Web of Science ID 000255380500044

  View details for PubMedID 18396862

• Perturbations to the geometric and electronic structure of the CUA site: Factors that influence delocalization and their contributions to electron transfer JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Xie, X., Gorelsky, S. I., Sarangi, R., Garner, D. K., Hwang, H. J., Hodgsont, K. O., Hedman, B., Lu, Y., Solornon, E. I. 2008; 130 (15): 5194-5205

  Abstract

  Using a combination of electronic spectroscopies and DFT calculations, the effect of pH perturbation on the geometric and electronic structure of the CuA site has been defined. Descriptions are developed for high pH (pH = 7) and low pH (pH = 4) forms of CuA azurin and its H120A mutant which address the discrepancies concerning the extent of delocalization indicated by multifrequency EPR and ENDOR data (J. Am. Chem. Soc. 2005, 127, 7274; Biophys. J. 2002, 82, 2758). Our resonance Raman and MCD spectra demonstrate that the low pH and H120A mutant forms are essentially identical and are the perturbed forms of the completely delocalized high pH CuA site. However, in going from high pH to low pH, a seven-line hyperfine coupling pattern associated with complete delocalization of the electron (S = 1/2) over two Cu coppers (I(Cu) = 3/2) changes into a four-line pattern reflecting apparent localization. DFT calculations show that the unpaired electron is delocalized in the low pH form and reveal that its four-line hyperfine pattern results from the large EPR spectral effects of approximately 1% 4s orbital contribution of one Cu to the ground-state spin wave function upon protonative loss of its His ligand. The contribution of the Cu-Cu interaction to electron delocalization in this low symmetry protein site is evaluated, and the possible functional significance of the pH-dependent transition in regulating proton-coupled electron transfer in cytochrome c oxidase is discussed.

  View details for DOI 10.1021/ja7102668

  View details for Web of Science ID 000254933000044

  View details for PubMedID 18348522

• INOR 477-Nonheme iron/oxygen intermediates Solomon, E. I. AMER CHEMICAL SOC. 2008

  View details for Web of Science ID 000271775106152

• INOR 37-Catecholate - Fe(III) bonds Solomon, E. I. AMER CHEMICAL SOC. 2008

  View details for Web of Science ID 000271775106150

• Spectroscopic and density functional theory studies of the blue-copper site in M121SeM and C112SeC azurin: Cu-Se versus Cu-S bonding JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sarangi, R., Gorelsky, S. I., Basumallick, L., Hwang, H. J., Pratt, R. C., Stack, T. D., Lu, Y., Hodgson, K. O., Hedman, B., Solomon, E. I. 2008; 130 (12): 3866-3877

  Abstract

  S K-edge X-ray absorption, UV-vis absorption, magnetic circular dichroism (MCD), and resonance Raman spectroscopies are used to investigate the electronic structure differences among WT, M121SeM, and C112SeC Pseudomonas aeruginosa (P.a) azurin. A comparison of S K-edge XAS of WT and M121SeM azurin and a CuII-thioether model complex shows that the 38% S character in the ground state wave function of the blue-copper (BC) sites solely reflects the Cu-SCys bond. Resonance Raman (rR) data on WT and C112SeC azurin give direct evidence for the kinematic coupling between the Cu-SCys stretch and the cysteine deformation modes in WT azurin, which leads to multiple features in the rR spectrum of the BC site. The UV-vis absorption and MCD data on WT, M121SeM, and C112SeC give very similar C0/D0 ratios, indicating that the C-term MCD intensity mechanism involves Cu-centered spin-orbit coupling (SOC). The spectroscopic data combined with density functional theory (DFT) calculations indicate that SCys and SeCys have similar covalent interactions with Cu at their respective bond lengths of 2.1 and 2.3 A. This reflects the similar electronegativites of S and Se in the thiolate/selenolate ligand fragment and explains the strong spectroscopic similarities between WT and C112SeC azurin.

  View details for DOI 10.1021/ja076495a

  View details for Web of Science ID 000254173600045

  View details for PubMedID 18314977

  View details for PubMedCentralID PMC2713798

• Spectroscopic studies of perturbed T1 Cu sites in the multicopper oxidases Saccharomyces cerevisiae Fet3p and Rhus vernicifera laccase: Allosteric coupling between the T1 and trinuclear Cu sites BIOCHEMISTRY Augustine, A. J., Kragh, M. E., Sarangi, R., Fujii, S., Liboiron, B. D., Stoj, C. S., Kosman, D. J., Hodgson, K. O., Hedman, B., Solomon, E. I. 2008; 47 (7): 2036-2045

  Abstract

  The multicopper oxidases catalyze the 4e- reduction of O2 to H2O coupled to the 1e- oxidation of 4 equiv of substrate. This activity requires four Cu atoms, including T1, T2, and coupled binuclear T3 sites. The T2 and T3 sites form a trinuclear cluster (TNC) where O2 is reduced. The T1 is coupled to the TNC through a T1-Cys-His-T3 electron transfer (ET) pathway. In this study the two T3 Cu coordinating His residues which lie in this pathway in Fet3 have been mutated, H483Q, H483C, H485Q, and H485C, to study how perturbation at the TNC impacts the T1 Cu site. Spectroscopic methods, in particular resonance Raman (rR), show that the change from His to Gln to Cys increases the covalency of the T1 Cu-S Cys bond and decreases its redox potential. This study of T1-TNC interactions is then extended to Rhus vernicifera laccase where a number of well-defined species including the catalytically relevant native intermediate (NI) can be trapped for spectroscopic study. The T1 Cu-S covalency and potential do not change in these species relative to resting oxidized enzyme, but interestingly the differences in the structure of the TNC in these species do lead to changes in the T1 Cu rR spectrum. This helps to confirm that vibrations in the cysteine side chain of the T1 Cu site and the protein backbone couple to the Cu-S vibration. These changes in the side chain and backbone provide a possible mechanism for regulating intramolecular T1 to TNC ET in NI and partially reduced enzyme forms for efficient turnover.

  View details for DOI 10.1021/bi7020052

  View details for Web of Science ID 000253102000021

  View details for PubMedID 18197705

• Near-IR MCD of the nonheme ferrous active site in naphthalene 1,2-dioxygenase: Correlation to crystallography and structural insight into the mechanism of Rieske dioxygenases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ohta, T., Chakrabarty, S., Lipscomb, J. D., Solomon, E. I. 2008; 130 (5): 1601-1610

  Abstract

  Near-IR MCD and variable temperature, variable field (VTVH) MCD have been applied to naphthalene 1,2-dioxygenase (NDO) to describe the coordination geometry and electronic structure of the mononuclear nonheme ferrous catalytic site in the resting and substrate-bound forms with the Rieske 2Fe2S cluster oxidized and reduced. The structural results are correlated with the crystallographic studies of NDO and other related Rieske nonheme iron oxygenases to develop molecular level insights into the structure/function correlation for this class of enzymes. The MCD data for resting NDO with the Rieske center oxidized indicate the presence of a six-coordinate high-spin ferrous site with a weak axial ligand which becomes more tightly coordinated when the Rieske center is reduced. Binding of naphthalene to resting NDO (Rieske oxidized and reduced) converts the six-coordinate sites into five-coordinate (5c) sites with elimination of a water ligand. In the Rieske oxidized form the 5c sites are square pyramidal but transform to a 1:2 mixture of trigonal bipyramial/square pyramidal sites when the Rieske center is reduced. Thus the geometric and electronic structure of the catalytic site in the presence of substrate can be significantly affected by the redox state of the Rieske center. The catalytic ferrous site is primed for the O2 reaction when substrate is bound in the active site in the presence of the reduced Rieske site. These structural changes ensure that two electrons and the substrate are present before the binding and activation of O2, which avoids the uncontrolled formation and release of reactive oxygen species.

  View details for DOI 10.1021/ja074769o

  View details for Web of Science ID 000253100100031

  View details for PubMedID 18189388

  View details for PubMedCentralID PMC2886598

• X-ray absorption spectroscopic and theoretical studies on (L)(2)[Cu-2(S-2)n](2+) complexes: Disulfide versus disulfide(center dot 1-) bonding JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sarangi, R., York, J. T., Helton, M. E., Fujisawa, K., Karlin, K. D., Tolman, W. B., Hodgson, K. O., Hedman, B., Solomon, E. I. 2008; 130 (2): 676-686

  Abstract

  Cu K-, Cu L-, and S K-edge X-ray absorption spectroscopic (XAS) data have been combined with density functional theory (DFT) calculations on [{(TMPA)Cu}2S2](ClO4)2 (1), [{Cu[HB(3,5-Pr(i)2pz)3]}2(S2)] (2), and [{(TMEDA)Cu}2(S2)2](OTf)2 (3) to obtain a quantitative description of their ground state wavefunctions. The Cu L-edge intensities give 63 and 37% Cu d-character in the ground state of 1 and 2, respectively, whereas the S K-pre-edge intensities reflect 20 and 48% S character in their ground states, respectively. These data indicate a more than 2-fold increase in the total disulfide bonding character in 2 relative to 1. The increase in the number of Cu-S bonds in 2 (mu-eta(2):eta(2) S2(2-) bridge) compared to 1 ((mu-eta(1):eta(1) S2(2-) bridge) dominantly determines the large increase in covalency and Cu-disulfide bond strength in 2. Cu K- and L- and S K-pre-edge energy positions directly demonstrate the Cu(II)/(S2(-))2 nature of 3. The two disulfide(*1-)'s in 3 undergo strong bonding interactions that destabilize the resultant filled antibonding pi* orbitals of the (S2(-))2 fragment relative to the Cu 3d levels. This leads to an inverted bonding scheme in 3 with dominantly ligand-based holes in its ground state, consistent with its description as a dicopper(II)-bis-disulfide(*1-) complex.

  View details for DOI 10.1021/ja0762745

  View details for Web of Science ID 000252292500063

  View details for PubMedID 18076173

  View details for PubMedCentralID PMC2570853

• A Combined NRVS and DFT Study of Fe-IV=O Model Complexes: A Diagnostic Method for the Elucidation of Non-Heme Iron Enzyme Intermediates ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Bell, C. B., Wong, S. D., Xiao, Y., Klinker, E. J., Tenderholt, A. L., Smith, M. C., Rohde, J., Que, L., Cramer, S. P., Solomon, E. I. 2008; 47 (47): 9071-9074

  View details for DOI 10.1002/anie.200803740

  View details for Web of Science ID 000261038700013

  View details for PubMedID 18925598

  View details for PubMedCentralID PMC2662738

• Extended charge decomposition analysis and its application for the investigation of electronic relaxation THEORETICAL CHEMISTRY ACCOUNTS Gorelsky, S. I., Solomon, E. I. 2008; 119 (1-3): 57-67

  View details for DOI 10.1007/s00214-007-0270-1

  View details for Web of Science ID 000251871900007

• O-2 Reduction to H2O by the multicopper oxidases DALTON TRANSACTIONS Solomon, E. I., Augustine, A. J., Yoon, J. 2008: 3921-3932

  Abstract

  In nature the four electron reduction of O2 to H2O is carried out by Cytochrome c oxidase (CcO) and the multicopper oxidases (MCOs). In the former, Cytochrome c provides electrons for pumping protons to produce a gradient for ATP synthesis, while in the MCOs the function is the oxidation of substrates, either organic or metal ions. In the MCOs the reduction of O2 is carried out at a trinuclear Cu cluster (TNC). Oxygen intermediates have been trapped which exhibit unique spectroscopic features that reflect novel geometric and electronic structures. These intermediates have both intact and cleaved O-O bonds, allowing the reductive cleavage of the O-O bond to be studied in detail both experimentally and computationally. These studies show that the topology of the TNC provides a unique geometric and electronic structure particularly suited to carry out this key reaction in nature.

  View details for DOI 10.1039/b800799c

  View details for Web of Science ID 000257875200001

  View details for PubMedID 18648693

  View details for PubMedCentralID PMC2854021

• Mixed valent sites in biological electron transfer CHEMICAL SOCIETY REVIEWS Solomon, E. I., Xie, X., Dey, A. 2008; 37 (4): 623-638

  Abstract

  Many of the active sites involved in electron transfer (ET) in biology have more than one metal and are mixed valent in at least one redox state. These include Cu(A), and the polynuclear Fe-S clusters which vary in their extent of delocalization. In this tutorial review the relative contributions to delocalization are evaluated using S K-edge X-ray absorption, magnetic circular dichroism and other spectroscopic methods. The role of intra-site delocalization in ET is considered.

  View details for DOI 10.1039/b714577m

  View details for Web of Science ID 000254315700001

  View details for PubMedID 18362972

• Spectroscopic and quantum chemical studies on low-spin Fe-IV=O complexes: Fe-O bonding and its contributions to reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Decker, A., Rohde, J., Klinker, E. J., Wong, S. D., Que, L., Solomon, E. I. 2007; 129 (51): 15983-15996

  Abstract

  High-valent FeIV=O species are key intermediates in the catalytic cycles of many mononuclear non-heme iron enzymes and have been structurally defined in model systems. Variable-temperature magnetic circular dichroism (VT-MCD) spectroscopy has been used to evaluate the electronic structures and in particular the Fe-O bonds of three FeIV=O (S = 1) model complexes, [FeIV(O)(TMC)(NCMe)]2+, [FeIV(O)(TMC)(OC(O)CF3)]+, and [FeIV(O)(N4Py)]2+. These complexes are characterized by their strong and covalent Fe-O pi-bonds. The MCD spectra show a vibronic progression in the nonbonding --> pi* excited state, providing the Fe-O stretching frequency and the Fe-O bond length in this excited state and quantifying the pi-contribution to the total Fe-O bond. Correlation of these experimental data to reactivity shows that the [FeIV(O)(N4Py)]2+ complex, with the highest reactivity toward hydrogen-atom abstraction among the three, has the strongest Fe-O pi-bond. Density functional calculations were correlated to the data and support the experimental analysis. The strength and covalency of the Fe-O pi-bond result in high oxygen character in the important frontier molecular orbitals (FMOs) for this reaction, the unoccupied beta-spin d(xz/yz) orbitals, that activates these for electrophilic attack. An extension to biologically relevant FeIV=O (S = 2) enzyme intermediates shows that these can perform electrophilic attack reactions along the same mechanistic pathway (pi-FMO pathway) with similar reactivity but also have an additional reaction channel involving the unoccupied alpha-spin d(z2) orbital (sigma-FMO pathway). These studies experimentally probe the FMOs involved in the reactivity of FeIV=O (S = 1) model complexes resulting in a detailed understanding of the Fe-O bond and its contributions to reactivity.

  View details for DOI 10.1021/ja074900s

  View details for Web of Science ID 000251974000049

  View details for PubMedID 18052249

  View details for PubMedCentralID PMC2547486

• Solvent tuning of electrochemical potentials in the active sites of HiPIP versus ferredoxin SCIENCE Dey, A., Jenney, F. E., Adams, M. W., Babini, E., Takahashi, Y., Fukuyama, K., Hodgson, K. O., Hedman, B., Solomon, E. I. 2007; 318 (5855): 1464-1468

  Abstract

  A persistent puzzle in the field of biological electron transfer is the conserved iron-sulfur cluster motif in both high potential iron-sulfur protein (HiPIP) and ferredoxin (Fd) active sites. Despite this structural similarity, HiPIPs react oxidatively at physiological potentials, whereas Fds are reduced. Sulfur K-edge x-ray absorption spectroscopy uncovers the substantial influence of hydration on this variation in reactivity. Fe-S covalency is much lower in natively hydrated Fd active sites than in HiPIPs but increases upon water removal; similarly, HiPIP covalency decreases when unfolding exposes an otherwise hydrophobically shielded active site to water. Studies on model compounds and accompanying density functional theory calculations support a correlation of Fe-S covalency with ease of oxidation and therefore suggest that hydration accounts for most of the difference between Fd and HiPIP reduction potentials.

  View details for DOI 10.1126/science.1147753

  View details for Web of Science ID 000251246100050

  View details for PubMedID 18048692

• CD and MCD of CytC3 and taurine dioxygenase: Role of the facial triad in alpha-KG-dependent oxygenases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Neidig, M. L., Brown, C. D., Light, K. M., Fujimori, D. G., Nolan, E. M., Price, J. C., Barr, E. W., Bollinger, J. M., Krebs, C., Walsh, C. T., Solomon, E. I. 2007; 129 (46): 14224-14231

  Abstract

  The alpha-ketoglutarate (alpha-KG)-dependent oxygenases are a large and diverse class of mononuclear non-heme iron enzymes that require FeII, alpha-KG, and dioxygen for catalysis with the alpha-KG cosubstrate supplying the additional reducing equivalents for oxygen activation. While these systems exhibit a diverse array of reactivities (i.e., hydroxylation, desaturation, ring closure, etc.), they all share a common structural motif at the FeII active site, termed the 2-His-1-carboxylate facial triad. Recently, a new subclass of alpha-KG-dependent oxygenases has been identified that exhibits novel reactivity, the oxidative halogenation of unactivated carbon centers. These enzymes are also structurally unique in that they do not contain the standard facial triad, as a Cl- ligand is coordinated in place of the carboxylate. An FeII methodology involving CD, MCD, and VTVH MCD spectroscopies was applied to CytC3 to elucidate the active-site structural effects of this perturbation of the coordination sphere. A significant decrease in the affinity of FeII for apo-CytC3 was observed, supporting the necessity of the facial triad for iron coordination to form the resting site. In addition, interesting differences observed in the FeII/alpha-KG complex relative to the cognate complex in other alpha-KG-dependent oxygenases indicate the presence of a distorted 6C site with a weak water ligand. Combined with parallel studies of taurine dioxygenase and past studies of clavaminate synthase, these results define a role of the carboxylate ligand of the facial triad in stabilizing water coordination via a H-bonding interaction between the noncoordinating oxygen of the carboxylate and the coordinated water. These studies provide initial insight into the active-site features that favor chlorination by CytC3 over the hydroxylation reactions occurring in related enzymes.

  View details for DOI 10.1021/ja074557r

  View details for Web of Science ID 000251182000047

  View details for PubMedID 17967013

  View details for PubMedCentralID PMC2525739

• Substrate activation for O-2 reactions by oxidized metal centers in biology PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Pau, M. Y., Lipscomb, J. D., Solomon, E. 2007; 104 (47): 18355-18362

  Abstract

  The uncatalyzed reactions of O(2) (S = 1) with organic substrates (S = 0) are thermodynamically favorable but kinetically slow because they are spin-forbidden and the one-electron reduction potential of O(2) is unfavorable. In nature, many of these important O(2) reactions are catalyzed by metalloenzymes. In the case of mononuclear non-heme iron enzymes, either Fe(II) or Fe(III) can play the catalytic role in these spin-forbidden reactions. Whereas the ferrous enzymes activate O(2) directly for reaction, the ferric enzymes activate the substrate for O(2) attack. The enzyme-substrate complex of the ferric intradiol dioxygenases exhibits a low-energy catecholate to Fe(III) charge transfer transition that provides a mechanism by which both the Fe center and the catecholic substrate are activated for the reaction with O(2). In this Perspective, we evaluate how the coupling between this experimentally observed charge transfer and the change in geometry and ligand field of the oxidized metal center along the reaction coordinate can overcome the spin-forbidden nature of the O(2) reaction.

  View details for DOI 10.1073/pnas.0704191104

  View details for Web of Science ID 000251292500005

  View details for PubMedID 18003930

  View details for PubMedCentralID PMC2141783

• SK-Edge XAS and DFT calculations on square-planar NiII-thiolate complexes: Effects of active and passive H-bonding INORGANIC CHEMISTRY Dey, A., Green, K. N., Jenkins, R. M., Jeffrey, S. P., Darensbourg, M., Hodgson, K. O., Hedman, B., Solomon, E. I. 2007; 46 (23): 9655-9660

  Abstract

  S K-edge XAS for a low-spin NiII-thiolate complex shows a 0.2 eV shift to higher pre-edge energy but no change in Ni-S bond covalency upon H-bonding. This is different from the H-bonding effect we observed in high-spin FeIII-thiolate complexes where there is a significant decrease in Fe-S bond covalency but no change in energy due to H-bonding (Dey, A.; Okamura, T.-A.; Ueyama, N.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 2005, 127, 12046-12053). These differences were analyzed using DFT calculations, and the results indicate that two different types of H-bonding interactions are possible in metal-thiolate systems. In the high-spin FeIII-thiolate case, the H-bonding involves a thiolate donor orbital which is also involved in bonding with the metal (active), while in the low-spin NiII-thiolate, the orbital involved in H-bonding is nonbonding with respect to the M-S bonding (passive). The contributions of active and passive H-bonds to the reduction potential and Lewis acid properties of a metal center are evaluated.

  View details for DOI 10.1021/ic7006292

  View details for Web of Science ID 000250732000024

  View details for PubMedID 17949080

  View details for PubMedCentralID PMC2536514

• Sulfur K-edge XAS of W-V=O vs. Mo-V=O bis(dithiolene) complexes: Contributions of relativistic effects to electronic structure and reactivity of tungsten enzymes JOURNAL OF INORGANIC BIOCHEMISTRY Tenderholt, A. L., Szilagyi, R. K., Holm, R. H., Hodgson, K., Hedman, B., Solomon, E. 2007; 101 (11-12): 1594-1600

  Abstract

  Molybdenum- or tungsten-containing enzymes catalyze oxygen atom transfer reactions involved in carbon, sulfur, or nitrogen metabolism. It has been observed that reduction potentials and oxygen atom transfer rates are different for W relative to Mo enzymes and the isostructural Mo/W complexes. Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations on [Mo(V)O(bdt)(2)](-) and [W(V)O(bdt)(2)](-), where bdt=benzene-1,2-dithiolate(2-), have been used to determine that the energies of the half-filled redox-active orbital, and thus the reduction potentials and MO bond strengths, are different for these complexes due to relativistic effects in the W sites.

  View details for DOI 10.1016/j.jinorgbio.2007.07.011

  View details for Web of Science ID 000251523100008

  View details for PubMedID 17720249

  View details for PubMedCentralID PMC2940715

• Polarized X-ray absorption spectroscopy of single-crystal Mn(V) complexes relevant to the oxygen-evolving complex of photosystem II JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yano, J., Robblee, J., Pushkar, Y., Marcus, M. A., Bendix, J., Workman, J. M., Collins, T. J., Solomon, E. I., George, S. D., Yachandra, V. K. 2007; 129 (43): 12989-13000

  Abstract

  High-valent Mn-oxo species have been suggested to have a catalytically important role in the water splitting reaction which occurs in the Photosystem II membrane protein. In this study, five- and six-coordinate mononuclear Mn(V) compounds were investigated by polarized X-ray absorption spectroscopy in order to understand the electronic structure and spectroscopic characteristics of high-valent Mn species. Single crystals of the Mn(V)-nitrido and Mn(V)-oxo compounds were aligned along selected molecular vectors with respect to the X-ray polarization vector using X-ray diffraction. The local electronic structure of the metal site was then studied by measuring the polarization dependence of X-ray absorption near-edge spectroscopy (XANES) pre-edge spectra (1s to 3d transition) and comparing with the results of density functional theory (DFT) calculations. The Mn(V)-nitrido compound, in which the manganese is coordinated in a tetragonally distorted octahedral environment, showed a single dominant pre-edge peak along the MnN axis that can be assigned to a strong 3d(z(2))-4p(z) mixing mechanism. In the square pyramidal Mn(V)-oxo system, on the other hand, an additional peak was observed at 1 eV below the main pre-edge peak. This component was interpreted as a 1s to 3d(xz,yz) transition with 4px,y mixing, due to the displacement of the Mn atom out of the equatorial plane. The XANES results have been correlated to DFT calculations, and the spectra have been simulated using a TD (time-dependent)-DFT approach. The relevance of these results to understanding the mechanism of the photosynthetic water oxidation is discussed.

  View details for DOI 10.1021/ja071286b

  View details for Web of Science ID 000250818900032

  View details for PubMedID 17918832

• Spectroscopic and kinetic studies of perturbed trinuclear copper clusters: The role of protons in reductive cleavage of the O-O bond in the multicopper oxidase Fet3p JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Augustine, A. J., Quintanar, L., Stoj, C. S., Kosman, D. J., Solomon, E. I. 2007; 129 (43): 13118-13126

  Abstract

  The multicopper oxidase Fet3p couples four 1e(-) oxidations of substrate to the 4e(-) reduction of O2 to H2O. Fet3p uses four Cu atoms to accomplish this reaction: the type 1, type 2, and coupled binuclear type 3 sites. The type 2 and type 3 sites together form a trinuclear Cu cluster (TNC) which is the site of O2 reduction. This study focuses on mutants of two residues, E487 and D94, which lie in the second coordination sphere of the TNC and defines the role that each plays in the structural integrity of the TNC, its reactivity with O2, and in the directional movement of protons during reductive cleavage of the O-O bond. The E487D, E487A, and D94E mutants have been studied in the holo and type 1 depleted (T1D) forms. Residue E487, located near the T3 center, is found to be responsible for donation of a proton during the reductive cleavage of the O-O bond in the peroxide intermediate and an inverse kinetic solvent isotope effect, which indicates that this proton is already transferred when the O-O bond is cleaved. Residue D94, near the T2 site, plays a key role in the reaction of the reduced TNC with O2 and drives electron transfer from the T2 Cu to cleave the O-O bond by deprotonating the T2 Cu water ligand. A mechanism is developed where these second sphere residues participate in the proton assisted reductive cleavage of the O-O bond at the TNC.

  View details for DOI 10.1021/ja073905m

  View details for Web of Science ID 000250818900046

  View details for PubMedID 17918838

  View details for PubMedCentralID PMC2556285

• Electronic structure of the peroxy intermediate and its correlation to the native intermediate in the multicopper oxidases: Insights into the reductive cleavage of the O-O bond JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yoon, J., Solomon, E. I. 2007; 129 (43): 13127-13136

  Abstract

  The multicopper oxidases (MCOs) utilize a blue type 1 (T1) copper site and a trinuclear Cu cluster composed of a type 2 (T2) and a binuclear type 3 (T3) site that together catalyze the four-electron reduction of O2 to H2O. Reaction of the fully reduced enzyme with O2 proceeds via two sequential two-electron steps generating the peroxy intermediate (PI) and the native intermediate (NI). While a detailed description of the geometric and electronic structure of NI has been developed, this has been more elusive for PI largely due to the diamagnetic nature of its ground state. Density functional theory (DFT) calculations have been used to correlate to spectroscopic data to generate a description of the geometric and electronic structure of PI. A highly conserved carboxylate residue near the T2 site is found to play a critical role in stabilizing the PI structure, which induces oxidation of the T2 and one T3 Cu center and strong superexchange stabilization via the peroxide bridge, allowing irreversible binding of O2 at the trinuclear Cu site. Correlation of PI to NI is achieved using a two-dimensional potential energy surface generated to describe the catalytic two-electron reduction of the peroxide O-O bond by the MCOs. It is found that the reaction is thermodynamically driven by the relative stability of NI and the involvement of the simultaneous two-electron-transfer process. A low activation barrier (calculated approximately 5-6 kcal/mol and experimental approximately 3-5 kcal/mol) is produced by the triangular topology of the trinuclear Cu cluster site, as this symmetry provides good donor-acceptor frontier molecular orbital (FMO) overlap. Finally, the O-O bond cleavage in the trinuclear Cu cluster can be achieved via either a proton-assisted or a proton-unassisted process, allowing the MCOs to function over a wide range of pH. It is found that while the proton helps to stabilize the acceptor O22- sigma* orbital in the proton-assisted process for better donor-acceptor FMO overlap, the third oxidized Cu center in the trinuclear site assumes the role as a Lewis acid in the proton-unassisted process for similarly efficient O-O bond cleavage.

  View details for DOI 10.1021/ja073947a

  View details for Web of Science ID 000250818900047

  View details for PubMedID 17918839

  View details for PubMedCentralID PMC2532529

• Sulfur K-edge X-ray absorption Spectroscopy and density functional theory calculations on superoxide reductase: Role of the axial thiolate in reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Jenney, F. E., Adams, M. W., Johnson, M. K., Hodgson, K. O., Hedman, B., Solomon, E. I. 2007; 129 (41): 12418-12431

  Abstract

  Superoxide reductase (SOR) is a non-heme iron enzyme that reduces superoxide to peroxide at a diffusion-controlled rate. Sulfur K-edge X-ray absorption spectroscopy (XAS) is used to investigate the ground-state electronic structure of the resting high-spin and CN- bound low-spin FeIII forms of the 1Fe SOR from Pyrococcus furiosus. A computational model with constrained imidazole rings (necessary for reproducing spin states), H-bonding interaction to the thiolate (necessary for reproducing Fe-S bond covalency of the high-spin and low-spin forms), and H-bonding to the exchangeable axial ligand (necessary to reproduce the ground state of the low-spin form) was developed and then used to investigate the enzymatic reaction mechanism. Reaction of the resting ferrous site with superoxide and protonation leading to a high-spin FeIII-OOH species and its subsequent protonation resulting in H2O2 release is calculated to be the most energetically favorable reaction pathway. Our results suggest that the thiolate acts as a covalent anionic ligand. Replacing the thiolate with a neutral noncovalent ligand makes protonation very endothermic and greatly raises the reduction potential. The covalent nature of the thiolate weakens the FeIII bond to the proximal oxygen of this hydroperoxo species, which raises its pKa by an additional 5 log units relative to the pKa of a primarily anionic ligand, facilitating its protonation. A comparison with cytochrome P450 indicates that the stronger equatorial ligand field from the porphyrin results in a low-spin FeIII-OOH species that would not be capable of efficient H2O2 release due to a spin-crossing barrier associated with formation of a high-spin 5C FeIII product. Additionally, the presence of the dianionic porphyrin pi ring in cytochrome P450 allows O-O heterolysis, forming an FeIV-oxo porphyrin radical species, which is calculated to be extremely unfavorable for the non-heme SOR ligand environment. Finally, the 5C FeIII site that results from the product release at the end of the O2- reduction cycle is calculated to be capable of reacting with a second O2-, resulting in superoxide dismutase (SOD) activity. However, in contrast to FeSOD, the 5C FeIII site of SOR, which is more positively charged, is calculated to have a high affinity for binding a sixth anionic ligand, which would inhibit its SOD activity.

  View details for DOI 10.1021/ja064167p

  View details for Web of Science ID 000250105500039

  View details for PubMedID 17887751

  View details for PubMedCentralID PMC2533108

• Resolution of the spectroscopy versus crystallography issue for NO intermediates of nitrite reductase from Rhodobacter sphaeroides JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ghosh, S., Dey, A., Usov, O. M., Sun, Y., Grigoryants, V. M., Scholes, C. P., Solomon, E. I. 2007; 129 (34): 10310-?

  View details for DOI 10.1021/ja072841c

  View details for Web of Science ID 000249035200006

  View details for PubMedID 17685522

  View details for PubMedCentralID PMC2532526

• The two oxidized forms of the trinuclear Cu cluster in the multicopper oxidases and mechanism for the decay of the native intermediate PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Yoon, J., Liboiron, B. D., Sarangi, R., Hodgson, K. O., Hedman, B., Solomona, E. I. 2007; 104 (34): 13609-13614

  Abstract

  Multicopper oxidases (MCOs) catalyze the 4e(-) reduction of O(2) to H(2)O. The reaction of the fully reduced enzyme with O(2) generates the native intermediate (NI), which undergoes a slow decay to the resting enzyme in the absence of substrate. NI is a fully oxidized form, but its spectral features are very different from those of the resting form (also fully oxidized), because the type 2 and the coupled-binuclear type 3 Cu centers in the O(2)-reducing trinuclear Cu cluster site are isolated in the resting enzyme, whereas these are all bridged by a micro(3)-oxo ligand in NI. Notably, the one azide-bound NI (NI(Az)) exhibits spectral features very similar to those of NI, in which the micro(3)-oxo ligand in NI has been replaced by a micro(3)-bridged azide. Comparison of the spectral features of NI and NI(Az), combined with density functional theory (DFT) calculations, allows refinement of the NI structure. The decay of NI to the resting enzyme proceeds via successive proton-assisted steps, whereas the rate-limiting step involves structural rearrangement of the micro(3)-oxo-bridge from inside to outside the cluster. This phenomenon is consistent with the slow rate of NI decay that uncouples the resting enzyme from the catalytic cycle, leaving NI as the catalytically relevant fully oxidized form of the MCO active site. The all-bridged structure of NI would facilitate electron transfer to all three Cu centers of the trinuclear cluster for rapid proton-coupled reduction of NI to the fully reduced form for catalytic turnover.

  View details for Web of Science ID 000249064700017

  View details for PubMedID 17702865

• INOR 22-Differential studies of the kinetics, mechanisms, and active site structures for truncated and full-length phenylalanine hydroxylases Anarat, G., Chow, M. S., Hertzler, S. M., Datta, S., Solomon, E. I., Caradonna, J. P. AMER CHEMICAL SOC. 2007

  View details for Web of Science ID 000207593909392

• COMP 112-Sulfur K-edge XAS and DFT studies of Fe-S bonds in models and protein active sites: Effects of H-bonds on covalency and redox properties Solomon, E. I., Dey, A. AMER CHEMICAL SOC. 2007

  View details for Web of Science ID 000207593906449

• INOR 522-Mixed valancy in bioinorganic electron transfer Solomon, E. I. AMER CHEMICAL SOC. 2007

  View details for Web of Science ID 000207593909195

• INOR 84-Sulfur K-edge XAS and DFT studies of Fe-S bonds in models and protein active sites: Effects of H-bonds on covalency and redox properties Dey, A., Solomon, E. I. AMER CHEMICAL SOC. 2007

  View details for Web of Science ID 000207593909711

• Spectroscopic and electronic structure studies of intermediate X in ribonucleotide reductase R2 and two variants: A description of the Fe-IV-Oxo bond in the Fe-III-O-Fe-IV dimer JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mitic, N., Clay, M. D., Saleh, L., Bollinger, J. M., Solomon, E. I. 2007; 129 (29): 9049-9065

  Abstract

  Spectroscopic and electronic structure studies of the class I Escherichia coli ribonucleotide reductase (RNR) intermediate X and three computationally derived model complexes are presented, compared, and evaluated to determine the electronic and geometric structure of the FeIII-FeIV active site of intermediate X. Rapid freeze-quench (RFQ) EPR, absorption, and MCD were used to trap intermediate X in R2 wild-type (WT) and two variants, W48A and Y122F/Y356F. RFQ-EPR spin quantitation was used to determine the relative contributions of intermediate X and radicals present, while RFQ-MCD was used to specifically probe the FeIII/FeIV active site, which displayed three FeIV d-d transitions between 16,700 and 22,600 cm(-1), two FeIV d-d spin-flip transitions between 23,500 and 24,300 cm(-1), and five oxo to FeIV and FeIII charge transfer (CT) transitions between 25,000 and 32,000 cm(-1). The FeIV d-d transitions were perturbed in the two variants, confirming that all three d-d transitions derive from the d-pi manifold. Furthermore, the FeIV d-pi splittings in the WT are too large to correlate with a bis-mu-oxo structure. The assignment of the FeIV d-d transitions in WT intermediate X best correlates with a bridged mu-oxo/mu-hydroxo [FeIII(mu-O)(mu-OH)FeIV] structure. The mu-oxo/mu-hydroxo core structure provides an important sigma/pi superexchange pathway, which is not present in the bis-mu-oxo structure, to promote facile electron transfer from Y122 to the remote FeIV through the bent oxo bridge, thereby generating the tyrosyl radical for catalysis.

  View details for DOI 10.1021/ja070909i

  View details for Web of Science ID 000248185500035

  View details for PubMedID 17602477

  View details for PubMedCentralID PMC2565590

• Copper(I) complex O-2-reactivity with a N3S thioether ligand: A copper-dioxygen adduct including sulfur ligation, ligand oxygenation, and comparisons with all nitrogen ligand analogues INORGANIC CHEMISTRY Lee, D., Hatcher, L. Q., Vance, M. A., Sarangi, R., Milligan, A. E., Sarjeant, A. A., Incarvito, C. D., Rheingold, A. L., Hodgson, K. O., Hedman, B., Solomon, E. I., Karlin, K. D. 2007; 46 (15): 6056-6068

  Abstract

  In order to contribute to an understanding of the effects of thioether sulfur ligation in copper-O(2) reactivity, the tetradentate ligands L(N3S) (2-ethylthio-N,N-bis(pyridin-2-yl)methylethanamine) and L(N3S')(2-ethylthio-N,N-bis(pyridin-2-yl)ethylethanamine) have been synthesized. Corresponding copper(I) complexes, [CuI(L(N3S))]ClO(4) (1-ClO(4)), [CuI(L(N3S))]B(C(6)F(5))(4) (1-B(C(6)F(5))(4)), and [CuI(L(N3S'))]ClO(4) (2), were generated, and their redox properties, CO binding, and O(2)-reactivity were compared to the situation with analogous compounds having all nitrogen donor ligands, [CuI(TMPA)(MeCN)](+) and [Cu(I)(PMAP)](+) (TMPA = tris(2-pyridylmethyl)amine; PMAP = bis[2-(2-pyridyl)ethyl]-(2-pyridyl)methylamine). X-ray structures of 1-B(C(6)F(5))(4), a dimer, and copper(II) complex [Cu(II)(L(N3S))(MeOH)](ClO(4))(2) (3) were obtained; the latter possesses axial thioether coordination. At low temperature in CH(2)Cl(2), acetone, or 2-methyltetrahydrofuran (MeTHF), 1 reacts with O(2) and generates an adduct formulated as an end-on peroxodicopper(II) complex [{Cu(II)(L(N3S))}(2)(mu-1,2-O(2)(2-))](2+) (4)){lambda(max) = 530 (epsilon approximately 9200 M(-1) cm(-1)) and 605 nm (epsilon approximately 11,800 M(-1) cm(-1))}; the number and relative intensity of LMCT UV-vis bands vary from those for [{Cu(II)(TMPA)}(2)(O(2)(2-))](2+) {lambda(max) = 524 nm (epsilon = 11,300 M(-1) cm(-1)) and 615 nm (epsilon = 5800 M(-1) cm(-1))} and are ascribed to electronic structure variation due to coordination geometry changes with the L(N3S) ligand. Resonance Raman spectroscopy confirms the end-on peroxo-formulation {nu(O-O) = 817 cm(-1) (16-18O(2) Delta = 46 cm(-1)) and nu(Cu-O) = 545 cm(-1) (16-18O(2) Delta = 26 cm(-1)); these values are lower in energy than those for [{Cu(II)(TMPA)}(2)(O(2)(2-))](2+) {nu(Cu-O) = 561 cm(-1) and nu(O-O) = 827 cm(-1)} and can be attributed to less electron density donation from the peroxide pi* orbitals to the Cu(II) ion. Complex 4 is the first copper-dioxygen adduct with thioether ligation; direct evidence comes from EXAFS spectroscopy {Cu K-edge; Cu-S = 2.4 Angstrom}. Following a [Cu(I)(L(N3S))](+)/O(2) reaction and warming, the L(N3S) thioether ligand is oxidized to the sulfoxide in a reaction modeling copper monooxygenase activity. By contrast, 2 is unreactive toward dioxygen probably due to its significantly increased Cu(II)/Cu(I) redox potential, an effect of ligand chelate ring size (in comparison to 1). Discussion of the relevance of the chemistry to copper enzyme O(2)-activation, and situations of biological stress involving methionine oxidation, is provided.

  View details for DOI 10.1021/ic700541k

  View details for Web of Science ID 000248011300034

  View details for PubMedID 17580938

• Copper(I)/S-8 reversible reactions leading to an end-on bound dicopper(II) disulfide complex: Nucleophilic reactivity and analogies to copper-dioxygen chemistry JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Maiti, D., Woertink, J. S., Vance, M. A., Milligan, A. E., Sarjeant, A. A., Solomon, E. I., Karlin, K. D. 2007; 129 (28): 8882-8892

  Abstract

  Elemental sulfur (S8) reacts reversibly with the copper(I) complex [(TMPA')CuI](+) (1), where TMPA' is a TMPA (tris(2-pyridylmethyl)amine) analogue with a 6-CH2OCH3 substituent on one pyridyl ligand arm, affording a spectroscopically pure end-on bound disulfido-dicopper(II) complex [{(TMPA')Cu(II)}2(mu-1,2-S2(2-))](2+) (2) {nu(S-S) = 492 cm(-1); nu(Cu-S)sym = 309 cm(-1)}; by contrast, [(TMPA)Cu(I)(CH3CN)](+) (3)/S8 chemistry produces an equilibrium mixture of at least three complexes. The reaction of excess PPh3 with 2 leads to formal "release" of zerovalent sulfur and reduction of copper ion to give the corresponding complex [(TMPA')Cu(I)(PPh3)](+) (11) along with S=PPh3 as products. Dioxygen displaces the disulfur moiety from 2 to produce the end-on Cu2O2 complex, [{(TMPA')Cu(II)}2(mu-1,2-O2(2-)](2+) (9). Addition of the tetradentate ligand TMPA to 2 generates the apparently more thermodynamically stable [{(TMPA)Cu(II)}2(mu-1,2-S2(2-))](2+) (4) and expected mixture of other species. Bubbling 2 with CO leads to the formation of the carbonyl adduct [(TMPA')CuI(CO)](+) (8). Carbonylation/sulfur-release/CO-removal cycles can be repeated several times. Sulfur atom transfer from 2 also occurs in a near quantitative manner when it is treated with 2,6-dimethylphenyl isocyanide (ArNC), leading to the corresponding isothiocyanate (ArNCS) and [(TMPA')Cu(I)(CNAr)](+) (12). Complex 2 readily reacts with PhCH2Br: [{(TMPA')Cu(II)}2(mu-1,2-S(2)(2-)](2+) (2) + 2 PhCH2Br --> [{(TMPA')Cu(II)(Br)}2](2+) (6) + PhCH2SSCH2Ph. The unprecedented substrate reactivity studies reveal that end-on bound mu-1,2-disulfide-dicopper(II) complex 2 provides a nucleophilic S2(2-) moiety, in striking contrast to the electrophilic behavior of a recently described side-on bound mu-eta(2):eta(2)-disulfido-dicopper(II) complex, [{(N3)Cu(II)}(2)(mu-eta(2):eta(2)-S2(2-))](2+) (5) with tridentate N3 ligand. The investigation thus reveals striking analogies of copper/sulfur and copper/dioxygen chemistries, with regard to structure type formation and specific substrate reactivity patterns.

  View details for DOI 10.1021/ja071968z

  View details for Web of Science ID 000247966200043

  View details for PubMedID 17592845

• O-2 and N2O activation by bi-, tri-, and tetranuclear Cu clusters in biology ACCOUNTS OF CHEMICAL RESEARCH Solomon, E. I., Sarangi, R., Woertink, J. S., Augustine, A. J., Yoon, J., Ghosh, S. 2007; 40 (7): 581-591

  Abstract

  Copper-cluster sites in biology exhibit unique spectroscopic features reflecting exchange coupling between oxidized Cu's and e (-) delocalization in mixed valent sites. These novel electronic structures play critical roles in O 2 binding and activation for electrophilic aromatic attack and H-atom abstraction, the 4e (-)/4H (+) reduction of O 2 to H 2O, and in the 2e (-)/2H (+) reduction of N 2O. These electronic structure/reactivity correlations are summarized below.

  View details for DOI 10.1021/ar600060t

  View details for Web of Science ID 000248074000014

  View details for PubMedID 17472331

  View details for PubMedCentralID PMC2532530

• Kinetic and spectroscopic studies of N694C lipoxygenase: A probe of the substrate activation mechanism of a nonheme ferric enzyme JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Neidig, M. L., Wecksler, A. T., Schenk, G., Holman, T. R., Solomon, E. I. 2007; 129 (24): 7531-7537

  Abstract

  Lipoxygenases (LOs) comprise a class of substrate activating mononuclear nonheme iron enzymes which catalyze the hydroperoxidation of unsaturated fatty acids. A commonly proposed mechanism for LO catalysis involves H-atom abstraction by an FeIII-OH- site, best described as a proton coupled electron transfer (PCET) process, followed by direct reaction of O2 with the resulting substrate radical to yield product. An alternative mechanism that has also been discussed involves the abstraction of a proton from the substrate by the FeIII-OH leading to a sigma-organoiron intermediate, where the subsequent sigma bond insertion of dioxygen into the C-Fe bond completes the reaction. H-atom abstraction is favored by a high E(o) of the FeII/FeIII couple and high pK(a) of water bound to the ferrous state, while an organoiron mechanism would be favored by a low E(o) (to keep the site oxidized) and a high pK(a) of water bound to the ferric state (to deprotonate the substrate). A first coordination sphere mutant of soybean LO (N694C) has been prepared and characterized by near-infrared circular dichroism (CD) and variable-temperature, variable-field (VTVH) magnetic circular dichroism (MCD) spectroscopies (FeII site), as well as UV/vis absorption, UV/vis CD, and electron paramagnetic resonance (EPR) spectroscopies (FeIII site). These studies suggest that N694C has a lowered E degrees of the FeII/FeIII couple and a raised pKa of water bound to the ferric site relative to wild type soybean lipoxygenase-1 (WT sLO-1) which would favor the organoiron mechanism. However, the observation in N694C of a significant deuterium isotope effect, anaerobic reduction of iron by substrate, and a substantial decrease in k(cat) (approximately 3000-fold) support H-atom abstraction as the relevant substrate-activation mechanism in sLO-1.

  View details for DOI 10.1021/ja068503d

  View details for Web of Science ID 000247240500022

  View details for PubMedID 17523638

  View details for PubMedCentralID PMC2896304

• VTVH-MCD and DFT studies of thiolate bonding to {FeNO}(7)/{FeO2}(8) complexes of isopenicillin N synthase: Substrate determination of oxidase versus oxygenase activity in nonheme Fe enzymes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Brown, C. D., Neidig, M. L., Neibergall, M. B., Lipscomb, J. D., Solomon, E. I. 2007; 129 (23): 7427-7438

  Abstract

  Isopenicillin N synthase (IPNS) is a unique mononuclear nonheme Fe enzyme that catalyzes the four-electron oxidative double ring closure of its substrate ACV. A combination of spectroscopic techniques including EPR, absorbance, circular dichroism (CD), magnetic CD, and variable-temperature, variable-field MCD (VTVH-MCD) were used to evaluate the geometric and electronic structure of the [FeNO]7 complex of IPNS coordinated with the ACV thiolate ligand. Density Function Theory (DFT) calculations correlated to the spectroscopic data were used to generate an experimentally calibrated bonding description of the Fe-IPNS-ACV-NO complex. New spectroscopic features introduced by the binding of the ACV thiolate at 13 100 and 19 800 cm-1 are assigned as the NO pi*(ip) --> Fe dx2-y2 and S pi--> Fe dx2-y2 charge transfer (CT) transitions, respectively. Configuration interaction mixes S CT character into the NO pi*(ip) --> Fe dx2-y2 CT transition, which is observed experimentally from the VTVH-MCD data from this transition. Calculations on the hypothetical {FeO2}8 complex of Fe-IPNS-ACV reveal that the configuration interaction present in the [FeNO]7 complex results in an unoccupied frontier molecular orbital (FMO) with correct orientation and distal O character for H-atom abstraction from the ACV substrate. The energetics of NO/O2 binding to Fe-IPNS-ACV were evaluated and demonstrate that charge donation from the ACV thiolate ligand renders the formation of the FeIII-superoxide complex energetically favorable, driving the reaction at the Fe center. This single center reaction allows IPNS to avoid the O2 bridged binding generally invoked in other nonheme Fe enzymes that leads to oxygen insertion (i.e., oxygenase function) and determines the oxidase activity of IPNS.

  View details for DOI 10.1021/ja071364v

  View details for Web of Science ID 000247072300053

  View details for PubMedID 17506560

  View details for PubMedCentralID PMC2536647

• Sulfur K-edge XAS and DFT studies on Ni-II complexes with oxidized thiolate ligands: Implications for the roles of oxidized thiolates in the active sites of Fe and Co nitrile hydratase INORGANIC CHEMISTRY Dey, A., Jeffrey, S. P., Darensbourg, M., Hodgson, K. O., Hedman, B., Solomon, E. I. 2007; 46 (12): 4989-4996

  Abstract

  S K-edge X-ray absorption spectroscopy data on a series of NiII complexes with thiolate (RS-) and oxidized thiolate (RSO2-) ligands are used to quantify Ni-S bond covalency and its change upon ligand oxidation. Analyses of these results using geometry-optimized density functional theory (DFT) calculations suggest that the Ni-S sigma bonds do not weaken on ligand oxidation. Molecular orbital analysis indicates that these oxidized thiolate ligands use filled high-lying S-O pi* orbitals for strong sigma donation. However, the RSO2- ligands are poor pi donors, as the orbital required for pi interaction is used in the S-O sigma-bond formation. The oxidation of the thiolate reduces the repulsion between electrons in the filled Ni t2 orbital and the thiolate out-of-plane pi-donor orbital leading to shorter Ni-S bond length relative to that of the thiolate donor. The insights obtained from these results are relevant to the active sites of Fe- and Co-type nitrile hydratases (Nhase) that also have oxidized thiolate ligands. DFT calculations on models of the active site indicate that whereas the oxidation of these thiolates has a major effect in the axial ligand-binding affinity of the Fe-type Nhase (where there is both sigma and pi donation from the S ligands), it has only a limited effect on the sixth-ligand-binding affinity of the Co-type Nhases (where there is only sigma donation). These oxidized residues may also play a role in substrate binding and proton shuttling at the active site.

  View details for DOI 10.1021/ic070244l

  View details for Web of Science ID 000246907800034

  View details for PubMedID 17500514

  View details for PubMedCentralID PMC2565589

• Further insights into the spectroscopic properties, electronic structure, and kinetics of formation of the heme-peroxo-copper complex [(F8TPP)Fe-III-(O-2(2-))-Cu-II(TMPA)](+) INORGANIC CHEMISTRY Ghiladi, R. A., Chufan, E. E., del Rio, D., Solomon, E. I., Krebs, C., Huynh, B. H., Huang, H., Moenne-Loccoz, P., Kaderli, S., Honecker, M., Zuberbuehler, A. D., Marzilli, L., Cotter, R. J., Karlin, K. D. 2007; 46 (10): 3889-3902

  Abstract

  In the further development and understanding of heme-copper O2-reduction chemistry inspired by the active-site chemistry in cytochrome c oxidase, we describe a dioxygen adduct, [(F8TPP)FeIII-(O22-)-CuII(TMPA)](ClO4) (3), formed by addition of O2 to a 1:1 mixture of the porphyrinate-iron(II) complex (F8TPP)FeII (1a) {F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate dianion} and the copper(I) complex [(TMPA)CuI(MeCN)](ClO4) (1b) {TMPA = tris(2-pyridylmethyl)amine}. Complex 3 forms in preference to heme-only or copper-only binuclear products, is remarkably stable {t1/2 (RT; MeCN) approximately 20 min; lambda max = 412 (Soret), 558 nm; EPR silent}, and is formulated as a peroxo complex on the basis of manometry {1a/1b/O2 = 1:1:1}, MALDI-TOF mass spectrometry {16O2, m/z 1239 [(3 + MeCN)+]; 18O2, m/z 1243}, and resonance Raman spectroscopy {nu(O-O) = 808 cm-1; Delta16O2/18O2 = 46 cm-1; Delta16O2/16/18O2 = 23 cm-1}. Consistent with a mu-eta2:eta1 bridging peroxide ligand, two metal-O stretching frequencies are observed {nu(Fe-O) = 533 cm-1, nu(Fe-O-Cu) = 511 cm-1}, and supporting normal coordinate analysis is presented. 2H and 19F NMR spectroscopies reveal that 3 is high-spin {also muB = 5.1 +/- 0.2, Evans method} with downfield-shifted pyrrole and upfield-shifted TMPA resonances, similar to the pattern observed for the structurally characterized mu-oxo complex [(F8TPP)FeIII-O-CuII(TMPA)]+ (4) (known S = 2 system, antiferromagnetically coupled high-spin FeIII and CuII). Mössbauer spectroscopy exhibits a sharp quadrupole doublet (zero field; delta = 0.57 mm/s, |DeltaEQ| = 1.14 mm/s) for 3, with isomer shift and magnetic field dependence data indicative of a peroxide ligand and S = 2 formulation. Both UV-visible-monitored stopped-flow kinetics and Mössbauer spectroscopic studies reveal the formation of heme-only superoxide complex (S)(F8TPP)FeIII-(O2-) (2a) (S = solvent molecule) prior to 3. Thermal decomposition of mu-peroxo complex 3 yields mu-oxo complex 4 with concomitant release of approximately 0.5 mol O2 per mol 3. Characterization of the reaction 1a/1b + O2 --> 2 --> 3 --> 4, presented here, advances our understanding and provides new insights to heme/Cu dioxygen-binding and reduction.

  View details for DOI 10.1021/ic061726k

  View details for Web of Science ID 000246209800017

  View details for PubMedID 17444630

• Intramolecular single-turnover reaction in a cytochrome c oxidase model bearing a Tyr244 mimic JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Collman, J. P., Decreau, R. A., Yan, Y., Yoon, J., Solomon, E. I. 2007; 129 (18): 5794-?

  View details for DOI 10.1021/ja0690969

  View details for Web of Science ID 000246180200005

  View details for PubMedID 17429972

  View details for PubMedCentralID PMC2512969

• Spectroscopic, computational, and kinetic studies of the mu(4)-sulfide-bridged tetranuclear Cu-Z cluster in N2O reductase: pH effect on the edge ligand and its contribution to reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ghosh, S., Gorelsky, S. I., George, S. D., Chan, J. M., Cabrito, I., Dooley, D. M., Moura, J. J., Moura, I., Solomon, E. I. 2007; 129 (13): 3955-3965

  Abstract

  A combination of spectroscopy and density functional theory (DFT) calculations has been used to evaluate the pH effect at the CuZ site in Pseudomonas nautica (Pn) nitrous oxide reductase (N2OR) and Achromobacter cycloclastes (Ac) N2OR and its relevance to catalysis. Absorption, magnetic circular dichroism, and electron paramagnetic resonance with sulfur K-edge X-ray absorption spectra of the enzymes at high and low pH show minor changes. However, resonance Raman (rR) spectroscopy of PnN2OR at high pH shows that the 415 cm-1 Cu-S vibration (observed at low pH) shifts to higher frequency, loses intensity, and obtains a 9 cm-1 18O shift, implying significant Cu-O character, demonstrating the presence of a OH- ligand at the CuICuIV edge. From DFT calculations, protonation of either the OH- to H2O or the mu4-S2- to mu4-SH- would produce large spectral changes which are not observed. Alternatively, DFT calculations including a lysine residue at an H-bonding distance from the CuICuIV edge ligand show that the position of the OH- ligand depends on the protonation state of the lysine. This would change the coupling of the Cu-(OH) stretch with the Cu-S stretch, as observed in the rR spectrum. Thus, the observed pH effect (pKa approximately 9.2) likely reflects protonation equilibrium of the lysine residue, which would both raise E degrees and provide a proton for lowering the barrier for the N-O cleavage and for reduction of the [Cu4S(im)7OH]2+ to the fully reduced 4CuI active form for turnover.

  View details for DOI 10.1021/ja066059e

  View details for PubMedID 17352474

• EPR as a measure of hydrogen bonding in low-spin heme-thiolate proteins Pazicni, S., Dey, A., Linck, R. C., Solomon, E. I., Burstyn, J. N. AMER CHEMICAL SOC. 2007: 515

  View details for Web of Science ID 000207722805503

• O2 and N2O activation by binuclear, trinuclear, and tetranuclear copper clusters Solomon, E. I. AMER CHEMICAL SOC. 2007: 138

  View details for Web of Science ID 000207722805128

• Role of inner and outer coordination sphere residues in the structure and oxygen reactivity of the trinuclear copper cluster in the multicopper oxidases Augustine, A. J., Stoj, C., Kosman, D., Solomon, E. I. AMER CHEMICAL SOC. 2007: 275

  View details for Web of Science ID 000207722805264

• The CuZ active site of nitrous oxide reductase: Geometric and electronic structure and role in N2O reduction Ghosh, S., Gorelsky, S. I., George, S., Cabrito, I., Chan, J., Dooley, D. M., Moura, I., Moura, J. G., Solomon, E. I. AMER CHEMICAL SOC. 2007: 583

  View details for Web of Science ID 000207722805571

• INOR 474-{FeNO}7 complexes of mononuclear non-heme iron enzymes: Exploring reaction pathways in alpha KG-dependent and related enzymes Brown, C. D., Neidig, M. L., Neibergall, M. B., Lipscomb, J. D., Solomon, E. I. AMER CHEMICAL SOC. 2007: 10

  View details for Web of Science ID 000207722806010

• INOR 451-Overcoming the spin forbidden nature in the O2 reaction of the intradiol dioxygenases Pau, M. M., Lipscomb, J. D., Solomon, E. I. AMER CHEMICAL SOC. 2007: 223

  View details for Web of Science ID 000207722806223

• INOR 466-The structural basis of the ferrous iron specificity of the yeast ferroxidase, Fet3p Kosman, D., Stoj, C. S., Quintanar, L., Solomon, E. I. AMER CHEMICAL SOC. 2007: 332

  View details for Web of Science ID 000207722806332

• Understanding how the thiolate sulfur contributes to the function of the non-heme iron enzyme superoxide reductase (SOR) Kovacs, J. A., Kitagawa, T., Nam, E., Dey, A., Lugo-Mas, P., Brines, L. M., Villar, G., Alokolaro, P., Solomon, E. I. AMER CHEMICAL SOC. 2007: 578

  View details for Web of Science ID 000207722805566

• Structure/function correlations of mononuclear non-heme ferrous enzymes: Spectroscopic and DFT studies of pterin-dependent hydroxylases Chow, M. S., Wilson, S., Anarat, G., Datta, S., Eser, B., Lee, A., Abu-Omar, M. M., Caradonna, J. P., Fitzpatrick, P. F., Solomon, E. I. AMER CHEMICAL SOC. 2007: 714

  View details for Web of Science ID 000207722805701

• Oxygen activation by the non-coupled binuclear copper enzymes Woertink, J. S., Solomon, E. I. AMER CHEMICAL SOC. 2007: 520

  View details for Web of Science ID 000207722805508

• Sulfur K-edge X-ray absorption spectroscopy as a probe of ligand-metal bond covalency: Metal vs ligand oxidation in copper and nickel dithiolene complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sarangi, R., George, S. D., Rudd, D. J., Szilagyi, R. K., Ribas, X., Rovira, C., Almeida, M., Hodgson, K. O., Hedman, B., Solomon, E. I. 2007; 129 (8): 2316-2326

  Abstract

  A combination of Cu L-edge and S K-edge X-ray absorption data and density functional theory (DFT) calculations has been correlated with 33S electron paramagnetic resonance superhyperfine results to obtain the dipole integral (Is) for the S 1s-->3p transition for the dithiolene ligand maleonitriledithiolate (MNT) in (TBA)2[Cu(MNT)2] (TBA= tetra-n-butylammonium). The results have been combined with the Is of sulfide derived from XPS studies to experimentally obtain a relation between the S 1s-->4p transition energy (which reflects the charge on the S atom, QSmol) and the dipole integral over a large range of QSmol. The results show that, for high charges on S, Is can vary from the previously reported Is values, calculated using data over a limited range of QSmol. A combination of S K-edge and Cu K- and L-edge X-ray absorption data and DFT calculations has been used to investigate the one-electron oxidation of [Cu(MNT)2]2- and [Ni(MNT)2]2-. The conversion of [Cu(MNT)2]2- to [Cu(MNT)2]- results in a large change in the charge on the Cu atom in the molecule (QCumol) and is consistent with a metal-based oxidation. This is accompanied by extensive charge donation from the ligands to compensate the high charge on the Cu in [Cu(MNT)2]- based on the increased S K-edge and decreased Cu L-edge intensity, respectively. In contrast, the oxidation of [Ni(MNT)2]2- to [Ni(MNT)2]- results in a small change in QNimol, indicating a ligand-based oxidation consistent with oxidation of a molecular orbital, psiSOMO (singly occupied molecular orbital), with predominant ligand character.

  View details for DOI 10.1021/ja0665949

  View details for Web of Science ID 000244330800033

  View details for PubMedID 17269767

  View details for PubMedCentralID PMC2880206

• Spectroscopic and electronic structure study of the enzyme-substrate complex of intradiol dioxygenases: Substrate activation by a high-spin ferric non-heme iron site JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pau, M. Y., Davis, M. I., Orville, A. M., Lipscomb, J. D., Solomon, E. I. 2007; 129 (7): 1944-1958

  Abstract

  Various mechanisms have been proposed for the initial O(2) attack in intradiol dioxygenases based on different electronic descriptions of the enzyme-substrate (ES) complex. We have examined the geometric and electronic structure of the high-spin ferric ES complex of protocatechuate 3,4-dioxygenase (3,4-PCD) with UV/visible absorption, circular dichroism (CD), magnetic CD (MCD), and variable-temperature variable-field (VTVH) MCD spectroscopies. The experimental data were coupled with DFT and INDO/S-CI calculations, and an experimentally calibrated bonding description was obtained. The broad absorption spectrum for the ES complex in the 6000-31000 cm(-1) region was resolved into at least five individual transitions, assigned as ligand-to-metal charge transfer (LMCT) from the protocatechuate (PCA) substrate and Tyr408. From our DFT calculations, all five LMCT transitions originate from the PCA and Tyr piop orbitals to the ferric dpi orbitals. The strong pi covalent donor interactions dominate the bonding in the ES complex. Using hypothetical Ga(3+)-catecholate/semiquinone complexes as references, 3,4-PCD-PCA was found to be best described as a highly covalent Fe(3+)-catecholate complex. The covalency is distributed unevenly among the four PCA valence orbitals, with the strongest interaction between the piop-sym and Fe dxz orbitals. This strong pi interaction, as reflected in the lowest energy PCA-to-Fe(3+) LMCT transition, is responsible for substrate activation for the O(2) reaction of intradiol dioxygenases. This involves a multi-electron-transfer (one beta and two alpha) mechanism, with Fe3+ acting as a buffer for the spin-forbidden two-electron redox process between PCA and O(2) in the formation of the peroxy-bridged ESO2 intermediate. The Fe ligand field overcomes the spin-forbidden nature of the triplet O(2) reaction, which potentially results in an intermediate spin state (S = 3/2) on the Fe(3+) center which is stabilized by a change in coordination along the reaction coordinate.

  View details for DOI 10.1021/ja065671x

  View details for Web of Science ID 000244206400042

  View details for PubMedID 17256852

  View details for PubMedCentralID PMC2536531

• Synthesis, characterization, and reactivities of manganese(V)-oxo porphyrin complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Song, W. J., Seo, M. S., George, S. D., Ohta, T., Song, R., Kang, M., Tosha, T., Kitagawa, T., Solomon, E. I., Nam, W. 2007; 129 (5): 1268-1277

  Abstract

  The reactions of manganese(III) porphyrin complexes with terminal oxidants, such as m-chloroperbenzoic acid, iodosylarenes, and H(2)O(2), produced high-valent manganese(V)-oxo porphyrins in the presence of base in organic solvents at room temperature. The manganese(V)-oxo porphyrins have been characterized with various spectroscopic techniques, including UV-vis, EPR, 1H and 19F NMR, resonance Raman, and X-ray absorption spectroscopy. The combined spectroscopic results indicate that the manganese(V)-oxo porphyrins are diamagnetic low-spin (S = 0) species with a longer, weaker Mn-O bond than in previously reported Mn(V)-oxo complexes of non-porphyrin ligands. This is indicative of double-bond character between the manganese(V) ion and the oxygen atom and may be attributed to the presence of a trans axial ligand. The [(Porp)Mn(V)=O](+) species are stable in the presence of base at room temperature. The stability of the intermediates is dependent on base concentration. In the absence of base, (Porp)Mn(IV)=O is generated instead of the [(Porp)Mn(V)=O](+) species. The stability of the [(Porp)Mn(V)=O](+) species also depends on the electronic nature of the porphyrin ligands: [(Porp)Mn(V)=O](+) complexes bearing electron-deficient porphyrin ligands are more stable than those bearing electron-rich porphyrins. Reactivity studies of manganese(V)-oxo porphyrins revealed that the intermediates are capable of oxygenating PPh(3) and thioanisoles, but not olefins and alkanes at room temperature. These results indicate that the oxidizing power of [(Porp)Mn(V)=O](+) is low in the presence of base. However, when the [(Porp)Mn(V)=O](+) complexes were associated with iodosylbenzene in the presence of olefins and alkanes, high yields of oxygenated products were obtained in the catalytic olefin epoxidation and alkane hydroxylation reactions. Mechanistic aspects, such as oxygen exchange between [(Porp)Mn(V)=16O](+) and H(2)(18)O, are also discussed.

  View details for DOI 10.1021/ja066460v

  View details for Web of Science ID 000243840100045

  View details for PubMedID 17263410

  View details for PubMedCentralID PMC2915770

• Identification of the peroxy adduct in multicopper oxidases by a combination of computational chemistry and extended X-ray absorption fine-structure measurements JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ryde, U., Hsiao, Y., Rulisek, L., Solomon, E. I. 2007; 129 (4): 726-727

  View details for DOI 10.1021/ja062954g

  View details for Web of Science ID 000243683800001

  View details for PubMedID 17243785

• Fe L-edge x-ray absorption spectroscopy of low-spin heme relative to non-heme Fe complexes: Delocalization of Fe d-electrons into the porphyrin ligand JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hocking, R. K., Wasinger, E. C., Yan, Y., deGroot, F. M., Walker, F. A., Hodgson, K. O., Hedman, B., Solomon, E. I. 2007; 129 (1): 113-125

  Abstract

  Hemes (iron porphyrins) are involved in a range of functions in biology, including electron transfer, small-molecule binding and transport, and O2 activation. The delocalization of the Fe d-electrons into the porphyrin ring and its effect on the redox chemistry and reactivity of these systems has been difficult to study by optical spectroscopies due to the dominant porphyrin pi-->pi(*) transitions, which obscure the metal center. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., differences in mixing of the d-orbitals with ligand orbitals) using a valence bond configuration interaction (VBCI) model. Applied to low-spin heme systems, this methodology allows experimental determination of the delocalization of the Fe d-electrons into the porphyrin (P) ring in terms of both P-->Fe sigma and pi-donation and Fe-->P pi back-bonding. We find that pi-donation to Fe(III) is much larger than pi back-bonding from Fe(II), indicating that a hole superexchange pathway dominates electron transfer. The implications of the results are also discussed in terms of the differences between heme and non-heme oxygen activation chemistry.

  View details for DOI 10.1021/ja065627h

  View details for Web of Science ID 000243195100032

  View details for PubMedID 17199290

  View details for PubMedCentralID PMC2890250

• Description of the ground-state covalencies of the bis(dithiolato) transition-metal complexes from X-ray absorption spectroscopy and time-dependent density-functional calculations CHEMISTRY-A EUROPEAN JOURNAL Ray, K., George, S. D., Solomon, E. I., Wieghardt, K., Neese, F. 2007; 13 (10): 2783-2797

  Abstract

  The electronic structures of [M(L(Bu))(2)](-) (L(Bu)=3,5-di-tert-butyl-1,2-benzenedithiol; M=Ni, Pd, Pt, Cu, Co, Au) complexes and their electrochemically generated oxidized and reduced forms have been investigated by using sulfur K-edge as well as metal K- and L-edge X-ray absorption spectroscopy. The electronic structure content of the sulfur K-edge spectra was determined through detailed comparison of experimental and theoretically calculated spectra. The calculations were based on a new simplified scheme based on quasi-relativistic time-dependent density functional theory (TD-DFT) and proved to be successful in the interpretation of the experimental data. It is shown that dithiolene ligands act as noninnocent ligands that are readily oxidized to the dithiosemiquinonate(-) forms. The extent of electron transfer strongly depends on the effective nuclear charge of the central metal, which in turn is influenced by its formal oxidation state, its position in the periodic table, and scalar relativistic effects for the heavier metals. Thus, the complexes [M(L(Bu))(2)](-) (M=Ni, Pd, Pt) and [Au(L(Bu))(2)] are best described as delocalized class III mixed-valence ligand radicals bound to low-spin d(8) central metal ions while [M(L(Bu))(2)](-) (M=Cu, Au) and [M(L(Bu))(2)](2-) (M=Ni, Pd, Pt) contain completely reduced dithiolato(2-) ligands. The case of [Co(L(Bu))(2)](-) remains ambiguous. On the methodological side, the calculation led to the new result that the transition dipole moment integral is noticeably different for S(1s)-->valence-pi versus S(1s)-->valence-sigma transitions, which is explained on the basis of the differences in radial distortion that accompany chemical bond formation. This is of importance in determining experimental covalencies for complexes with highly covalent metal-sulfur bonds from ligand K-edge absorption spectroscopy.

  View details for DOI 10.1002/chem.200601425

  View details for Web of Science ID 000245389700003

  View details for PubMedID 17290468

• The two-state issue in the mixed-valence binuclear Cu-A center in cytochrome c oxidase and N2O reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Gorelsky, S. I., Xie, X., Chen, Y., Fee, J. A., Solomon, E. I. 2006; 128 (51): 16452-16453

  Abstract

  For the CuA site in the protein, sigmau* and piu are the ground and lowest energy excited-states, respectively. EPR data on CuA proteins show a low g parallel value of 2.19 which derives from spin-orbital coupling between sigmau* and piu which requires an energy gap between sigmau* and piu of 3000-4500 cm-1. On the other hand, from paramagnetic NMR studies, it has been observed that the first excited-state is thermally accessible and the energy gap between the ground state and the thermally accessible state is approximately 350 cm-1. This study addressed this apparent discrepancy and evaluated the roles of the two electronic states, sigmau* and piu, in electron transfer (ET) of CuA. The potential energy surface calculations show that both NMR and EPR results are consistent with the electronic/geometric structure of CuA. The anti-Curie behavior observed in paramagnetic NMR studies of CuA results from the thermal equilibrium between the sigmau* and piu states which are at very close energies in their respective equilibrium geometries. Alternatively, the EPR g-value analysis involves the sigmau* ground state in the geometry with a short dCu-Cu where the piu state is a Frank-Condon excited-state with the energy of 3200 cm-1. The protein environment plays a role in maintaining CuA in the sigmau* state as a lowest-energy state with the lowest reorganization energy and high-covalent coupling to the Cys and His ligands for efficient intra- and intermolecular ET with a low-driving force.

  View details for DOI 10.1021/ja067583i

  View details for Web of Science ID 000242941600020

  View details for PubMedID 17177365

• Multireference ab initio calculations on reaction intermediates of the multicopper oxidases INORGANIC CHEMISTRY Chalupsky, J., Neese, F., Solomon, E. I., Ryde, U., Rulisek, L. 2006; 45 (26): 11051-11059

  Abstract

  The multicopper oxidases (MCOs) couple the four-electron reduction of dioxygen to water with four one-electron oxidations of various substrates. Extensive spectroscopic studies have identified several intermediates in the MCO catalytic cycle, but they have not been able to settle the structures of three of the intermediates, viz. the native intermediate (NI), the peroxy intermediate (PI), and the peroxy adduct (PA). The suggested structures have been further refined and characterized by quantum mechanical/molecular mechanical (QM/MM) calculations. In this paper, we try to establish a direct link between theory and experiment, by calculating spectroscopic parameters for these intermediates using multireference wave functions from the multistate CASPT2 and MRDDCI2 methods. Thereby, we have been able to reproduce low-spin ground states (S = 0 or S = 1/2) for all the MCO intermediates, as well as a low-lying (approximately 150 cm-1) doublet state and a doublet-quartet energy gap of approximately 780 cm-1 for the NI. Moreover, we reproduce the zero-field splitting (approximately 70 cm-1) of the ground 2E state in a D3 symmetric hydroxy-bridged trinuclear Cu(II) model of the NI and obtain a quantitatively correct quartet-doublet splitting (164 cm-1) for a mu3-oxo-bridged trinuclear Cu(II) cluster. All results support the suggestion that the NI has an O2- atom in the center of the trinuclear cluster, whereas both the PI and PA have an O22- ion in the center of the cluster, in agreement with the QM/MM results and spectroscopic measurements.

  View details for DOI 10.1021/ic0619512

  View details for Web of Science ID 000242899400082

  View details for PubMedID 17173465

• Spectroscopic and electronic structure studies of the role of active site interactions in the decarboxylation reaction of alpha-keto acid-dependent dioxygenases JOURNAL OF INORGANIC BIOCHEMISTRY Neidig, M. L., Brown, C. D., Kavana, M., Choroba, O. W., Spencer, J. B., Moran, G. R., Solomon, E. I. 2006; 100 (12): 2108-2116

  Abstract

  The alpha-ketoglutate (alpha-KG)-dependent dioxygenases are a large class of mononuclear non-heme iron enzymes that require Fe(II), alpha-KG and dioxygen for catalysis, with the alpha-KG cosubstrate supplying the two additional electrons required for dioxygen activation. A sub-class of these enzymes exists in which the alpha-keto acid is covalently attached to the substrate, including (4-hydroxy)mandelate synthase (HmaS) and (4-hydroxyphenyl)pyruvate dioxygenase (HPPD) which utilize the same substrate but exhibit two different general reactivities (H-atom abstraction and electrophilic attack). Previous kinetic studies of Streptomyces avermitilis HPPD have shown that the substrate analog phenylpyruvate (PPA), which only differs from the normal substrate (4-hydroxyphenyl)pyruvate (HPP) by the absence of a para-hydroxyl group on the aromatic ring, does not induce a reaction with dioxygen. While an Fe(IV)O intermediate is proposed to be the reactive species in converting substrate to product, the key step utilizing O(2) to generate this species is the decarboxylation of the alpha-keto acid. It has been generally proposed that the two requirements for decarboxylation are bidentate coordination of the alpha-keto acid to Fe(II) and the presence of a 5C Fe(II) site for the O(2) reaction. Circular dichroism and magnetic circular dichroism studies have been performed and indicate that both enzyme complexes with PPA are similar with bidentate alpha-KG coordination and a 5C Fe(II) site. However, kinetic studies indicate that while HmaS reacts with PPA in a coupled reaction similar to the reaction with HPP, HPPD reacts with PPA in an uncoupled reaction at an approximately 10(5)-fold decreased rate compared to the reaction with HPP. A key difference is spectroscopically observed in the n-->pi( *) transition of the HPPD/Fe(II)/PPA complex which, based upon correlation to density functional theory calculations, is suggested to result from H-bonding between a nearby residue and the carboxylate group of the alpha-keto acid. Such an interaction would disfavor the decarboxylation reaction by stabilizing electron density on the carboxylate group such that the oxidative cleavage to yield CO(2) is disfavored.

  View details for DOI 10.1016/j.jinorgbio.2006.08.021

  View details for Web of Science ID 000242919600023

  View details for PubMedID 17070917

• Circular dichroism and magnetic circular dichroism studies of the active site of p53R2 from human and mouse: Iron binding and nature of the biferrous site relative to other ribonucleotide reductases BIOCHEMISTRY Wei, P., Tomter, A. B., Rohr, A. K., Andersson, K. K., Solomon, E. I. 2006; 45 (47): 14043-14051

  Abstract

  Ribonucleotide reductases (RNR) catalyze the rate-limiting step in the synthesis of deoxyribonucleotides from the corresponding ribonucleotides in the synthesis of DNA. Class I RNR has two subunits: R1 with the substrate binding and active site and R2 with a stable tyrosyl radical and diiron cluster. Biferrous R2 reacts with oxygen to form the tyrosyl radical needed for enzymatic activity. A novel R2 form, p53R2, is a 351-amino acid protein induced by the "tumor suppressor gene" p53. p53R2 has been studied using a combination of circular dichroism, magnetic circular dichroism, variable-temperature variable-field MCD, and EPR spectroscopies. The active site of biferrous p53R2 in both the human (hp53R2) and mouse (mp53R2) forms is found to have one five-coordinate and one four-coordinate iron, which are weakly antiferromagnetically coupled through mu-1,3-carboxylate bridges. These spectroscopic data are very similar to those of Escherichia coli R2, and mouse R2, with a stronger resemblance to data of the former. Titrations of apo-hp53R2 and apo-mp53R2 with Fe(II) were pursued for the purpose of comparing their metal binding affinities to those of other R2s. Both p53R2s were found to have a high affinity for Fe(II), which is different from that of mouse R2 and may reflect differences in the regulation of enzymatic activity, as p53R2 is mainly triggered during DNA repair. The difference in ferrous affinity between mammalian R2 and p53R2 suggests the possibility of specific inhibition of DNA precursor synthesis during cell division.

  View details for DOI 10.1021/bi061127p

  View details for Web of Science ID 000242179100012

  View details for PubMedID 17115699

• A functional model for the cysteinate-ligated non-heme iron enzyme superoxide reductase (SOR) JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Kitagawa, T., Dey, A., Lugo-Mas, P., Benedict, J. B., Kaminsky, W., Solomon, E., Kovacs, J. A. 2006; 128 (45): 14448-14449

  Abstract

  Superoxide reductases (SORs) are cysteine-ligated, non-heme iron enzymes that reduce toxic superoxide radicals (O2-). The functional role of the trans cysteinate, as well as the mechanism by which SOR reduces O2-, is unknown. Herein is described a rare example of a functional metalloenzyme analogue, which catalytically reduces superoxide in a proton-dependent mechanism, via a trans thiolate-ligated iron-peroxo intermediate, the first example of its type. Acetic-acid-promoted H2O2 release, followed by Cp2Co reduction, regenerates the active Fe(II) catalyst. The thiolate ligand and its trans positioning relative to the substrate are shown to contribute significantly to the catalyst's function, by lowering the redox potential, changing the spin state, and dramatically lowering the nuFe-O stretching frequency well-below that of any other reported iron-peroxo, while leaving nuO-O high, so as to favor superoxide reduction and Fe-O, as opposed to O-O, bond cleavage. Thus we provide critical insight into the relationship between the SOR structure and its function, as well as important benchmark parameters for characterizing highly unstable thiolate-ligated iron-peroxo intermediates.

  View details for DOI 10.1021/ja064870d

  View details for Web of Science ID 000241857200016

  View details for PubMedID 17090014

  View details for PubMedCentralID PMC2532059

• Spectroscopic methods in bioinorganic chemistry: Blue to green to red copper sites INORGANIC CHEMISTRY Solomon, E. I. 2006; 45 (20): 8012-8025

  Abstract

  A wide variety of spectroscopic methods are now available that provide complimentary insights into the electronic structures of transition-metal complexes. Combined with calculations, these define key bonding interactions, enable the evaluation of reaction coordinates, and determine the origins of unique spectroscopic features/electronic structures that can activate metal centers for catalysis. This presentation will summarize the contributions of a range of spectroscopic methods combined with calculations in elucidating the electronic structure of an active site using the blue copper site as an example. The contribution of electronic structure to electron-transfer reactivity will be considered in terms of anisotropic covalency, electron-transfer pathways, reorganization energy, and protein contributions to the geometric and electronic structures of blue-copper-related active sites.

  View details for DOI 10.1021/ic060450d

  View details for Web of Science ID 000240711500010

  View details for PubMedID 16999398

• INOR 633-Mononuclear non-heme Fe enzymes and redox-active cosubstrates: Exploring reaction pathways in alpha KG-dependent and related enzymes Brown, C. D., Neidig, M. L., Neibergall, M. B., Lipscomb, J. D., Solomon, E. I. AMER CHEMICAL SOC. 2006

  View details for Web of Science ID 000207781606849

• INOR 12-Excited state spectroscopic methods Solomon, E. I. AMER CHEMICAL SOC. 2006

  View details for Web of Science ID 000207781606015

• INOR 142-Spectroscopic and kinetic studies of systematically perturbed trinuclear copper sites in multicopper oxidases Augustine, A. J., Stoj, C., Kosman, D., Solomon, E. I. AMER CHEMICAL SOC. 2006

  View details for Web of Science ID 000207781606447

• INOR 620-Spectroscopic and electronic structure studies of the non-heme iron enzymes HPPD and HmaS: Aromatic electrophilic attack vs. H-atom abstraction Neidig, M. L., Decker, A., Choroba, O. W., Huang, F., Kavana, M., Moran, G. R., Spencer, J. B., Solomon, E. I. AMER CHEMICAL SOC. 2006

  View details for Web of Science ID 000207781606520

• Metal-thiolate bonds in bioinorganic chemistry JOURNAL OF COMPUTATIONAL CHEMISTRY Solomon, E. I., Gorelsky, S. I., Dey, A. 2006; 27 (12): 1415-1428

  Abstract

  Metal-thiolate active sites play major roles in bioinorganic chemistry. The M--S(thiolate) bonds can be very covalent, and involve different orbital interactions. Spectroscopic features of these active sites (intense, low-energy charge transfer transitions) reflect the high covalency of the M--S(thiolate) bonds. The energy of the metal-thiolate bond is fairly insensitive to its ionic/covalent and pi/sigma nature as increasing M--S covalency reduces the charge distribution, hence the ionic term, and these contributions can compensate. Thus, trends observed in stability constants (i.e., the Irving-Williams series) mostly reflect the dominantly ionic contribution to bonding of the innocent ligand being replaced by the thiolate. Due to high effective nuclear charges of the Cu(II) and Fe(III) ions, the cupric- and ferric-thiolate bonds are very covalent, with the former having strong pi and the latter having more sigma character. For the blue copper site, the high pi covalency couples the metal ion into the protein for rapid directional long range electron transfer. For rubredoxins, because the redox active molecular orbital is pi in nature, electron transfer tends to be more localized in the vicinity of the active site. Although the energy of hydrogen bonding of the protein environment to the thiolate ligands tends to be fairly small, H-bonding can significantly affect the covalency of the metal-thiolate bond and contribute to redox tuning by the protein environment.

  View details for DOI 10.1002/jcc.20451

  View details for Web of Science ID 000239072600015

  View details for PubMedID 16807974

• How does single oxygen atom addition affect the properties of an Fe-nitrile hydratase analogue? The compensatory role of the unmodified thiolate JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lugo-Mas, P., Dey, A., Xu, L., Davin, S. D., Benedict, J., Kaminsky, W., Hodgson, K. O., Hedman, B., Solomon, E. I., Kovacs, J. A. 2006; 128 (34): 11211-11221

  Abstract

  Nitrile hydratase (NHase) is one of a growing number of enzymes shown to contain post-translationally modified cysteine sulfenic acids (Cys-SOH). Cysteine sulfenic acids have been shown to play diverse roles in cellular processes, including transcriptional regulation, signal transduction, and the regulation of oxygen metabolism and oxidative stress responses. The function of the cysteine sulfenic acid coordinated to the iron active site of NHase is unknown. Herein we report the first example of a sulfenate-ligated iron complex, [Fe(III)(ADIT)(ADIT-O)](+) (5), and compare its electronic and magnetic properties with those of structurally related complexes in which the sulfur oxidation state and protonation state have been systematically altered. Oxygen atom addition was found to decrease the unmodified thiolate Fe-S bond length and blue-shift the ligand-to-metal charge-transfer band (without loss of intensity). S K-edge X-ray absorption spectroscopy and density functional theory calculations show that, although the modified RS-O(-) fragment is incapable of forming a pi bond with the Fe(III) center, the unmodified thiolate compensates for this loss of pi bonding by increasing its covalent bond strength. The redox potential shifts only slightly (75 mV), and the magnetic properties are not affected (the S = (1)/(2) spin state is maintained). The coordinated sulfenate S-O bond is activated and fairly polarized (S(+)-O(-)). Addition of strong acids at low temperatures results in the reversible protonation of sulfenate-ligated 5. An X-ray structure demonstrates that Zn(2+) binds to the sulfenate oxygen to afford [Fe(III)(ADIT)(ADIT-O-ZnCl(3))] (6). The coordination of ZnCl(3)(-) to the RS-O(-) unit causes the covalent overlap with the unmodified thiolate to increase further. A possible catalytic role for the unmodified NHase thiolate, involving its ability to "tune" the electronics in response to protonation of the sulfenate (RS-O(-)) oxygen and/or substrate binding, is discussed.

  View details for DOI 10.1021/ja062706k

  View details for Web of Science ID 000239932500051

  View details for PubMedID 16925440

• Spectroscopic and electronic structure studies of aromatic electrophilic attack and hydrogen-atom abstraction by non-heme iron enzymes PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Neidig, M. L., Decker, A., Choroba, O. W., Huang, F., Kavana, M., Moran, G. R., Spencer, J. B., Solomon, E. I. 2006; 103 (35): 12966-12973

  Abstract

  (4-Hydroxy)mandelate synthase (HmaS) and (4-hydroxyphenyl)pyruvate dioxygenase (HPPD) are two alpha-keto acid dependent mononuclear non-heme iron enzymes that use the same substrate, (4-hydroxyphenyl)pyruvate, but exhibit two different general reactivities. HmaS performs hydrogen-atom abstraction to yield benzylic hydroxylated product (S)-(4-hydroxy)mandelate, whereas HPPD utilizes an electrophilic attack mechanism that results in aromatic hydroxylated product homogentisate. These enzymes provide a unique opportunity to directly evaluate the similarities and differences in the reaction pathways used for these two reactivities. An Fe(II) methodology using CD, magnetic CD, and variable-temperature, variable-field magnetic CD spectroscopies was applied to HmaS and compared with that for HPPD to evaluate the factors that affect substrate interactions at the active site and to correlate these to the different reactivities exhibited by HmaS and HPPD to the same substrate. Combined with density functional theory calculations, we found that HmaS and HPPD have similar substrate-bound complexes and that the role of the protein pocket in determining the different reactivities exhibited by these enzymes (hydrogen-atom abstraction vs. aromatic electrophilic attack) is to properly orient the substrate, allowing for ligand field geometric changes along the reaction coordinate. Elongation of the Fe(IV) O bond in the transition state leads to dominant Fe(III) O(*-) character, which significantly contributes to the reactivity with either the aromatic pi-system or the C H sigma-bond.

  View details for DOI 10.1073/pnas.0605067103

  View details for Web of Science ID 000240380800006

  View details for PubMedID 16920789

  View details for PubMedCentralID PMC1559736

• Fe L-edge XAS studies of K-4[Fe(CN)(6)] and K-3[Fe(CN)(6)]: A direct probe of back-bonding JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hocking, R. K., Wasinger, E. C., de Groot, F. M., Hodgson, K. O., Hedman, B., Solomon, E. I. 2006; 128 (32): 10442-10451

  Abstract

  Distinct spectral features at the Fe L-edge of the two compounds K3[Fe(CN)6] and K4[Fe(CN)6] have been identified and characterized as arising from contributions of the ligand pi orbitals due to metal-to-ligand back-bonding. In addition, the L-edge energy shifts and total intensities allow changes in the ligand field and effective nuclear charge to be determined. It is found that the ligand field term dominates the edge energy shift. The results of the experimental analysis were compared to BP86 DFT calculations. The overall agreement between the calculations and experiment is good; however, a larger difference in the amount of pi back-donation between Fe(II) and Fe(III) is found experimentally. The analysis of L-edge spectral shape, energy shift, and total intensity demonstrates that Fe L-edge X-ray absorption spectroscopy provides a direct probe of metal-to-ligand back-bonding.

  View details for DOI 10.1021/ja061802i

  View details for Web of Science ID 000239618700027

  View details for PubMedID 16895409

• X-ray absorption spectroscopy and density functional theory studies of [(H(3)buea)Fe-III-X](n-) (X = S2-, O2-, OH-): Comparison of bonding and hydrogen bonding in oxo and sulfido complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Hocking, R. K., Larsen, P., Borovik, A. S., Hodgson, K. O., Hedman, B., Solomon, E. I. 2006; 128 (30): 9825-9833

  Abstract

  Iron L-edge, iron K-edge, and sulfur K-edge X-ray absorption spectroscopy was performed on a series of compounds [Fe(III)H(3)buea(X)](n-) (X = S(2-), O(2-), OH(-)). The experimentally determined electronic structures were used to correlate to density functional theory calculations. Calculations supported by the data were then used to compare the metal-ligand bonding and to evaluate the effects of H-bonding in Fe(III)(-)O vs Fe(III)(-)S complexes. It was found that the Fe(III)(-)O bond, while less covalent, is stronger than the Fe(III)(-)S bond. This dominantly reflects the larger ionic contribution to the Fe(III)(-)O bond. The H-bonding energy (for three H-bonds) was estimated to be -25 kcal/mol for the oxo as compared to -12 kcal/mol for the sulfide ligand. This difference is attributed to the larger charge density on the oxo ligand resulting from the lower covalency of the Fe-O bond. These results were extended to consider an Fe(IV)(-)O complex with the same ligand environment. It was found that hydrogen bonding to Fe(IV)(-)O is less energetically favorable than that to Fe(III)(-)O, which reflects the highly covalent nature of the Fe(IV)(-)O bond.

  View details for DOI 10.1021/ja061618x

  View details for Web of Science ID 000239278600060

  View details for PubMedID 16866539

• X-ray absorption edge spectroscopy and computational studies on LCuO2 species: Superoxide-Cu-II versus peroxide-Cu-III bonding JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sarangi, R., Aboelella, N., Fujisawa, K., Tolman, W. B., Hedman, B., Hodgson, K. O., Solomon, E. I. 2006; 128 (25): 8286-8296

  Abstract

  The geometric and electronic structures of two mononuclear CuO2 complexes, [Cu(O2){HB(3-Ad-5-(i)Prpz)3}] (1) and [Cu(O2)(beta-diketiminate)] (2), have been evaluated using Cu K- and L-edge X-ray absorption spectroscopy (XAS) studies in combination with valence bond configuration interaction (VBCI) simulations and spin-unrestricted broken symmetry density functional theory (DFT) calculations. Cu K- and L-edge XAS data indicate the Cu(II) and Cu(III) nature of 1 and 2, respectively. The total integrated intensity under the L-edges shows that the 's in 1 and 2 contain 20% and 28% Cu character, respectively, indicative of very covalent ground states in both complexes, although more so in 1. Two-state VBCI simulations also indicate that the ground state in 2 has more Cu (/3d8) character. DFT calculations show that the in both complexes is dominated by O2(n-) character, although the O2(n-) character is higher in 1. It is shown that the ligand L plays an important role in modulating Cu-O2 bonding in these LCuO2 systems and tunes the ground states of 1 and 2 to have dominant Cu(II)-superoxide-like and Cu(III)-peroxide-like character, respectively. The contributions of ligand field (LF) and the charge on the absorbing atom in the molecule (Q(mol)M) to L- and K-edge energy shifts are evaluated using DFT and time-dependent DFT calculations. It is found that LF makes a dominant contribution to the edge energy shift, while the effect of Q(mol)M is minor. The charge on the Cu in the Cu(III) complex is found to be similar to that in Cu(II) complexes, which indicates a much stronger interaction with the ligand, leading to extensive charge transfer.

  View details for DOI 10.1021/ja0615223

  View details for Web of Science ID 000238418000045

  View details for PubMedID 16787093

  View details for PubMedCentralID PMC2556900

• Metal and ligand K-edge XAS of titanium-TEMPO complexes: Determination of oxidation states and insights into Ti-O bond homolysis INORGANIC CHEMISTRY George, S. D., Huang, K., Waymouth, R. M., Solomon, E. I. 2006; 45 (11): 4468-4477

  Abstract

  Ti-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) provides a means for generating Ti(III) complexes by homolysis of the Ti-O bond. It has been determined that bis-Cp-Ti-TEMPO complexes readily undergo homolytic cleavage while the mono-Cp-Ti-TEMPO complexes do not. Here Ti K- and Cl K-edge XAS are applied to directly determine the oxidation state of TiCl3TEMPO, TiCpCl2TEMPO, and TiCp2ClTEMPO, with reference to Ti(III) and Ti(IV) complexes of known oxidation state. The Ti K-edge data show that Ti(III) complexes exhibit a pre-edge feature approximately 1 eV lower than any of the Ti(IV) complexes; while the Cl K-edges show that Ti(III) complexes have a Cl K- pre-edge feature to approximately 1 eV higher energy than any of the Ti(IV) complexes. Taken together, the Ti and Cl K-edge data indicate that the Ti-TEMPO complexes are best described as Ti(IV)-TEMPO anions (rather than Ti(III)-nitroxyl radicals). In addition, the Cl K-edges indicate that replacement of Cl by Cp weakens the bonding with the remaining ligands, with the Cl 3p covalency decreasing from 25% to 21% to 17% on going from TiCl3TEMPO to TiCpCl2TEMPO to TiCp2ClTEMPO. DFT calculations also show that the electronic structures of the Ti-TEMPO complexes are modulated by the replacement of chloride by Cp. The effect of the Cp on the ancillary ligation is one factor that contributes to facile Ti-O bond homolysis in TiCp2ClTEMPO. However, the results indicate the primary contribution to the energetics of Ti-O bond homolysis in TiCp2ClTEMPO is stabilization of the three-coordinate product by Cp.

  View details for DOI 10.1021/ic060402t

  View details for Web of Science ID 000237690700029

  View details for PubMedID 16711697

• Direct hydrogen-atom abstraction by activated bleomycin: An experimental and computational study JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Decker, A., Chow, M. S., Kemsley, J. N., Lehnert, N., Solomon, E. I. 2006; 128 (14): 4719-4733

  Abstract

  Bleomycin (BLM), a glycopeptide antibiotic chemotherapy agent, is capable of single- and double-strand DNA damage. Activated bleomycin (ABLM), a low-spin Fe(III)-OOH complex, is the last intermediate detected prior to DNA cleavage following hydrogen-atom abstraction from the C-4' of a deoxyribose sugar moiety. The mechanism of this C-H bond cleavage reaction and the nature of the active oxidizing species are still open issues. We have used kinetic measurements in combination with density functional calculations to study the reactivity of ABLM and the mechanism of the initial attack on DNA. Circular dichroism spectroscopy was used to directly monitor the kinetics of the ABLM reaction. These experiments yield a deuterium isotope effect, kH/kD approximately 3 for ABLM decay, indicating the involvement of a hydrogen atom in the rate-determining step. H-atom donors with relatively weak X-H bonds accelerate the reaction rate, establishing that ABLM is capable of hydrogen-atom abstraction. Density functional calculations were used to evaluate the two-dimensional potential energy surface for the direct hydrogen-atom abstraction reaction of the deoxyribose 4'-H by ABLM. The calculations confirm that ABLM is thermodynamically and kinetically competent for H-atom abstraction. The activation and reaction energies for this pathway are favored over both homolytic and heterolytic O-O bond cleavage. Direct H-atom abstraction by ABLM would generate a reactive Fe(IV)=O species, which would be capable of a second DNA strand cleavage, as observed in vivo. This study provides experimental and theoretical evidence for direct H-atom abstraction by ABLM and proposes an attractive mechanism for the role of ABLM in double-strand cleavage.

  View details for DOI 10.1021/ja057378n

  View details for Web of Science ID 000236770300063

  View details for PubMedID 16594709

• Spectroscopy and electronic structures of mono- and binuclear high-valent non-heme iron-oxo systems JOURNAL OF INORGANIC BIOCHEMISTRY Decker, A., Clay, M. D., Solomon, E. I. 2006; 100 (4): 697-706

  Abstract

  High-valent iron-oxo intermediates are known or believed to be key oxidizing species in the catalytic mechanisms of many mononuclear and binuclear non-heme iron enzymes. So far only limited experimental data on their electronic structures are available. In this study we extend knowledge from the experimentally well characterized mononuclear Fe(IV)=O (S=1) biomimetic model system to computational insight into the spectroscopy and electronic structures of mono-and binuclear high-valent iron-oxo enzyme intermediates. In the mononuclear Fe(IV)=O complexes, we predict the spectroscopy and energies of the electronic transitions to be very different for the S=1 and S=2 spin states, but the iron-oxo bonding for both spin states to be very similar. A comparison of the S=2 mono- and binuclear high-valent iron-sites predicts similar electronic transitions. However, the bent iron-oxo bridge and interactions with the second iron-center in the dimer shift the transitions to higher energies and splits the d(xz/yz) orbital set. These electronic structure and TD-DFT results provide a basis for understanding the spectroscopy and electronic structures of high-valent intermediates in mono- and binuclear non-heme iron enzymes.

  View details for DOI 10.1016/j.jinorgbio.2006.01.013

  View details for Web of Science ID 000237829000025

  View details for PubMedID 16510189

• Reinvestigation of the method used to map the electronic structure of blue copper proteins by NMR relaxation JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY Hansen, D. F., Gorelsky, S. I., Sarangi, R., Hodgson, K. O., Hedman, B., Christensen, H. E., SOLOMON, E. I., Led, J. J. 2006; 11 (3): 277-285

  Abstract

  A previous method for mapping the electron spin distribution in blue copper proteins by paramagnetic nuclear magnetic resonance (NMR) relaxation (Hansen DF, Led JJ, 2004, J Am Chem Soc 126:1247-1253) suggested that the blue copper site of plastocyanin from Anabaena variabilis (A.v.) is less covalent than those found for other plastocyanins by other experimental methods, such as X-ray absorption spectroscopy. Here, a detailed spectroscopic study revealed that the electronic structure of A.v. plastocyanin is similar to those of other plastocyanins. Therefore, the NMR approach was reinvestigated using a more accurate geometric structure as the basis for the mapping, in contrast to the previous approach, as well as a more complete spin distribution model including Gaussian-type natural atomic orbitals instead of Slater-type hydrogen-like atomic orbitals. The refinement results in a good agreement between the electron spin density derived from paramagnetic NMR and the electronic structure description obtained by the other experimental methods. The refined approach was evaluated against density functional theory (DFT) calculations on a model complex of the metal site of plastocyanin in the crystal phase. In general, the agreement between the experimental paramagnetic relaxation rates and the corresponding rates obtained by the DFT calculations is good. Small deviations are attributed to minor differences between the solution structure and the crystal structure outside the first coordination sphere. Overall, the refined approach provides a complementary experimental method for determining the electronic structure of paramagnetic metalloproteins, provided that an accurate geometric structure is available.

  View details for DOI 10.1007/s00775-005-0070-9

  View details for Web of Science ID 000236586000003

  View details for PubMedID 16432723

• Towards understanding the O-2 chemistry of intradiol dioxygenases Pau, M. M., Davis, M. I., Orville, A. M., Lipscomb, J. D., Solomon, E. I. AMER CHEMICAL SOC. 2006

  View details for Web of Science ID 000238125906148

• Spectroscopic methods in bioinorganic chemistry: Blue to green to red Copper sites Solomon, E. I. AMER CHEMICAL SOC. 2006

  View details for Web of Science ID 000238125905313

• The CuZ cluster of nitrous oxide reductase: Geometric and electronic structure and role in N2O reduction Ghosh, S., Gorelsky, S. I., Solomon, E. I. AMER CHEMICAL SOC. 2006

  View details for Web of Science ID 000238125905588

• Reactivity of activated bleomycin: An experimental and computational study Decker, A., Chow, M. S., Kemsley, J. N., Lehnert, N., Solomon, E. I. AMER CHEMICAL SOC. 2006

  View details for Web of Science ID 000238125906146

• mu-eta(2):eta(2)-Peroxodicopper(II) complex with a secondary diamine ligand: A functional model of tyrosinase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mirica, L. M., Rudd, D. J., Vance, M. A., SOLOMON, E. I., Hodgson, K. O., Hedman, B., Stack, T. D. 2006; 128 (8): 2654-2665

  Abstract

  The activation of dioxygen (O(2)) by Cu(I) complexes is an important process in biological systems and industrial applications. In tyrosinase, a binuclear copper enzyme, a mu-eta(2):eta(2)-peroxodicopper(II) species is accepted generally to be the active oxidant. Reported here is the characterization and reactivity of a mu-eta(2):eta(2)-peroxodicopper(II) complex synthesized by reacting the Cu(I) complex of the secondary diamine ligand N,N'-di-tert-butyl-ethylenediamine (DBED), [(DBED)Cu(MeCN)](X) (1.X, X = CF(3)SO(3)(-), CH(3)SO(3)(-), SbF(6)(-), BF(4)(-)), with O(2) at 193 K to give [[Cu(DBED)](2)(O(2))](X)(2) (2.X(2)). The UV-vis and resonance Raman spectroscopic features of 2 vary with the counteranion employed yet are invariant with change of solvent. These results implicate an intimate interaction of the counteranions with the Cu(2)O(2) core. Such interactions are supported further by extended X-ray absorption fine structure (EXAFS) analyses of solutions that reveal weak copper-counteranion interactions. The accessibility of the Cu(2)O(2) core to exogenous ligands such as these counteranions is manifest further in the reactivity of 2 with externally added substrates. Most notable is the hydroxylation reactivity with phenolates to give catechol and quinone products. Thus the strategy of using simple bidentate ligands at low temperatures provides not only spectroscopic models of tyrosinase but also functional models.

  View details for DOI 10.1021/ja056740v

  View details for PubMedID 16492052

• Sulfur K-Edge XAS and DFT calculations on nitrile hydratase: Geometric and electronic structure of the non-heme iron active site JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., CHOW, M., Taniguchi, K., Lugo-Mas, P., Davin, S., Maeda, M., Kovacs, J. A., Odaka, M., Hodgson, K. O., Hedman, B., Solomon, E. I. 2006; 128 (2): 533-541

  Abstract

  The geometric and electronic structure of the active site of the non-heme iron enzyme nitrile hydratase (NHase) is studied using sulfur K-edge XAS and DFT calculations. Using thiolate (RS(-))-, sulfenate (RSO(-))-, and sulfinate (RSO(2)(-))-ligated model complexes to provide benchmark spectral parameters, the results show that the S K-edge XAS is sensitive to the oxidation state of S-containing ligands and that the spectrum of the RSO(-) species changes upon protonation as the S-O bond is elongated (by approximately 0.1 A). These signature features are used to identify the three cysteine residues coordinated to the low-spin Fe(III) in the active site of NHase as CysS(-), CysSOH, and CysSO(2)(-) both in the NO-bound inactive form and in the photolyzed active form. These results are correlated to geometry-optimized DFT calculations. The pre-edge region of the X-ray absorption spectrum is sensitive to the Z(eff) of the Fe and reveals that the Fe in [FeNO](6) NHase species has a Z(eff) very similar to that of its photolyzed Fe(III) counterpart. DFT calculations reveal that this results from the strong pi back-bonding into the pi antibonding orbital of NO, which shifts significant charge from the formally t(2)(6) low-spin metal to the coordinated NO.

  View details for DOI 10.1021/ja0549695

  View details for PubMedID 16402841

• Mechanism of N2O reduction by the mu(4)-S tetranuclear Cu-z cluster of nitrous oxide reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Gorelsky, S. I., Ghosh, S., Solomon, E. I. 2006; 128 (1): 278-290

  Abstract

  Reaction thermodynamics and potential energy surfaces are calculated using density functional theory to investigate the mechanism of the reductive cleavage of the N-O bond by the mu(4)-sulfide-bridged tetranuclear Cu(Z) site of nitrous oxide reductase. The Cu(Z) cluster provides an exogenous ligand-binding site, and, in its fully reduced 4Cu(I) state, the cluster turns off binding of stronger donor ligands while enabling the formation of the Cu(Z)-N(2)O complex through enhanced Cu(Z) --> N(2)O back-donation. The two copper atoms (Cu(I) and Cu(IV)) at the ligand-binding site of the cluster play a crucial role in the enzymatic function, as these atoms are directly involved in bridged N(2)O binding, bending the ligand to a configuration that resembles the transition state (TS) and contributing the two electrons for N(2)O reduction. The other atoms of the Cu(Z) cluster are required for extensive back-bonding with minimal sigma ligand-to-metal donation for the N(2)O activation. The low reaction barrier (18 kcal mol(-)(1)) of the direct cleavage of the N-O bond in the Cu(Z)-N(2)O complex is due to the stabilization of the TS by a strong Cu(IV)(2+)-O(-) bond. Due to the charge transfer from the Cu(Z) cluster to the N(2)O ligand, noncovalent interactions with the protein environment stabilize the polar TS and reduce the activation energy to an extent dependent on the strength of proton donor. After the N-O bond cleavage, the catalytic cycle consists of a sequence of alternating protonation/one-electron reduction steps which return the Cu(Z) cluster to the fully reduced (4Cu(I)) state for future turnover.

  View details for DOI 10.1021/ja055856o

  View details for Web of Science ID 000234547700067

  View details for PubMedID 16390158

• Oxygen binding of water-soluble cobalt porphyrins in aqueous solution INORGANIC CHEMISTRY Collman, J. P., Yan, Y. L., Eberspacher, T., Xie, X. J., SOLOMON, E. I. 2005; 44 (26): 9628-9630

  Abstract

  Water-soluble cobalt porphyrin 1Co and imidazole ligand 2 were synthesized. 1Co binds dioxygen in the presence of imidazole ligand 2 in aqueous solution. The formation of the oxygen adduct 2-1Co(O(2)) was studied using UV-vis and EPR spectroscopy. The impact of pH on the kinetic stability of the oxygen adduct was examined.

  View details for DOI 10.1021/ic0516717

  View details for Web of Science ID 000234192300011

  View details for PubMedID 16363827

• MXAN analysis of the XANES energy region of a mononuclear copper complex: Applications to bioinorganic systems INORGANIC CHEMISTRY Sarangi, R., Benfatto, M., Hayakawa, K., Bubacco, L., SOLOMON, E. I., Hodgson, K. O., Hedman, B. 2005; 44 (26): 9652-9659

  Abstract

  The near edge XAS spectra of the mononuclear copper complex [Cu(TMPA)(OH(2))](ClO(4))(2) (1) have been simulated using the multiple scattering edge simulation package MXAN (or Minuit XANes). These simulations, which employ the muffin-tin (MT) approximation, have been compared to simulations generated using the finite-difference method (FDM) to evaluate the effect of MT corrections. The sensitivity of the MXAN method was tested using structural models that included several different variations on the bond angles and bond distances for the first-shell atoms of 1. The sensitivity to small structural changes was also evaluated by comparing MXAN simulations of 1 and of structurally modified [Cu(TMPA)(L)](n)(+) complexes [where L = -O-(F(8)TPP)Fe(III), -F, -OPO(2)(O-p-nitrophenyl)Zn(II)(TMPA), and -NCMe] to the experimental data. The accuracy of the bond distances obtained from the MXAN simulations was then examined by comparison to the metrics of the crystal structures. The results show that MXAN can successfully extract geometric information from the edge structure of an XAS spectrum. The systematic application of MXAN to 1 indicates that this approach is sensitive to small structural changes in the molecule that are manifested in the XAS edge spectrum. These results represent the first step toward the application of this methodology to bioinorganic and biological systems.

  View details for DOI 10.1021/ic050703n

  View details for PubMedID 16363833

• Spectroscopic and computational studies of NTBC bound to the non-heme iron enzyme (4-hydroxyphenyl)pyruvate dioxygenase: Active site contributions to drug inhibition BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Neidig, M. L., Decker, A., Kavana, M., Moran, G. R., Solomon, E. I. 2005; 338 (1): 206-214

  Abstract

  (4-Hydroxyphenyl)pyruvate dioxygenase (HPPD) is an alpha-keto-acid-dependent dioxygenase which catalyzes the conversion of (4-hydroxyphenyl)pyruvate (HPP) to homogentisate as part of tyrosine catabolism. While several di- and tri-ketone alkaloids are known as inhibitors of HPPD and used commercially as herbicides, one such inhibitor, [2-nitro-4-(trifluoromethyl)benzoyl]-1,3-cyclohexanedione (NTBC), has also been used therapeutically to treat type I tyrosinemia and alkaptonuria in humans. To gain further insight into the mechanism of inhibition by NTBC, a combination of CD/MCD spectroscopy and DFT calculations of HPPD/Fe(II)/NTBC has been performed to evaluate the contribution of the Fe(II)-NTBC bonding interaction to the high affinity of this drug for the enzyme. The results indicate that the bonding of NTBC to Fe(II) is very similar to that for HPP, both involving similar pi-backbonding interactions between NTBC/HPP and Fe(II). Combined with the result that the calculated binding energy of NTBC is, in fact, approximately 3 kcal/mol less than that for HPP, the bidentate coordination of NTBC to Fe(II) is not solely responsible for its extremely high affinity for the enzyme. Thus, the pi-stacking interactions between the aromatic rings of NTBC and two phenyalanine residues, as observed in the crystallography of the HPPD/Fe(II)/NTBC complex, appear to be responsible for the observed high affinity of drug binding.

  View details for DOI 10.1016/j.bbrc.2005.08.242

  View details for Web of Science ID 000233296700030

  View details for PubMedID 16197918

• Spectroscopic and computational studies of the de novo designed protein DF2t: Correlation to the biferrous active site of ribonucleotide reductase and factors that affect O-2 reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Wei, P. P., Skulan, A. J., Wade, H., DeGrado, W. F., Solomon, E. I. 2005; 127 (46): 16098-16106

  Abstract

  DF2t, a de novo designed protein that mimics the active-site structure of many non-heme biferrous enzymes, has been studied using a combination of circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature variable-field (VTVH) MCD. The active site of DF2t is found to have one five-coordinate iron and one four-coordinate iron, which are weakly antiferromagnetically coupled through a mu-1,3 carboxylate bridge. These results bear a strong resemblance to the spectra of Escherichia coli ribonucleotide reductase (R2), and density functional theory calculations were conducted on the W48F/D84E R2 mutant in order to determine the energetics of formation of a monodentate end-on-bound O2 to one iron in the binuclear site. The mu-1,3 carboxylate bridges found in O2-activating enzymes lack efficient superexchange pathways for the second electron transfer (i.e., the OH/oxo bridge in hemerythrin), and simulations of the binding of O2 in a monodentate end-on manner revealed that the bridging carboxylate ligands do not appear capable of transferring an electron to O2 from the remote Fe. Comparison of the results from previous studies of the mu-1,2 biferric-peroxo structure, which bridges both irons, finds that the end-on superoxide mixed-valent species is considerably higher in energy than the bridging peroxo-diferric species. Thus, one of the differences between O2-activating and O2-binding proteins appears to be the ability of O2 to bridge both Fe centers to generate a peroxo intermediate capable of further reactivity.

  View details for DOI 10.1021/ja053661a

  View details for Web of Science ID 000233445900033

  View details for PubMedID 16287296

• Sulfur K-edge XAS and DFT calculations on [Fe4S4](2+) clusters: Effects of H-bonding and structural distortion on covalency and spin topology INORGANIC CHEMISTRY Dey, A., Roche, C. L., Walters, M. A., Hodgson, K. O., Hedman, B., Solomon, E. I. 2005; 44 (23): 8349-8354

  Abstract

  Sulfur K-edge X-ray absorption spectroscopy of a hydrogen-bonded elongated [Fe4S4]2+ cube is reported. The data show that this synthetic cube is less covalent than a normal compressed cube with no hydrogen bonding. DFT calculations reveal that the observed difference in electronic structure has significant contributions from both the cluster distortion and from hydrogen bonding. The elongated and compressed Fe4S4 structures are found to have different spin topologies (i.e., orientation of the delocalized Fe2S2 subclusters which are antiferromagnetically coupled to each other). It is suggested that the H-bonding interaction with the counterion does not contribute to the cluster elongation. A magneto-structural correlation is developed for the Fe4S4 cube that is used to identify the redox-active Fe2S2 subclusters in active sites of HiPIP and ferredoxin proteins involving these clusters.

  View details for DOI 10.1021/ic050981m

  View details for Web of Science ID 000233180600029

  View details for PubMedID 16270973

• Ground-state electronic and magnetic properties of a mu(3)-Oxo-bridged trinuclear Cu(II) complex: Correlation to the native intermediate of the multicopper oxidases INORGANIC CHEMISTRY Yoon, J., Solomon, E. I. 2005; 44 (22): 8076-8086

  Abstract

  The ground-state electronic and magnetic properties of one of the possible structures of the trinuclear Cu(II) site in the native intermediate (NI) of the multicopper oxidases, the mu(3)-oxo-bridged structure, are evaluated using the C(3)-symmetric Cu(3)(II) complex, mu(3)O. mu(3)O is unique in that no ligand, other than the oxo, contributes to the exchange coupling. However, mu(3)O has a ferromagnetic ground state, inconsistent with that of NI. Therefore, two perturbations have been considered: protonation of the mu(3)-oxo ligand and relaxation of the mu(3)-oxo ligand into the Cu(3) plane. Notably, when the oxo ligand is sufficiently close to the Cu(3) plane (<0.3 Angstroms), the ground state of mu(3)O becomes antiferromagnetic and can be correlated to that of NI. In addition, the ferromagnetic (4)A ground state of mu(3)O is found from variable-temperature EPR to undergo a zero-field splitting (ZFS) of 2D = -5.0 cm(-1), which derives from the second-order anisotropic exchange. This allows evaluation of the sigma-to-pi excited-state exchange pathways and provides experimental evidence that the orbitally degenerate (2)E ground state of the antiferromagnetic mu(3)O would also undergo a ZFS by the first-order antisymmetric exchange that has the same physical origin as the anisotropic exchange. The important contribution of the mu(3)-oxo bridge to the ground-to-ground and ground-to-excited-state superexchange pathways that are responsible for the isotropic, antisymmetric, and anisotropic exchanges are discussed.

  View details for DOI 10.1021/ic0507870

  View details for Web of Science ID 000232898800046

  View details for PubMedID 16241158

  View details for PubMedCentralID PMC2630029

• Normal mode analysis of Pyrococcus furiosus rubredoxin via nuclear resonance vibrational spectroscopy (NRVS) and resonance Raman spectroscopy JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Xiao, Y. M., Wang, H. X., George, S. J., Smith, M. C., Adams, M. W., Jenney, F. E., Sturhahn, W., Alp, E. E., Zhao, J. O., Yoda, Y., Dey, A., SOLOMON, E. I., Cramer, S. P. 2005; 127 (42): 14596-14606

  Abstract

  We have used (57)Fe nuclear resonance vibrational spectroscopy (NRVS) to study the Fe(S(cys))(4) site in reduced and oxidized rubredoxin (Rd) from Pyrococcus furiosus (Pf). The oxidized form has also been investigated by resonance Raman spectroscopy. In the oxidized Rd NRVS, strong asymmetric Fe-S stretching modes are observed between 355 and 375 cm(-1); upon reduction these modes shift to 300-320 cm(-1). This is the first observation of Fe-S stretching modes in a reduced Rd. The peak in S-Fe-S bend mode intensity is at approximately 150 cm(-1) for the oxidized protein and only slightly lower in the reduced case. A third band occurs near 70 cm(-1) for both samples; this is assigned primarily as a collective motion of entire cysteine residues with respect to the central Fe. The (57)Fe partial vibrational density of states (PVDOS) were interpreted by normal mode analysis with optimization of Urey-Bradley force fields. The three main bands were qualitatively reproduced using a D(2)(d) Fe(SC)(4) model. A C(1) Fe(SCC)(4) model based on crystallographic coordinates was then used to simulate the splitting of the asymmetric stretching band into at least 3 components. Finally, a model employing complete cysteines and 2 additional neighboring atoms was used to reproduce the detailed structure of the PVDOS in the Fe-S stretch region. These results confirm the delocalization of the dynamic properties of the redox-active Fe site. Depending on the molecular model employed, the force constant K(Fe-S) for Fe-S stretching modes ranged from 1.24 to 1.32 mdyn/A. K(Fe-S) is clearly diminished in reduced Rd; values from approximately 0.89 to 1.00 mdyn/A were derived from different models. In contrast, in the final models the force constants for S-Fe-S bending motion, H(S-Fe-S), were 0.18 mdyn/A for oxidized Rd and 0.15 mdyn/A for reduced Rd. The NRVS technique demonstrates great promise for the observation and quantitative interpretation of the dynamical properties of Fe-S proteins.

  View details for DOI 10.1021/ja042960h

  View details for Web of Science ID 000232780900034

  View details for PubMedID 16231912

• Spectroscopic and electronic structure studies of the trinuclear Cu cluster active site of the multicopper oxidase laccase: Nature of its coordination unsaturation JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Quintanar, L., Yoon, J. J., Aznar, C. P., Palmer, A. E., Andersson, K. K., Britt, R. D., Solomon, E. I. 2005; 127 (40): 13832-13845

  Abstract

  Laccase is a multicopper oxidase that contains four Cu ions, one type 1 (T1), one type 2 (T2), and a coupled binuclear type 3 Cu pair (T3). The T2 and T3 centers form a trinuclear Cu cluster that is the active site for O2 reduction to H2O. A combination of spectroscopic and DFT studies on a derivative where the T1 Cu has been replaced by a spectroscopically innocent Hg2+ ion has led to a detailed geometric and electronic structure description of the resting trinuclear Cu cluster, complementing crystallographic results. The nature of the T2 Cu ligation has been elucidated; this site is three-coordinate with two histidines and a hydroxide over its functional pH range (stabilized by a large inductive effect, cluster charge, and a hydrogen-bonding network). Both the T2 and T3 Cu centers have open coordination positions oriented toward the center of the cluster. DFT calculations show that the negative protein pocket (four conserved Asp/Glu residues within 12 A) and the dielectric of the protein play important roles in the electrostatic stability and integrity of the highly charged, coordinatively unsaturated trinuclear cupric cluster. These tune the ligand binding properties of the cluster, leading to its high affinity for fluoride and its coordination unsaturation in aqueous media, which play a key role in its O2 reactivity.

  View details for DOI 10.1021/ja0421405

  View details for Web of Science ID 000232413300038

  View details for PubMedID 16201804

• Variable-temperature, variable-field magnetic circular dichroism studies of tris-hydroxy- and mu(3)-oxo-bridged trinuclear Cu(II) complexes: Evaluation of proposed structures of the native intermediate of the multicopper oxidases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yoon, J., Mirica, L. M., Stack, T. D., Solomon, E. I. 2005; 127 (39): 13680-13693

  Abstract

  Multicopper oxidases catalyze the 4e- reduction of O2 to H2O. Reaction of the fully reduced enzyme with O2 produces the native intermediate (NI) that consists of four oxidized Cu centers, three of which form a trinuclear cluster site, all bridged by the product of full O2 reduction. The most characteristic feature of NI is the intense magnetic circular dichroism pseudo-A feature (a pair of temperature-dependent C-terms with opposite signs) associated with O --> Cu(II) ligand-to-metal charge transfer (LMCT) that derives from the strong Cu-O bonds in the trinuclear site. In this study, the two most plausible Cu-O structures of the trinuclear site, the tris-mu2-hydroxy-bridged and the mu3-oxo-bridged structures, are evaluated through spectroscopic and electronic structure studies on relevant model complexes, TrisOH and mu3O. It is found that the two components of a pseudo-A-term for TrisOH are associated with LMCT to the same Cu that are coupled by a metal-centered excited-state spin-orbit coupling (SOC), whereas for mu3O they are associated with LMCT to different Cu centers that are coupled by oxo-centered excited state SOC. Based on this analysis of the two candidate models, only the mu3-oxo-bridged structure is consistent with the spectroscopic properties of NI. The Cu-O sigma-bonds in the mu3-oxo-bridged structure would provide the thermodynamic driving force for the 4e- reduction of O2 and would allow the facile electron transfer to all Cu centers in the trinuclear cluster that is consistent with its involvement in the catalytic cycle.

  View details for DOI 10.1021/ja0525152

  View details for Web of Science ID 000232257100058

  View details for PubMedID 16190734

• Geometric and electronic structure of the heme-peroxo-copper complex [(F8TPP)Fe-III-(O-2(2-))-Cu-II(TMPA)](CIO4) JOURNAL OF THE AMERICAN CHEMICAL SOCIETY del Rio, D., Sarangi, R., Chufan, E. E., Karlin, K. D., Hedman, B., Hodgson, K. O., Solomon, E. I. 2005; 127 (34): 11969-11978

  Abstract

  The geometric and electronic structure of the untethered heme-peroxo-copper model complex [(F(8)TPP)Fe(III)-(O(2)(2)(-))-Cu(II)(TMPA)](ClO(4)) (1) has been investigated using Cu and Fe K-edge EXAFS spectroscopy and density functional theory calculations in order to describe its geometric and electronic structure. The Fe and Cu K-edge EXAFS data were fit with a Cu...Fe distance of approximately 3.72 A. Spin-unrestricted DFT calculations for the S(T) = 2 spin state were performed on [(P)Fe(III)-(O(2)(2)(-))-Cu(II)(TMPA)](+) as a model of 1. The peroxo unit is bound end-on to the copper, and side-on to the high-spin iron, for an overall mu-eta(1):eta(2) coordination mode. The calculated Cu...Fe distance is approximately 0.3 A longer than that observed experimentally. Reoptimization of [(P)Fe(III)-(O(2)(2)(-))-Cu(II)(TMPA)](+) with a 3.7 A Cu...Fe constrained distance results in a similar energy and structure that retains the overall mu-eta(1):eta(2)-peroxo coordination mode. The primary bonding interaction between the copper and the peroxide involves electron donation into the half-occupied Cu d(z)2 orbital from the peroxide pi(sigma) orbital. In the case of the Fe(III)-peroxide eta(2) bond, the two major components arise from the donor interactions of the peroxide pi*(sigma) and pi*(v) orbitals with the Fe d(xz) and d(xy) orbitals, which give rise to sigma and delta bonds, respectively. The pi*(sigma) interaction with both the half-occupied d(z)2 orbital on the copper (eta(1)) and the d(xz) orbital on the iron (eta(2)), provides an effective superexchange pathway for strong antiferromagnetic coupling between the metal centers.

  View details for DOI 10.1021/ja043374r

  View details for PubMedID 16117536

• Sulfur K-edge XAS and DFT calculations on P450 model complexes: Effects of hydrogen bonding on electronic structure and redox potentials JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Okamura, T., Ueyama, N., Hedman, B., Hodgson, K. O., Solomon, E. I. 2005; 127 (34): 12046-12053

  Abstract

  Hydrogen bonding (H-bonding) is generally thought to play an important role in tuning the electronic structure and reactivity of metal-sulfur sites in proteins. To develop a quantitative understanding of this effect, S K-edge X-ray absorption spectroscopy (XAS) has been employed to directly probe ligand-metal bond covalency, where it has been found that protein active sites are significantly less covalent than their related model complexes. Sulfur K-edge XAS data are reported here on a series of P450 model complexes with increasing H-bonding to the ligated thiolate from its substituent. The XAS spectroscopic results show a dramatic decrease in preedge intensity. DFT calculations reproduce these effects and show that the observed changes are in fact solely due to H-bonding and not from the inductive effect of the substituent on the thiolate. These calculations also indicate that the H-bonding interaction in these systems is mainly dipolar in nature. The -2.5 kcal/mol energy of the H-bonding interaction was small relative to the large change in ligand-metal bond covalency (30%) observed in the data. A bond decomposition analysis of the total energy is developed to correlate the preedge intensity change to the change in Fe-S bonding interaction on H-bonding. This effect is greater for the reduced than the oxidized state, leading to a 260 mV increase in the redox potential. A simple model shows that E degrees should vary approximately linearly with the covalency of the Fe-S bond in the oxidized state, which can be determined directly from S K-edge XAS.

  View details for DOI 10.1021/ja0519031

  View details for PubMedID 16117545

• A combined quantum and molecular mechanical study of the O-2 reductive cleavage in the catalytic cycle of multicopper oxidases INORGANIC CHEMISTRY Rulisek, L., SOLOMON, E. I., Ryde, U. 2005; 44 (16): 5612-5628

  Abstract

  The four-electron reduction of dioxygen to water in multicopper oxidases takes place in a trinuclear copper cluster, which is linked to a mononuclear blue copper site, where the substrates are oxidized. Recently, several intermediates in the catalytic cycle have been spectroscopically characterized, and two possible structural models have been suggested for both the peroxy and native intermediates. In this study, these spectroscopic results are complemented by hybrid quantum and molecular mechanical (QM/MM) calculations, taking advantage of recently available crystal structures with a full complement of copper ions. Thereby, we obtain optimized molecular structures for all of the experimentally studied intermediates involved in the reductive cleavage of the O(2) molecule and energy profiles for individual reaction steps. This allows identification of the experimentally observed intermediates and further insight into the reaction mechanism that is probably relevant for the whole class of multicopper oxidases. We suggest that the peroxy intermediate contains an O(2)(2-) ion, in which one oxygen atom bridges the type 2 copper ion and one of the type 3 copper ions, whereas the other one coordinates to the other type 3 copper ion. One-electron reduction of this intermediate triggers the cleavage of the O-O bond, which involves the uptake of a proton. The product of this cleavage is the observed native intermediate, which we suggest to contain a O(2-) ion coordinated to all three of the copper ions in the center of the cluster.

  View details for DOI 10.1021/ic050092z

  View details for Web of Science ID 000231030900012

  View details for PubMedID 16060610

• Spectroscopic and DFT investigation of [M{HB(3,5-(i)Pr(2)pz)(3)}(SC6F5)] (M = Mn, Fe, Co, Ni, Cu, and Zn) model complexes: Periodic trends in metal-thiolate bonding INORGANIC CHEMISTRY Gorelsky, S. I., Basumallick, L., Vura-Weis, J., Sarangi, R., Hodgson, K. O., Hedman, B., Fujisawa, K., Solomon, E. I. 2005; 44 (14): 4947-4960

  Abstract

  A series of metal-varied [ML(SC6F5)] model complexes (where L = hydrotris(3,5-diisopropyl-1-pyrazolyl)borate and M = Mn, Fe, Co, Ni, Cu, and Zn) related to blue copper proteins has been studied by a combination of absorption, MCD, resonance Raman, and S K-edge X-ray absorption spectroscopies. Density functional calculations have been used to characterize these complexes and calculate their spectra. The observed variations in geometry, spectra, and bond energies are interpreted in terms of changes in the nature of metal-ligand bonding interactions. The metal 3d-ligand orbital interaction, which contributes to covalent bonding in these complexes, becomes stronger going from Mn(II) to Co(II) (the sigma contribution) and to Cu(II) (the pi contribution). This change in the covalency results from the increased effective nuclear charge of the metal atom in going from Mn(II) to Zn(II) and the change in the 3d orbital populations (d5-->d10). Ionic bonding also plays an important role in determining the overall strength of the ML(+)-SC6F5(-) interaction. However, there is a compensating effect: as the covalent contribution to the metal-ligand bonding increases, the ionic contribution decreases. These results provide insight into the Irving-Williams series, where it is found that the bonding of the ligand being replaced by the thiolate makes a major contribution to the observed order of the stability constants over the series of metal ions.

  View details for DOI 10.1021/ic050371m

  View details for PubMedID 15998022

• Tyrosinase reactivity in a model complex: An alternative hydroxylation mechanism SCIENCE Mirica, L. M., Vance, M., Rudd, D. J., Hedman, B., Hodgson, K. O., Solomon, E. I., Stack, T. D. 2005; 308 (5730): 1890-1892

  Abstract

  The binuclear copper enzyme tyrosinase activates O2 to form a mu-eta2:eta2-peroxodicopper(II) complex, which oxidizes phenols to catechols. Here, a synthetic mu-eta2:eta2-peroxodicopper(II) complex, with an absorption spectrum similar to that of the enzymatic active oxidant, is reported to rapidly hydroxylate phenolates at -80 degrees C. Upon phenolate addition at extreme temperature in solution (-120 degrees C), a reactive intermediate consistent with a bis-mu-oxodicopper(III)-phenolate complex, with the O-O bond fully cleaved, is observed experimentally. The subsequent hydroxylation step has the hallmarks of an electrophilic aromatic substitution mechanism, similar to tyrosinase. Overall, the evidence for sequential O-O bond cleavage and C-O bond formation in this synthetic complex suggests an alternative intimate mechanism to the concerted or late stage O-O bond scission generally accepted for the phenol hydroxylation reaction performed by tyrosinase.

  View details for DOI 10.1126/science.1112081

  View details for Web of Science ID 000230120000034

  View details for PubMedID 15976297

• Role of aspartate 94 in the decay of the peroxide intermediate in the multicopper oxidase Fet3p BIOCHEMISTRY Quintanar, L., Stoj, C., Wang, T. P., Kosman, D. J., Solomon, E. J. 2005; 44 (16): 6081-6091

  Abstract

  Fet3p is a multicopper oxidase that contains four Cu ions: one type 1, one type 2, and a coupled binuclear type 3 site. The type 2 and type 3 centers form a trinuclear cluster that is the active site for O(2) reduction to H(2)O. When the type 1 Cu is depleted (C484S mutation), the reaction of the reduced trinuclear cluster with O(2) generates a peroxide intermediate. Kinetic studies of the decay of the peroxide intermediate suggest that a carboxyl residue (D94 in Fet3p) assists the reductive cleavage of the O-O bond at low pH. Mutations at the D94 residue (D94A, D94N, and D94E) have been studied to evaluate its role in the decay of the peroxide intermediate. Spectroscopic studies show that the D94 mutations affect the geometric and electronic structure of the trinuclear cluster in a way that is consistent with the hydrogen bond connectivity of D94. While the D94E mutation does not affect the initial reaction of the cluster with O(2), the D94A mutation causes larger structural changes that render the trinuclear cluster unreactive toward O(2), demonstrating a structural role for the D94 residue. The decay of the peroxide intermediate is markedly affected by the D94E mutation, confirming the involvement of D94 in this reaction. The D94 residue appears to activate a proton of the type 2 Cu(+)-bound water for participation in the transition state. These studies provide new insight into the role of D94 and proton involvement in the reductive cleavage of the O-O bond.

  View details for DOI 10.1021/bi047379c

  View details for Web of Science ID 000228678900014

  View details for PubMedID 15835897

• Dioxygen activation by copper, heme and non-heme iron enzymes: comparison of electronic structures and reactivities CURRENT OPINION IN CHEMICAL BIOLOGY Decker, A., Solomon, E. I. 2005; 9 (2): 152-163

  Abstract

  Enzymes containing heme, non-heme iron and copper active sites play important roles in the activation of dioxygen for substrate oxidation. One key reaction step is CH bond cleavage through H-atom abstraction. On the basis of the ligand environment and the redox properties of the metal, these enzymes employ different methods of dioxygen activation. Heme enzymes are able to stabilize the very reactive iron(IV)-oxo porphyrin-radical intermediate. This is generally not accessible for non-heme iron systems, which can instead use low-spin ferric-hydroperoxo and iron(IV)-oxo species as reactive oxidants. Copper enzymes employ still a different strategy and achieve H-atom abstraction potentially through a superoxo intermediate. This review compares and contrasts the electronic structures and reactivities of these various oxygen intermediates.

  View details for DOI 10.1016/j,cbpa.2005.02.012

  View details for Web of Science ID 000228607700009

  View details for PubMedID 15811799

• Spectroscopy of non-heme iron thiolate complexes: Insight into the electronic structure of the low-spin active site of nitrile hydratase INORGANIC CHEMISTRY Kennepohl, P., Neese, F., Schweitzer, D., Jackson, H. L., Kovacs, J. A., Solomon, E. I. 2005; 44 (6): 1826-1836

  Abstract

  Detailed spectroscopic and computational studies of the low-spin iron complexes [Fe(III)(S2(Me2)N3 (Pr,Pr))(N3)] (1) and [Fe(III)(S2(Me2)N3 (Pr,Pr))]1+ (2) were performed to investigate the unique electronic features of these species and their relation to the low-spin ferric active sites of nitrile hydratases. Low-temperature UV/vis/NIR and MCD spectra of 1 and 2 reflect electronic structures that are dominated by antibonding interactions of the Fe 3d manifold and the equatorial thiolate S 3p orbitals. The six-coordinate complex 1 exhibits a low-energy S(pi) --> Fe 3d(xy) (approximately 13,000 cm(-1)) charge-transfer transition that results predominantly from the low energy of the singly occupied Fe 3d(xy) orbital, due to pure pi interactions between this acceptor orbital and both thiolate donor ligands in the equatorial plane. The 3d(pi) --> 3d(sigma) ligand-field transitions in this species occur at higher energies (>15,000 cm(-1)), reflecting its near-octahedral symmetry. The Fe 3d(xz,yz) --> Fe 3d(xy) (d(pi) --> d(pi)) transition occurs in the near-IR and probes the Fe(III)-S pi-donor bond; this transition reveals vibronic structure that reflects the strength of this bond (nu(e) approximately 340 cm(-1)). In contrast, the ligand-field transitions of the five-coordinate complex 2 are generally at low energy, and the S(pi) --> Fe charge-transfer transitions occur at much higher energies relative to those in 1. This reflects changes in thiolate bonding in the equatorial plane involving the Fe 3d(xy) and Fe 3d(x2-y2) orbitals. The spectroscopic data lead to a simple bonding model that focuses on the sigma and pi interactions between the ferric ion and the equatorial thiolate ligands, which depend on the S-Fe-S bond angle in each of the complexes. These electronic descriptions provide insight into the unusual S = 1/2 ground spin state of these complexes: the orientation of the thiolate ligands in these complexes restricts their pi-donor interactions to the equatorial plane and enforces a low-spin state. These anisotropic orbital considerations provide some intriguing insights into the possible electronic interactions at the active site of nitrile hydratases and form the foundation for further studies into these low-spin ferric enzymes.

  View details for DOI 10.1021/ic0487068

  View details for Web of Science ID 000227764700020

  View details for PubMedID 15762709

• Spectroscopic and density functional studies of the red copper site in nitrosocyanin: Role of the protein in determining active site geometric and electronic structure JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Basumallick, L., Sarangi, R., George, S. D., Elmore, B., Hooper, A. B., Hedman, B., Hodgson, K. O., Solomon, E. I. 2005; 127 (10): 3531-3544

  Abstract

  The electronic structure of the red copper site in nitrosocyanin is defined relative to that of the well understood blue copper site of plastocyanin by using low-temperature absorption, circular dichroism, magnetic circular dichroism, resonance Raman, EPR and X-ray absorption spectroscopies, combined with DFT calculations. These studies indicate that the principal electronic structure change in the red copper site is the sigma rather than the pi donor interaction of the cysteine sulfur with the Cu 3d(x2-y2) redox active molecular orbital (RAMO). Further, MCD data show that there is an increase in ligand field strength due to an increase in coordination number, whereas resonance Raman spectra indicate a weaker Cu-S bond. The latter is supported by the S K-edge data, which demonstrate a less covalent thiolate interaction with the RAMO of nitrosocyanin at 20% relative to plastocyanin at 38%. EXAFS results give a longer Cu-S(Cys) bond distance in nitrosocyanin (2.28 A) compared to plastocyanin (2.08 A) and also show a large change in structure with reduction of the red copper site. The red copper site is the only presently known blue copper-related site with an exogenous water coordinated to the copper. Density functional calculations reproduce the experimental properties and are used to determine the specific protein structure contributions to exogenous ligand binding in red copper. The relative orientation of the CuNNS and the CuSC(beta) planes (determined by the protein sequence) is found to be key in generating an exchangeable coordination position at the red copper active site. The exogenous water ligation at the red copper active site greatly increases the reorganization energy (by approximately 1.0 eV) relative to that of the blue copper protein site, making the red site unfavorable for fast outer-sphere electron transfer, while providing an exchangeable coordination position for inner-sphere electron transfer.

  View details for DOI 10.1021/ja044412+

  View details for PubMedID 15755175

• Preface forum: "Functional insight from physical methods on metalloenzymes" INORGANIC CHEMISTRY SOLOMON, E. I. 2005; 44 (4): 723-726

  View details for DOI 10.1021/ic040127f

  View details for Web of Science ID 000227172200001

  View details for PubMedID 15859241

• Metal and ligand K-Edge XAS of organotitanium complexes: Metal 4p and 3d contributions to pre-edge intensity and their contributions to bonding JOURNAL OF THE AMERICAN CHEMICAL SOCIETY George, S. D., Brant, P., Solomon, E. I. 2005; 127 (2): 667-674

  Abstract

  Titanium cyclopentadienyl (Cp) complexes play important roles as homogeneous polymerization catalysts and have recently received attention as potential anticancer agents. To systematically probe the contribution of the Cp to bonding in organotitanium complexes, Ti K-edge XAS has been applied to TiCl(4) and then to the mono- and bis-Cp complexes, TiCpCl(3) and TiCp(2)Cl(2). Ti K-edge XAS is used as a direct probe of metal 3d-4p mixing and provides insight into the contribution of the Cp to bonding. These data are complimented by Cl K-edge XAS data, which provide a direct probe of the effect of the Cp on the bonding to the spectator chloride ligand. The experimental results are correlated to DFT calculations. A model for metal 3d-4p mixing is proposed, which is based on covalent interactions with the ligands and demonstrates that metal K-pre-edge intensities may be used as a measure of ligand-metal covalency in molecular Ti(IV) systems in noncentrosymmetric environments.

  View details for DOI 10.1021/ja044827v

  View details for PubMedID 15643891

• Comparison of Fe-IV = O heme and non-heme species: Electronic structures, bonding, and reactivities ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Decker, A., Solomon, E. I. 2005; 44 (15): 2252-2255

  View details for DOI 10.1002/anie.200462182

  View details for Web of Science ID 000228415900016

  View details for PubMedID 15719352

• Investigation of the local structure of Fe(II) bleomycin and peplomycins using theoretical analysis of XANES PHYSICA SCRIPTA Smolentsev, G., Soldatov, A. V., Wasinger, E., Solomon, E., Hodgson, K., Hedman, B. 2005; T115: 862-863

  View details for Web of Science ID 000204272100258

• Structure-function correlations in oxygen activating non-heme iron enzymes CHEMICAL COMMUNICATIONS Neidig, M. L., Solomon, E. I. 2005: 5843-5863

  Abstract

  A large group of mononuclear non-heme iron enzymes exist which activate dioxygen to catalyze key biochemical transformations, including many of medical, pharmaceutical and environmental significance. These enzymes utilize high-spin Fe(II) active sites and additional reducing equivalents from cofactors or substrates to react with O2 to yield iron-oxygen intermediates competent to transform substrate to product. While Fe(II) sites have been difficult to study due to the lack of dominant spectroscopic features, a spectroscopic methodology has been developed which allows the elucidation of the geometric and electronic structures of these active sites and provides molecular level insight into the mechanisms of catalysis. This review provides a summary of this methodology with emphasis on its application to the determination of important active site structure-function correlations in mononuclear non-heme iron enzymes. These studies provide key insight into the mechanisms of oxygen activation, active site features that contribute to differences in reactivity and, combined with theoretical calculations and model studies, the nature of oxygen intermediates active in catalysis.

  View details for DOI 10.1039/b510233m

  View details for Web of Science ID 000233775600004

  View details for PubMedID 16317455

• Ligand K-edge X-ray absorption spectroscopy and DFT calculations on [Fe3S4](0,+) clusters: Delocalization, redox, and effect of the protein environment JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Glaser, T., Moura, J. J., HOLM, R. H., Hedman, B., Hodgson, K. O., Solomon, E. I. 2004; 126 (51): 16868-16878

  Abstract

  Ligand K-edge XAS of an [Fe3S4]0 model complex is reported. The pre-edge can be resolved into contributions from the mu(2)S(sulfide), mu(3)S(sulfide), and S(thiolate) ligands. The average ligand-metal bond covalencies obtained from these pre-edges are further distributed between Fe(3+) and Fe(2.5+) components using DFT calculations. The bridging ligand covalency in the [Fe2S2]+ subsite of the [Fe3S4]0 cluster is found to be significantly lower than its value in a reduced [Fe2S2] cluster (38% vs 61%, respectively). This lowered bridging ligand covalency reduces the superexchange coupling parameter J relative to its value in a reduced [Fe2S2]+ site (-146 cm(-1) vs -360 cm(-1), respectively). This decrease in J, along with estimates of the double exchange parameter B and vibronic coupling parameter lambda2/k(-), leads to an S = 2 delocalized ground state in the [Fe3S4]0 cluster. The S K-edge XAS of the protein ferredoxin II (Fd II) from the D. gigas active site shows a decrease in covalency compared to the model complex, in the same oxidation state, which correlates with the number of H-bonding interactions to specific sulfur ligands present in the active site. The changes in ligand-metal bond covalencies upon redox compared with DFT calculations indicate that the redox reaction involves a two-electron change (one-electron ionization plus a spin change of a second electron) with significant electronic relaxation. The presence of the redox inactive Fe(3+) center is found to decrease the barrier of the redox process in the [Fe3S4] cluster due to its strong antiferromagnetic coupling with the redox active Fe2S2 subsite.

  View details for DOI 10.1021/ja0466208

  View details for Web of Science ID 000225910400045

  View details for PubMedID 15612726

• Spectroscopic demonstration of a large antisymmetric exchange contribution to the spin-frustrated ground state of a D-3 symmetric hydroxy-bridged trinuclear Cu(II) complex: Ground-to-excited state superexchange pathways JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yoon, J., Mirica, L. M., Stack, T. D., Solomon, E. I. 2004; 126 (39): 12586-12595

  Abstract

  The magnetic and electronic properties of a spin-frustrated ground state of an antiferromagnetically coupled 3-fold symmetric trinuclear copper complex (TrisOH) is investigated using a combination of variable-temperature variable-field magnetic circular dichroism (VTVH MCD) and powder/single-crystal EPR. Direct evidence for a low-lying excited S = (1)/(2) state from the zero-field split ground (2)E state is provided by the nonlinear dependence of the MCD intensity on 1/T and the nesting of the VTVH MCD isotherms. A consistent zero-field splitting (Delta) value of approximately 65 cm(-1) is obtained from both approaches. In addition, the strong angular dependence of the single-crystal EPR spectrum, with effective g-values from 2.32 down to an unprecedented 1.2, requires in-state spin-orbit coupling of the (2)E state via antisymmetric exchange. The observable EPR intensities also require lowering of the symmetry of the trimer structure, likely reflecting a magnetic Jahn-Teller effect. Thus, the Delta of the ground (2)E state is shown to be governed by the competing effects of antisymmetric exchange (G = 36.0 +/- 0.8 cm(-1)) and symmetry lowering (delta = 17.5 +/- 5.0 cm(-1)). G and delta have opposite effects on the spin distribution over the three metal sites where the former tends to delocalize and the latter tends to localize the spin of the S(tot) = (1)/(2) ground state on one metal center. The combined effects lead to partial delocalization, reflected by the observed EPR parallel hyperfine splitting of 74 x 10(-4) cm(-1). The origin of the large G value derives from the efficient superexchange pathway available between the ground d(x2-y2) and excited d(xy) orbitals of adjacent Cu sites, via strong sigma-type bonds with the in-plane p-orbitals of the bridging hydroxy ligands. This study provides significant insight into the orbital origin of the spin Hamiltonian parameters of a spin-frustrated ground state of a trigonal copper cluster.

  View details for DOI 10.1021/ja046380w

  View details for Web of Science ID 000224219900078

  View details for PubMedID 15453791

• O-2 activation by binuclear Cu sites: Noncoupled versus exchange coupled reaction mechanisms PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Chen, P., Solomon, E. I. 2004; 101 (36): 13105-13110

  Abstract

  Binuclear Cu proteins play vital roles in O(2) binding and activation in biology and can be classified into coupled and noncoupled binuclear sites based on the magnetic interaction between the two Cu centers. Coupled binuclear Cu proteins include hemocyanin, tyrosinase, and catechol oxidase. These proteins have two Cu centers strongly magnetically coupled through direct bridging ligands that provide a mechanism for the 2-electron reduction of O(2) to a mu-eta(2):eta(2) side-on peroxide bridged Cu(II)(2)(O(2)(2-)) species. This side-on bridged peroxo-Cu(II)(2) species is activated for electrophilic attack on the phenolic ring of substrates. Noncoupled binuclear Cu proteins include peptidylglycine alpha-hydroxylating monooxygenase and dopamine beta-monooxygenase. These proteins have binuclear Cu active sites that are distant, that exhibit no exchange interaction, and that activate O(2) at a single Cu center to generate a reactive Cu(II)/O(2) species for H-atom abstraction from the C-H bond of substrates. O(2) intermediates in the coupled binuclear Cu enzymes can be trapped and studied spectroscopically. Possible intermediates in noncoupled binuclear Cu proteins can be defined through correlation to mononuclear Cu(II)/O(2) model complexes. The different intermediates in these two classes of binuclear Cu proteins exhibit different reactivities that correlate with their different electronic structures and exchange coupling interactions between the binuclear Cu centers. These studies provide insight into the role of exchange coupling between the Cu centers in their reaction mechanisms.

  View details for DOI 10.1073/pnas.0402114101

  View details for Web of Science ID 000223799100003

  View details for PubMedID 15340147

  View details for PubMedCentralID PMC516532

• O-2 and N2O activation by copper active sites in biology. Meeting of the Division of Chemical Toxicology of the American-Chemical-Society held at the 228th National Meeting of the American-Chemical-Society Chen, P. AMER CHEMICAL SOC. 2004: U39–U39

  View details for Web of Science ID 000223712800100

• Nature of the peroxo intermediate of the W48F/D84E ribonucleotide reductase variant: Implications for O-2 activation by binuclear non-heme iron enzymes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Skulan, A. J., Brunold, T. C., Baldwin, J., Saleh, L., Bollinger, J. M., SOLOMON, E. I. 2004; 126 (28): 8842-8855

  Abstract

  Analysis of the spectroscopic signatures of the R2-W48F/D84E biferric peroxo intermediate identifies a cis mu-1,2 peroxo coordination geometry. DFT geometry optimizations on both R2-W48F/D84E and R2-wild-type peroxo intermediate models including constraints imposed by the protein also identify the cis mu-1,2 peroxo geometry as the most stable peroxo intermediate structure. This study provides significant insight into the electronic structure and reactivity of the R2-W48F/D84E peroxo intermediate, structurally related cis mu-1,2 peroxo model complexes, and other enzymatic biferric peroxo intermediates.

  View details for DOI 10.1021/ja049105a

  View details for Web of Science ID 000222704700061

  View details for PubMedID 15250738

• Solvent effects on the conversion of dicopper(II) mu-eta(2):eta(2)-peroxo to bis-mu-oxo dicopper(III) complexes: Direct probing of the solvent interaction INORGANIC CHEMISTRY Liang, H. C., Henson, M. J., Hatcher, L. Q., Vance, M. A., Zhang, C. X., Lahti, D., Kaderli, S., Sommer, R. D., Rheingold, A. L., Zuberbuhler, A. D., SOLOMON, E. I., Karlin, K. D. 2004; 43 (14): 4115-4117

  Abstract

  A new tridentate ligand, PYAN, is employed to investigate solvent influences for dioxygen reactivity with [Cu(PYAN)(MeCN)]B(C(6)F(5))(4) (1). Stopped-flow kinetic studies confirm that the adducts [[u(II)(PYAN)]2)(O(2))][B(C(6)F(5))(4)](2) (2(Peroxo)) and [[u(III)(PYAN)]2)(O)(2)][B(C(6)F(5))(4)](2) (2(Oxo)) are in rapid equilibrium. Thermodynamic parameters for the equilibrium between 2(Peroxo) and 2(Oxo) re as follows: THF, deltaH degrees approximately -15.7 kJ/mol, deltaS degrees approximately -83 J/K.mol; acetone, deltaH degrees approximately -15.8 kJ/mol, deltaS degrees approximately -76 J/K.mol. UV-visible absorption and resonance Raman spectroscopic signatures demonstrate that the equilibrium is highly solvent dependent; the mixture is mostly 2(Peroxo) in CH(2)Cl(2), but there are significantly increasing quantities of 2(Oxo) along the series methylene chloride --> diethyl ether --> acetone --> tetrahydrofuran (THF). Copper(II)-N(eq) stretches (239, 243, 244, and 246 cm(-)(1) in CH(2)Cl(2), Et(2)O, acetone, and THF, respectively) are identified for 2(Peroxo), but they are not seen in 2(Oxo), revealing for the first time direct evidence for solvent coordination in the more open 2(Peroxo) structure.

  View details for DOI 10.1021/ic0498283

  View details for Web of Science ID 000222541700006

  View details for PubMedID 15236520

• Ligand K-Edge X-ray absorption spectroscopy of [Fe4S4](1+,2+,3+) clusters: Changes in bonding and electronic relaxation upon redox JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Glaser, T., Couture, M. M., Eltis, L. D., HOLM, R. H., Hedman, B., Hodgson, K. O., Solomon, E. I. 2004; 126 (26): 8320-8328

  Abstract

  Sulfur K-edge X-ray absorption spectroscopy (XAS) is reported for [Fe(4)S(4)](1+,2+,3+) clusters. The results are quantitatively and qualitatively compared with DFT calculations. The change in covalency upon redox in both the [Fe(4)S(4)](1+/2+) (ferredoxin) and the [Fe(4)S(4)](2+/3+) (HiPIP) couple are much larger than that expected from just the change in number of 3d holes. Moreover, the change in the HiPIP couple is higher than that of the ferredoxin couple. These changes in electronic structure are analyzed using DFT calculations in terms of contributions from the nature of the redox active molecular orbital (RAMO) and electronic relaxation. The results indicate that the RAMO of HiPIP has 50% ligand character, and hence, the HiPIP redox couple involves limited electronic relaxation. Alternatively, the RAMO of the ferredoxin couple is metal-based, and the ferredoxin redox couple involves extensive electronic relaxation. The contributions of these RAMO differences to ET processes in the different proteins are discussed.

  View details for Web of Science ID 000222405400058

  View details for PubMedID 15225075

• Ferrous binding to the multicopper oxidases Saccharomyces cerevisiae Fet3p and human ceruloplasmin: Contributions to ferroxidase activity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Quintanar, L., Gebhard, M., Wang, T. P., Kosman, D. J., Solomon, E. I. 2004; 126 (21): 6579-6589

  Abstract

  The multicopper oxidases are a family of enzymes that couple the reduction of O(2) to H(2)O with the oxidation of a range of substrates. Saccharomyces cerevisiae Fet3p and human ceruloplasmin (hCp) are members of this family that exhibit ferroxidase activity. Their high specificity for Fe(II) has been attributed to the existence of a binding site for iron. In this study, mutations at the E185 and Y354 residues, which are putative ligands for iron in Fet3p, have been generated and characterized. The effects of these mutations on the electronic structure of the T1 Cu site have been assessed, and the reactivities of this site toward 1,4-hydroquinone (a weak binding substrate) and Fe(II) have been evaluated and interpreted in terms of the semiclassical Marcus theory for electron transfer. The electronic and geometric structure of the Fe(II) substrate bound to Fet3p and hCp has been studied for the first time, using variable-temperature variable field magnetic circular dichroism (VTVH MCD) spectroscopy. The iron binding sites in Fet3p and hCp appear to be very similar in nature, and their contributions to the ferroxidase activity of these proteins have been analyzed. It is found that these iron binding sites play a major role in tuning the reduction potential of iron to provide a large driving force for the ferroxidase reaction, while still supporting the delivery of the Fe(III) product to the acceptor protein. Finally, the analysis of possible electron-transfer (ET) pathways from the protein-bound Fe(II) to the T1 Cu site indicates that the E185 residue not only plays a role in iron binding, but also provides the dominant ET pathway to the T1 Cu site.

  View details for DOI 10.1021/ja049220t

  View details for Web of Science ID 000221671400038

  View details for PubMedID 15161286

• Photoelectron spectroscopic and electronic structure studies of CH2O bonding and reactivity on ZnO surfaces: Steps in the methanol synthesis reaction INORGANIC CHEMISTRY Jones, P. M., May, J. A., Reitz, J. B., Solomon, E. I. 2004; 43 (11): 3349-3370

  Abstract

  Adsorption of CH(2)O on ZnO(0001) has been investigated using XPS, NEXAFS, variable-energy photoelectron spectroscopy (PES), and density functional theory (DFT) calculations. CH(2)O is chemisorbed on the (0001) surface at 130 K. Its C1s XPS peak position at 292.7 eV and NEXAFS sigma shape resonance at 302.6 eV are consistent with an eta(1) bound surface geometry. Geometry optimized DFT calculations also indicate that CH(2)O is bound to the Zn(II) site in an eta(1) configuration through its oxygen atom. The variable-energy PES of the eta(1) bound CH(2)O/ZnO(0001) complex exhibits four valence band features at 21.2, 16.4, 13.8, and 10.7 eV below the vacuum level providing an experimental and theoretical description of this surface interaction. Annealing the ZnO(0001)/CH(2)O surface complex to 220 K decomposes the chemisorbed CH(2)O, producing formyl (291.5 eV), methoxide (290.2 eV), and formate (293.6 eV) intermediates. Thus this reaction coordinate involves the conversion of an oxygen bound formaldehyde to a carbon bound formyl species on ZnO(0001). Only formate is formed on the ZnO(100) surface. DFT is used to explore surface intermediates and the transition state in the methanol synthesis reaction (MSR). The bonding interactions of H(2), CO, CH(3)O(-), HCO(-), and trans-HCOH to the ZnO(0001) surface are elucidated using geometry optimization. H(2) was found to be heterolytically cleaved on the ZnO(0001) surface, and carbon monoxide, formyl, and methoxide are calculated to be eta(1) bound. These results are consistent with observed metal oxide surface reactivity where heterolytic bond cleavage is dominant. The oxygen atom in the bound formyl was found to be activated for attack by a proton. This results in the planar eta(1) bound trans-HCOH surface species. The transition state in the gas phase rearrangement of trans-HCOH to formaldehyde was calculated to have a barrier of 31 kcal/mol. The correlation diagram for this rearrangement in the gas phase indicates that configuration interaction at the crossing of two levels helps to lower the barrier. A transition state calculation was also performed for this rearrangement on the ZnO(0001) surface. The surface transition state geometry is significantly different than the gas phase. The surface geometry is no longer planar (23 degrees dihedral angle) and is displaced parallel to the surface. Interaction with the Zn(II) site at the crossing of surface bound CH(2)O and trans-HCOH levels further lowers the barrier to rearrangement relative to gas phase by 9 kcal/mol. The rearrangement of trans-HCOH (carbon bound) to CH(2)O (oxygen bound) on ZnO(0001) was calculated to be the overall barrier of the MSR reaction.

  View details for DOI 10.1021/ic035252q

  View details for Web of Science ID 000221684900009

  View details for PubMedID 15154797

• Oxygen activation by the noncoupled binuclear copper site in peptidylglycine alpha-hydroxylating monooxygenase. Spectroscopic definition of the resting sites and the putative Cu-M(II)-OOH intermediate BIOCHEMISTRY Chen, P., Bell, J., EIPPER, B. A., Solomon, E. I. 2004; 43 (19): 5735-5747

  Abstract

  Spectroscopic methods, density functional calculations, and ligand field analyses are combined to define the geometric models and electronic structure descriptions of the Cu(M) and Cu(H) sites in the oxidized form of the noncoupled binuclear copper protein peptidylglycine alpha-hydroxylating monooxygenase (PHM). The Cu(M) site has a square pyramidal geometry with a long axial Cu-methionine bond and two histidines, H(2)O, and OH(-) as equatorial ligands. The Cu(H) site has a slightly D(2)(d) distorted square planar geometry with three histidines and H(2)O ligands. The structurally inequivalent Cu(M) and Cu(H) sites do not exhibit measurable differences in optical and electron paramagnetic resonance (EPR) spectra, which result from their similar ligand field transition energies and ground-state Cu covalencies. The additional axial methionine ligand interaction and associated square pyramidal distortion of the Cu(M) site have the opposite effect of the strong equatorial OH(-) donor ligand on the Cu d orbital splitting pattern relative to the Cu(H) site leading to similar ligand field transition energies for both sites. The small molecule NO(2)(-) binds in different coordination modes to the Cu(M) and Cu(H) site because of differences in their exchangeable coordination positions resulting in these Cu(II) sites being spectroscopically distinguishable. Azide binding to PHM is used as a spectroscopic and electronic structure analogue to OOH(-) binding to provide a starting point for developing a geometric and electronic structural model for the putative Cu(II)(M)-OOH intermediate in the H-atom abstraction reaction of PHM. Possible electronic structure contributions of the Cu(II)(M)-OOH intermediate to reactivity are considered by correlation to the well-studied L3Cu(II)-OOH model complex (L3 = [HB[3-tBu-5-iPrpz](3)]). The Met-S ligand of the Cu(M) site is found to contribute to the stabilization of the Cu(II)(M)-oxyl species, which would be a product of Cu(II)(M)-OOH H-atom abstraction reaction. This Met-S contribution could have a significant effect on the energetics of a H-atom abstraction reaction by the Cu(II)(M)-OOH intermediate.

  View details for DOI 10.1021/bi0362830

  View details for Web of Science ID 000221365600018

  View details for PubMedID 15134448

• Spectroscopic and quantum chemical characterization of the electronic structure and bonding in a non-heme FeIV[double bond]O complex. Journal of the American Chemical Society Decker, A., Rohde, J., Que, L., Solomon, E. I. 2004; 126 (17): 5378-5379

  Abstract

  High valent FeIV=O species are key intermediates in the catalytic cycles of many mononuclear non-heme iron enzymes involving the binding and activation of dioxygen. Using variable temperature magnetic circular dichroism (VT MCD) spectroscopy and experimentally calibrated density functional calculations, we are able to present the first detailed description of the electronic structure of a non-heme FeIV=O S = 1 complex. These studies define the nature of the FeIV=O bond and present the basis for understanding high-valent oxygen intermediates in non-heme iron enzymes.

  View details for PubMedID 15113207

• Spectroscopic and quantum chemical characterization of the electronic structure and bonding in a non-heme Fe-IV=O complex JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Decker, A., Rohde, J. U., Que, L., Solomon, E. I. 2004; 126 (17): 5378-5379

  Abstract

  High valent FeIV=O species are key intermediates in the catalytic cycles of many mononuclear non-heme iron enzymes involving the binding and activation of dioxygen. Using variable temperature magnetic circular dichroism (VT MCD) spectroscopy and experimentally calibrated density functional calculations, we are able to present the first detailed description of the electronic structure of a non-heme FeIV=O S = 1 complex. These studies define the nature of the FeIV=O bond and present the basis for understanding high-valent oxygen intermediates in non-heme iron enzymes.

  View details for DOI 10.1021/ja0498033

  View details for Web of Science ID 000221135400022

• Oxygen activation by the noncoupled binuclear copper site in peptidylglycine alpha-hydroxylating monooxygenase. Reaction mechanism and role of the noncoupled nature of the active site JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, P., Solomon, E. I. 2004; 126 (15): 4991-5000

  Abstract

  Reaction thermodynamics and potential energy surfaces are calculated using density functional methods to investigate possible reactive Cu/O(2) species for H-atom abstraction in peptidylglycine alpha-hydroxylating monooxygenase (PHM), which has a noncoupled binuclear Cu active site. Two possible mononuclear Cu/O(2) species have been evaluated, the 2-electron reduced Cu(II)(M)-OOH intermediate and the 1-electron reduced side-on Cu(II)(M)-superoxo intermediate, which could form with comparable thermodynamics at the catalytic Cu(M) site. The substrate H-atom abstraction reaction by the Cu(II)(M)-OOH intermediate is found to be thermodynamically accessible due to the contribution of the methionine ligand, but with a high activation barrier ( approximately 37 kcal/mol, at a 3.0-A active site/substrate distance), arguing against the Cu(II)(M)-OOH species as the reactive Cu/O(2) intermediate in PHM. In contrast, H-atom abstraction from substrate by the side-on Cu(II)(M)-superoxo intermediate is a nearly isoenergetic process with a low reaction barrier at a comparable active site/substrate distance ( approximately 14 kcal/mol), suggesting that side-on Cu(II)(M)-superoxo is the reactive species in PHM. The differential reactivities of the Cu(II)(M)-OOH and Cu(II)(M)-superoxo species correlate to their different frontier molecular orbitals involved in the H-atom abstraction reaction. After the H-atom abstraction, a reasonable pathway for substrate hydroxylation involves a "water-assisted" direct OH transfer to the substrate radical, which generates a high-energy Cu(II)(M)-oxyl species. This provides the necessary driving force for intramolecular electron transfer from the Cu(H) site to complete the reaction in PHM. The differential reactivity pattern between the Cu(II)(M)-OOH and Cu(II)(M)-superoxo intermediates provides insight into the role of the noncoupled nature of PHM and dopamine beta-monooxygenase active sites, as compared to the coupled binuclear Cu active sites in hemocyanin, tyrosinase, and catechol oxidase, in O(2) activation.

  View details for DOI 10.1021/ja031564g

  View details for Web of Science ID 000220849900049

  View details for PubMedID 15080705

• CD and MCD studies of the non-heme ferrous active site in (4-hydroxyphenyl)pyruvate dioxygenase: Correlation between oxygen activation in the extradiol and alpha-KG-dependent dioxygenases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Neidig, M. L., Kavana, M., Moran, G. R., Solomon, E. I. 2004; 126 (14): 4486-4487

  Abstract

  (4-Hydroxyphenyl)pyruvate dioxygenase (HPPD) is an unusual alpha-keto acid-dependent non-heme iron dioxygenase as it incorporates both atoms of dioxygen into a single substrate, paralleling the extradiol dioxygenases. CD/MCD studies of the catalytically active ferrous site and its interaction with substrate reveal a geometic and electronic structure and mechanistic approach to oxygen activation which bridges those of the alpha-KG-dependent and the extradiol dioxygenases.

  View details for DOI 10.1021/ja0316521

  View details for Web of Science ID 000220752300013

  View details for PubMedID 15070344

• Electronic and spectroscopic studies of the non-heme reduced binuclear iron sites of two ribonucleotide reductase variants: Comparison to reduced methane monooxygenase and contributions to O-2 reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Wei, P. P., Skulan, A. J., Mitic, N., Yang, Y. S., Saleh, L., Bollinger, J. M., Solomon, E. I. 2004; 126 (12): 3777-3788

  Abstract

  Circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature variable-field (VTVH) MCD have been used to probe the biferrous active site of two variants of ribonucleotide reductase. The aspartate to glutamate substitution (R2-D84E) at the binuclear iron site modifies the endogenous ligand set of ribonucleotide reductase to match that of the binuclear center in the hydroxylase component of methane monooxygenase (MMOH). The crystal structure of chemically reduced R2-D84E suggests that the active-site structure parallels that of MMOH. However, CD, MCD, and VTVH MCD data combined with spin-Hamiltonian analysis of reduced R2-D84E indicate a different coordination environment relative to reduced MMOH, with no mu-(1,1)(eta(1),eta(2)) carboxylate bridge. To further understand the variations in geometry of the active site, which lead to differences in reactivity, density functional theory (DFT) calculations have been carried out to identify active-site structures for R2-wt and R2-D84E consistent with these spectroscopic data. The effects of varying the ligand set, positions of bound and free waters, and additional protein constraints on the geometry and energy of the binuclear site of both R2-wt and variant R2s are also explored to identify the contributions to their structural differences and their relation to reduced MMOH.

  View details for DOI 10.1021/ja0374731

  View details for Web of Science ID 000220440400038

  View details for PubMedID 15038731

• Oxygen intermediates in mononuclear non-heme iron sites: Electronic structure and reactivity. Decker, A., Solomon, E. I. AMER CHEMICAL SOC. 2004: U1510

  View details for Web of Science ID 000223655605038

• Structure/function correlations over non-heme iron enzymes. Solomon, E. I. AMER CHEMICAL SOC. 2004: U1423

  View details for Web of Science ID 000223655604722

• NO and O-2 reactivity of non-heme ferrous sites: A DFT study of the geometric and electronic structures of {FeNO}(7) and {FeO2}(8) complexes. Pau, M. Y., Schenk, G., Decker, A., Davis, M. I., Solomon, E. I. AMER CHEMICAL SOC. 2004: U1510

  View details for Web of Science ID 000223655605037

• Axial ligation of Fe(II)-bleomycin probed by XANES spectroscopy INORGANIC CHEMISTRY Smolentsev, G., Soldatov, A. V., Wasinger, E. C., SOLOMON, E. I. 2004; 43 (6): 1825-1827

  Abstract

  Full multiple scattering calculations of the Fe K-edge X-ray absorption near edge structure of bleomycin have been performed. Structural insight is based on the comparison between experimental and theoretical data calculated for different active site models coming from NMR-informed molecular dynamic simulations. In all models considered, the equatorial ligands (secondary amine in beta-aminoalanine, pyrimidine and imidazole rings and the beta-hydroxyhistidine) were left unchanged. Seven models with two axial ligands (the primary amine in beta-aminoalanine and the carbomoyl group of the mannose or a solvent molecule) were tested. The best agreement between theoretical and experimental spectra is achieved for the model of bleomycin with the primary amine and the oxygen of the mannose sugar occupying the axial positions. The coordination environment is characterized by serious distortions of the Fe octahedron, including the presence of one ligand with a very short bond length and significant angular distortions.

  View details for DOI 10.1021/ic0350537

  View details for Web of Science ID 000220295200007

  View details for PubMedID 15018497

• S K-edge X-ray absorption spectroscopic investigation of the Ni-containing superoxide dismutase active site: New structural insight into the mechanism JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Szilagyi, R. K., Bryngelson, P. A., Maroney, M. J., Hedman, B., Hodgson, K. O., Solomon, E. I. 2004; 126 (10): 3018-3019

  Abstract

  Superoxide dismutases protect cells from the toxic effects of reactive oxygen species derived from superoxide. Nickel-containing superoxide dismutases (NiSOD), found in Streptomyces species and in cyanobacteria, are distinct from Mn-, Fe-, or Cu/Zn-containing SODs in amino acid sequence and metal ligand environment. Sulfur K-edge X-ray absorption spectroscopic investigations were carried out for a series of mono- and binuclear Ni model compounds with varying sulfur ligation, and for oxidized and reduced NiSOD to elucidate the types of Ni-S interactions found in the two oxidation states. The S K-edge XAS spectra clearly indicate the presence of Ni(III)-bound terminal thiolate in the oxidized enzyme and the absence of such coordination to Ni(II) in the peroxide-reduced enzyme. This striking change in the S ligation for Ni with redox suggests that, upon peroxide reduction, an electron is transferred to the Ni(III) site and the terminal thiolate becomes protonated, providing an efficient mechanism for proton-coupled electron transfer.

  View details for DOI 10.1021/ja039106v

  View details for PubMedID 15012109

• Electronic structures of metal sites in proteins and models: Contributions to function in blue copper proteins CHEMICAL REVIEWS SOLOMON, E. I., Szilagyi, R. K., George, S. D., Basumallick, L. 2004; 104 (2): 419-458

  View details for DOI 10.1021/cr0206317

  View details for Web of Science ID 000188934400005

  View details for PubMedID 14871131

• Preface: Biomimetic inorganic chemistry CHEMICAL REVIEWS Holm, R. H., Solomon, E. I. 2004; 104 (2): 347–48

  View details for DOI 10.1021/cr0206364

  View details for Web of Science ID 000188934400001

  View details for PubMedID 14871127

• Comparison between the geometric and electronic structures and reactivities of {FeNO}(7) and {FeO2}(8) complexes: A density functional theory study JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Schenk, G., Pau, M. Y., Solomon, E. I. 2004; 126 (2): 505-515

  Abstract

  In a previous study, we analyzed the electronic structure of S = 3/2 [FeNO](7) model complexes [Brown et al. J. Am. Chem. Soc. 1995, 117, 715-732]. The combined spectroscopic data and SCF-X alpha-SW electronic structure calculations are best described in terms of Fe(III) (S = 5/2) antiferromagnetically coupled to NO(-) (S = 1). Many nitrosyl derivatives of non-heme iron enzymes have spectroscopic properties similar to those of these model complexes. These NO derivatives can serve as stable analogues of highly labile oxygen intermediates. It is thus essential to establish a reliable density functional theory (DFT) methodology for the geometry and energetics of [FeNO](7) complexes, based on detailed experimental data. This methodology can then be extended to the study of [FeO(2)](8) complexes, followed by investigations into the reaction mechanisms of non-heme iron enzymes. Here, we have used the model complex Fe(Me(3)TACN)(NO)(N(3))(2) as an experimental marker and determined that a pure density functional BP86 with 10% hybrid character and a mixed triple-zeta/double-zeta basis set lead to agreement between experimental and computational data. This methodology is then applied to optimize the hypothetical Fe(Me(3)TACN)(O(2))(N(3))(2) complex, where the NO moiety is replaced by O(2). The main geometric differences are an elongated Fe[bond]O(2) and a steeper Fe[bond]O[bond]O angle in the [FeO(2)](8) complex. The electronic structure of [FeO(2)](8) corresponds to Fe(III) (S = 5/2) antiferromagnetically coupled to O(2)(-) (S = 1/2), and, consistent with the extended bond length, the [FeO(2)](8) unit has only one Fe(III)-O(2)(-) bonding interaction, while the [FeNO](7) unit has both sigma and pi type Fe(III)-NO(-) bonds. This is in agreement with experiment as NO forms a more stable Fe(III)-NO(-) adduct relative to O(2)(-). Although NO is, in fact, harder to reduce, the resultant NO(-) species forms a more stable bond to Fe(III) relative to O(2)(-) due to the different bonding interactions.

  View details for DOI 10.1021/ja036715u

  View details for Web of Science ID 000188197800043

  View details for PubMedID 14719948

• N2O reduction by the mu(4)-sulfide-bridged tetranuclear Cu-Z cluster active site ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Chen, P., Gorelsky, S. I., Ghosh, S., Solomon, E. I. 2004; 43 (32): 4132-4140

  Abstract

  Nitrous oxide (N2O) reduction is a chemical challenge both in the selective oxidation of organic substrates by N2O and in the removal of N2O as a green-house gas. The reduction of N2O is thermodynamically favorable but kinetically inert, and requires activating transition-metal centers. In biological systems, N2O reduction is the last step in the denitrification process of the bacterial nitrogen cycle and is accomplished by the enzyme nitrous oxide reductase, whose active site consists of a micro4-sulfide-bridged tetranuclear CuZ cluster which has many unusual spectroscopic features. Recent studies have developed a detailed electronic-structure description of the resting CuZ cluster, determined its catalytically relevant state, and provided insight into the role of this tetranuclear copper cluster in N2O activation and reduction.

  View details for DOI 10.1002/anie.200301734

  View details for Web of Science ID 000223505600004

  View details for PubMedID 15307074

• Activation of N2O reduction by the fully reduced mu(4)-sulfide bridged tetranuclear Cu-Z cluster in nitrous oxide reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ghosh, S., Gorelsky, S. I., Chen, P., Cabrito, I., Moura, J. J., Moura, I., Solomon, E. I. 2003; 125 (51): 15708-15709

  Abstract

  The tetranuclear CuZ cluster catalyzes the two-electron reduction of N2O to N2 and H2O in the enzyme nitrous oxide reductase. This study shows that the fully reduced 4CuI form of the cluster correlates with the catalytic activity of the enzyme. This is the first demonstration that the S = 1/2 form of CuZ can be further reduced. Complementary DFT calculations support the experimental findings and demonstrate that N2O binding in a bent mu-1,3-bridging mode to the 4CuI form is most efficient due to strong back-bonding from two reduced copper atoms. This back-donation activates N2O for electrophilic attack by a proton.

  View details for DOI 10.1021/ja038344n

  View details for Web of Science ID 000187436200012

  View details for PubMedID 14677937

• Spectroscopic studies of the Met182Thr mutant of nitrite reductase: Role of the axial ligand in the geometric and electronic structure of blue and green copper sites JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Basumallick, L., Szilagyi, R. K., Zhao, Y. W., Shapleigh, J. P., Scholes, C. P., SOLOMON, E. I. 2003; 125 (48): 14784-14792

  Abstract

  A combination of spectroscopic methods and density functional calculations has been used to describe the electronic structure of the axial mutant (Met182Thr) of Rhodobacter sphaeroides nitrite reductase in which the axial methionine has been changed to a threonine. This mutation results in a dramatic change in the geometric and electronic structure of the copper site. The electronic absorption data imply that the type 1 site in the mutant is like a typical blue copper site in contrast to the wild-type site, which is green. Similar ligand field strength in the mutant and the wild type (from MCD spectra) explains the similar EPR parameters for very different electronic structures. Resonance Raman shows that the Cu-S(Cys) bond is stronger in the mutant relative to the wild type. From a combination of absorption, CD, MCD, and EPR data, the loss of the strong axial thioether (present in the wild-type site) results in an increase of the equatorial thiolate-Cu interaction and the site becomes less tetragonal. Spectroscopically calibrated density functional calculations were used to provide additional insight into the role of the axial ligand. The calculations reproduce well the experimental ground-state bonding and the changes in going from a green to a blue site along this coupled distortion coordinate. Geometry optimizations at the weak and strong axial ligand limits show that the bonding of the axial thioether is the key factor in determining the structure of the ground state. A comparison of plastocyanin (blue), wild-type nitrite reductase (green), and the Met182Thr mutant (blue) sites enables evaluation of the role of the axial ligand in the geometric and electronic structure of type 1 copper sites, which can affect the electron-transfer properties of these sites.

  View details for DOI 10.1021/ja037232t

  View details for Web of Science ID 000186834500045

  View details for PubMedID 14640653

• Circular dichroism and magnetic circular dichroism studies of the biferrous form of the R2 subunit of ribonucleotide reductase from mouse: Comparison to the R2 from Escherichia coli and other binuclear ferrous enzymes BIOCHEMISTRY Strand, K. R., Yang, Y. S., Andersson, K. K., SOLOMON, E. I. 2003; 42 (42): 12223-12234

  Abstract

  Ribonucleotide reductase (RNR) catalyzes the synthesis of the four deoxyribonucleotides needed for DNA synthesis and repair in living organisms. The reduced [Fe(II)Fe(II)] form of the model mammalian enzyme, mouse RNR R2, has been studied using a combination of circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature variable-field (VTVH) MCD spectroscopies. Titrations of ferrous ion to the apo-enzyme have been performed and analyzed to investigate the metal binding affinity of the metal-binding site. Spectral features of individual iron sites have been analyzed to obtain detailed geometric and electronic structural information. VTVH MCD data have been collected and analyzed using two complementary models to obtain detailed ground state information including the zero-field splitting (ZFS) of both ferrous centers and the exchange coupling (J) between the two sites. These ground and excited state results provide a complete description of the biferrous site of mouse R2. The biferrous site consists of one 4- and one 5-coordinate iron, with positive and negative ZFS values, respectively. Weak exchange coupling between the two ferrous centers is present, consistent with having carboxylate bridges. The two sites have highly cooperative and weak metal binding affinities. This may be a novel regulatory mechanism for RNR. These results are compared with those from reduced Escherichia coli R2 and reduced acyl-carrier protein Delta(9) desaturase to correlate to similarities and differences in their dioxygen reactivity.

  View details for DOI 10.1021/bi035248q

  View details for Web of Science ID 000186133900012

  View details for PubMedID 14567684

• L-edge X-ray absorption spectroscopy of non-heme iron sites: Experimental determination of differential orbital covalency JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Wasinger, E. C., de Groot, F. M., Hedman, B., Hodgson, K. O., Solomon, E. I. 2003; 125 (42): 12894-12906

  Abstract

  X-ray absorption spectroscopy has been utilized to obtain the L-edge multiplet spectra for a series of non-heme ferric and ferrous complexes. Using these data, a methodology for determining the total covalency and the differential orbital covalency (DOC), that is, differences in covalency in the different symmetry sets of the d orbitals, has been developed. The integrated L-edge intensity is proportional to the number of one-electron transition pathways to the unoccupied molecular orbitals as well as to the covalency of the iron site, which reduces the total L-edge intensity and redistributes intensity, producing shake-up satellites. Furthermore, differential orbital covalency leads to differences in intensity for the different symmetry sets of orbitals and, thus, further modifies the experimental spectra. The ligand field multiplet model commonly used to simulate L-edge spectra does not adequately reproduce the spectral features, especially the charge transfer satellites. The inclusion of charge transfer states with differences in covalency gives excellent fits to the data and experimental estimates of the different contributions of charge transfer shake-up pathways to the t(2g) and e(g) symmetry orbitals. The resulting experimentally determined DOC is compared to values calculated from density functional theory and used to understand chemical trends in high- and low-spin ferrous and ferric complexes with different covalent environments. The utility of this method toward problems in bioinorganic chemistry is discussed.

  View details for DOI 10.1021/ja034634s

  View details for PubMedID 14558838

• EPR spectroscopy of [Fe2O2(5-Et-3-TPA)(2)](3+): Electronic origin of the unique spin-hamiltonian parameters of the (Fe2O2)-O-III,IV diamond core INORGANIC CHEMISTRY Skulan, A. J., Hanson, M. A., Hsu, H. F., Dong, Y. H., Que, L., SOLOMON, E. I. 2003; 42 (20): 6489-6496

  Abstract

  The electronic origins of the magnetic signatures of [Fe(2)O(2)(5-Et(3)-TPA)(2)](ClO(4))(3), where 5-Et(3)-TPA = tris(5-ethyl-2-pyridylmethyl)amine, were investigated by density functional calculations. These signatures consist of a near-axial EPR spectrum, anisotropic superhyperfine broadening upon (17)O substitution in the Fe(2)O(2) core, and an unusually large, positive zero-field splitting parameter, D = 38 +/- 3 cm(-1). Density functional calculations identify the anisotropic (17)O superhyperfine broadening to be due to a preponderance of oxo 2p density perpendicular to the plane of the Fe(2)O(2) core in the three singly occupied molecular orbitals of the S = (3)/(2) ground state. The near-axial g-matrix arises from DeltaS = 0 spin-orbit mixing between the singly and doubly occupied d(pi) orbitals of the iron d-manifold. The large D is due to DeltaS = +/-1 spin-orbit mixing with low-lying d(pi) excited states. These experimental observables reflect the dominance of iron-oxo (rather than Fe-Fe) bonding in the Fe(2)O(2) core, and define the low-lying valence orbitals responsible for reactivity.

  View details for DOI 10.1021/ic034170z

  View details for Web of Science ID 000185697300041

  View details for PubMedID 14514326

• Spectroscopic and electronic structure studies of 2,3-dihydroxybiphenyl 1,2-dioxygenase: O-2 reactivity of the non-heme ferrous site in extradiol dioxygenases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Davis, M. I., Wasinger, E. C., Decker, A., Pau, M. Y., Vaillancourt, F. H., Bolin, J. T., Eltis, L. D., Hedman, B., Hodgson, K. O., Solomon, E. I. 2003; 125 (37): 11214-11227

  Abstract

  The extradiol dioxygenase, 2,3-dihydroxybiphenyl 1,2-dioxygenase (DHBD, EC 1.13.11.39), has been studied using magnetic circular dichroism (MCD), variable-temperature variable-field (VTVH) MCD, X-ray absorption (XAS) pre-edge, and extended X-ray absorption fine structure (EXAFS) spectroscopies, which are analogous to methods used in earlier studies on the extradiol dioxygenase catechol 2,3-dioxygenase [Mabrouk et al. J. Am. Chem Soc. 1991, 113, 4053-4061]. For DHBD, the spectroscopic data can be correlated to the results of crystallography and with the results from density functional calculations to obtain detailed geometric and electronic structure descriptions of the resting and substrate (DHB) bound forms of the enzyme. The geometry of the active site of the resting enzyme, square pyramidal with a strong Fe-glutamate bond in the equatorial plane, localizes the redox active orbital in an orientation appropriate for O(2) binding. However, the O(2) reaction is not favorable, as it would produce a ferric superoxide intermediate with a weak Fe-O bond. Substrate binding leads to a new square pyramidal structure with the strong Fe-glutamate bond in the axial direction as indicated by a decrease in the (5)E(g) and increase in the (5)T(2g) splitting. Electronic structure calculations provide insight into the relative lack of dioxygen reactivity for the resting enzyme and its activation upon substrate binding.

  View details for DOI 10.1021/ja029746i

  View details for PubMedID 16220940

• Rapid-freeze-quench magnetic circular dichroism of intermediate X in ribonucleotide reductase: New structural insight JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mitic, N., Saleh, L., Schenk, G., Bollinger, J. M., Solomon, E. I. 2003; 125 (37): 11200-11201

  Abstract

  To elucidate the electronic structure of intermediate X in the oxygen activation reaction of the R2 subunit of ribonucleotide reductase, a protocol has been developed to perform magnetic circular dichroism (MCD) on a rapid-freeze-quench, strain free optical sample. RFQ-MCD data have been collected on intermediate X in the double mutant of R2, Y122/Y356F. While X has been reported to exhibit a broad absorption band at 365 nm, there are at least 10 electronic transitions observed at low-temperature MCD. From C0/D0 ratios, the transitions of X can be divided into three regions: 16 000-22 000 cm-1 region involving spin-allowed ligand field transitions of the Fe(IV), 23 000-24 000 cm-1 region of spin-forbidden, spin-flip transitions on the Fe(IV), and the charge transfer (CT) region from 26 000 to 32 000 cm-1. The C0/D0 ratios from d --> d and CT transitions strongly support significant Fe(IV) character coupled into the paramagnetic center. Ligand field (spin-allowed d --> d region) analysis allows the bis-mu-oxo and mu-oxo plus other monoanionic bridge possibilities for the structure of intermediate X to be distinguished, providing new insight into the molecular mechanism of the cluster formation in R2.

  View details for DOI 10.1021/ja036556e

  View details for Web of Science ID 000185341800032

  View details for PubMedID 16220933

• Spectroscopic investigation of stellacyanin mutants: Axial ligand interactions at the blue copper site JOURNAL OF THE AMERICAN CHEMICAL SOCIETY George, S. D., Basumallick, L., Szilagyi, R. K., Randall, D. W., Hill, M. G., Nersissian, A. M., Valentine, J. S., Hedman, B., Hodgson, K. O., Solomon, E. I. 2003; 125 (37): 11314-11328

  Abstract

  Detailed electronic and geometric structural descriptions of the blue copper sites in wild-type (WT) stellacyanin and its Q99M and Q99L axial mutants have been obtained using a combination of XAS, resonance Raman, MCD, EPR, and DFT calculations. The results show that the origin of the short Cu-S(Cys) bond in blue copper proteins is the weakened axial interaction, which leads to a shorter (based on EXAFS results) and more covalent (based on S K-edge XAS) Cu-S bond. XAS pre-edge energies show that the effective nuclear charge on the copper increases going from O(Gln) to S(Met) to no axial (Leu) ligand, indicating that the weakened axial ligand is not fully compensated for by the increased donation from the thiolate. This is further supported by EPR results. MCD data show that the decreased axial interaction leads to an increase in the equatorial ligand field, indicating that the site acquires a more trigonally distorted tetrahedral structure. These geometric and electronic structural changes, which result from weakening the bonding interaction of the axial ligand, allow the site to maintain efficient electron transfer (high H(DA) and low reorganization energy), while modulating the redox potential of the site to the biologically relevant range. These spectroscopic studies are complemented by DFT calculations to obtain insight into the factors that allow stellacyanin to maintain a trigonally distorted tetrahedral structure with a relatively strong axial Cu(II)-oxygen bond.

  View details for DOI 10.1021/ja035802j

  View details for PubMedID 16220954

• Spectroscopic studies of the interaction of ferrous bleomycin with DNA JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Kemsley, J. N., Zaleski, K. L., Chow, M. S., Decker, A., Shishova, E. Y., Wasinger, E. C., Hedman, B., Hodgson, K. O., Solomon, E. I. 2003; 125 (36): 10810-10821

  Abstract

  Bleomycin is an antibiotic used in cancer chemotherapy for its ability to achieve both single- and double-strand cleavage of DNA through abstraction of the deoxyribose C4'-H. Magnetic circular dichroism (MCD) and X-ray absorption (XAS) spectroscopies have been used to study the interaction of the biologically relevant FeIIBLM complex with DNA. Calf thymus DNA was used as the substrate as well as short oligonucleotides, including one with a preferred 5'-G-pyrimidine-3' cleavage site [d(GGAAGCTTCC)2] and one without [d(GGAAATTTCC)2]. DNA binding to FeIIBLM significantly perturbs the FeII active site, resulting in a change in intensity ratio of the d d transitions and a decrease in excited-state orbital splitting (5Eg). Although this effect is somewhat dependent on length and composition of the oligonucleotide, it is not correlated to the presence of a 5'-G-pyrimidine-3' cleavage site. No effect is observed on the charge-transfer transitions, indicating that the H-bonding recognition between the pyrimidine and guanine base does not perturb Fe-pyrimidine backbonding. Azide binding studies indicate that FeIIBLM bound to either oligomer has the same affinity for N3-. Parallel studies of BLM structural derivatives indicate that FeIIiso-PEPLM, in which the carbamoyl group is shifted on the mannose sugar, forms the same DNA-bound species as FeIIBLM. In contrast, FeIIDP-PEPLM, in which the -aminoalanine group is absent, forms a new species upon DNA binding. These data are consistent with a model in which the primary amine from the -aminoalanine is an FeII ligand and the mannose carbamoyl provides either a ligand to the FeII or significant second-sphere effects on the FeII site; intercalation of the bithiazole tail into the double helix likely brings the metal-bound complex close enough to the DNA to create steric interactions that remove the sugar groups from interaction with the FeII. The fact that the FeII active site is perturbed regardless of DNA sequence is consistent with the fact that cleavage is observed for both 5'-GC-3' and nonspecific oligomers and indicates that different reaction coordinates may be active, depending on orientation of the deoxyribose C4'-H.

  View details for DOI 10.1021/ja034579n

  View details for Web of Science ID 000185154300021

  View details for PubMedID 12952460

• Description of the ground state wave functions of Ni dithiolenes using sulfur K-edge X-ray absorption spectroscopy JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Szilagyi, R. K., Lim, B. S., Glaser, T., Holm, R. H., Hedman, B., Hodgson, K. O., Solomon, E. I. 2003; 125 (30): 9158-9169

  Abstract

  The pterin-dithiolene cofactor is an essential component of the catalytic sites of all molybdoenzymes except nitrogenase. Understanding its bonding to transition metals allows for development of electronic structure/function correlations in catalysis. The electronic structure description for a series of bis(dithiolene) complexes ([NiL(2)](Z)(), L = 1,2-Me(2)C(2)S(2); Z = 2-, 1-, 0) using sulfur XAS provides the basis for extension to the biologically relevant metal-containing dithiolenes. The transition dipole integral has been developed for the dithiolene sulfur through correlation of XAS pre-edge energy positions of sulfide-, thiolate-, and enedithiolate-S. The ground state wave functions of all three NiL(2) complexes have more than 50% S character experimentally demonstrating the noninnocent behavior of the dithiolene ligand. The S K-edge experimental results are correlated with spin-unrestricted, broken-symmetry density functional calculations. These show only limited spin polarization in the neutral complex and delocalized, ligand based ground states for the mono- and dianionic complexes. These XAS and DFT results are correlated with other spectroscopic features and provide insight into reactivity.

  View details for DOI 10.1021/ja029806k

  View details for Web of Science ID 000184364500049

  View details for PubMedID 15369373

• Interactions of mononuclear non-heme iron sites with dioxygen: Studies on electronic structure and reactivity Decker, A., Lehnert, N., Chow, M. S., Solomon, E. I. ELSEVIER SCIENCE INC. 2003: 124

  View details for DOI 10.1016/S0162-0134(03)80603-7

  View details for Web of Science ID 000184009800192

• Structure/function correlations over non-heme iron enzymes Solomon, E. I. ELSEVIER SCIENCE INC. 2003: 62

  View details for DOI 10.1016/S0162-0134(03)80509-3

  View details for Web of Science ID 000184009800098

• First MCD characterization of intermediate X in ribonucleotide reductase: Insight into the geometric and electronic structure description of X Mitic, N., Saleh, L., Bollinger, J. M., Solomon, E. I. ELSEVIER SCIENCE INC. 2003: 191

  View details for DOI 10.1016/S0162-0134(03)80710-9

  View details for Web of Science ID 000184009800299

• Dioxygen activation by copper(I) complexes: Electronic effects on the formation and substrate reactivity of the Cu2O2 core. Quant, L. D., Shearer, J., Liang, H. C., Zhang, C. X., Karlin, K. D., Vance, M., Henson, M. J., Solomon, E. I. ELSEVIER SCIENCE INC. 2003: 214

  View details for Web of Science ID 000184009800333

• Electronic structure of the oxygen activating intermediates of ribonucleotide reductase: the peroxo intermediate and a model complex of the high-valent intermediate, X Skulan, A. J., Brunold, T. C., Hanson, M. A., Solomon, E. I., Baldwin, J., Bollinger, J. M., Hsu, H. F., Que, L. ELSEVIER SCIENCE INC. 2003: 231

  View details for DOI 10.1016/S0162-0134(03)80771-7

  View details for Web of Science ID 000184009800360

• Spectroscopic characterization of soybean lipoxygenase-1: The role of second coordination sphere residues in the regulation of enzyme activity Schenk, G., Neidig, M. L., Zhou, J., Holman, T. R., Solomon, E. I. ELSEVIER SCIENCE INC. 2003: 225

  View details for Web of Science ID 000184009800351

• Spectroscopic characterization of the Leu513His variant of fungal laccase: Effect of increased axial ligand interaction on the geometric and electronic structure of the type 1 Cu site INORGANIC CHEMISTRY Palmer, A. E., Szilagyi, R. K., Cherry, J. R., Jones, A., Xu, F., Solomon, E. I. 2003; 42 (13): 4006-4017

  Abstract

  A variety of spectroscopic techniques, combined with density functional calculations, are used to describe the electronic structure of the Leu513His variant of the type 1 Cu site in Myceliophthora thermophila laccase. This mutation changes the type 1 Cu from a blue to a green site. Electron paramagnetic resonance (EPR), optical absorption, circular dichroism, and magnetic circular dichroism (MCD) spectroscopies reveal that, relative to the trigonal planar blue type 1 Cu site in wild-type fungal laccase, the covalency and the ligand field strength at the Leu513His green type 1 Cu center decrease. Additionally, there is a significant reorientation of the d(x)()()2(-)(y)()()2( )singly occupied MO, such that the overlap with the Cys sulfur valence orbital changes from pi to sigma. A density functional study in which internal coordinates are systematically altered reveals that these changes are due to the increased strength of the axial ligand (none to His), leading to a tetragonal distortion and elongation of the equatorial Cu-ligand bonds. These calculations provide insight into the experimental differences in the EPR parameters, charge-transfer absorption spectrum, and ligand-field MCD spectrum between the axial-His variant and blue Cu centers (plastocyanin and the type 1 site in fungal laccase). There are also significant differences between the green site in the Leu513His variant and other naturally occurring, green type 1 Cu sites such as in nitrite reductase, which have short axial Cu-S(Met) bonds. The large difference in EPR parameters between these green type 1 sites derives from a change in ligand field excitation energies observed by MCD, which reflects a decrease in ligand field strength. This is associated with different steric interactions of a His vs an axial Met ligand in a tetragonally distorted type 1 site. Changes in the electronic structure of the Cu site correlate with the difference in reactivity of the green His variant relative to blue wild-type fungal laccase.

  View details for DOI 10.1021/ic026099n

  View details for Web of Science ID 000183945100009

  View details for PubMedID 12817956

• Spectroscopic characterization of soybean lipoxygenase-1 mutants: the role of second coordination sphere residues in the regulation of enzyme activity BIOCHEMISTRY Schenk, G., Neidig, M. L., Zhou, J., Holman, T. R., SOLOMON, E. I. 2003; 42 (24): 7294-7302

  Abstract

  Lipoxygenases are non-heme iron enzymes, which catalyze the stereo- and regiospecific hydroperoxidation of unsaturated fatty acids. Spectroscopic studies on soybean lipoxygenase have shown that the ferrous form of the enzyme is a mixture of five- and six-coordinate species (40 and 60%, respectively). Addition of substrate leads to a purely six-coordinate form. A series of mutations in the second coordination sphere (Q697E, Q697N, Q495A, and Q495E) were generated, and the structures of the mutants were solved by crystallography [Tomchick et al. (2001) Biochemistry 40, 7509-7517]. While this study clearly showed the contribution of H-bond interactions between the first and the second coordination spheres in catalysis, no correlation with the coordination environment of the Fe(II) was observed. A recent study using density-functional theory [Lehnert and Solomon (2002) J. Biol. Inorg. Chem. 8, 294-305] indicated that coordination flexibility, involving the Asn694 ligand, is regulated via H-bond interactions. In this paper, we investigate the solution structures of the second coordination sphere mutants using CD and MCD spectroscopy since these techniques are more sensitive indicators of the first coordination sphere ligation of Fe(II) systems. Our data demonstrate that the iron coordination environment directly relates to activity, with the mutations that have the ability to form a five-coordinate/six-coordinate mixture being more active. We propose that the H-bond between the weak Asn694 ligand and the Gln697 plays a key role in the modulation of the coordination flexibility of Asn694, and thus, is crucial for the regulation of enzyme reactivity.

  View details for DOI 10.1021/bi027380g

  View details for Web of Science ID 000183624800004

  View details for PubMedID 12809485

• Spectroscopic study of [Fe2O2(5-Et-3-TPA)(2)](3+): Nature of the Fe2O2 diamond core and its possible relevance to high-valent binuclear non-heme enzyme intermediates JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Skulan, A. J., Hanson, M. A., Hsu, H. F., Que, L., Solomon, E. I. 2003; 125 (24): 7344-7356

  Abstract

  The spectroscopic properties and electronic structure of an Fe(2)(III,IV) bis-mu-oxo complex, [Fe(2)O(2)(5-Et(3)-TPA)(2)](ClO(4))(3) where 5-Et(3)-TPA = tris(5-ethyl-2-pyridylmethyl)amine, are explored to determine the molecular origins of the unique electronic and geometric features of the Fe(2)O(2) diamond core. Low-temperature magnetic circular dichroism (MCD) allows the two features in the broad absorption envelope (4000-30000 cm(-)(1)) to be resolved into 13 transitions. Their C/D ratios and transition polarizations from variable temperature-variable field MCD saturation behavior indicate that these divide into three types of electronic transitions; t(2) --> t(2) involving excitations between metal-based orbitals with pi Fe-O overlap (4000-10000 cm(-)(1)), t(2)/t(2) --> e involving excitations to metal-based orbitals with sigma Fe-O overlap (12500-17000 cm(-)(1)) and LMCT (17000-30000 cm(-)(1)) and allows transition assignments and calibration of density functional calculations. Resonance Raman profiles show the C(2)(h)() geometric distortion of the Fe(2)O(2) core results in different stretching force constants for adjacent Fe-O bonds (k(str)(Fe-O(long)) = 1.66 and k(str)(Fe-O(short)) = 2.72 mdyn/A) and a small ( approximately 20%) difference in bond strength between adjacent Fe-O bonds. The three singly occupied pi-metal-based orbitals form strong superexchange pathways which lead to the valence delocalization and the S = (3)/(2) ground state. These orbitals are key to the observed reactivity of this complex as they overlap with the substrate C-H bonding orbital in the best trajectory for hydrogen atom abstraction. The electronic structure implications of these results for the high-valent enzyme intermediates X and Q are discussed.

  View details for DOI 10.1021/ja021137n

  View details for Web of Science ID 000183503500044

  View details for PubMedID 12797809

• Spectroscopic evidence for a heme-superoxide/Cu(I) intermediate in a functional model of cytochrome C oxidase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Collman, J. P., Sunderland, C. J., Berg, K. E., Vance, M. A., SOLOMON, E. I. 2003; 125 (22): 6648-6649

  Abstract

  A superstructured tetraphenylporphyrin with a covalently attached proximal imidazole axial base and three distal imidazole pickets has been developed as a model for the active site of terminal oxidases such as cytochrome c oxidase. The oxygen adduct of the Fe-only heme (at low temperature) has a diamagnetic NMR and is EPR silent, which taken together with a resonance Raman oxygen isotope sensitive band (nuFe-O) at 575/554 cm-1 (16O2/18O2) indicates formation of a six-coordinate heme-superoxide complex. Unexpectedly, the Fe/Cu complex, where the copper is in a trisimidazole environment approximately 5 A above the heme plane, displays similar characteristics: a diamagnetic NMR, EPR silence, and nuFe-O at 570/544 cm-1. This indicates the dioxygen adduct of this Fe/Cu system is also a superoxide. This contrasts with previously characterized partially reduced dioxygen intermediates of binuclear heme/copper complexes that form Fe/Cu mu-peroxo complexes.

  View details for DOI 10.1021/ja034382v

  View details for Web of Science ID 000183314000025

  View details for PubMedID 12769571

• Spectroscopy and bonding in side-on and end-on Cu-2(S-2) cores: Comparison to peroxide analogues JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, P., Fujisawa, K., Helton, M. E., Karlin, K. D., SOLOMON, E. I. 2003; 125 (21): 6394-6408

  Abstract

  Spectroscopic methods combined with density functional calculations were used to study the disulfide-Cu(II) bonding interactions in the side-on micro -eta(2):eta(2)-bridged Cu(2)(S(2)) complex, [[Cu(II)[HB(3,5-Pr(i)(2)pz)(3)]](2)(S(2))], and the end-on trans- micro -1,2-bridged Cu(2)(S(2)) complex, [[Cu(II)(TMPA)](2)(S(2))](2+), in correlation to their peroxide structural analogues. Resonance Raman shows weaker S-S bonds and stronger Cu-S bonds in the disulfide complexes relative to the O-O and Cu-O bonds in the peroxide analogues. The weaker S-S bonds come from the more limited interaction between the S 3p orbitals relative to that of the O 2s/p hybrid orbitals. The stronger Cu-S bonds result from the more covalent Cu-disulfide interactions relative to the Cu-peroxide interactions. This is consistent with the higher energy of the disulfide valence level relative to that of the peroxide. The ground states of the side-on Cu(2)(S(2))/Cu(2)(O(2)) complexes are more covalent than those of the end-on Cu(2)(S(2))/Cu(2)(O(2)) complexes. This derives from the larger sigma-donor interactions in the side-on micro -eta(2):eta(2) structure, which has four Cu-disulfide/peroxide bonds, relative to the end-on trans- micro -1,2 structure, which forms two bonds to the Cu. The larger disulfide/peroxide sigma-donor interactions in the side-on complexes are reflected in their more intense higher energy disulfide/peroxide to Cu charge transfer transitions in the absorption spectra. The large ground-state covalencies of the side-on complexes result in significant nuclear distortions in the ligand-to-metal charge transfer excited states, which give rise to the strong resonance Raman enhancements of the metal-ligand and intraligand vibrations. Particularly, the large covalency of the Cu-disulfide interaction in the side-on Cu(2)(S(2)) complex leads to a different rR enhancement profile, relative to the peroxide analogues, reflecting a S-S bond distortion in the opposite directions in the disulfide/peroxide pi(sigma) to Cu charge transfer excited states. A ligand sigma back-bonding interaction exists only in the side-on complexes, and there is more sigma mixing in the side-on Cu(2)(S(2)) complex than in the side-on Cu(2)(O(2)) complex. This sigma back-bonding is shown to significantly weaken the S-S/O-O bond relative to that of the analogous end-on complex, leading to the low nu(S)(-)(S)/nu(O)(-)(O) vibrational frequencies observed in the resonance Raman spectra of the side-on complexes.

  View details for DOI 10.1021/ja0214678

  View details for Web of Science ID 000183031800025

  View details for PubMedID 12785779

• Spectroscopic and kinetic studies of PKU-inducing mutants of phenylalanine hydroxylase: Arg158Gln and Glu280Lys JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Kemsley, J. N., Wasinger, E. C., Datta, S., Mitic, N., Acharya, T., Hedman, B., Caradonna, J. P., Hodgson, K. O., SOLOMON, E. I. 2003; 125 (19): 5677-5686

  Abstract

  Phenylalanine hydroxylase (PAH) is a tetrahydrobiopterin-dependent, nonheme iron enzyme that catalyzes the hydroxylation of L-Phe to L-Tyr in the rate-limiting step of phenylalanine catabolism. This reaction is tightly coupled in the wild-type enzyme to oxidation of the tetrahydropterin cofactor. Dysfunction of PAH activity in humans leads to the disease phenylketonuria (PKU). We have investigated two PKU-inducing mutants, Arg158Gln and Glu280Lys, using kinetic methods, magnetic circular dichrosim (MCD) spectroscopy, and X-ray absorption spectroscopy (XAS). Analysis of the products produced by the mutant enzymes shows that although both oxidize pterin at more than twice the rate of wild-type enzyme, these reactions are only approximately 20% coupled to production of L-Tyr. Previous MCD and XAS studies had demonstrated that the resting Fe(II) site is six-coordinate in the wild-type enzyme and converts to a five-coordinate site when both L-Phe and reduced pterin are present in the active site. Although the Arg158Gln mutant forms the five-coordinate site when both cosubstrates are bound, the Fe(II) site of the Glu280Lys mutant remains six-coordinate. These results provide insight into the PAH reaction and disease mechanism at a molecular level, indicating that the first step of the mechanism is formation of a peroxy-pterin species, which subsequently reacts with the Fe(II) site if the pterin is properly oriented for formation of an Fe-OO-pterin bridge and an open coordination position is available on the Fe(II).

  View details for DOI 10.1021/ja029106f

  View details for PubMedID 12733906

• Resonance Raman investigation of equatorial ligand donor effects on the Cu2O22+ core in end-on and side-on mu-peroxo-dicopper(II) and bis-mu-oxo-dicopper(III) complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Henson, M. J., Vance, M. A., Zhang, C. X., Liang, H. C., Karlin, K. D., SOLOMON, E. I. 2003; 125 (17): 5186-5192

  Abstract

  The effect of endogenous donor strength on Cu(2)O(2) bonds was studied by electronically perturbing [[(R-TMPA)Cu(II)]](2)(O(2))](2+) and [[(R-MePY2)Cu](2)(O(2))](2+) (R = H, MeO, Me(2)N), which form the end-on mu-1,2 bound peroxide and an equilibrium mixture of side-on peroxo-dicopper(II) and bis-mu-oxo-dicopper(III) isomers, respectively. For [[(R-TMPA)Cu(II)](2)(O(2))](2+), nu(O-O) shifts from 827 to 822 to 812 cm(-1) and nu(Cu)(-)(O(sym)) shifts from 561 to 557 to 551 cm(-1), respectively, as R- varies from H to MeO to Me(2)N. Thus, increasing the N-donor strength to the copper decreases peroxide pi(sigma) donation to the copper, weakening the Cu-O and O-O bonds. A decrease in nu(Cu-O) of the bis-mu-oxo-dicopper(III) complex was also observed with increasing N-donor strength for the R-MePY2 ligand system. However, no change was observed for nu(O-O) of the side-on peroxo. This is attributed to a reduced charge donation from the peroxide pi(sigma) orbital with increased N-donor strength, which increases the negative charge on the peroxide and adversely affects the back-bonding from the Cu to the peroxide sigma orbital. However, an increase in the bis-mu-oxo-dicopper(III) isomer relative to side-on peroxo-dicopper(II) species is observed for R-MePY2 with R = H < MeO < Me(2)N. This effect is attributed to the thermodynamic stabilization of the bis-mu-oxo-dicopper(III) isomer relative to the side-on peroxo-dicopper(II) isomer by strong donor ligands. Thus, the side-on peroxo-dicopper(II)/bis-mu-oxo-dicopper(III) equilibrium can be controlled by electronic as well as steric effects.

  View details for DOI 10.1021/ja0276366

  View details for Web of Science ID 000182491700042

  View details for PubMedID 12708870

• Non-heme iron enzymes: Contrasts to heme catalysis PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA SOLOMON, E. I., Decker, A., Lehnert, N. 2003; 100 (7): 3589-3594

  Abstract

  Non-heme iron enzymes catalyze a wide range of O(2) reactions, paralleling those of heme systems. Non-heme iron active sites are, however, much more difficult to study because they do not exhibit the intense spectral features characteristic of the porphyrin ligand. A spectroscopic methodology was developed that provides significant mechanistic insight into the reactivity of non-heme ferrous active sites. These studies reveal a general mechanistic strategy used by these enzymes and differences in substrate and cofactor interactions dependent on their requirement for activation by iron. Contributions to O(2) activation have been elucidated for non-heme relative to heme ligand sets, and major differences in reactivity are defined with respect to the heterolytic and homolytic cleavage of O-O bonds.

  View details for DOI 10.1073/pnas.0336792100

  View details for Web of Science ID 000182058400014

  View details for PubMedID 12598659

  View details for PubMedCentralID PMC152966

• Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 3. Kinetics of electron transfer INORGANIC CHEMISTRY Kennepohl, P., SOLOMON, E. I. 2003; 42 (3): 696-708

  Abstract

  The kinetics of electron transfer for rubredoxins are examined using density functional methods to determine the electronic structure characteristics that influence and allow for fast electron self-exchange in these electron-transport proteins. Potential energy surfaces for [FeX(4)](2-,1-) models confirm that the inner-sphere reorganization energy is inherently small for tetrathiolates ( approximately 0.1 eV), as evidenced by the only small changes in the equilibrium Fe-S bond distance during redox (Deltar(redox) approximately 0.05 A). It is concluded that electronic relaxation and covalency in the reduced state allow for this small in this case relative to other redox couples, such as the tetrachloride. Using a large computational model to include the protein medium surrounding the [Fe(SCys)(4)](2-,1-) active site in Desulfovibrio vulgaris Rubredoxin, the electronic coupling matrix element for electron self-exchange is defined for direct active-site contact (H0(DA)). Simple Beratan-Onuchic model is used to extend coupling over the complete surface of the protein to provide an understanding of probable electron-transfer pathways. Regions of similar coupling properties are grouped together to define a surface coupling map, which reveals that very efficient self-exchange occurs only within 4 sigma-bonds of the active site. Longer-range electron transfer cannot support the fast rates of electron self-exchange observed experimentally. Pathways directly through the two surface cysteinate ligands dominate, but surface-accessible amides hydrogen-bonded to the cysteinates also contribute significantly to the rate of electron self-exchange.

  View details for DOI 10.1021/ic0303320

  View details for Web of Science ID 000180870300012

  View details for PubMedID 12562183

• Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 1. Photoelectron spectroscopic determination of electronic relaxation INORGANIC CHEMISTRY Kennepohl, P., SOLOMON, E. I. 2003; 42 (3): 679-688

  Abstract

  Electronic relaxation, the change in molecular electronic structure as a response to oxidation, is investigated in [FeX(4)](2)(-)(,1)(-) (X = Cl, SR) model complexes. Photoelectron spectroscopy, in conjunction with density functional methods, is used to define and evaluate the core and valence electronic relaxation upon ionization of [FeX(4)](2)(-). The presence of intense yet formally forbidden charge-transfer satellite peaks in the PES data is a direct reflection of electronic relaxation. The phenomenon is evaluated as a function of charge redistribution at the metal center (Deltaq(rlx)) resulting from changes in the electronic structure. This charge redistribution is calculated from experimental core and valence PES data using a valence bond configuration interaction (VBCI) model. It is found that electronic relaxation is very large for both core (Fe 2p) and valence (Fe 3d) ionization processes and that it is greater in [Fe(SR)(4)](2)(-) than in [FeCl(4)](2)(-). Similar results are obtained from DFT calculations. The results suggest that, although the lowest-energy valence ionization (from the redox-active molecular orbital) is metal-based, electronic relaxation causes a dramatic redistribution of electron density ( approximately 0.7ē) from the ligands to the metal center corresponding to a generalized increase in covalency over all M-L bonds. The more covalent tetrathiolate achieves a larger Deltaq(rlx) because the LMCT states responsible for relaxation are significantly lower in energy than those in the tetrachloride. The large observed electronic relaxation can make significant contributions to the thermodynamics and kinetics of electron transfer in inorganic systems.

  View details for DOI 10.1021/ic020330f

  View details for Web of Science ID 000180870300010

  View details for PubMedID 12562181

• Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 2. Reduction potentials INORGANIC CHEMISTRY Kennepohl, P., SOLOMON, E. I. 2003; 42 (3): 689-695

  Abstract

  This study utilizes photoelectron spectroscopy (PES) combined with theoretical methods to determine the electronic structure contributions to the large reduction potential difference between [FeCl(4)](2)(-)(,1)(-) and [Fe(SR)(4)](2)(-)(,1)(-) (DeltaE(0) approximately 1 V). Valence PES data confirm that this effect results from electronic structure differences because there is a similarly large shift in the onset of valence ionization between the two reduced species (DeltaI(vert) = 1.4 +/- 0.3 eV). Specific electronic contributions to DeltaI(vert) have been investigated and defined. Ligand field effects, which are often considered to be of great importance, contribute very little to DeltaI(vert) (DeltaE(LF) < -0.05 eV). By contrast, electronic relaxation, a factor that is often neglected in the analysis of chemical reactivity, strongly affects the valence ionization energies of both species. The larger electronic relaxation in the tetrathiolate allows it to more effectively stabilize the oxidized state and lowers its I(vert) relative to that of the chloride (DeltaE(rlx) = 0.2 eV). The largest contribution to the difference in redox potentials is the much lower effective charge () of the tetrathiolate in the reduced state, which results in a large difference in the energy of the Fe 3d manifold between the two redox couples (DeltaE(Fe)( )(3d) = 1.2 eV). This difference derives from the significantly higher covalency of the iron-thiolate bond, which decreases and significantly lowers its redox potential.

  View details for DOI 10.1021/ic0203318

  View details for Web of Science ID 000180870300011

  View details for PubMedID 12562182

• Reaction of elemental sulfur with a copper(I) complex forming a trans-mu-1,2 end-on disulfide complex: New directions in copper-sulfur chemistry JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Helton, M. E., Chen, P., Paul, P. P., Tyeklar, Z., Sommer, R. D., Zakharov, L. N., Rheingold, A. L., SOLOMON, E. I., Karlin, K. D. 2003; 125 (5): 1160-1161

  Abstract

  Elemental sulfur (S8) was found to react with [(TMPA)CuI(CH3CN)]+ to form the trans-mu-1,2 end-on disulfide complex [(TMPA)Cu-S-S-Cu(TMPA)]2+. The X-ray structure of this centrosymmetric disulfide complex shows a Cu(1)-S(1) bond length of 2.280(2) A and a S(1)-S(1A) bond length of 2.044(4) A.

  View details for Web of Science ID 000180713000026

  View details for PubMedID 12553805

• Density-functional investigation on the mechanism of H-atom abstraction by lipoxygenase JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY Lehnert, N., Solomon, E. I. 2003; 8 (3): 294-305

  Abstract

  Using experimentally calibrated density functional calculations on models of the active site of soybean lipoxygenase 1 (SLO-1), insight has been obtained into the coordination flexibility of the iron active site and its molecular mechanism of catalysis. The ferrous form of SLO-1 shows a variation in coordination number in solution that is related to a weakly coordinating Asn694 ligand. From the calculations it is determined that the weak Fe-O(694) bond associated with this coordination flexibility is due to a sideways tilted geometry of Asn694 that is imposed on the site by the protein. Release of this constraint (by altering the hydrogen bonding network) leads to a pure six-coordinate site. In contrast, the ferric form of the enzyme stays five-coordinate. In this case, deprotonation of a coordinated water gives a strong hydroxo donor in the cis position to Asn694, weakening the Fe-O(694) bond. Hence, Asn694 is a stronger ligand to the reduced relative to the oxidized site. Using these experimentally calibrated models, the reaction energy for H-atom transfer in SLO-1 has been calculated to be about -18 kcal/mol. The observed change in coordination number going from five-coordinate in ferric to six-coordinate in ferrous SLO-1 increases the reduction potential of the iron active site. Hence, the protein adjusts the active site for optimal reactivity. Analysis of the electronic structure along the reaction coordinate shows that the H-atom transfer in SLO-1 actually corresponds to a proton-coupled electron transfer (PCET). The transferred electron does not localize on the proton, but tunnels directly from the substrate to the ferric active site in a concerted proton tunneling-electron tunneling (PTET) process. The covalently linked Fe-O-H-C bridge in the transition state lowers the energy barrier and provides an efficient superexchange pathway for this tunneling. The thermal barrier for the PTET process is estimated from the calculations to be about +15 kcal/mol including zero-point energy corrections. This corresponds to a thermal reaction rate of k(therm) approximately 1 s(-1). In comparison, the rate of proton tunneling can be as high as 2 x 10(9) s(-1) under these conditions.

  View details for DOI 10.1007/s00775-002-0415-6

  View details for Web of Science ID 000181365200007

  View details for PubMedID 12589565

• Examples of high-frequency EPR studies in bioinorganic chemistry JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY Andersson, K. K., Schmidt, P. P., Katterle, B., Strand, K. R., Palmer, A. E., Lee, S. K., SOLOMON, E. I., Graslund, A., Barra, A. L. 2003; 8 (3): 235-247

  Abstract

  Low-temperature EPR spectroscopy with frequencies between 95 and 345 GHz and magnetic fields up to 12 T has been used to study metal sites in proteins or inorganic complexes and free radicals. The high-field EPR method was used to resolve g-value anisotropy by separating it from overlapping hyperfine couplings. The presence of hydrogen bonding interactions to the tyrosyl radical oxygens in ribonucleotide reductases were detected. At 285 GHz the g-value anisotropy from the rhombic type 2 Cu(II) signal in the enzyme laccase has its g-value anisotropy clearly resolved from slightly different overlapping axial species. Simple metal site systems with S>1/2 undergo a zero-field splitting, which can be described by the spin Hamiltonian. From high-frequency EPR, the D values that are small compared to the frequency (high-field limit) can be determined directly by measuring the distance of the outermost signal to the center of the spectrum, which corresponds to (2 S-1)* mid R: Dmid R: For example, D values of 0.8 and 0.3 cm(-1) are observed for S=5/2 Fe(III)-EDTA and transferrin, respectively. When D values are larger compared to the frequency and in the case of half-integer spin systems, they can be obtained from the frequency dependence of the shifts of g(eff), as observed for myoglobin in the presence ( D=5 cm(-1)) or absence ( D=9.5 cm(-1)) of fluoride. The 285 and 345 GHz spectra of the Fe(II)-NO-EDTA complex show that it is best described as a S=3/2 system with D=11.5 cm(-1), E=0.1 cm(-1), and g(x)= g(y)= g(z)=2.0. Finally, the effects of HF-EPR on X-band EPR silent states and weak magnetic interactions are demonstrated.

  View details for DOI 10.1007/s00775-002-0429-0

  View details for Web of Science ID 000181365200001

  View details for PubMedID 12589559

• Spectroscopic studies of the effect of ligand donor strength on the Fe-NO bond in intradiol dioxygenases INORGANIC CHEMISTRY Wasinger, E. C., Davis, M. I., Pau, M. Y., Orville, A. M., Zaleski, J. M., Hedman, B., Lipscomb, J. D., Hodgson, K. O., SOLOMON, E. I. 2003; 42 (2): 365-376

  Abstract

  The geometric and electronic structure of NO bound to reduced protocatechuate 3,4-dioxygenase and its substrate (3,4-dihydroxybenzoate, PCA) complex have been examined by X-ray absorption (XAS), UV-vis absorption (Abs), magnetic circular dichroism (MCD), and variable temperature variable field (VTVH) MCD spectroscopies. The results are compared to those previously published on model complexes described as [FeNO]7 systems in which an S = 5/2 ferric center is antiferromagnetically coupled to an S = 1 NO-. XAS pre-edge analysis indicates that the Fe-NO units in FeIIIPCD[NO-] and FeIIIPCD[PCA,NO-] lack the greatly increased pre-edge intensity representative of most [FeNO]7 model sites. Furthermore, from extended X-ray absorption fine structure (EXAFS) analysis, the FeIIIPCD[NO-] and FeIIIPCD[PCA,NO-] active sites are shown to have an Fe-NO distance of at least 1.91 A, approximately 0.2 A greater than those found in the model complexes. The weakened Fe-NO bond is consistent with the overall lengthening of the bond lengths and the fact that VTVH MCD data show that NO(-)-->FeIII CT transitions are no longer polarized along the z-axis of the zero-field splitting tensor. The weaker Fe-NO bond derives from the strong donor interaction of the endogenous phenolate and substrate catecholate ligands, which is observed from the increased intensity in the CT region relative to that of [FeNO]7 model complexes, and from the shift in XAS edge position to lower energy. As NO is an analogue of O2, the effect of endogenous ligand donor strength on the Fe-NO bond has important implications with respect to O2 activation by non-heme iron enzymes.

  View details for DOI 10.1021/ic025906f

  View details for PubMedID 12693216

• Electronic structure and reactivity of high-spin iron-alkyl- and -pterinperoxo complexes INORGANIC CHEMISTRY Lehnert, N., Fujisawa, K., SOLOMON, E. I. 2003; 42 (2): 469-481

  Abstract

  The spectroscopic properties and electronic structure of the four-coordinate high-spin [FeIII(L3)(OOtBu)]+ complex (1; L3 = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate; tBu = tert-butyl) are investigated and compared to the six-coordinated high-spin [Fe(6-Me3TPA)(OHx)(OOtBu)]x+ system (TPA = tris(2-pyridylmethyl)amine, x = 1 or 2) studied earlier [Lehnert, N.; Ho, R. Y. N.; Que, L., Jr.; Solomon, E. I. J. Am. Chem. Soc. 2001, 123, 12802-12816]. Complex 1 is characterized by Raman features at 889 and 830 cm-1 which are assigned to the O-O stretch (mixed with the symmetric C-C stretch) and a band at 625 cm-1 that corresponds to nu(Fe-O). The UV-vis spectrum shows a charge-transfer (CT) transition at 510 nm from the alkylperoxo pi v* (v = vertical to C-O-O plane) to a d orbital of Fe(III). A second CT is identified from MCD at 370 nm that is assigned to a transition from pi h* (h = horizontal to C-O-O plane) to an Fe(III) d orbital. For the TPA complex the pi v* CT is at 560 nm while the pi h* CT is to higher energy than 250 nm. These spectroscopic differences between four- and six-coordinate Fe(III)-OOR complexes are interpreted on the basis of their different ligand fields. In addition, the electronic structure of Fe-OOPtn complexes with the biologically relevant pterinperoxo ligand are investigated. Substitution of the tert-butyl group in 1 by pterin leads to the corresponding Fe(III)-OOPtn species (2), which shows a stronger electron donation from the peroxide to Fe(III) than 1. This is related to the lower ionization potential of pterin. Reduction of 2 by one electron leads to the Fe(II)-OOPtn complex (3), which is relevant as a model for potential intermediates in pterin-dependent hydroxylases. However, in the four-coordinate ligand field of 3, the additional electron is located in a nonbonding d orbital of iron. Hence, the pterinperoxo ligand is not activated for heterolytic cleavage of the O-O bond in this system. This is also evident from the calculated reaction energies that are endothermic by at least 20 kcal/mol.

  View details for DOI 10.1021/ic020496g

  View details for Web of Science ID 000180594400030

  View details for PubMedID 12693229

• Spectroscopic and electronic structure studies of the diamagnetic side-on Cu-II-superoxo complex Cu(O-2)[HB(3-R-5-(i)Prpz)(3)]: Antiferromagnetic coupling versus covalent delocalization JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, P., Root, D. E., Campochiaro, C., Fujisawa, K., SOLOMON, E. I. 2003; 125 (2): 466-474

  Abstract

  Magnetic, vibrational, and optical techniques are combined with density functional calculations to elucidate the electronic structure of the diamagnetic mononuclear side-on CuII-superoxo complex. The electronic nature of its lowest singlet/triplet states and the ground-state diamagnetism are explored. The triplet state is found to involve the interaction between the Cu xy and the superoxide pi v * orbitals, which are orthogonal to each other. The singlet ground state involves the interaction between the Cu xy and the in-plane superoxide pi v * orbitals, which have a large overlap and thus strong bonding. The ground-state singlet/triplet states are therefore fundamentally different in orbital origin and not appropriately described by an exchange model. The ground-state singlet is highly delocalized with no spin polarization.

  View details for DOI 10.1021/ja020969i

  View details for Web of Science ID 000180311800038

  View details for PubMedID 12517160

• Distal metal effects in cobalt porphyrins related to CcO INORGANIC CHEMISTRY Collman, J. P., Berg, K. E., Sunderland, C. J., Aukauloo, A., Vance, M. A., SOLOMON, E. I. 2002; 41 (25): 6583-6596

  Abstract

  Cobalt(II) porphyrins were studied to determine the influence of distal site metalation and superstructure upon dioxygen reactivity in active site models of cytochrome c oxidase (CcO). Monometallic, Co(II)(P) complexes when ligated by an axial imidazole react with dioxygen to form reversible Co-superoxide adducts, which were characterized by EPR and resonance Raman (RR). Unexpectedly, certain Co porphyrins with Cu(I) metalated imidazole pickets do not form mu-peroxo Co(III)/Cu(II) products even though the calculated intermetallic distance suggests this is possible. Instead, cobalt-porphyrin-superoxide complexes are obtained with the distal copper remaining as Cu(I). Moreover, distal metals (Cu(I) or Zn(II)) greatly enhance the stability of the dioxygen adduct, such that Co superoxides of bimetallic complexes demonstrate minimal reversibility. The "trapping" of dioxygen by a second metal is attributed to structural and electrostatic changes within the distal pocket upon metalation. EPR evidence suggests that the terminal oxygen in these bimetallic Co-superoxide systems is H-bonded to the NH of an imidazole picket amide linker, which may contribute to enthalpic stabilization of the dioxygen adduct. Stabilization of the dioxygen adduct in these bimetallic systems suggests one possible role for the distal copper in the Fe/Cu bimetallic active site of terminal oxidases, which form a heme-superoxide/copper(I) adduct upon oxygenation.

  View details for DOI 10.1021/ic020395i

  View details for Web of Science ID 000179797700009

  View details for PubMedID 12470053

• The EPR spectrum of a Cu(II/II/III) cluster: anisotropic exchange in a bent Cu(II)(2)O-2 core INORGANICA CHIMICA ACTA Machonkin, T. E., Mukherjee, P., Henson, M. J., Stack, T. D., SOLOMON, E. I. 2002; 341: 39-44

  View details for Web of Science ID 000179754500007

• Electronic structure and reactivity of low-spin Fe(III)-hydroperoxo complexes: Comparison to activated bleomycin JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lehnert, N., Neese, F., Ho, R. Y., Que, L., Solomon, E. I. 2002; 124 (36): 10810-10822

  Abstract

  The spectroscopic properties, electronic structure, and reactivity of the low-spin Fe(III)-hydroperoxo complex [Fe(N4Py)(OOH)](2+) (1, N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) are investigated in comparison to those of activated bleomycin (ABLM). Complex 1 is characterized by Raman features at 632 (Fe-O stretch) and 790 cm(-1) (O-O stretch), corresponding to a strong Fe-O bond (force constant 3.62 mdyn/A) and a weak O-O bond (3.05 mdyn/A). The UV-vis spectrum of 1 shows a broad absorption band around 550 nm that is assigned to a charge-transfer transition from the hydroperoxo to a t(2g) d orbital of Fe(III) using resonance Raman and MCD spectroscopies and density functional (DFT) calculations. Compared to low-spin [Fe(TPA)(OH(x))(OO(t)Bu)](x+)(TPA = tris(2-pyridylmethyl)amine, x = 1 or 2), an overall similar Fe-OOR bonding results for low-spin Fe(III)-alkylperoxo and -hydroperoxo species. Correspondingly, both systems show similar reactivities and undergo homolytic cleavage of the O-O bond. From the DFT calculations, this reaction is more endothermic for 1 due to the reduced stabilization of the .OH radical compared to .O(t)Bu and the absence of the hydroxo ligand that helps to stabilize the resulting Fe(IV)=O species. In contrast, ABLM has a somewhat different electronic structure where no pi donor bond between the hydroperoxo ligand and iron(III) is present [Neese, F.; Zaleski, J. M.; Loeb-Zaleski, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 11703]. Possible reaction pathways for ABLM are discussed in relation to known experimental results.

  View details for DOI 10.1021/ja012621d

  View details for Web of Science ID 000177872200039

  View details for PubMedID 12207537

• Spectroscopic and electronic structure studies of the mu(4)-sulfide bridged tetranuclear Cu-z cluster in N2O reductase: Molecular insight into the catalytic mechanism JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, P., Cabrito, I., Moura, J. J., Moura, I., Solomon, E. I. 2002; 124 (35): 10497-10507

  Abstract

  Spectroscopic methods combined with density functional calculations are used to develop a detailed bonding description of the mu(4)-sulfide bridged tetranuclear Cu(Z) cluster in N(2)O reductase. The ground state of Cu(Z) has the 1Cu(II)/3Cu(I) configuration. The single electron hole dominantly resides on one Cu atom (Cu(I)) and partially delocalizes onto a second Cu atom (Cu(II)) via a Cu(I)-S-Cu(II) sigma/sigma superexchange pathway which is manifested by a Cu(II) --> Cu(I) intervalence transfer transition in absorption. The observed excited-state spectral features of Cu(Z) are dominated by the S --> Cu(I) charge-transfer transitions and Cu(I) based d-d transitions. The intensity pattern of individual S --> Cu(I) charge-transfer transitions reflects different bonding interactions of the sulfur valence orbitals with the four Cu's in the Cu(Z) cluster, which are consistent with the individual Cu-S force constants obtained from a normal coordinate analysis of the Cu(Z) resonance Raman frequencies and profiles. The Cu(I) d orbital splitting pattern correlates with its distorted T-shaped ligand field geometry and accounts for the observed low g( parallel ) value of Cu(Z) in EPR. The dominantly localized electronic structure description of the Cu(Z) site results from interactions of Cu(II) with the two additional Cu's of the cluster (Cu(III)/Cu(IV)), where the Cu-Cu electrostatic interactions lead to hole localization with no metal-metal bonding. The substrate binding edge of Cu(Z) has a dominantly oxidized Cu(I) and a dominantly reduced Cu(IV). The electronic structure description of Cu(Z) provides a strategy to overcome the reaction barrier of N(2)O reduction at this Cu(I)/Cu(IV) edge by simultaneous two-electron transfer to N(2)O in a bridged binding mode. One electron can be donated directly from Cu(IV) and the other from Cu(II) through the Cu(II)-S-Cu(I) sigma/sigma superexchange pathway. A frontier orbital scheme provides molecular insight into the catalytic mechanism of N(2)O reduction by the Cu(Z) cluster.

  View details for DOI 10.1021/ja0205028

  View details for Web of Science ID 000177881300047

  View details for PubMedID 12197752

• A stabilized mu-eta(2):eta(2) peroxodicopper(II) complex with a secondary diamine ligand and its tyrosinase-like reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mirica, L. M., Vance, M., Rudd, D. J., Hedman, B., Hodgson, K. O., SOLOMON, E. I., Stack, T. D. 2002; 124 (32): 9332-9333

  Abstract

  The activation of dioxygen (O(2)) by Cu(I) complexes is an ubiquitous process in biology and industrial applications. In tyrosinase, a binuclear copper enzyme, a mu-eta(2):eta(2)-peroxodicopper(II) species is generally accepted to be the active oxidant. Reported here is the characterization and reactivity of a stable mu-eta(2):eta(2)-peroxodicopper(II) complex at -80 degrees C using a secondary diamine ligand, N,N'-di-tert-butyl-ethylenediamine (DBED). The spectroscopic characteristics of this complex (UV-vis, resonance Raman) prove to be strongly dependent on the counteranion employed and not on the solvent, suggesting an intimate interaction of the counteranions with the Cu-O(2) cores. This interaction is also supported by solution EXAFS data. This new complex exhibits hydroxylation reactivity by converting phenolates to catechols, proving to be a functional model of tyrosinase. Additional interest in this Cu/O(2) species results from the use of Cu(I)-DBED as a polymerization catalyst of phenols to polyphenylene oxide (PPO) with O(2) as the terminal oxidant.

  View details for DOI 10.1021/ja026905p

  View details for Web of Science ID 000177358600005

  View details for PubMedID 12167002

• Nature of the intermediate formed in the reduction of O-2 to H2O at the trinuclear copper cluster active site in native laccase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lee, S. K., George, S. D., Antholine, W. E., Hedman, B., Hodgson, K. O., Solomon, E. I. 2002; 124 (21): 6180-6193

  Abstract

  The multicopper oxidases contain at least four copper atoms and catalyze the four-electron reduction of O(2) to H(2)O at a trinuclear copper cluster. An intermediate, termed native intermediate, has been trapped by a rapid freeze-quench technique from Rhus vernicifera laccase when the fully reduced form reacts with dioxygen. This intermediate had been described as an oxygen-radical bound to the trinuclear copper cluster with one Cu site reduced. XAS, however, shows that all copper atoms are oxidized in this intermediate. A combination of EXAFS, multifrequency EPR, and VTVH MCD has been used to understand how this fully oxidized trinuclear Cu cluster relates to the fully oxidized resting form of the enzyme. It is determined that in the native intermediate all copper atoms of the cluster are bridged by the product of full O(2) reduction. In contrast, the resting form has one copper atom of the cluster (the T2 Cu) magnetically isolated from the others. The native intermediate decays to the resting oxidized form with a rate that is too slow to be in the catalytic cycle. Thus, the native intermediate appears to be the catalytically relevant fully oxidized form of the enzyme, and its role in catalysis is considered.

  View details for DOI 10.1021/ja0114052

  View details for PubMedID 12022853

• Spectroscopic characterization and O-2 reactivity of the trinuclear Cu cluster of mutants of the multicopper oxidase Fet3p BIOCHEMISTRY Palmer, A. E., Quintanar, L., Severance, S., Wang, T. P., Kosman, D. J., Solomon, E. I. 2002; 41 (20): 6438-6448

  Abstract

  Fet3p is a multicopper oxidase that uses four copper ions (one type 1, one type 2, and one type 3 binuclear site) to couple substrate oxidation to the reduction of O(2) to H(2)O. The type 1 Cu site shuttles electrons between the substrate and the type 2/type 3 Cu sites which form a trinuclear Cu cluster that is the active site for O(2) reduction. This study extends the spectroscopic and reactivity studies that have been conducted with type 1-substituted Hg (T1Hg) laccase to Fet3p and a mutant of Fet3p in which the trinuclear Cu cluster is perturbed. To examine the reaction between the trinuclear Cu cluster and O(2), the type 1 Cu Cys(484) was mutated to Ser, resulting in a type 1-depleted (T1D) form of the enzyme. Additional His to Gln mutations were made at the trinuclear cluster to further probe specific contributions to reactivity. One of these mutants (His(126)Gln) produces the first stable but perturbed trinuclear Cu cluster (T1DT3' Fet3p). Spectroscopic characterization (absorption, circular dichroism, magnetic circular dichroism, and electron paramagnetic resonance) of the resting trinuclear sites in T1D and T1DT3' Fet3p reveal that the His(126)Gln mutation changes the electronic structure of both the type 3 and type 2 Cu sites. The trinuclear clusters in T1D and T1DT3' Fet3p react with O(2) to produce peroxide intermediates analogous to that observed in T1Hg laccase. Spectroscopic data on the peroxide intermediates in the three forms provide further insight into the structure of this intermediate. In T1D Fet3p, the decay of this peroxide intermediate is pH-dependent, and the rate of decay is 10-fold higher at low pH. In T1DT3' Fet3p, the decay of the peroxide intermediate is pH-independent and is slow at all pH's. This change in the pH dependence provides new insight into the mechanism of intermediate decay involving reductive cleavage of the O-O bond.

  View details for DOI 10.1021/bi011979j

  View details for Web of Science ID 000175651400027

  View details for PubMedID 12009907

• X-ray absorption spectroscopic investigation of the resting ferrous and cosubstrate-bound active sites of phenylalanine hydroxylase BIOCHEMISTRY Wasinger, E. C., Mitic, N., Hedman, B., Caradonna, J., SOLOMON, E. I., Hodgson, K. O. 2002; 41 (20): 6211-6217

  Abstract

  Previous studies of ferrous wild-type phenylalanine hydroxylase, [Fe(2+)]PAH(T)[], have shown the active site to be a six-coordinate distorted octahedral site. After the substrate and cofactor bind to the enzyme ([Fe(2+)]PAH(R)[L-Phe,5-deaza-6-MPH(4)]), the active site converts to a five-coordinate square pyramidal structure in which the identity of the missing ligand had not been previously determined. X-ray absorption spectroscopy (XAS) at the Fe K-edge further supports this coordination number change with the binding of both cosubstrates to the enzyme, and determines this to be due to the loss of a water ligand.

  View details for DOI 10.1021/bi0121510

  View details for Web of Science ID 000175651400001

  View details for PubMedID 12009881

• Spectroscopic studies of 1-aminocyclopropane-1-carboxylic acid oxidase: Molecular mechanism and CO2 activation in the biosynthesis of ethylene JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Zhou, J., Rocklin, A. M., Lipscomb, J. D., Que, L., Solomon, E. I. 2002; 124 (17): 4602-4609

  Abstract

  1-Aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACCO) catalyzes the last step in the biosynthesis of the gaseous plant hormone ethylene, which is involved in development, including germination, fruit ripening, and senescence. ACCO is a mononuclear non-heme ferrous enzyme that couples the oxidation of the cosubstrate ascorbate to the oxidation of substrate ACC by dioxygen. In addition to substrate and cosubstrate, ACCO requires the activator CO(2) for continuous turnover. NIR circular dichroism and magnetic circular dichroism spectroscopies have been used to probe the geometric and electronic structure of the ferrous active site in ACCO to obtain molecular-level insight into its catalytic mechanism. Resting ACCO/Fe(II) is coordinatively saturated (six-coordinate). In the presence of CO(2), one ferrous ligand is displaced to yield a five-coordinate site only when both the substrate ACC and cosubstrate ascorbate are bound to the enzyme. The open coordination position allows rapid O(2) activation for the oxidation of both substrates. In the absence of CO(2), ACC binding alone converts the site to five-coordinate, which would react with O(2) in the absence of ascorbate and quickly deactivate the enzyme. These studies show that ACCO employs a general strategy similar to other non-heme iron enzymes in terms of opening iron coordination sites at the appropriate time in the reaction cycle and define the role of CO(2) as stabilizing the six-coordinate ACCO/Fe(II)/ACC complex, thus preventing the uncoupled reaction that inactivates the enzyme.

  View details for DOI 10.1021/ja017250f

  View details for Web of Science ID 000175227600027

  View details for PubMedID 11971707

• Contrasting copper-dioxygen chemistry arising from alike tridentate alkyltriamine copper(l) complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Liang, H. C., Zhang, C. X., Henson, M. J., Sommer, R. D., Hatwell, K. R., Kaderli, S., Zuberbuhler, A. D., Rheingold, A. L., SOLOMON, E. I., Karlin, K. D. 2002; 124 (16): 4170-4171

  Abstract

  Copper(I)-dioxygen interactions are of great interest due to their role in biological O2-processing as well as their importance in industrial oxidation processes. We describe here the study of systems which lead to new insights concerning the factors which govern Cu(II)-mu-eta2:eta2 (side-on) peroxo versus Cu(III)-bis-mu-oxo species formation. Drastic differences in O2-reactivity of Cu(I) complexes which differ only by a single -CH3 versus -H substituent on the central amine of the tridentate ligands employed are observed. [Cu(MeAN)]B(C6F5)4 (1) (MeAN = N,N,N',N',N'-pentamethyl-dipropylenetriamine) reacts with O2 at -80 degrees C to form almost exclusively the side-on peroxo complex [{CuII(MeAN)}2(O2)]2+ (3) in CH2Cl2, tetrahydrofuran, acetone, and diethyl ether solvents, as characterized by UV-vis and resonance Raman spectroscopies. In sharp contrast, [Cu(AN)]B(C6F5)4 (2) (AN = 3, 3'-iminobis(N,N-dimethyl-propylamine) can support either Cu2O2 structures in a strongly solvent-dependent manner. Extreme behavior is observed in CH2Cl2 solvent, where 1 reacts with O2 giving 3, while 2 forms exclusively the bis-mu-oxo species [{CuIII(AN)}2(O)2]2+ (4Oxo). Stopped-flow kinetics measurements also reveal significant variations in the oxygenation reactions of 1 versus 2, including the observations that 4Oxo forms much faster than does 3; the former decomposes quickly, while the latter is quite stable at 193 K. The solvent-dependence of the bis-mu-oxo versus side-on peroxo preference observed for 2 is opposite to that reported for other known copper(I) complexes; the factors which may be responsible for the unusual behavior of 1/O2 versus 2/O2 (possibly N-H hydrogen bonding in the AN chemistry) are suggested. The factors which affect bis-mu-oxo versus side-on peroxo formation continue to be of interest.

  View details for DOI 10.1021/ja0125265

  View details for Web of Science ID 000175088600001

  View details for PubMedID 11960420

• Electronic structure and its relation to function in copper proteins CURRENT OPINION IN CHEMICAL BIOLOGY Szilagyi, R. K., Solomon, E. I. 2002; 6 (2): 250-258

  Abstract

  Spectroscopic and theoretical investigations of the geometric and electronic structures of mononuclear and binuclear copper sites in proteins help in understanding the contributions of these proteins to biological electron transfer. Spectroscopically calibrated density functional theory calculations, which give reasonable bonding descriptions in both ground- and excited-states, define the role of the protein in determining the geometric and electronic structure of the active site.

  View details for Web of Science ID 000174821700020

  View details for PubMedID 12039012

• Electronic structure description of the mu(4)-sulfide bridged tetranuclear Cu-z center in N2O reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, P., George, S. D., Cabrito, I., Antholine, W. E., Moura, J. J., Moura, I., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 2002; 124 (5): 744-745

  Abstract

  Spectroscopy coupled with density functional calculations has been used to define the spin state, oxidation states, spin distribution, and ground state wave function of the mu4-sulfide bridged tetranuclear CuZ cluster of nitrous oxide reductase. Initial insight into the electronic contribution to N2O reduction is developed, which involves a sigma superexchange pathway through the bridging sulfide.

  View details for DOI 10.1021/ja0169623

  View details for Web of Science ID 000173628900010

  View details for PubMedID 11817937

• Frontier molecular orbital analysis of Cu-n-O-2 reactivity JOURNAL OF INORGANIC BIOCHEMISTRY Chen, P., Solomon, E. I. 2002; 88 (3-4): 368-374

  Abstract

  Frontier molecular orbital (FMO) theory coupled with density functional calculations has been applied to investigate the chemical reactivity of three key bioinorganic Cu(n)-O(2) complexes, the mononuclear end-on hydroperoxo-Cu(II), the side-on bridged mu-eta(2):eta(2)-O(2)(2-) Cu(II)(2) dimer and the bis-mu-oxo Cu(III)(2) dimer. Two acceptor orbitals (sigma* and pi*) of each complex and two types of donating substrates (sigma-substrate, phosphine; pi-substrate, alkylbenzene) are considered in the electrophilic attack mechanism. The angular dependences of different reaction pathways are determined using FMO theory and the angular overlap model. Including steric effects, the sigma*/sigma and pi*/pi pathways are found more reactive than the corresponding cross sigma*/pi and pi*/sigma pathways which have poor donor-acceptor orbital overlaps in the sterically constrained substrate access region.

  View details for Web of Science ID 000175319500018

  View details for PubMedID 11897352

• Spectroscopic and electronic structure studies of protocatechuate 3,4-dioxygenase: Nature of tyrosinate-Fe(III) bonds and their contribution to reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Davis, M. I., Orville, A. M., Neese, F., Zaleski, J. M., Lipscomb, J. D., SOLOMON, E. I. 2002; 124 (4): 602-614

  Abstract

  The geometric and electronic structure of the high-spin ferric active site of protocatechuate 3,4-dioxygenase (3,4-PCD) has been examined by absorption (Abs), circular dichroism (CD), magnetic CD (MCD), and variable-temperature-variable-field (VTVH) MCD spectroscopies. Density functional (DFT) and INDO/S-CI molecular orbital calculations provide complementary insight into the electronic structure of 3,4-PCD and allow an experimentally calibrated bonding scheme to be developed. Abs, CD, and MCD indicate that there are at least seven transitions below 35 000 cm(-1) which arise from tyrosinate ligand-to-metal-charge transfer (LMCT) transitions. VTVH MCD spectroscopy gives the polarizations of these LMCT bands in the principal axis system of the D-tensor, which is oriented relative to the molecular structure from the INDO/S-CI calculations. Three transitions are associated with the equatorial tyrosinate and four with the axial tyrosinate. This large number of transitions per tyrosinate is due to the pi and importantly the sigma overlap of the two tyrosinate valence orbitals with the metal d orbitals and is governed by the Fe-O-C angle and the Fe-O-C-C dihedral angles. The previously reported crystal structure indicates that the Fe-O-C angles are 133 degrees and 148 degrees for the equatorial and axial tyrosinate, respectively. Each tyrosinate has transitions at different energies with different intensities, which correlate with differences in geometry that reflect pseudo-sigma bonding to the Fe(III) and relate to reactivity. These factors reflect the metal-ligand bond strength and indicate that the axial tyrosinate-Fe(III) bond is weaker than the equatorial tyrosinate-Fe(III) bond. Furthermore, it is found that the differences in geometry, and hence electronic structure, are imposed by the protein. The consequences to catalysis are significant because the axial tyrosinate has been shown to dissociate upon substrate binding and the equatorial tyrosinate in the enzyme-substrate complex is thought to influence asymmetric binding of the chelated substrate moiety via a strong trans influence which activates the substrate for reaction with O2.

  View details for DOI 10.1021/ja011945z

  View details for Web of Science ID 000173456800022

  View details for PubMedID 11804491

• X-ray absorption spectroscopic investigation of Fe(II)-peplomycin and peplomycin derivatives: the effect of axial ligation on Fe-pyrimidine back-bonding JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY Wasinger, E. C., Zaleski, K. L., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 2002; 7 (1-2): 157-164

  Abstract

  X-ray absorption spectroscopy (XAS) is used to study ferrous complexes of a bleomycin (BLM) congener, peplomycin (PEP), and two of its derivatives, iso-peplomycin (ISO) and depyruvamide peplomycin (DP), in which potential axial ligands have been perturbed and removed, respectively. Application of extended X-ray absorption fine structure analysis shows an elongation of the short-distance component of the first coordination sphere in DP and ISO relative to PEP. The XAS pre-edge intensity concomitantly decreases with increased axial perturbation. The short-distance component of PEP is correlated to the Fe-pyrimidine bond and is related to the amount of pi-back-bonding. Thus, the XAS analysis of these complexes provides structural information relevant to their differences in O2 reactivity.

  View details for DOI 10.1007/s007750100283

  View details for Web of Science ID 000173024100018

  View details for PubMedID 11862552

• Electronic structure of high-spin iron(III)-alkylperoxo complexes and its relation to low-spin analogues: Reaction coordinate of O-O bond homolysis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lehnert, N., Ho, R. Y., Que, L., SOLOMON, E. I. 2001; 123 (51): 12802-12816

  Abstract

  The spectroscopic properties of the high-spin Fe(III)-alkylperoxo model complex [Fe(6-Me(3)TPA)(OH(x))(OO(t)Bu)](x)(+) (1; TPA = tris(2-pyridylmethyl)amine, (t)Bu = tert-butyl, x = 1 or 2) are defined and related to density functional calculations of corresponding models in order to determine the electronic structure and reactivity of this system. The Raman spectra of 1 show four peaks at 876, 842, 637, and 469 cm(-1) that are assigned with the help of normal coordinate analysis, and corresponding force constants have been determined to be 3.55 mdyn/A for the O-O and 2.87 mdyn/A for the Fe-O bond. Complex 1 has a broad absorption feature around 560 nm that is assigned to a charge-transfer (CT) transition from the alkylperoxo to a t(2g) d orbital of Fe(III) with the help of resonance Raman profiles and MCD spectroscopy. An additional contribution to the Fe-O bond arises from a sigma interaction between and an e(g) d orbital of iron. The electronic structure of 1 is compared to the related low-spin model complex [Fe(TPA)(OH(x))(OO(t)Bu)](x)(+) and the reaction coordinate for O-O homolysis is explored for both the low-spin and the high-spin Fe(III)-alkylperoxo systems. Importantly, there is a barrier for homolytic cleavage of the O-O bond on the high-spin potential energy surface that is not present for the low-spin complex, which is therefore nicely set up for O-O homolysis. This is reflected by the electronic structure of the low-spin complex having a strong Fe-O and a weak O-O bond due to a strong Fe-O sigma interaction. In addition, the reaction coordinate of the Fe-O homolysis has been investigated, which is a possible decay pathway for the high-spin system, but which is thermodynamically unfavorable for the low-spin complex.

  View details for DOI 10.1021/ja011450+

  View details for Web of Science ID 000172939700008

  View details for PubMedID 11749538

• Oxygen Binding, Activation, and Reduction to Water by Copper Proteins. Angewandte Chemie (International ed. in English) Solomon, E. I., Chen, P., Metz, M., Lee, S. K., Palmer, A. E. 2001; 40 (24): 4570-4590

  Abstract

  Copper active sites play a major role in biological and abiological dioxygen activation. Oxygen intermediates have been studied in detail for the proteins and enzymes involved in reversible O(2) binding (hemocyanin), activation (tyrosinase), and four-electron reduction to water (multicopper oxidases). These oxygen intermediates exhibit unique spectroscopic features indicative of new geometric and electronic structures involved in oxygen activation. The spectroscopic and quantum-mechanical study of these intermediates has defined geometric- and electronic-structure/function correlations, and developed detailed reaction coordinates for the reversible binding of O(2), hydroxylation, and H-atom abstraction from different substrates, and the reductive cleavage of the O-O bond in the formation water.

  View details for PubMedID 12404359

• A new Cu(II) side-on peroxo model clarifies the assignment of the oxyhemocyanin Raman spectrum INORGANIC CHEMISTRY Henson, M. J., Mahadevan, V., Stack, T. D., SOLOMON, E. I. 2001; 40 (20): 5068-?

  View details for Web of Science ID 000171177500007

  View details for PubMedID 11559060

• Excited electronic states of transition-metal dimers and the VBCI model: an overview COORDINATION CHEMISTRY REVIEWS Tuczek, F., SOLOMON, E. I. 2001; 219: 1075-1112

  View details for Web of Science ID 000171343600037

• Spectroscopic properties and electronic structure of low-spin Fe(III)-alkylperoxo complexes: Homolytic cleavage of the O-O bond JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lehnert, N., Ho, R. Y., Que, L., SOLOMON, E. I. 2001; 123 (34): 8271-8290

  Abstract

  The spectroscopic properties, electronic structure, and reactivity of the low-spin Fe(III)-alkylperoxo model complex [Fe(TPA)(OH(x))(OO(t)Bu)](x+) (1; TPA = tris(2-pyridylmethyl)amine, (t)Bu = tert-butyl, x = 1 or 2) are explored. The vibrational spectra of 1 show three peaks that are assigned to the O-O stretch (796 cm(-1)), the Fe-O stretch (696 cm(-)(1)), and a combined O-C-C/C-C-C bending mode (490 cm(-1)) that is mixed with upsilon(FeO). The corresponding force constants have been determined to be 2.92 mdyn/A for the O-O bond which is small and 3.53 mdyn/A for the Fe-O bond which is large. Complex 1 is characterized by a broad absorption band around 600 nm that is assigned to a charge-transfer (CT) transition from the alkylperoxo pi*(upsilon) to a t(2g) d orbital of Fe(III). This metal-ligand pi bond is probed by MCD and resonance Raman spectroscopies which show that the CT state is mixed with a ligand field state (t(2g) --> e(g)) by configuration interaction. This gives rise to two intense transitions under the broad 600 nm envelope with CT character which are manifested by a pseudo-A term in the MCD spectrum and by the shapes of the resonance Raman profiles of the 796, 696, and 490 cm(-1) vibrations. Additional contributions to the Fe-O bond arise from sigma interactions between mainly O-O bonding donor orbitals of the alkylperoxo ligand and an e(g) d orbital of Fe(III), which explains the observed O-O and Fe-O force constants. The observed homolytic cleavage of the O-O bond of 1 is explored with experimentally calibrated density functional (DFT) calculations. The O-O bond homolysis is found to be endothermic by only 15 to 20 kcal/mol due to the fact that the Fe(IV)=O species formed is highly stabilized (for spin states S = 1 and 2) by two strong pi and a strong sigma bond between Fe(IV) and the oxo ligand. This low endothermicity is compensated by the entropy gain upon splitting the O-O bond. In comparison, Cu(II)-alkylperoxo complexes studied before [Chen, P.; Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 10177] are much less suited for O-O bond homolysis, because the resulting Cu(III)=O species is less stable. This difference in metal-oxo intermediate stability enables the O-O homolysis in the case of iron but directs the copper complex toward alternative reaction channels.

  View details for DOI 10.1021/ja010165n

  View details for Web of Science ID 000170730000013

  View details for PubMedID 11516278

• Spectroscopic studies of substrate interactions with clavaminate synthase 2, a multifunctional alpha-KG-dependent non-heme iron enzyme: Correlation with mechanisms and reactivities JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Zhou, J., Kelly, W. L., Bachmann, B. O., Gunsior, M., TOWNSEND, C. A., SOLOMON, E. I. 2001; 123 (30): 7388-7398

  Abstract

  Using a single ferrous active site, clavaminate synthase 2 (CS2) activates O(2) and catalyzes the hydroxylation of deoxyguanidinoproclavaminic acid (DGPC), the oxidative ring closure of proclavaminic acid (PC), and the desaturation of dihydroclavaminic acid (and a substrate analogue, deoxyproclavaminic acid (DPC)), each coupled to the oxidative decarboxylation of cosubstrate, alpha-ketoglutarate (alpha-KG). CS2 can also catalyze an uncoupled decarboxylation of alpha-KG both in the absence and in the presence of substrate, which results in enzyme deactivation. Resting CS2/Fe(II) has a six-coordinate Fe(II) site, and alpha-KG binds to the iron in a bidentate mode. The active site becomes five-coordinate only when both substrate and alpha-KG are bound, the latter still in a bidentate mode. Absorption, CD, MCD, and VTVH MCD studies of the interaction of CS2 with DGPC, PC, and DPC provide significant molecular level insight into the structure/function correlations of this multifunctional enzyme. There are varying amounts of six-coordinate ferrous species in the substrate complexes, which correlate to the uncoupled reaction. Five-coordinate ferrous species with similar geometric and electronic structures are present for all three substrate/alpha-KG complexes. Coordinative unsaturation of the Fe(II) in the presence of both cosubstrate and substrate appears to be critical for the coupling of the oxidative decarboxylation of alpha-KG to the different substrate oxidation reactions. In addition to the substrate orientation relative to the open coordination position on the iron site, it is hypothesized that the enzyme can affect the nature of the reactivity by further regulating the binding energy of the water to the ferrous species in the enzyme/succinate/product complex.

  View details for DOI 10.1021/ja004025+

  View details for Web of Science ID 000170111600024

  View details for PubMedID 11472170

• Invited award contribution for ACS Award in Inorganic Chemistry. Geometric and electronic structure contributions to function in bioinorganic chemistry: active sites in non-heme iron enzymes. Inorganic chemistry SOLOMON, E. I. 2001; 40 (15): 3656-3669

  Abstract

  Spectroscopy has played a major role in the definition of structure/function correlations in bioinorganic chemistry. The importance of spectroscopy combined with electronic structure calculations is clearly demonstrated by the non-heme iron enzymes. Many members of this large class of enzymes activate dioxygen using a ferrous active site that has generally been difficult to study with most spectroscopic methods. A new spectroscopic methodology has been developed utilizing variable temperature, variable field magnetic circular dichroism, which enables one to obtain detailed insight into the geometric and electronic structure of the non-heme ferrous active site and probe its reaction mechanism on a molecular level. This spectroscopic methodology is presented and applied to a number of key mononuclear non-heme iron enzymes leading to a general mechanistic strategy for O2 activation. These studies are then extended to consider the new features present in the binuclear non-heme iron enzymes and applied to understand (1) the mechanism of the two electron/coupled proton transfer to dioxygen binding to a single iron center in hemerythrin and (2) structure/function correlations over the oxygen-activating enzymes stearoyl-ACP Delta9-desaturase, ribonucleotide reductase, and methane monooxygenase. Electronic structure/reactivity correlations for O2 activation by non-heme relative to heme iron enzymes will also be developed.

  View details for PubMedID 11442362

• Decay of the peroxide intermediate in laccase: Reductive cleavage of the O-O bond JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Palmer, A. E., Lee, S. K., SOLOMON, E. I. 2001; 123 (27): 6591-6599

  Abstract

  Laccase is a multicopper oxidase that contains four Cu ions, one type 1, one type 2, and a coupled binuclear type 3 Cu pair. The type 2 and type 3 centers form a trinuclear Cu cluster that is the active site for O(2) reduction to H(2)O. To examine the reaction between the type 2/type 3 trinuclear cluster and dioxygen, the type 1 Cu was removed and replaced with Hg(2+), producing the T1Hg derivative. When reduced T1Hg laccase is reacted with dioxygen, a peroxide intermediate (P) is formed. The present study examines the kinetics and mechanism of formation and decay of P in T1HgLc. The formation of P was found to be independent of pH and did not involve a kinetic solvent isotope effect, indicating that no proton is involved in the rate-determining step of formation of P. Alternatively, pH and isotope studies on the decay of P revealed that a proton enhances the rate of decay by 10-fold at low pH. This process shows an inverse k(H)/k(D) kinetic solvent isotope effect and involves protonation of a nearby residue that assists in catalysis, rather than direct protonation of the peroxide. Decay of P also involves a significant oxygen isotope effect (k(16)O(2)/k(18)O(2)) of 1.11 +/- 0.05, indicating that reductive cleavage of the O-O bond is the rate-determining step in the decay of P. The activation energy for this process was found to be approximately 9.0 kcal/mol. The exceptionally slow rate of decay of P is explained by the fact that this process involves a 1e(-) reductive cleavage of the O-O bond and there is a large Franck-Condon barrier associated with this process. Alternatively, the 2e(-) reductive cleavage of the O-O bond has a much larger driving force which minimizes this barrier and accelerates the rate of this reaction by approximately 10(7) in the native enzyme. This large difference in rate for the 2e(-) versus 1e(-) process supports a molecular mechanism for multicopper oxidases in which O(2) is reduced to H(2)O in two 2e(-) steps.

  View details for PubMedID 11439045

• A quantitative description of the ground-state wave function of Cu-A by X-ray absorption spectroscopy: Comparison to plastocyanin and relevance to electron transfer JOURNAL OF THE AMERICAN CHEMICAL SOCIETY George, S. D., Metz, M., Szilagyi, R. K., Wang, H. X., Cramer, S. P., Lu, Y., Tolman, W. B., Hedman, B., Hodgson, K. O., Solomon, E. I. 2001; 123 (24): 5757-5767

  Abstract

  To evaluate the importance of the electronic structure of Cu(A) to its electron-transfer (ET) function, a quantitative description of the ground-state wave function of the mixed-valence (MV) binuclear Cu(A) center engineered into Pseudomonas aeruginosa azurin has been developed, using a combination of S K-edge and Cu L-edge X-ray absorption spectroscopies (XAS). Parallel descriptions have been developed for a binuclear thiolate-bridged MV reference model complex ([(L(i)(PrdacoS)Cu)(2)](+)) and a homovalent (II,II) analogue ([L(i)(Pr2tacnS)Cu)(2)](2+), where L(i)(PrdacoS) and L(i)(Pr2tacnS) are macrocyclic ligands with attached thiolates that bridge the Cu ions. Previous studies have qualitatively defined the ground-state wave function of Cu(A) in terms of ligand field effects on the orbital orientation and the presence of a metal--metal bond. The studies presented here provide further evidence for a direct Cu--Cu interaction and, importantly, experimentally quantify the covalency of the ground-state wave function. The experimental results are further supported by DFT calculations. The nature of the ground-state wave function of Cu(A) is compared to that of the well-defined blue copper site in plastocyanin, and the importance of this wave function to the lower reorganization energy and ET function of Cu(A) is discussed. This wave function incorporates anisotropic covalency into the intra- and intermolecular ET pathways in cytochrome c oxidase. Thus, the high covalency of the Cys--Cu bond allows a path through this ligand to become competitive with a shorter His path in the intramolecular ET from Cu(A) to heme a and is particularly important for activating the intermolecular ET path from heme c to Cu(A).

  View details for DOI 10.1021/ja004109i

  View details for PubMedID 11403610

• Spectroscopy and reactivity of the type 1 copper site in Fet3p from Saccharomyces cerevisiae: Correlation of structure with reactivity in the multicopper oxidases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Machonkin, T. E., Quintanar, L., Palmer, A. E., Hassett, R., Severance, S., Kosman, D. J., SOLOMON, E. I. 2001; 123 (23): 5507-5517

  Abstract

  Fet3p is a multicopper oxidase recently isolated from the yeast, Saccharomyces cerevisiae. Fet3p is functionally homologous to ceruloplasmin (Cp) in that both are ferroxidases. However, by sequence homology Fet3p is more similar to fungal laccase, and both contain a type 1 Cu site that lacks the axial methionine ligand present in the functional type 1 sites of Cp. To determine the contribution of the electronic structure of the type 1 Cu site of Fet3p to the ferroxidase mechanism, we have examined the absorption, circular dichroism, magnetic circular dichroism, electron paramagnetic resonance, and resonance Raman spectra of wild-type Fet3p and type 1 and type 2 Cu-depleted mutants. The spectroscopic features of the type 1 Cu site of Fet3p are nearly identical to those of fungal laccase, indicating a very similar three-coordinate geometry. We have also examined the reactivity of the type 1 Cu site by means of redox titrations and stopped-flow kinetics. From poised potential redox titrations, the E degrees of the type 1 Cu site is 427 mV, which is low for a three-coordinate type 1 Cu site. The kinetics of reduction of the type 1 Cu sites of four different multicopper oxidases with two different substrates were compared. The type 1 site of a plant laccase (Rhus vernicifera) is reduced moderately slowly by both Fe(II) and a bulky organic substrate, 1,4-hydroquinone (with 6 equiv of substrate, k(obs) = 0.029 and 0.013 s(-)(1), respectively). On the other hand, the type 1 site of a fungal laccase (Coprinus cinereus) is reduced very rapidly by both substrates (k(obs) > 23 s(-)(1)). In contrast, both Fet3p and Cp are rapidly reduced by Fe(II) (k(obs) > 23 s(-)(1)), but only very slowly by 1,4-hydroquinone (10- and 100-fold more slowly than plant laccase, respectively). Semiclassical theory is used to analyze the origin of these differences in reactivity in terms of type 1 Cu site accessibility to specific substrates.

  View details for Web of Science ID 000169176300018

  View details for PubMedID 11389633

• Sulfur K-edge X-ray absorption spectroscopy of 2Fe-2S ferredoxin: Covalency of the oxidized and reduced 2Fe forms and comparison to model complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Anxolabehere-Mallart, E., Glaser, T., Frank, P., Aliverti, A., Zanetti, G., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 2001; 123 (23): 5444-5452

  Abstract

  Ligand K-edge X-ray absorption spectroscopy (XAS) provides a direct experimental probe of ligand-metal bonding. In previous studies, this method has been applied to mononuclear Fe-S and binuclear 2Fe-2S model compounds as well as to rubredoxins and the Rieske protein. These studies are now extended to the oxidized and reduced forms of ferredoxin I from spinach. Because of its high instability, the mixed-valence state was generated electrochemically in the protein matrix, and ligand K-edge absorption spectra were recorded using an XAS spectroelectrochemical cell. The experimental setup is described. The XAS edge data are analyzed to independently determine the covalencies of the iron-sulfide and -thiolate bonds. The results are compared with those obtained previously for the Rieske protein and for 2Fe-2S model compounds. It is found that the sulfide covalency is significantly lower in oxidized FdI compared to that of the oxidized model complex. This decrease is interpreted in terms of H bonding present in the protein, and its contribution to the reduction potential E degrees is estimated. Further, a significant increase in covalency for the Fe(III)-sulfide bond and a decrease of the Fe(II)-sulfide bond are observed in the reduced Fe(III)Fe(II) mixed-valence species compared to those of the Fe(III)Fe(III) homovalent site. This demonstrates that, upon reduction, the sulfide interactions with the ferrous site decrease, allowing greater charge donation to the remaining ferric center. That is the dominant change in electronic structure of the Fe(2)S(2)RS(4) center upon reduction and can contribute to the redox properties of this active site.

  View details for Web of Science ID 000169176300010

  View details for PubMedID 11389625

• Dioxygen binding to deoxyhemocyanin: Electronic structure and mechanism of the spin-forbidden two-electron reduction of O-2 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Metz, M., SOLOMON, E. I. 2001; 123 (21): 4938-4950

  Abstract

  Spectroscopically calibrated DFT is used to investigate the reaction coordinate of O(2) binding to Hemocyanin (Hc). A reaction path is calculated in which O(2) approaches the binuclear copper site with increasing metal-ligand overlap, which switches the coordination mode from end-on eta(1)-eta(1), to mu-eta(1):eta(2), then to butterfly, and finally to the planar [Cu(2)(mu-eta(2):eta(2)O(2))] structure. Analysis of the electronic structures during O(2) binding reveals that simultaneous two-electron transfer (ET) takes place. At early stages of O(2) binding the energy difference between the triplet and the singlet state is reduced by charge transfer (CT), which delocalizes the unpaired electrons and thus lowers the exchange stabilization onto the separated copper centers. The electron spins on the copper(II) ions are initially ferromagnetically coupled due to close to orthogonal magnetic orbital pathways through the dioxygen bridging ligand, and a change in the structure of the Cu(2)O(2) core turns on the superexchange coupling between the coppers. This favors the singlet state over the triplet state enabling intersystem crossing. Comparison with mononuclear model complexes indicates that the protein matrix holds the two copper(I) centers in close proximity, which enthalpically and entropically favors O(2) binding due to destabilization of the reduced binuclear site. This also allows regulation of the enthalpy by the change of the Cu--Cu distance in deoxyHc, which provides an explanation for the O(2) binding cooperativity in Hc. These results are compared to our earlier studies of Hemerythrin (Hr) and a common theme emerges where the spin forbiddeness of O(2) binding is overcome through delocalization of unpaired electrons onto the metal centers and the superexchange coupling of the metal centers via a ligand bridge.

  View details for Web of Science ID 000168914400009

  View details for PubMedID 11457321

• Protein effects on the electronic structure of the [Fe4S4](2+) cluster in ferredoxin and HiPIP JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Glaser, T., Bertini, I., Moura, J. J., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 2001; 123 (20): 4859-4860

  View details for DOI 10.1021/ja0155940

  View details for Web of Science ID 000168912000033

  View details for PubMedID 11457306

• Raman and extended X-ray absorption fine structure characterization of a sulfur-ligated Cu(I) ethylene complex: Modeling the proposed ethylene binding site of Arabidopsis thaliana ETR1 INORGANIC CHEMISTRY Hirsch, J., George, S. D., SOLOMON, E. I., Hedman, B., Hodgson, K. O., Burstyn, J. N. 2001; 40 (10): 2439-?

  View details for Web of Science ID 000168524100040

  View details for PubMedID 11327928

• Recent advances in bioinorganic spectroscopy CURRENT OPINION IN CHEMICAL BIOLOGY Lehnert, N., George, S. D., Solomon, E. I. 2001; 5 (2): 176-187

  Abstract

  Spectroscopic methods covering many energy regions together provide complementary insight into metalloenzyme active sites. These methods probe geometric and electronic structure and define these contributions to reactivity. Two recent advances--determination of the polarizations of electronic transitions in solution using magnetic circular dichroism, electron paramagnetic resonance and quantum chemistry, and experimental estimation of covalency using metal L-edges and ligand K-edges--are particularly important.

  View details for PubMedID 11282345

• Electronic spectral studies of molybdenyl complexes. 2. MCD spectroscopy of [MoOS4](-) centers INORGANIC CHEMISTRY McMaster, J., Carducci, M. D., Yang, Y. S., SOLOMON, E. I., ENEMARK, J. H. 2001; 40 (4): 687-702

  Abstract

  Magnetic circular dichroism (MCD) and absorption spectroscopies have been used to probe the electronic structure of [PPh4][MoO(p-SC6H4X)4] (X = H, Cl, OMe) and [PPh4][MoO(edt)2] complexes (edt = ethane-1,2-dithiolate). The results of density functional calculations (DFT) on [MoO(SMe)4]- and [MoO(edt)2]- model complexes were used to provide a framework for the interpretation of the spectra. Our analysis shows that the lowest energy transitions in [MoVOS4] chromophores (S4 = sulfur donor ligand) result from S-->Mo charge transfer transitions from S valence orbitals that lie close to the ligand field manifold. The energies of these transitions are strongly dependent on the orientation of the S lone-pair orbitals with respect to the Mo atom that is determined by the geometry of the ligand backbone. Thus, the lowest energy transition in the MCD spectrum of [PPh4][MoO(p-SC6H4X)4] (X = H) occurs at 14,800 cm-1, while that in [PPh4][MoO(edt)2] occurs at 11,900 cm-1. The identification of three bands in the absorption spectrum of [PPh4][MoO(edt)2] arising from LMCT from S pseudo-sigma combinations to the singly occupied Mo 4d orbital in the xy plane suggests that there is considerable covalency in the ground-state electronic structures of [MoOS4] complexes. DFT calculations on [MoO(SMe)4]- reveal that the singly occupied HOMO is 53% Mo 4dxy and 35% S p for the equilibrium C4 geometry. For [MoO(edt)2]- the steric constraints imposed by the edt ligands result in the S pi orbitals being of similar energy to the Mo 4d manifold. Significant S pseudo-sigma and pi donation may also weaken the Mo identical to O bond in [MoOS4] centers, a requirement for facile active site regeneration in the catalytic cycle of the DMSO reductases. The strong dependence of the energies of the bands in the absorption and MCD spectra of [PPh4][MoO(p-SC6H4X)4] (X = H, Cl, OMe) and [PPh4][MoO(edt)2] on the ligand geometry suggests that the structural features of the active sites of the DMSO reductases may result in an electronic structure that is optimized for facile oxygen atom transfer.

  View details for DOI 10.1021/ic0005846

  View details for Web of Science ID 000166971600016

  View details for PubMedID 11225111

• SK-edge X-ray absorption studies of tetranuclear iron-sulfur clusters: mu-sulfide bonding and its contribution to electron delocalization JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Glaser, T., Rose, K., Shadle, S. E., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 2001; 123 (3): 442-454

  Abstract

  X-ray absorption spectroscopy (XAS) at the sulfur ( approximately 2470 eV) and chlorine ( approximately 2822 eV) K-edges has been applied to a series of 4Fe-4S model complexes. These are compared to 2Fe-2S model complexes to obtain insight into the localized ground state in the mixed-valence dimer versus the delocalized ground state in the mixed-valence tetramer. The preedges of hypothetical delocalized mixed-valence dimers [Fe(2)S(2)](+) are estimated using trends from experimental data and density functional calculations, for comparison to the delocalized mixed-valence tetramer [Fe(4)S(4)](2+). The differences between these two mixed-valence sites are due to the change of the sulfide-bridging mode from micro(2) to micro(3). The terminal chloride and thiolate ligands are used as spectator ligands for the electron density of the iron center. From the intensity of the preedge, the covalency of the terminal ligands is found to increase in the tetramer as compared to the dimer. This is associated with a higher effective nuclear charge on the iron in the tetramer (derived from the energies of the preedge). The micro(3)-bridging sulfide in the tetramer has a reduced covalency per bond (39%) as compared to the micro(2)-bridging sulfide in the dimer (51%). A simple perturbation model is used to derive a quadratic dependence of the superexchange coupling constant J on the covalency of the metal ions with the bridging ligands. This relationship is used to estimate the superexchange contribution in the tetramer (J = -156 cm(-)(1)) as compared to the mixed-valence dimer (J = -360 cm(-)(1)). These results, combined with estimates for the double exchange and the vibronic coupling contributions of the dimer sub-site of the tetramer, lead to a delocalized S(t) = (9)/(2) spin ground state for the mixed-valence dimer in the tetramer. Thus, the decrease in the covalency, hence the superexchange pathway associated with changing the bridging mode of the sulfides from micro(2) to micro(3) on going from the dimer to the tetramer, significantly contributes to the delocalization of the excess electron over the dimer sub-site in the tetramer.

  View details for DOI 10.1021/ja002183v

  View details for Web of Science ID 000166698000011

  View details for PubMedID 11456546

• The thermodynamics, kinetics, and molecular mechanism of intramolecular electron transfer in human ceruloplasmin JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Machonkin, T. E., SOLOMON, E. I. 2000; 122 (50): 12547-12560

  View details for Web of Science ID 000166154100017

• Ligand K-edge X-ray absorption spectroscopy: A direct probe of ligand-metal covalency ACCOUNTS OF CHEMICAL RESEARCH Glaser, T., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 2000; 33 (12): 859-868

  Abstract

  Ligand K-edge X-ray absorption spectroscopy (XAS) is a new experimental probe of the covalency of a metal-ligand bond. The intensity of the ligand pre-edge feature is proportional to the mixing of ligand orbitals into the metal d orbitals. The methodology to determine covalencies in one-electron (hole) and many-electron systems is described and demonstrated for a series of metal tetrachlorides [MCl(4)](n)(-), metal tetrathiolates [M(SR)(4)](n)(-), and dimeric iron-sulfur (Fe-S) clusters [Fe(2)S(2)(SR)(4)](2-). It is then applied to blue Cu proteins, the Cu(A) site, hydrogen bonding in Fe-S clusters, and the delocalization behavior in [2Fe-2S] vs [4Fe-4S] clusters. The covalencies determined in these studies provide important electronic structure insight into function.

  View details for Web of Science ID 000166180700006

  View details for PubMedID 11123885

• Electronic structure of activated bleomycin: Oxygen intermediates in heme versus non-heme iron JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Neese, F., Zaleski, J. M., Zaleski, K. L., SOLOMON, E. I. 2000; 122 (47): 11703-11724

  View details for DOI 10.1021/ja001812y

  View details for Web of Science ID 000165696800014

• Spectroscopic and electronic structural studies of blue copper model complexes. 1. Perturbation of the thiolate-Cu bond JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Randall, D. W., George, S. D., Hedman, B., Hodgson, K. O., Fujisawa, K., SOLOMON, E. I. 2000; 122 (47): 11620-11631

  View details for Web of Science ID 000165696800007

• Differential reactivity between interconvertible side-on peroxo and bis-mu-oxodicopper isomers using peralkylated diamine ligands JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mahadevan, V., Henson, M. J., SOLOMON, E. I., Stack, T. D. 2000; 122 (41): 10249-10250

  View details for Web of Science ID 000090107600060

• Spectroscopic and theoretical studies of mononuclear copper(II) alkyl- and hydroperoxo complexes: Electronic structure contributions to reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, P., Fujisawa, K., SOLOMON, E. I. 2000; 122 (41): 10177-10193

  View details for DOI 10.1021/ja0016755

  View details for Web of Science ID 000090107600036

• Excited-state exchange coupling in bent Mn(III)-O-Mn(III) complexes: Dominance of the pi/sigma superexchange pathway and its possible contributions to the reactivities of binuclear metalloproteins JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Brunold, T. C., Gamelin, D. R., SOLOMON, E. I. 2000; 122 (35): 8511-8523

  View details for DOI 10.1021/ja000264l

  View details for Web of Science ID 000089285300021

• Spectroscopic and electronic structure description of the reduced binuclear non-heme iron active site in ribonucleotide reductase from E. coli: Comparison to reduced Delta(9) desaturase and electronic structure contributions to differences in O-2 reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yang, Y. S., Baldwin, J., Ley, B. A., Bollinger, J. M., SOLOMON, E. I. 2000; 122 (35): 8495-8510

  View details for DOI 10.1021/ja994406r

  View details for Web of Science ID 000089285300020

• A systematic K-edge X-ray absorption spectroscopic study of Cu(III) sites JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dubois, J. L., Mukherjee, P., Stack, T. D., Hedman, B., SOLOMON, E. I., Hodgson, K. O. 2000; 122 (24): 5775-5787

  View details for DOI 10.1021/ja993134p

  View details for Web of Science ID 000087845700012

• Electronic structure contributions to electron transfer in blue Cu and Cu-A JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY Randall, D. W., Gamelin, D. R., LaCroix, L. B., Solomon, E. I. 2000; 5 (1): 16-29

  Abstract

  The experimentally determined electronic structures of mononuclear blue Cu and binuclear Cu(A) centers are summarized and their relation to intra- and inter-protein electron transfer (ET) kinetics are described. Specific contributions of the electronic structures of these two broad classes of Cu ET proteins to H(AB), lambda, and deltaE degrees are discussed. Also, the role of the protein structure in determining key geometric features which define the electronic structures of the metal sites in these proteins is considered.

  View details for Web of Science ID 000085954900002

  View details for PubMedID 10766432

• Geometric and electronic structure/function correlations in non-heme iron enzymes. Chemical reviews Solomon, E. I., Brunold, T. C., Davis, M. I., Kemsley, J. N., Lee, S. K., Lehnert, N., Neese, F., Skulan, A. J., Yang, Y. S., Zhou, J. 2000; 100 (1): 235-350

  View details for PubMedID 11749238

• Geometric and electronic structure/function correlations in non-heme iron enzymes CHEMICAL REVIEWS SOLOMON, E. I., Brunold, T. C., Davis, M. I., Kemsley, J. N., Lee, S. K., Lehnert, N., Neese, F., Skulan, A. J., Yang, Y. S., Zhou, J. 2000; 100 (1): 235-349

  View details for Web of Science ID 000085235400011

• X-ray absorption spectroscopy of folded and unfolded copper(I) azurin INORGANICA CHIMICA ACTA DeBeer, S., Wittung-Stafshede, P., Leckner, J., Karlsson, G., Winkler, J. R., Gray, H. B., MALMSTROM, B. G., SOLOMON, E. I., Hedman, B., Hodgson, K. O. 2000; 297 (1-2): 278-282

  View details for Web of Science ID 000084949800034

• Spectroscopic and electronic structural studies of the Cu(III)(2) bis-mu-oxo core and its relation to the side-on peroxo-bridged dimer JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Henson, M. J., Mukherjee, P., Root, D. E., Stack, T. D., SOLOMON, E. I. 1999; 121 (44): 10332-10345

  View details for Web of Science ID 000083719800011

• Ligand K-edge and metal L-edge X-ray absorption spectroscopy and density functional calculations of oxomolybdenum complexes with thiolate and related ligands: Implications for sulfite oxidase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Izumi, Y., Glaser, T., Rose, K., McMaster, J., Basu, P., ENEMARK, J. H., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1999; 121 (43): 10035-10046

  View details for Web of Science ID 000083641500019

• Relationship between the Dipole Strength of Ligand Pre-Edge Transitions and Metal-Ligand Covalency. Inorganic chemistry Neese, F., Hedman, B., Hodgson, K. O., Solomon, E. I. 1999; 38 (21): 4854-4860

  Abstract

  The electric dipole contributions to the observed pre-edge intensities in ligand K-edge X-ray absorption (XAS) spectra are analyzed in terms of covalent-bonding contributions between the metal and ligand for a prototype system with one hole in the d shell. One- and two-center contributions to the intensity are identified. By direct evaluation of the integrals involved in the intensity expression, the two-center terms are shown to be at least 1 order of magnitude smaller than the one-center terms and can be ignored to a reasonable approximation. The one-center terms reflect the amount of ligand character in the partially occupied metal-based MOs and are proportional to the intrinsic transition moment of a ligand-centered 1s --> np transition. The final intensity does not contain terms proportional to the square of the metal-ligand distance as might have been expected on the basis of the analogy between ligand K-edge and ligand-to-metal charge transfer (LMCT) transitions that both formally lead to transfer of electron density from the ligand to the metal. This is due to the fact that the transition density is completely localized on the ligand in the case of a ligand K-edge transition but is delocalized over the metal and the ligand in the case of a LMCT transition. The effective nuclear charge dependence of the one-center transition moment integral was studied by Hartree-Fock level calculations and was found to be small. Electronic relaxation effects were considered and found to be small from a Hartree-Fock calculation on a cupric chloride model.

  View details for PubMedID 11671216

• Relationship between the dipole strength of ligand pre-edge transitions and metal-ligand covalency INORGANIC CHEMISTRY Neese, F., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1999; 38 (21): 4854-4860

  Abstract

  The electric dipole contributions to the observed pre-edge intensities in ligand K-edge X-ray absorption (XAS) spectra are analyzed in terms of covalent-bonding contributions between the metal and ligand for a prototype system with one hole in the d shell. One- and two-center contributions to the intensity are identified. By direct evaluation of the integrals involved in the intensity expression, the two-center terms are shown to be at least 1 order of magnitude smaller than the one-center terms and can be ignored to a reasonable approximation. The one-center terms reflect the amount of ligand character in the partially occupied metal-based MOs and are proportional to the intrinsic transition moment of a ligand-centered 1s --> np transition. The final intensity does not contain terms proportional to the square of the metal-ligand distance as might have been expected on the basis of the analogy between ligand K-edge and ligand-to-metal charge transfer (LMCT) transitions that both formally lead to transfer of electron density from the ligand to the metal. This is due to the fact that the transition density is completely localized on the ligand in the case of a ligand K-edge transition but is delocalized over the metal and the ligand in the case of a LMCT transition. The effective nuclear charge dependence of the one-center transition moment integral was studied by Hartree-Fock level calculations and was found to be small. Electronic relaxation effects were considered and found to be small from a Hartree-Fock calculation on a cupric chloride model.

  View details for Web of Science ID 000083257300036

• Reversible dioxygen binding to hemerythrin. 1. Electronic structures of deoxy- and oxyhemerythrin JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Brunold, T. C., SOLOMON, E. I. 1999; 121 (36): 8277-8287

  View details for Web of Science ID 000082768300018

• Reversible dioxygen binding to hemerythrin. 2. Mechanism of the proton-coupled two-electron transfer to O-2 at a single iron center JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Brunold, T. C., SOLOMON, E. I. 1999; 121 (36): 8288-8295

  View details for Web of Science ID 000082768300019

• Investigation of the anomalous spectroscopic features of the copper sites in chicken ceruloplasmin: Comparison to human ceruloplasmin BIOCHEMISTRY Machonkin, T. E., Musci, G., Zhang, H. H., di Patti, M. C., Calabrese, L., Hedman, B., Hodgson, K. O., Solomon, E. I. 1999; 38 (34): 11093-11102

  Abstract

  Chicken ceruloplasmin has been previously reported to display a number of key differences relative to human ceruloplasmin: a lower copper content and a lack of a type 2 copper signal by electron paramagnetic resonance (EPR) spectroscopy. We have studied the copper sites of chicken ceruloplasmin in order to probe the origin of these differences, focusing on two forms of the enzyme: "resting" (as isolated by a fast, one-step procedure) and "peroxide-oxidized". From X-ray absorption, EPR, and UV/visible absorption spectroscopies, we have shown that all of the copper sites are oxidized in peroxide-oxidized chicken ceruloplasmin and that none of the type 1 copper sites display the EPR features typical for type 1 copper sites that lack an axial methionine. In the resting form, the type 2 copper center is reduced. Upon oxidation, it does not appear in the EPR spectrum at 77 K, but it can be observed by using magnetic susceptibility, EPR at approximately 8 K, and magnetic circular dichroism spectroscopy. It displays unusually fast relaxation, indicative of coupling with the adjacent type 3 copper pair of the trinuclear copper cluster. From reductive titrations, we have found that the reduction potential of the type 2 center is higher than those of the other copper sites, thus explaining why it is reduced in the resting form. These results provide new insight into the nature of the additional type 1 copper sites and the redox distribution among copper sites in the different ceruloplasmins relative to other multicopper oxidases.

  View details for Web of Science ID 000082342600021

  View details for PubMedID 10460165

• Spectroscopic Investigation of Reduced Protocatechuate 3,4-Dioxygenase: Charge-Induced Alterations in the Active Site Iron Coordination Environment. Inorganic chemistry Davis, M. I., Wasinger, E. C., Westre, T. E., Zaleski, J. M., Orville, A. M., Lipscomb, J. D., Hedman, B., Hodgson, K. O., Solomon, E. I. 1999; 38 (16): 3676-3683

  Abstract

  Chemical reduction of the mononuclear ferric active site in the bacterial intradiol cleaving catecholic dioxygenase protocatechuate 3,4-dioxygenase (3,4-PCD, Brevibacterium fuscum) produces a high-spin ferrous center. We have applied circular dichroism (CD), magnetic circular dichroism (MCD), variable-temperature-variable-field (VTVH) MCD, X-ray absorption (XAS) pre-edge, and extended X-ray absorption fine structure (EXAFS) spectroscopies to investigate the geometric and electronic structure of the reduced iron center. Excited-state ligand field CD and MCD data indicate that the site is six-coordinate where the (5)E(g) excited-state splitting is 2033 cm(-)(1). VTVH MCD analysis of the ground state indicates that the site has negative zero-field splitting with a small rhombic splitting of the lowest doublet (delta = 1.6 +/- 0.3 cm(-)(1)). XAS pre-edge analysis also indicates a six-coordinate site while EXAFS analysis provides accurate bond lengths. Since previous spectroscopic analysis and the crystal structure of oxidized 3,4-PCD indicate a five-coordinate ferric active site, the results presented here show that the coordination number increases upon reduction. This is attributed to the coordination of a second solvent ligand. The coordination number increase relative to the oxidized site also appears to be associated with a large decrease in the ligand donor strength in the reduced enzyme due to protonation of the original hydroxide ligand.

  View details for PubMedID 11671125

• Spectroscopic investigation of reduced protocatechuate 3,4-dioxygenase: Charge-induced alterations in the active site iron coordination environment INORGANIC CHEMISTRY Davis, M. I., Wasinger, E. C., Westre, T. E., Zaleski, J. M., Orville, A. M., Lipscomb, J. D., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1999; 38 (16): 3676-3683

  Abstract

  Chemical reduction of the mononuclear ferric active site in the bacterial intradiol cleaving catecholic dioxygenase protocatechuate 3,4-dioxygenase (3,4-PCD, Brevibacterium fuscum) produces a high-spin ferrous center. We have applied circular dichroism (CD), magnetic circular dichroism (MCD), variable-temperature-variable-field (VTVH) MCD, X-ray absorption (XAS) pre-edge, and extended X-ray absorption fine structure (EXAFS) spectroscopies to investigate the geometric and electronic structure of the reduced iron center. Excited-state ligand field CD and MCD data indicate that the site is six-coordinate where the (5)E(g) excited-state splitting is 2033 cm(-)(1). VTVH MCD analysis of the ground state indicates that the site has negative zero-field splitting with a small rhombic splitting of the lowest doublet (delta = 1.6 +/- 0.3 cm(-)(1)). XAS pre-edge analysis also indicates a six-coordinate site while EXAFS analysis provides accurate bond lengths. Since previous spectroscopic analysis and the crystal structure of oxidized 3,4-PCD indicate a five-coordinate ferric active site, the results presented here show that the coordination number increases upon reduction. This is attributed to the coordination of a second solvent ligand. The coordination number increase relative to the oxidized site also appears to be associated with a large decrease in the ligand donor strength in the reduced enzyme due to protonation of the original hydroxide ligand.

  View details for Web of Science ID 000082017600015

• Spectroscopic studies and electronic structure description of the high potential type 1 copper site in fungal laccase: Insight into the effect of the axial ligand JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Palmer, A. E., Randall, D. W., Xu, F., SOLOMON, E. I. 1999; 121 (30): 7138-7149

  View details for Web of Science ID 000082098000020

• Resonance Raman evidence for a hydrogen-bonded oxo bridge in the R2 protein of ribonucleotide reductase from mouse JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hanson, M. A., Schmidt, P. P., Strand, K. R., Graslund, A., SOLOMON, E. I., Andersson, K. K. 1999; 121 (28): 6755-6756

  View details for Web of Science ID 000081603000029

• Targeted mutations in a Trametes villosa laccase - Axial perturbations of the T1 copper JOURNAL OF BIOLOGICAL CHEMISTRY Xu, F., Palmer, A. E., Yaver, D. S., Berka, R. M., Gambetta, G. A., Brown, S. H., SOLOMON, E. I. 1999; 274 (18): 12372-12375

  Abstract

  Trametes villosa laccase was mutated on a tetrapeptide segment near the type 1 site. The mutations F463M and F463L were at the position corresponding to the type 1 copper axial methionine (M517) ligand in Zucchini ascorbate oxidase. The mutations E460S and A461E were near the T1 copper site. The mutated Trametes laccases were expressed in an Aspergillus oryzae host and characterized. The E460S mutation failed to produce a transformant with meaningful expression. The F463L and A461E mutations did not significantly alter the molecular and enzymological properties of the laccase. In contrast, the F463M mutation resulted in a type 1 copper site with an EPR signal intermediate between that of the wild type laccase and plastocyanin, an altered UV-visible spectrum, and a decreased redox potential (by 0.1 V). In oxidizing phenolic substrate, the mutation led to a more basic optimal pH as well as an increase in kcat and Km. These effects are attributed to a significant perturbation of the T1 copper center caused by the coordination of the axial methionine (M463) ligand.

  View details for Web of Science ID 000080056800029

  View details for PubMedID 10212209

• Investigation into the properties and reactivity of a mixed-state dicopper-peroxo complex Kovaleski, K. M., Obias, H. V., Neuhold, Y. M., Zuberbuhler, A. D., Pidcock, E., Solomon, E. I., Karlin, K. D. ELSEVIER SCIENCE INC. 1999: 194

  View details for Web of Science ID 000081157700364

• Spectroscopic and theoretical studies of substrate binding to the non-heme iron active site of protocatechuate 3,4-dioxygenase Davis, M. I., Orville, A. M., Neese, F., Zaleski, J. M., Lipscomb, J. D., Solomon, E. I. ELSEVIER SCIENCE INC. 1999: 109

  View details for Web of Science ID 000081157700207

• Electronic structure of redox-active high spin iron complexes: electronic relaxation and redox reactivity Kennepohl, P., Solomon, E. I. ELSEVIER SCIENCE INC. 1999: 190

  View details for Web of Science ID 000081157700356

• Interaction of non-heme ferrous active sites with substrates: structure function correlations for phenylalanine hydroxylase and bleomycin Kemsley, J. N., Zaleski, K. L., Mitic, N., Caradonna, J. P., Hecht, S. M., Solomon, E. I. ELSEVIER SCIENCE INC. 1999: 189

  View details for Web of Science ID 000081157700354

• Spectroscopic analysis of dioxygen activation by the trinuclear copper cluster in laccase Lee, S. K., Solomon, E. I. ELSEVIER SCIENCE INC. 1999: 204

  View details for Web of Science ID 000081157700381

• Spectroscopic studies and electronic structure description of the high potential T1 Cu site in fungal laccase: the role of the axial ligand Palmer, A. E., Randall, D., Xu, F., Solomon, E. I. ELSEVIER SCIENCE INC. 1999: 259

  View details for Web of Science ID 000081157700484

• Electronic spectroscopy of high valent manganese bis-mu-oxo dimers Skulan, A. J., Michelsen, K., Solomon, E. I. ELSEVIER SCIENCE INC. 1999: 299

  View details for Web of Science ID 000081157700557

• Electronic structure contributions to reactivity in mononuclear non-heme ferric enzymes. Spectroscopic and theoretical studies Neese, F., Solomon, E. I. ELSEVIER SCIENCE INC. 1999: 247

  View details for Web of Science ID 000081157700463

• Electronic structure contributions to reversible binding and activation of dioxygen by binuclear non-heme iron proteins Brunold, T. C., Solomon, E. I. ELSEVIER SCIENCE INC. 1999: 85

  View details for Web of Science ID 000081157700158

• Spectroscopic studies of fully-reduced non-heme iron enzymes and some model complexes Horner, O., Yang, Y. S., Lee, D., Hanson, M., Lippard, S. J., Solomon, E. I. ELSEVIER SCIENCE INC. 1999: 167

  View details for Web of Science ID 000081157700317

• Spectroscopic and theoretical studies of fully-reduced non-heme iron enzymes: structure function correlations Yang, Y. S., Broadwater, J. A., Fox, B. G., Solomon, E. I. ELSEVIER SCIENCE INC. 1999: 344

  View details for Web of Science ID 000081157700638

• Geometric and electronic structure contributions to function in non-heme iron enzymes Solomon, E. I. ELSEVIER SCIENCE INC. 1999: 5

  View details for Web of Science ID 000081157700007

• MCD C-Term Signs, Saturation Behavior, and Determination of Band Polarizations in Randomly Oriented Systems with Spin S >/= (1)/(2). Applications to S = (1)/(2) and S = (5)/(2). Inorganic chemistry Neese, F., Solomon, E. I. 1999; 38 (8): 1847-1865

  Abstract

  The magnetic circular dichroism (MCD) properties of a spin-allowed transition from an orbitally nondegenerate ground state manifold A to an orbitally nondegenerate excited state manifold J in the presence of spin-orbit coupling (SOC) are derived for any S >/= (1)/(2). Three physically distinct mechanisms are identified that lead to MCD intensity and depend on SOC between excited states which leads to a sum rule and SOC between the ground state and other excited states that leads to deviations from the sum rule. The model is valid for any symmetry of the magnetic coupling tensors and arbitrary transition polarizations. The S = (1)/(2) case is analytically solved, and the determination of linear polarizations from MCD saturation magnetization data is discussed. For all mechanisms the MCD intensity is proportional to the spin-expectation values of the ground state sublevels which are conveniently generated from a spin-Hamiltonian (SH). For Kramers systems with large zero-field splittings (ZFSs) this allows the contribution from each Kramers doublet to the total MCD intensity to be related through their effective g-values, therefore significantly reducing the number of parameters required to analyze experimental data. The behavior of high-spin systems is discussed in the limits of weak, intermediate, and strong ZFS relative to the Zeeman energy. The model remains valid in the important case of intermediate ZFS where the ground state sublevels may cross as a function of applied magnetic field and there are significant off-axis contributions to the MCD intensity due to a change of the electron spin quantization axis. The model permits calculation of MCD C-term signs from molecular wave functions, and explicit expressions are derived in terms of MOs for S = (1)/(2) and S = (5)/(2). Two examples from the literature are analyzed to demonstrate how the C-term signs can be evaluated by a graphical method that gives insight into their physical origin.

  View details for PubMedID 11670957

• MCD C-term signs, saturation behavior, and determination of band polarizations in randomly oriented systems with spin S >= 1/2. Applications to S = 1/2 and S = 5/2 INORGANIC CHEMISTRY Neese, F., SOLOMON, E. I. 1999; 38 (8): 1847-1865

  Abstract

  The magnetic circular dichroism (MCD) properties of a spin-allowed transition from an orbitally nondegenerate ground state manifold A to an orbitally nondegenerate excited state manifold J in the presence of spin-orbit coupling (SOC) are derived for any S >/= (1)/(2). Three physically distinct mechanisms are identified that lead to MCD intensity and depend on SOC between excited states which leads to a sum rule and SOC between the ground state and other excited states that leads to deviations from the sum rule. The model is valid for any symmetry of the magnetic coupling tensors and arbitrary transition polarizations. The S = (1)/(2) case is analytically solved, and the determination of linear polarizations from MCD saturation magnetization data is discussed. For all mechanisms the MCD intensity is proportional to the spin-expectation values of the ground state sublevels which are conveniently generated from a spin-Hamiltonian (SH). For Kramers systems with large zero-field splittings (ZFSs) this allows the contribution from each Kramers doublet to the total MCD intensity to be related through their effective g-values, therefore significantly reducing the number of parameters required to analyze experimental data. The behavior of high-spin systems is discussed in the limits of weak, intermediate, and strong ZFS relative to the Zeeman energy. The model remains valid in the important case of intermediate ZFS where the ground state sublevels may cross as a function of applied magnetic field and there are significant off-axis contributions to the MCD intensity due to a change of the electron spin quantization axis. The model permits calculation of MCD C-term signs from molecular wave functions, and explicit expressions are derived in terms of MOs for S = (1)/(2) and S = (5)/(2). Two examples from the literature are analyzed to demonstrate how the C-term signs can be evaluated by a graphical method that gives insight into their physical origin.

  View details for Web of Science ID 000079849200028

• Circular dichroism and magnetic circular dichroism studies of the reduced binuclear non-heme iron site of stearoyl-ACP Delta(9)-desaturase: Substrate binding and comparison to ribonucleotide reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yang, Y. S., Broadwater, J. A., Pulver, S. C., Fox, B. G., SOLOMON, E. I. 1999; 121 (12): 2770-2783

  View details for Web of Science ID 000079510100015

• Investigation of the electronic structure of 2Fe-2S model complexes and the Rieske protein using ligand K-edge X-ray absorption spectroscopy JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Rose, K., Shadle, S. E., Glaser, T., de Vries, S., Cherepanov, A., Canters, G. W., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1999; 121 (11): 2353-2363

  View details for Web of Science ID 000079363300003

• Circular dichroism and magnetic circular dichroism spectroscopy of the catalytically competent ferrous active site of phenylalanine hydroxylase and its interaction with pterin cofactor JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Kemsley, J. N., Mitic, N., Zaleski, K. L., Caradonna, J. P., SOLOMON, E. I. 1999; 121 (7): 1528-1536

  View details for Web of Science ID 000078974400014

• X-ray Absorption Spectra of the Oxidized and Reduced Forms of C112D Azurin from Pseudomonas aeruginosa. Inorganic chemistry DeBeer, S., Kiser, C. N., Mines, G. A., Richards, J. H., Gray, H. B., Solomon, E. I., Hedman, B., Hodgson, K. O. 1999; 38 (3): 433-438

  Abstract

  The oxidized and reduced forms of a mutant of Pseudomonas aeruginosa azurin, in which the Cys112 has been replaced by an aspartate, have been studied by X-ray absorption spectroscopy. It is well established that the characteristic approximately 600 nm absorption feature of blue copper proteins is due to the S(Cys112) 3ppi --> Cu 3d(x)()()2(-)(y)()()2 charge-transfer transition. While other mutagenesis studies have involved the creation of an artificial blue copper site, the present work involves a mutant in which the native blue copper site has been destroyed, thus serving as a direct probe of the importance of the copper-thiolate bond to the spectroscopy, active site structure, and electron-transfer function of azurin. Of particular interest is the dramatic decrease in electron-transfer rates, both electron self-exchange (k(ese) approximately 10(5) M(-)(1) s(-)(1) wild-type azurin vs k(ese) approximately 20 M(-)(1) s(-)(1) C112D azurin) and intramolecular electron transfer to ruthenium-labeled sites (k(et) approximately 10(6) s(-)(1) wild-type azurin vs k(et)

  View details for PubMedID 11673945

• X-ray absorption spectra of the oxidized and reduced forms of C112D azurin from Pseudomonas aeruginosa INORGANIC CHEMISTRY DeBeer, S., Kiser, C. N., Mines, G. A., Richards, J. H., Gray, H. B., SOLOMON, E. I., Hedman, B., Hodgson, K. O. 1999; 38 (3): 433-438

  Abstract

  The oxidized and reduced forms of a mutant of Pseudomonas aeruginosa azurin, in which the Cys112 has been replaced by an aspartate, have been studied by X-ray absorption spectroscopy. It is well established that the characteristic approximately 600 nm absorption feature of blue copper proteins is due to the S(Cys112) 3ppi --> Cu 3d(x)()()2(-)(y)()()2 charge-transfer transition. While other mutagenesis studies have involved the creation of an artificial blue copper site, the present work involves a mutant in which the native blue copper site has been destroyed, thus serving as a direct probe of the importance of the copper-thiolate bond to the spectroscopy, active site structure, and electron-transfer function of azurin. Of particular interest is the dramatic decrease in electron-transfer rates, both electron self-exchange (k(ese) approximately 10(5) M(-)(1) s(-)(1) wild-type azurin vs k(ese) approximately 20 M(-)(1) s(-)(1) C112D azurin) and intramolecular electron transfer to ruthenium-labeled sites (k(et) approximately 10(6) s(-)(1) wild-type azurin vs k(et)

  View details for Web of Science ID 000078616700006

• Substrate binding to the alpha-ketoglutarate-dependent non-heme iron enzyme clavaminate synthase 2: Coupling mechanism of oxidative decarboxylation and hydroxylation JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Zhou, J., Gunsior, M., Bachmann, B. O., TOWNSEND, C. A., SOLOMON, E. I. 1998; 120 (51): 13539-13540

  View details for Web of Science ID 000077874300043

• Calculation of Zero-Field Splittings, g-Values, and the Relativistic Nephelauxetic Effect in Transition Metal Complexes. Application to High-Spin Ferric Complexes. Inorganic chemistry Neese, F., Solomon, E. I. 1998; 37 (26): 6568-6582

  Abstract

  Equations are derived and discussed that allow the computation of zero-field splitting (ZFS) tensors in transition metal complexes for any value of the ground-state total spin S. An effective Hamiltonian technique is used and the calculation is carried to second order for orbitally nondegenerate ground states. The theory includes contributions from excited states of spin S and S +/- 1. This makes the theory more general than earlier treatments. Explicit equations are derived for the case where all states are well described by single-determinantal wave functions, for example restricted open shell Hartree-Fock (HF) and spin-polarized HF or density functional (DFT) calculation schemes. Matrix elements are evaluated for many electron wave functions that result from a molecular orbital (MO) treatment including configuration interaction (CI). A computational implementation in terms of bonded functions is outlined. The problem of ZFS in high-spin ferric complexes is treated at some length, and contributions due to low-symmetry distortions, anisotropic covalency, charge-transfer states, and ligand spin-orbit coupling are discussed. ROHF-INDO/S-CI results are presented for FeCl(4)(-) and used to evaluate the importance of the various terms. Finally, contributions to the experimentally observed reduction of the metal spin-orbit coupling constants (the relativistic nephelauxetic effect) are discussed. B3LYP and Hartree-Fock calculations for FeCl(4)(-) are used to characterize the change of the iron 3d radial function upon complex formation. It is found that the iron 3d radial distribution function is significantly expanded and that the expansion is anisotropic. This is interpreted as a combination of reduction in effective charge on the metal 3d electrons (central field covalence) together with expansive promotion effects that are a necessary consequence of chemical bond formation. The (3d) values that are important in the interpretation of magnetic data are up to 15% reduced from their free-ion value before any metal-ligand orbital mixing (symmetry-restricted covalency) is taken into account. Thus the use of free-ion values for spin-orbit coupling and related constants in the analysis of experimental data leads to values for MO coefficients that overestimate the metal-ligand covalency.

  View details for PubMedID 11670788

• Calculation of zero-field splittings, g-values, and the relativistic nephelauxetic effect in transition metal complexes. Application to high-spin ferric complexes INORGANIC CHEMISTRY Neese, F., SOLOMON, E. I. 1998; 37 (26): 6568-6582

  Abstract

  Equations are derived and discussed that allow the computation of zero-field splitting (ZFS) tensors in transition metal complexes for any value of the ground-state total spin S. An effective Hamiltonian technique is used and the calculation is carried to second order for orbitally nondegenerate ground states. The theory includes contributions from excited states of spin S and S +/- 1. This makes the theory more general than earlier treatments. Explicit equations are derived for the case where all states are well described by single-determinantal wave functions, for example restricted open shell Hartree-Fock (HF) and spin-polarized HF or density functional (DFT) calculation schemes. Matrix elements are evaluated for many electron wave functions that result from a molecular orbital (MO) treatment including configuration interaction (CI). A computational implementation in terms of bonded functions is outlined. The problem of ZFS in high-spin ferric complexes is treated at some length, and contributions due to low-symmetry distortions, anisotropic covalency, charge-transfer states, and ligand spin-orbit coupling are discussed. ROHF-INDO/S-CI results are presented for FeCl(4)(-) and used to evaluate the importance of the various terms. Finally, contributions to the experimentally observed reduction of the metal spin-orbit coupling constants (the relativistic nephelauxetic effect) are discussed. B3LYP and Hartree-Fock calculations for FeCl(4)(-) are used to characterize the change of the iron 3d radial function upon complex formation. It is found that the iron 3d radial distribution function is significantly expanded and that the expansion is anisotropic. This is interpreted as a combination of reduction in effective charge on the metal 3d electrons (central field covalence) together with expansive promotion effects that are a necessary consequence of chemical bond formation. The (3d) values that are important in the interpretation of magnetic data are up to 15% reduced from their free-ion value before any metal-ligand orbital mixing (symmetry-restricted covalency) is taken into account. Thus the use of free-ion values for spin-orbit coupling and related constants in the analysis of experimental data leads to values for MO coefficients that overestimate the metal-ligand covalency.

  View details for Web of Science ID 000077839100007

• Peroxo-, oxo-, and hydroxo-bridged dicopper complexes: Observation of exogenous hydrocarbon substrate oxidation JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Obias, H. V., Lin, Y., Murthy, N. N., Pidcock, E., SOLOMON, E. I., Ralle, M., Blackburn, N. J., Neuhold, Y. M., Zuberbuhler, A. D., Karlin, K. D. 1998; 120 (49): 12960-12961

  View details for Web of Science ID 000077610100031

• Detailed spectroscopic and theoretical studies on [Fe(EDTA)(O-2)](3-): Electronic structure of the side-on ferric-peroxide bond and its relevance to reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Neese, F., SOLOMON, E. I. 1998; 120 (49): 12829-12848

  View details for Web of Science ID 000077610100015

• Spectroscopic and functional characterization of a ligand coordination mutant of soybean lipoxygenase-1: First coordination sphere analogue of human 15-lipoxygenase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Holman, T. R., Zhou, J., SOLOMON, E. I. 1998; 120 (48): 12564-12572

  View details for Web of Science ID 000077463800021

• Propylene oxidation on copper oxide surfaces: Electronic and geometric contributions to reactivity and selectivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Reitz, J. B., SOLOMON, E. I. 1998; 120 (44): 11467-11478

  View details for Web of Science ID 000076987100031

• Investigation of iron-sulfur covalency in rubredoxins and a model system using sulfur K-edge X-ray absorption spectroscopy JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Rose, K., Shadle, S. E., Eidsness, M. K., Kurtz, D. M., Scott, R. A., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1998; 120 (41): 10743-10747

  View details for Web of Science ID 000076666800023

• Spectroscopic and geometric variations in perturbed blue copper centers: Electronic structures of stellacyanin and cucumber basic protein JOURNAL OF THE AMERICAN CHEMICAL SOCIETY LaCroix, L. B., Randall, D. W., Nersissian, A. M., Hoitink, C. W., Canters, G. W., Valentine, J. S., SOLOMON, E. I. 1998; 120 (37): 9621-9631

  View details for Web of Science ID 000076117200028

• Effect of Protonation on Peroxo-Copper Bonding: Spectroscopic and Electronic Structure Study of [Cu(2)((UN-O-)(OOH)](2+). Inorganic chemistry Root, D. E., Mahroof-Tahir, M., Karlin, K. D., Solomon, E. I. 1998; 37 (19): 4838-4848

  Abstract

  Spectroscopic studies of a &mgr;-1,1-hydroperoxo-bridged copper dimer are combined with SCF-Xalpha-SW molecular orbital calculations to describe the vibrational and electronic structure of the hydroperoxo-copper complex and compare it to that of previously studied peroxo-copper species. Four vibrational modes of the Cu(2)OOH unit in the resonance Raman and infrared spectra are assigned on the basis of isotope shifts: nu(O-O) = 892 cm(-)(1), nu(as)(Cu-O) = 506 cm(-)(1), nu(s)(Cu-O) = 322 cm(-)(1), and nu(O-H) = 3495 cm(-)(1). The 892 cm(-)(1) O-O stretch of the &mgr;-1,1-hydroperoxo-bridged copper dimer is 89 cm(-)(1) higher than that of the unprotonated complex. Resonance Raman profiles of the 892 cm(-)(1) O-O stretch are used to assign an electronic absorption band at 25 200 cm(-)(1) (epsilon = 6700 M(-)(1) cm(-)(1)) to a hydroperoxide pi-to-Cu charge transfer (CT) transition. This band is approximately 5000 cm(-)(1) higher in energy than the corresponding transition in the unprotonated complex. The pi-to-Cu CT transition intensity defines the degree of hydroperoxide-to-copper charge donation, which is lower than in the unprotonated complex due to the increased electronegativity of the peroxide with protonation. The lower Cu-O covalency of this hydroperoxo-copper complex shows that the high O-O stretching frequency is not due to increased pi-to-Cu charge donation but rather reflects the direct effect of protonation on intra-peroxide bonding. Density functional calculations are used to describe changes in intra-peroxide and Cu-O bonding upon protonation of the peroxo-copper complex and to relate these changes to changes in reactivity.

  View details for PubMedID 11670647

• Effect of protonation on peroxo-copper bonding: Spectroscopic and electronic structure study of [Cu-2((UN-O-)(OOH)](2+) INORGANIC CHEMISTRY Root, D. E., Mahroof-Tahir, M., Karlin, K. D., SOLOMON, E. I. 1998; 37 (19): 4838-4848

  Abstract

  Spectroscopic studies of a &mgr;-1,1-hydroperoxo-bridged copper dimer are combined with SCF-Xalpha-SW molecular orbital calculations to describe the vibrational and electronic structure of the hydroperoxo-copper complex and compare it to that of previously studied peroxo-copper species. Four vibrational modes of the Cu(2)OOH unit in the resonance Raman and infrared spectra are assigned on the basis of isotope shifts: nu(O-O) = 892 cm(-)(1), nu(as)(Cu-O) = 506 cm(-)(1), nu(s)(Cu-O) = 322 cm(-)(1), and nu(O-H) = 3495 cm(-)(1). The 892 cm(-)(1) O-O stretch of the &mgr;-1,1-hydroperoxo-bridged copper dimer is 89 cm(-)(1) higher than that of the unprotonated complex. Resonance Raman profiles of the 892 cm(-)(1) O-O stretch are used to assign an electronic absorption band at 25 200 cm(-)(1) (epsilon = 6700 M(-)(1) cm(-)(1)) to a hydroperoxide pi-to-Cu charge transfer (CT) transition. This band is approximately 5000 cm(-)(1) higher in energy than the corresponding transition in the unprotonated complex. The pi-to-Cu CT transition intensity defines the degree of hydroperoxide-to-copper charge donation, which is lower than in the unprotonated complex due to the increased electronegativity of the peroxide with protonation. The lower Cu-O covalency of this hydroperoxo-copper complex shows that the high O-O stretching frequency is not due to increased pi-to-Cu charge donation but rather reflects the direct effect of protonation on intra-peroxide bonding. Density functional calculations are used to describe changes in intra-peroxide and Cu-O bonding upon protonation of the peroxo-copper complex and to relate these changes to changes in reactivity.

  View details for Web of Science ID 000076104800014

• Spectroscopic studies of oxidized manganese catalase and mu-oxo-bridged dimanganese(III) model complexes: Electronic structure of the active site and its relation to catalysis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Brunold, T. C., Gamelin, D. R., Stemmler, T. L., Mandal, S. K., Armstrong, W. H., Penner-Hahn, J. E., SOLOMON, E. I. 1998; 120 (34): 8724-8738

  View details for Web of Science ID 000075741300019

• Site-directed mutations in fungal laccase: effect on redox potential, activity and pH profile BIOCHEMICAL JOURNAL Xu, F., Berka, R. M., Wahleithner, J. A., Nelson, B. A., Shuster, J. R., Brown, S. H., Palmer, A. E., SOLOMON, E. I. 1998; 334: 63-70

  Abstract

  A Myceliophthora thermophila laccase and a Rhizoctonia solani laccase were mutated on a pentapeptide segment believed to be near the type-1 Cu site. The mutation L513F in Myceliophthora laccase and the mutation L470F in Rhizoctonia laccase took place at a position corresponding to the type-1 Cu axial methionine (M517) ligand in Zucchini ascorbate oxidase. The triple mutations V509L,S510E,G511A in Myceliophthora laccase and L466V,E467S,A468G in Rhizoctonia laccase involved a sequence segment whose homologue in ascorbate oxidase is flanked by the M517 and a type-1 Cu-ligating histidine (H512). The single mutation did not yield significant changes in the enzymic properties (including any significant increase in the redox potential of the type-1 Cu). In contrast, the triple mutation resulted in several significant changes. In comparison with the wild type, the Rhizoctonia and Myceliophthora laccase triple mutants had a phenol-oxidase activity whose pH optimum shifted 1 unit lower and higher, respectively. Although the redox potentials were not significantly altered, the Km, kcat and fluoride inhibition of the laccases were greatly changed by the mutations. The observed effects are interpreted as possible mutation-induced structural perturbations on the molecular recognition between the reducing substrate and laccase and on the electron transfer from the substrate to the type-1 Cu centre.

  View details for Web of Science ID 000075558200010

  View details for PubMedID 9693103

  View details for PubMedCentralID PMC1219662

• Investigation of the reactive oxygen intermediate in an arene hydroxylation reaction performed by xylyl-bridged binuclear copper complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pidcock, E., Obias, H. V., Zhang, C. X., Karlin, K. D., SOLOMON, E. I. 1998; 120 (31): 7841-7847

  View details for Web of Science ID 000075420100020

• Spectroscopic and magnetic studies of human ceruloplasmin: Identification of a redox-inactive reduced Type 1 copper site BIOCHEMISTRY Machonkin, T. E., Zhang, H. H., Hedman, B., Hodgson, K. O., Solomon, E. I. 1998; 37 (26): 9570-9578

  Abstract

  Ceruloplasmin is unique among the multicopper oxidases in that in addition to the usual copper stoichiometry of one Type 1 copper site and a Type 2/Type 3 trinuclear copper cluster, it contains two other Type 1 sites. This assignment of copper sites, based on copper quantitation, sequence alignment, and crystallography, is difficult to reconcile with the observed spectroscopy. Furthermore, some chemical or spectroscopic differences in ceruloplasmin have been reported depending on the method of purification. We have studied the resting (as isolated by a fast, one-step procedure) and peroxide-oxidized forms of human ceruloplasmin. Using a combination of X-ray absorption spectroscopy, a chemical assay, magnetic susceptibility, electron paramagnetic resonance spectroscopy, and absorption spectroscopy, we have determined that peroxide-oxidized ceruloplasmin contains one permanently reduced Type 1 site. This site is shown to have a reduction potential of approximately 1.0 V. Thus, one of the additional Type 1 sites in ceruloplasmin cannot be catalytically relevant in the form of the enzyme studied. Furthermore, the resting form of the enzyme contains an additional reducing equivalent, which is distributed among the remaining five copper sites as expected from their relative potentials. This may indicate that the resting form of ceruloplasmin in plasma under aerobic conditions is a four-electron oxidized form, which is consistent with its function in the four-electron reduction of dioxygen to water.

  View details for Web of Science ID 000074585100040

  View details for PubMedID 9649340

• Spectroscopic study of [Fe-2(O-2)(OBz)(2){HB(pz ')3}(2)]: Nature of the mu-1,2 peroxide-Fe(III) bond and its possible relevance to O-2 activation by non-heme iron enzymes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Brunold, T. C., Tamura, N., Kitajima, N., Moro-oka, Y., SOLOMON, E. I. 1998; 120 (23): 5674-5690

  View details for Web of Science ID 000074301600009

• Spectroscopy of mixed-valence Cu-A-type centers: Ligand-field control of ground-state properties related to electron transfer JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Gamelin, D. R., Randall, D. W., Hay, M. T., Houser, R. P., Mulder, T. C., Canters, G. W., de Vries, S., Tolman, W. B., Lu, Y., SOLOMON, E. I. 1998; 120 (21): 5246-5263

  View details for Web of Science ID 000074039100015

• Electronic and geometric structure of a trinuclear mixed-valence copper(II,II,III) cluster JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Root, D. E., Henson, M. J., Machonkin, T., Mukherjee, P., Stack, T. D., SOLOMON, E. I. 1998; 120 (20): 4982-4990

  View details for Web of Science ID 000073887300009

• Magnetic circular dichroism spectroscopic studies of mononuclear non-heme ferrous model complexes. Correlation of excited- and ground-state electronic structure with geometry JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pavel, E. G., Kitajima, N., SOLOMON, E. I. 1998; 120 (16): 3949-3962

  View details for Web of Science ID 000073399300014

• Electron spectroscopic studies of CH3OH chemisorption on Cu2O and ZnO single-crystal surfaces: Methoxide bonding and reactivity related to methanol synthesis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Jones, P. M., May, J. A., Reitz, J. B., SOLOMON, E. I. 1998; 120 (7): 1506-1516

  View details for Web of Science ID 000072246500024

• Spectroscopic investigation of the metal ligation and reactivity of the ferrous active sites of bleomycin and bleomycin derivatives JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Loeb, K. E., Zaleski, J. M., Hess, C. D., Hecht, S. M., SOLOMON, E. I. 1998; 120 (6): 1249-1259

  View details for Web of Science ID 000072121800018

• Circular dichroism and magnetic circular dichroism spectroscopic studies of the non-heme ferrous active site in clavaminate synthase and its interaction with alpha-ketoglutarate cosubstrate JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pavel, E. G., Zhou, J., Busby, R. W., Gunsior, M., TOWNSEND, C. A., SOLOMON, E. I. 1998; 120 (4): 743-753

  View details for Web of Science ID 000071889300016

• Spectroscopic characterization of an engineered purple Cu-A center in azurin INORGANIC CHEMISTRY Hay, M. T., Ang, M. C., Gamelin, D. R., SOLOMON, E. I., Antholine, W. E., Ralle, M., Blackburn, N. J., Massey, P. D., Wang, X. T., Kwon, A. H., Lu, Y. 1998; 37 (2): 191-198

  View details for Web of Science ID 000071713300007

• Spectroscopic investigation of peroxide binding to the trinuclear copper cluster site in laccase: Correlation with the peroxy-level intermediate and relevance to catalysis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sundaram, U. M., Zhang, H. H., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1997; 119 (51): 12525-12540

  View details for Web of Science ID 000071272500012

• Irreversible reduction of dioxygen by simple peralkylated diamine-copper(I) complexes: Characterization and thermal stability of a [Cu-2(mu-O)(2)](2+) core JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mahadevan, V., Hou, Z. G., Cole, A. P., Root, D. E., Lal, T. K., SOLOMON, E. I., Stack, T. D. 1997; 119 (49): 11996-11997

  View details for Web of Science ID A1997YK70800036

• Circular dichroism and magnetic circular dichroism studies of the mixed-valence binuclear non-heme iron active site in uteroferrin and its anion complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yang, Y. S., McCormick, J. M., SOLOMON, E. I. 1997; 119 (49): 11832-11842

  View details for Web of Science ID A1997YK70800014

• New insights from spectroscopy into the structure/function relationships of lipoxygenases CHEMISTRY & BIOLOGY SOLOMON, E. I., Zhou, J., Neese, F., Pavel, E. G. 1997; 4 (11): 795-808

  Abstract

  Spectroscopic properties of the redox-active iron in the active site of plant and mammalian lipoxygenases can now be combined with recent crystal structure determinations to obtain new insights into lipoxygenase reaction mechanisms.

  View details for Web of Science ID A1997YJ93500003

  View details for PubMedID 9384534

• Ligand K-edge X-ray absorption spectroscopic studies: metal-ligand covalency in transition metal tetrathiolates INORGANICA CHIMICA ACTA Williams, K. R., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1997; 263 (1-2): 315-321

  View details for Web of Science ID A1997YD82100039

• Cu K-edge XAS study of the [Cu-2(mu-O)(2)] core: Direct experimental evidence for the presence of Cu(III) JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dubois, J. L., Mukherjee, P., Collier, A. M., Mayer, J. M., SOLOMON, E. I., Hedman, B., Stack, T. D., Hodgson, K. O. 1997; 119 (36): 8578-8579

  View details for Web of Science ID A1997XV70000028

• Oxygen intermediates tn the multicopper oxidases Solomon, E. I. FEDERATION AMER SOC EXP BIOL. 1997: A871

  View details for Web of Science ID 000073305600569

• A multiplet analysis of Fe K-edge 1s->3d pre-edge features of iron complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Westre, T. E., Kennepohl, P., DeWitt, J. G., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1997; 119 (27): 6297-6314

  View details for Web of Science ID A1997XJ83300011

• Spectroscopic characterization of the catalytically competent ferrous site of the resting, activated, and substrate-bound forms of phenylalanine hydroxylase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Loeb, K. E., Westre, T. E., Kappock, T. J., Mitic, N., Glasfeld, E., Caradonna, J. P., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1997; 119 (8): 1901-1915

  View details for Web of Science ID A1997WK42200010

• Influence of copper-sulfur covalency and copper-copper bonding on valence delocalization and electron transfer in the Cu-A site of cytochrome c oxidase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Williams, K. R., Gamelin, D. R., LaCroix, L. B., Houser, R. P., Tolman, W. B., Mulder, T. C., DeVries, S., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1997; 119 (3): 613-614

  View details for Web of Science ID A1997WD86700022

• Ligand field circular dichroism and magnetic circular dichroism studies of component B and substrate binding to the hydroxylase component of methane monooxygenase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pulver, S. C., Froland, W. A., Lipscomb, J. D., SOLOMON, E. I. 1997; 119 (2): 387-395

  View details for Web of Science ID A1997WC77200016

• Structural and Functional Aspects of Metal Sites in Biology. Chemical reviews Holm, R. H., Kennepohl, P., Solomon, E. I. 1996; 96 (7): 2239-2314

  View details for PubMedID 11848828

• Multicopper Oxidases and Oxygenases. Chemical reviews Solomon, E. I., Sundaram, U. M., Machonkin, T. E. 1996; 96 (7): 2563-2606

  View details for PubMedID 11848837

• Multicopper oxidases and oxygenases CHEMICAL REVIEWS SOLOMON, E. I., Sundaram, U. M., Machonkin, T. E. 1996; 96 (7): 2563-2605

  View details for Web of Science ID A1996VT05300011

• Structural and functional aspects of metal sites in biology CHEMICAL REVIEWS HOLM, R. H., Kennepohl, P., SOLOMON, E. I. 1996; 96 (7): 2239-2314

  View details for Web of Science ID A1996VT05300002

• Trinuclear intermediate in the copper-mediated reduction of O-2: Four electrons from three coppers SCIENCE Cole, A. P., Root, D. E., Mukherjee, P., SOLOMON, E. I., Stack, T. D. 1996; 273 (5283): 1848-1850

  Abstract

  The reaction of metal complexes with dioxygen (O2) generally proceeds in 1:1, 21, or 41 (metal:O2) stoichiometry. A discrete, structurally characterized 31 product is presented. This mixed-valence trinuclear copper cluster, which contains copper in the highly oxidized trivalent oxidation state, exhibits O2 bond scission and intriguing structural, spectroscopic, and redox properties. The relevance of this synthetic complex to the reduction of O2 at the trinuclear active sites of multicopper oxidases is discussed.

  View details for Web of Science ID A1996VJ71300041

  View details for PubMedID 8791587

• Excited-state contributions to ground-state properties of mixed-valence dimers: Spectral and electronic-structural studies of [Fe-2(OH)(3)(tmtacn)(2)](2+) related to the [Fe2S2](+) active sites of plant-type ferredoxins JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Gamelin, D. R., Bominaar, E. L., Kirk, M. L., Wieghardt, K., SOLOMON, E. I. 1996; 118 (34): 8085-8097

  View details for Web of Science ID A1996VE26600023

• Electronic structure of the perturbed blue copper site in nitrite reductase: Spectroscopic properties, bonding, and implications for the entatic/rack state JOURNAL OF THE AMERICAN CHEMICAL SOCIETY LaCroix, L. B., Shadle, S. E., Wang, Y. N., Averill, B. A., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1996; 118 (33): 7755-7768

  View details for Web of Science ID A1996VD32800015

• Excited-State Distortions and Electron Delocalization in Mixed-Valence Dimers: Vibronic Analysis of the Near-IR Absorption and Resonance Raman Profiles of [Fe(2)(OH)(3)(tmtacn)(2)](2+). Inorganic chemistry Gamelin, D. R., Bominaar, E. L., Mathonière, C., Kirk, M. L., Wieghardt, K., Girerd, J. J., Solomon, E. I. 1996; 35 (15): 4323-4335

  Abstract

  The near-IR transition associated with valence delocalization in the class III mixed-valence dimer [Fe(2)(OH)(3)(tmtacn)(2)](2+) is studied using variable-temperature (VT) electronic absorption and resonance Raman (RR) spectroscopies to gain insight into the properties of electron delocalization in this dimer. Laser excitation into this absorption band leads to dominant resonance Raman enhancement of totally-symmetric [Fe(2)(OH)(3)](2+) core vibrational modes at 316 and 124 cm(-)(1), descriptions of which are calculated from a normal coordinate analysis. Vibronic analysis of the near-IR resonance Raman excitation profiles and VT-absorption bandshapes using an anharmonic excited-state model provides a description of the geometric distortions accompanying this excitation. The excited-state distortion is dominated by expansion of the [Fe(2)(OH)(3)](2+) core along the Fe.Fe axis, reflecting the significant Fe-Fe sigma --> sigma character of this transition. The ground-state sigma-interaction between the two metals has been identified as the orbital pathway for valence delocalization, and the sigma --> sigma distortion analysis is used to quantify the structural dependence of the electronic-coupling matrix element, H(AB), associated with this pathway. The dominant role of totally-symmetric nuclear coordinates in the absorption and RR spectroscopies of [Fe(2)(OH)(3)(tmtacn)(2)](2+) is also discussed in relation to the Q(-) vibrational coordinate and the vibronic spectroscopies of other class II and class III mixed-valence dimers. It is shown that intensity contributions from the Q(-) coordinate to the absorption and RR spectra of [Fe(2)(OH)(3)(tmtacn)(2)](2+) are small relative to those of the totally-symmetric coordinates due to the inefficient change-in-curvature mechanism by which the Q(-) coordinate gains intensity, compared to the efficient excited-state displacement mechanism allowed for totally-symmetric coordinates. This is in contrast with the dominance of the Q(-) coordinate over other totally-symmetric coordinates observed in intervalence transfer (IT) absorption and RR spectroscopies of class II mixed-valence complexes.

  View details for PubMedID 11666647

• Excited-state distortions and electron delocalization in mixed-valence dimers: Vibronic analysis of the near-IR absorption and resonance Raman profiles of [Fe-2(OH)(3)(tmtacn)(2)](2+) INORGANIC CHEMISTRY Gamelin, D. R., Bominaar, E. L., Mathoniere, C., Kirk, M. L., Wieghardt, K., Girerd, J. J., SOLOMON, E. I. 1996; 35 (15): 4323-4335

  Abstract

  The near-IR transition associated with valence delocalization in the class III mixed-valence dimer [Fe(2)(OH)(3)(tmtacn)(2)](2+) is studied using variable-temperature (VT) electronic absorption and resonance Raman (RR) spectroscopies to gain insight into the properties of electron delocalization in this dimer. Laser excitation into this absorption band leads to dominant resonance Raman enhancement of totally-symmetric [Fe(2)(OH)(3)](2+) core vibrational modes at 316 and 124 cm(-)(1), descriptions of which are calculated from a normal coordinate analysis. Vibronic analysis of the near-IR resonance Raman excitation profiles and VT-absorption bandshapes using an anharmonic excited-state model provides a description of the geometric distortions accompanying this excitation. The excited-state distortion is dominated by expansion of the [Fe(2)(OH)(3)](2+) core along the Fe.Fe axis, reflecting the significant Fe-Fe sigma --> sigma character of this transition. The ground-state sigma-interaction between the two metals has been identified as the orbital pathway for valence delocalization, and the sigma --> sigma distortion analysis is used to quantify the structural dependence of the electronic-coupling matrix element, H(AB), associated with this pathway. The dominant role of totally-symmetric nuclear coordinates in the absorption and RR spectroscopies of [Fe(2)(OH)(3)(tmtacn)(2)](2+) is also discussed in relation to the Q(-) vibrational coordinate and the vibronic spectroscopies of other class II and class III mixed-valence dimers. It is shown that intensity contributions from the Q(-) coordinate to the absorption and RR spectra of [Fe(2)(OH)(3)(tmtacn)(2)](2+) are small relative to those of the totally-symmetric coordinates due to the inefficient change-in-curvature mechanism by which the Q(-) coordinate gains intensity, compared to the efficient excited-state displacement mechanism allowed for totally-symmetric coordinates. This is in contrast with the dominance of the Q(-) coordinate over other totally-symmetric coordinates observed in intervalence transfer (IT) absorption and RR spectroscopies of class II mixed-valence complexes.

  View details for Web of Science ID A1996UX98000006

• Chemical and spectroscopic definition of the peroxide-level intermediate in the multicopper oxidases: Relevance to the catalytic mechanism of dioxygen reduction to water JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Shin, W., Sundaram, U. M., Cole, J. L., Zhang, H. H., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1996; 118 (13): 3202-3215

  View details for Web of Science ID A1996UD46400015

• X-Ray magnetic circular dichroism at temperatures <1K: Demonstration with the blue copper site in plastocyanin INORGANICA CHIMICA ACTA Christiansen, J., Peng, G., Young, A. T., LaCroix, L. B., SOLOMON, E. I., Cramer, S. P. 1996; 243 (1-2): 229-232

  View details for Web of Science ID A1996UM71300030

• Electronic structure of the oxidized and reduced blue copper sites: Contributions to the electron transfer pathway, reduction potential, and geometry INORGANICA CHIMICA ACTA SOLOMON, E. I., Penfield, K. W., Gewirth, A. A., LOWERY, M. D., Shadle, S. E., GUCKERT, J. A., LaCroix, L. B. 1996; 243 (1-2): 67-78

  View details for Web of Science ID A1996UM71300011

• A study of a series of recombinant fungal laccases and bilirubin oxidase that exhibit significant differences in redox potential, substrate specificity, and stability BIOCHIMICA ET BIOPHYSICA ACTA-PROTEIN STRUCTURE AND MOLECULAR ENZYMOLOGY Xu, F., Shin, W. S., Brown, S. H., Wahleithner, J. A., Sundaram, U. M., SOLOMON, E. I. 1996; 1292 (2): 303-311

  Abstract

  A series of fungal laccases (Polyporus pinsitus, Rhizoctonia solani, Myceliophthora thermophila, Scytalidium thermophilum) and one bilirubin oxidase (Myrothecium verrucaria) have been studied to determine their redox potential, specificity, and stability. Polyporus and Rhizoctonia laccases possess potentials near 0.7-0.8 V (vs. NHE), while other oxidases have potentials near 0.5 V. It is observed that higher redox potential correlates with higher activity. By EPR, no significant change in the geometry of type 1 copper (II) site is observed over this series. At the optimal pH, the two substrates studied, 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonic acid) and syringaldazine, show Km values ranging form 10 to 120 and from 1 to 45 microM; and kcat values ranging from 50 to 16 000 and 200 to 3000 per min, respectively. The enzymes are more stable in the neutral-alkaline pH range. The thermal stability is in the order of bilirubin oxidase equivalent to Myceliophthora laccase equivalent to Scytalidium laccase > Polyporus laccase > Rhizoctonia laccase. Based on these results and the sequence alignments made against Zucchini ascorbate oxidase it is speculated that structural differences in the substrate-activation site (a 'blue', type 1 copper center) control the redox potential range as well as substrate specificity, and the cystine content contributes to stability.

  View details for Web of Science ID A1996TX49700013

  View details for PubMedID 8597577

• Circular dichroism and magnetic circular dichroism studies of the fully reduced binuclear non-heme iron active site in the Escherichia coli R2 subunit of ribonucleoside diphosphate reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pulver, S. C., Tong, W. H., Bollinger, J. M., Stubbe, J., SOLOMON, E. I. 1995; 117 (51): 12664-12678

  View details for Web of Science ID A1995TM48300002

• MAGNETIC CIRCULAR-DICHROISM SPECTROSCOPY AS A PROBE OF THE GEOMETRIC AND ELECTRONIC-STRUCTURE OF NONHEME FERROUS ENZYMES COORDINATION CHEMISTRY REVIEWS SOLOMON, E. I., Pavel, E. G., Loeb, K. E., Campochiaro, C. 1995; 144: 369-460

  View details for Web of Science ID A1995TF86100011

• SINGLE-CRYSTAL MORPHOLOGY OF THE COPPER-ACETATE DIMERS CU-2(CH3COO)(4)CENTER-DOT-2H(2)O AND CU-2(CH3COO)(4)PZ JOURNAL OF CRYSTAL GROWTH Bell, S. J., Jennings, K. L., DANIELSON, E. D., SOLOMON, E. I., Musselman, R. L. 1995; 154 (1-2): 108-112

  View details for Web of Science ID A1995RU56000017

• SATURATION MAGNETIZATION MAGNETIC CIRCULAR-DICHROISM SPECTROSCOPY OF SYSTEMS WITH POSITIVE ZERO-FIELD SPLITTINGS - APPLICATION TO FESIF6-CENTER-DOT-6H(2)O INORGANIC CHEMISTRY Campochiaro, C., Pavel, E. G., SOLOMON, E. I. 1995; 34 (18): 4669-4675

  View details for Web of Science ID A1995RT11300025

• EPR DEFINITION OF THE NONHEME FERRIC ACTIVE-SITES OF MAMMALIAN 15-LIPOXYGENASE - MAJOR SPECTRAL DIFFERENCE RELATIVE TO HUMAN 5-LIPOXYGENASES AND PLANT LIPOXYGENASES AND THEIR LIGAND-FIELD ORIGIN JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Zhang, Y., Gan, Q. F., Pavel, E. G., Sigal, E., SOLOMON, E. I. 1995; 117 (28): 7422-7427

  View details for Web of Science ID A1995RK42600015

• SPECTROSCOPIC DEFINITION OF THE GEOMETRIC AND ELECTRONIC-STRUCTURE OF THE NONHEME IRON ACTIVE-SITE IN IRON(II) BLEOMYCIN - CORRELATION WITH OXYGEN REACTIVITY JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Loeb, K. E., Zaleski, J. M., Westre, T. E., GUAJARDO, R. J., Mascharak, P. K., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1995; 117 (16): 4545-4561

  View details for Web of Science ID A1995QV14700013

• NEAR-INFRARED CIRCULAR-DICHROISM, MAGNETIC CIRCULAR-DICHROISM, AND X-RAY-ABSORPTION SPECTRAL COMPARISON OF THE NONHEME FERROUS ACTIVE-SITES OF PLANT AND MAMMALIAN 15-LIPOXYGENASES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pavlosky, M. A., Zhang, Y., Westre, T. E., Gan, Q. F., Pavel, E. G., Campochiaro, C., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1995; 117 (15): 4316-4327

  View details for Web of Science ID A1995QU25700013

• ELECTRONIC-STRUCTURE OF THE REDUCED BLUE COPPER ACTIVE-SITE - CONTRIBUTIONS TO REDUCTION POTENTIALS AND GEOMETRY JOURNAL OF THE AMERICAN CHEMICAL SOCIETY GUCKERT, J. A., LOWERY, M. D., SOLOMON, E. I. 1995; 117 (10): 2817-2844

  View details for Web of Science ID A1995QM73800016

• LIGAND K-EDGE X-RAY-ABSORPTION SPECTROSCOPIC STUDIES - METAL-LIGAND COVALENCY IN A SERIES OF TRANSITION-METAL TETRACHLORIDES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Shadle, S. E., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1995; 117 (8): 2259-2272

  View details for Web of Science ID A1995QK08200015

• GNXAS, A MULTIPLE-SCATTERING APPROACH TO EXAFS ANALYSIS - METHODOLOGY AND APPLICATIONS TO IRON COMPLEXES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Westre, T. E., DICICCO, A., Filipponi, A., Natoli, C. R., Hedman, B., SOLOMON, E. I., Hodgson, K. O. 1995; 117 (5): 1566-1583

  View details for Web of Science ID A1995QF53600012

• SPECTROSCOPIC AND ELECTRONIC-STRUCTURE STUDIES OF MET-HEMERYTHRIN MODEL COMPLEXES - A DESCRIPTION OF THE FERRIC-OXO DIMER BOND INORGANIC CHEMISTRY Brown, C. A., REMAR, G. J., Musselman, R. L., SOLOMON, E. I. 1995; 34 (3): 688-717

  View details for Web of Science ID A1995QE84000024

• DETERMINATION OF THE GEOMETRIC AND ELECTRONIC-STRUCTURE OF ACTIVATED BLEOMYCIN USING X-RAY-ABSORPTION SPECTROSCOPY JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Westre, T. E., Loeb, K. E., Zaleski, J. M., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1995; 117 (4): 1309-1313

  View details for Web of Science ID A1995QE73000014

• SPECTROSCOPIC AND THEORETICAL DESCRIPTION OF THE ELECTRONIC-STRUCTURE OF S=3/2 IRON-NITROSYL COMPLEXES AND THEIR RELATION TO O-2 ACTIVATION BY NONHEME TRON ENZYME ACTIVE-SITES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Brown, C. A., Pavlosky, M. A., Westre, T. E., Zhang, Y., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1995; 117 (2): 715-732

  View details for Web of Science ID A1995QC77600015

• BIOINORGANIC SPECTROSCOPY BIOCHEMICAL SPECTROSCOPY SOLOMON, E. I., Kirk, M. L., Gamelin, D. R., PULVER, S. 1995; 246: 71-110

  View details for Web of Science ID A1995BC90V00005

  View details for PubMedID 7752944

• ELECTRONIC SPECTRAL STUDIES OF MOLYBDENYL COMPLEXES - IMPLICATIONS FOR OXOMOLYBDENUM ENZYMES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Carducci, M. D., Brown, C., SOLOMON, E. I., ENEMARK, J. H. 1994; 116 (26): 11856-11868

  View details for Web of Science ID A1994QA28800028

• NEAR-IR CD MCD SPECTRAL ELUCIDATION OF 2 FORMS OF THE NONHEME ACTIVE-SITE IN NATIVE FERROUS SOYBEAN LIPOXYGENASE-1 - CORRELATION TO CRYSTAL-STRUCTURES AND REACTIVITY JOURNAL OF THE AMERICAN CHEMICAL SOCIETY PAVLOSKY, M. A., SOLOMON, E. I. 1994; 116 (25): 11610–11

  View details for DOI 10.1021/ja00104a068

  View details for Web of Science ID A1994PX38600068

• Magnetic circular dichroism studies of exogenous ligand and substrate binding to the non-heme ferrous active site in phthalate dioxygenase. Chemistry & biology Pavel, E. G., Martins, L. J., ELLIS, W. R., SOLOMON, E. I. 1994; 1 (3): 173-183

  Abstract

  Mononuclear non-heme iron centers are found in the active sites of a variety of enzymes that require molecular oxygen for catalysis. The mononuclear non-heme iron is believed to be the active site for catalysis, and is presumed to bind and activate molecular oxygen. The mechanism of this reaction is not understood. Phthalate dioxygenase is one such enzyme. Because it also contains a second iron site, the Rieske site, it is difficult to obtain information on the structure of the active site. We therefore used magnetic circular dichroism (MCD) spectroscopy to probe the mononuclear, non-heme Fe2+ site in this biodegradative enzyme.The MCD spectrum of the resting enzyme shows features indicative of one six-coordinate Fe2+ site; substrate binding converts the site to two different five-coordinate species, opening up a coordination position for O2 binding. MCD spectra of the corresponding apoenzyme have been subtracted to account for temperature-independent contributions from the Rieske site. Azide binds both to the resting enzyme to produce a new six-coordinate species, showing that one of the ferrous ligands is exchangeable, and also to the enzyme-substrate complex to form a ternary species. The low azide binding constant for the substrate-enzyme species relative to the resting enzyme indicates steric interaction and close proximity between exogenous ligand and the substrate.We have been able to provide some detailed structural insight into exogenous ligand and substrate binding to the non-heme Fe2+ site, even in the presence of the enzyme's [2Fe-2S] Rieske center. Further mechanistic studies are now required to maximize the molecular-level detail available from these spectroscopic studies.

  View details for PubMedID 9383387

• LIGAND K-EDGE X-RAY-ABSORPTION SPECTROSCOPY AS A PROBE OF LIGAND-METAL BONDING - CHARGE DONATION AND COVALENCY IN COPPER-CHLORIDE SYSTEMS INORGANIC CHEMISTRY Shadle, S. E., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1994; 33 (19): 4235-4244

  View details for Web of Science ID A1994PG75400009

• DETERMINATION OF THE FE-N-O ANGLE IN (FENO)(7) COMPLEXES USING MULTIPLE-SCATTERING EXAFS ANALYSIS BY GNXAS JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Westre, T. E., DICICCO, A., Filipponi, A., Natoli, C. R., Hedman, B., SOLOMON, E. I., Hodgson, K. O. 1994; 116 (15): 6757-6768

  View details for Web of Science ID A1994NZ54700035

• CHARGE-TRANSFER STATES AND ANTIFERROMAGNETISM OF BRIDGED CU DIMERS - APPLICATION TO OXYHEMOCYANIN JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Tuczek, F., SOLOMON, E. I. 1994; 116 (15): 6916-6924

  View details for Web of Science ID A1994NZ54700052

• SPECTROSCOPY OF BINUCLEAR DIOXYGEN COMPLEXES CHEMICAL REVIEWS SOLOMON, E. I., Tuczek, F., Root, D. E., Brown, C. A. 1994; 94 (3): 827-856

  View details for Web of Science ID A1994NM24200013

• ELECTRONIC-STRUCTURE AND SPECTROSCOPY OF MANGANESE CATALASE AND DI-MU-OXO [MN(III)MN(IV)] MODEL COMPLEXES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Gamelin, D. R., Kirk, M. L., Stemmler, T. L., Pal, S., Armstrong, W. H., PENNERHAHN, J. E., SOLOMON, E. I. 1994; 116 (6): 2392-2399

  View details for Web of Science ID A1994NE02600019

• SPECTROSCOPIC STUDIES OF THE COUPLED BINUCLEAR NONHEME IRON ACTIVE-SITE IN THE FULLY REDUCED HYDROXYLASE COMPONENT OF METHANE MONOOXYGENASE - COMPARISON TO DEOXY AND DEOXY-AZIDE HEMERYTHRIN JOURNAL OF THE AMERICAN CHEMICAL SOCIETY PULVER, S., Froland, W. A., Fox, B. G., Lipscomb, J. D., SOLOMON, E. I. 1993; 115 (26): 12409-12422

  View details for Web of Science ID A1993MQ10000024

• ELECTRONIC-STRUCTURES OF ACTIVE-SITES ON METAL-OXIDE SURFACES - DEFINITION OF THE CU/ZNO METHANOL SYNTHESIS CATALYST BY PHOTOELECTRON-SPECTROSCOPY CHEMICAL REVIEWS SOLOMON, E. I., Jones, P. M., May, J. A. 1993; 93 (8): 2623-2644

  View details for Web of Science ID A1993MN94100003

• CONSTRUCTION OF A BLUE COPPER SITE AT THE NATIVE ZINC SITE OF YEAST COPPER-ZINC SUPEROXIDE-DISMUTASE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lu, Y., LaCroix, L. B., LOWERY, M. D., SOLOMON, E. I., Bender, C. J., Peisach, J., Roe, J. A., Gralla, E. B., Valentine, J. S. 1993; 115 (14): 5907-5918

  View details for Web of Science ID A1993LT17300003

• CHARGE-TRANSFER STATES OF BRIDGED TRANSITION-METAL DIMERS - MONOCLEAR VS BINUCLEAR COPPER AZIDE SYSTEMS WITH RELEVANCE TO OXY-HEMOCYANIN INORGANIC CHEMISTRY Tuczek, F., SOLOMON, E. I. 1993; 32 (13): 2850-2862

  View details for Web of Science ID A1993LJ34800011

• ACTIVE-SITE ELECTRONIC-STRUCTURE CONTRIBUTIONS TO ELECTRON-TRANSFER PATHWAYS IN RUBREDOXIN AND PLASTOCYANIN - DIRECT VERSUS SUPEREXCHANGE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY LOWERY, M. D., GUCKERT, J. A., GEBHARD, M. S., SOLOMON, E. I. 1993; 115 (7): 3012–13

  View details for DOI 10.1021/ja00060a074

  View details for Web of Science ID A1993KW98000074

• ELECTRONIC-STRUCTURE CONTRIBUTIONS TO FUNCTION IN BIOINORGANIC CHEMISTRY SCIENCE SOLOMON, E. I., LOWERY, M. D. 1993; 259 (5101): 1575-1581

  Abstract

  Many metalloenzymes exhibit distinctive spectral features that are now becoming well understood. These reflect active site electronic structures that can make significant contributions to catalysis. Copper proteins provide well-characterized examples in which the unusual electronic structures of their active sites contribute to rapid, long-range electron transfer reactivity, oxygen binding and activation, and the multielectron reduction of dioxygen to water.

  View details for Web of Science ID A1993KR54300029

  View details for PubMedID 8384374

• X-RAY ABSORPTION SPECTROSCOPIC STUDIES OF THE BLUE COPPER SITE - METAL AND LIGAND K-EDGE STUDIES TO PROBE THE ORIGIN OF THE EPR HYPERFINE SPLITTING IN PLASTOCYANIN JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Shadle, S. E., PENNERHAHN, J. E., Schugar, H. J., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1993; 115 (2): 767-776

  View details for Web of Science ID A1993KJ68900057

• ELECTRONIC ABSORPTION-SPECTROSCOPY OF COPPER PROTEINS METALLOBIOCHEMISTRY, PART C SOLOMON, E. I., LOWERY, M. D., LaCroix, L. B., Root, D. E. 1993; 226: 1-33

  Abstract

  We have seen from the previous discussion that absorption spectral studies in the ligand field region probe the energy splittings of the d orbitals and that this relates to the geometry of the metal center. The energies and intensities of ligand-to-metal charge transfer transitions sensitively probe bonding interactions of the ligand with the metal center. Charge transfer transitions can be used both qualitatively to observe ligand binding to a metal center, owing to the requirement of orbital overlap for significant charge transfer intensity, and quantitatively to define the electron donor ability of that ligand and experimentally evaluate the results of electronic structure calculations. Studies of the intensities of peaks at the ligand K edge can define the covalent interaction of the ligand with the metal valence orbitals, whereas copper K-edge spectroscopy is a powerful probe of metal ion oxidation state and the ligand field geometry of d10 cuprous sites that are inaccessible through other spectroscopic methods. Absorption spectral studies in all regions are strongly complemented by CD, variable temperature MCD, and single-crystal polarized absorption spectroscopies, which should also be pursued whenever possible to obtain detailed electronic structural insight of relevance to catalysis.

  View details for Web of Science ID A1993BZ58Z00001

  View details for PubMedID 8277862

• GROUND-STATE ELECTRONIC-STRUCTURE OF THE DIMER-OF-DIMERS COMPLEX [(MN2O2)2(TPHPN)2]4+ - POTENTIAL RELEVANCE TO THE PHOTOSYSTEM-II WATER OXIDATION CATALYST JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Kirk, M. L., Chan, M. K., Armstrong, W. H., SOLOMON, E. I. 1992; 114 (26): 10432-10440

  View details for Web of Science ID A1992KD71700044

• SPECTROSCOPIC STUDIES OF SIDE-ON PEROXIDE-BRIDGED BINUCLEAR COPPER(II) MODEL COMPLEXES OF RELEVANCE TO OXYHEMOCYANIN AND OXYTYROSINASE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Baldwin, M. J., Root, D. E., Pate, J. E., Fujisawa, K., Kitajima, N., SOLOMON, E. I. 1992; 114 (26): 10421-10431

  View details for Web of Science ID A1992KD71700043

• SPECTROSCOPIC AND THEORETICAL DESCRIPTION OF THE ELECTRONIC-STRUCTURE OF THE S = 3/2 NITROSYL COMPLEX OF NONHEME IRON ENZYMES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Zhang, Y., Pavlosky, M. A., Brown, C. A., Westre, T. E., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1992; 114 (23): 9189-9191

  View details for Web of Science ID A1992JW79700062

• AXIAL LIGAND BONDING IN BLUE COPPER PROTEINS INORGANICA CHIMICA ACTA LOWERY, M. D., SOLOMON, E. I. 1992; 198: 233-243

  View details for Web of Science ID A1992JK24300025

• VARIABLE-ENERGY PHOTOELECTRON SPECTROSCOPIC STUDIES OF H2S CHEMISORPTION ON CU2O AND ZNO SINGLE-CRYSTAL SURFACES - HS- BONDING TO COPPER(I) AND ZINC(II) SITES RELATED TO CATALYTIC POISONING JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lin, J. Y., May, J. A., Didziulis, S. V., SOLOMON, E. I. 1992; 114 (12): 4718-4727

  View details for Web of Science ID A1992HX81500039

• ELECTRONIC-STRUCTURES OF ACTIVE-SITES IN COPPER PROTEINS - CONTRIBUTIONS TO REACTIVITY CHEMICAL REVIEWS SOLOMON, E. I., Baldwin, M. J., LOWERY, M. D. 1992; 92 (4): 521-542

  View details for Web of Science ID A1992JA89100003

• SINGLE-CRYSTAL POLARIZED ABSORPTION SPECTROSCOPIC STUDY OF THE ELECTRONIC-STRUCTURE OF MU-1,2-PEROXO BINUCLEAR COBALT COMPLEXES INORGANIC CHEMISTRY Tuczek, F., SOLOMON, E. I. 1992; 31 (6): 944-953

  View details for Web of Science ID A1992HK15800005

• COORDINATION CHEMISTRY OF NH3 ON ZNO(0001) AND CUCL(111) SURFACES - SIGMA-BONDING INTERACTIONS WITH D-10 METAL-ION SITES INORGANIC CHEMISTRY Lin, J. Y., Jones, P. M., LOWERY, M. D., Gay, R. R., Cohen, S. L., SOLOMON, E. I. 1992; 31 (4): 686-695

  View details for Web of Science ID A1992HF01400030

• SPECTROSCOPIC AND CHEMICAL STUDIES OF THE ASCORBATE OXIDASE TRINUCLEAR COPPER ACTIVE-SITE - COMPARISON TO LACCASE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Cole, J. L., Avigliano, L., Morpurgo, L., SOLOMON, E. I. 1991; 113 (24): 9080-9089

  View details for Web of Science ID A1991GQ93100008

• CHEMICAL AND SPECTROSCOPIC STUDIES OF THE MIXED-VALENT DERIVATIVES OF THE NONHEME IRON PROTEIN HEMERYTHRIN JOURNAL OF THE AMERICAN CHEMICAL SOCIETY McCormick, J. M., REEM, R. C., SOLOMON, E. I. 1991; 113 (24): 9066-9079

  View details for Web of Science ID A1991GQ93100007

• SPECTROSCOPIC AND THEORETICAL-STUDIES OF AN END-ON PEROXIDE-BRIDGED COUPLED BINUCLEAR COPPER(II) MODEL COMPLEX OF RELEVANCE TO THE ACTIVE-SITES IN HEMOCYANIN AND TYROSINASE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Baldwin, M. J., ROSS, P. K., Pate, J. E., Tyeklar, Z., Karlin, K. D., SOLOMON, E. I. 1991; 113 (23): 8671-8679

  View details for Web of Science ID A1991GP03700014

• VARIABLE PHOTON ENERGY PHOTOELECTRON SPECTROSCOPIC STUDY OF CO ADSORPTION TO COORDINATIVELY UNSATURATED TETRAHEDRON CU(I) AND ZN(II) SITES ON CUCL(111) AND ZNO(1010) SURFACES - D10 CONTRIBUTIONS TO CO BONDING AND ACTIVATION JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lin, J. Y., Jones, P., GUCKERT, J., SOLOMON, E. I. 1991; 113 (22): 8312-8326

  View details for Web of Science ID A1991GM03800018

• SPECTROSCOPIC CHARACTERIZATION OF THE PEROXIDE INTERMEDIATE IN THE REDUCTION OF DIOXYGEN CATALYZED BY THE MULTICOPPER OXIDASES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Cole, J. L., Ballou, D. P., SOLOMON, E. I. 1991; 113 (22): 8544-8546

  View details for Web of Science ID A1991GM03800064

• SPECTROSCOPIC STUDIES OF THE NONHEME FERRIC ACTIVE-SITE IN SOYBEAN LIPOXYGENASE - MAGNETIC CIRCULAR-DICHROISM AS A PROBE OF ELECTRONIC AND GEOMETRIC STRUCTURE - LIGAND-FIELD ORIGIN OF ZERO-FIELD SPLITTING JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Zhang, Y., Gebhard, M. S., SOLOMON, E. I. 1991; 113 (14): 5162-5175

  View details for Web of Science ID A1991FU90000004

• EXCITED-STATE SPECTRAL FEATURES OF THE RADICAL REDUCED, NATIVE AND FULLY REDUCED FORMS OF THE COUPLED BINUCLEAR NONHEME IRON CENTER IN RIBONUCLEOTIDE REDUCTASE - ACTIVE-SITE DIFFERENCES RELATIVE TO HEMERYTHRIN NEW JOURNAL OF CHEMISTRY McCormick, J. M., REEM, R. C., FOROUGHI, J., Bollinger, J. M., Jensen, G. M., Stephens, P. J., Stubbe, J., SOLOMON, E. I. 1991; 15 (6): 439-444

  View details for Web of Science ID A1991FZ33400005

• VARIABLE-TEMPERATURE VARIABLE-FIELD MAGNETIC CIRCULAR-DICHROISM STUDIES OF THE FE(II) ACTIVE-SITE IN METAPYROCATECHASE - IMPLICATIONS FOR THE MOLECULAR MECHANISM OF EXTRADIOL DIOXYGENASES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mabrouk, P. A., Orville, A. M., Lipscomb, J. D., SOLOMON, E. I. 1991; 113 (11): 4053-4061

  View details for Web of Science ID A1991FN00500001

• AN ELECTRONIC STRUCTURAL COMPARISON OF COPPER PEROXIDE COMPLEXES OF RELEVANCE TO HEMOCYANIN AND TYROSINASE ACTIVE-SITES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY ROSS, P. K., SOLOMON, E. I. 1991; 113 (9): 3246-3259

  View details for Web of Science ID A1991FJ12100005

• SPECTROSCOPIC STUDIES OF THE ELECTRONIC-STRUCTURE OF IRON(III) TRIS(CATECHOLATES) JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Karpishin, T. B., Gebhard, M. S., SOLOMON, E. I., Raymond, K. N. 1991; 113 (8): 2977-2984

  View details for Web of Science ID A1991FG03200028

• SINGLE-CRYSTAL SPECTROSCOPIC STUDIES OF FE(SC6H4PH-2)4(2-) ELECTRONIC-STRUCTURE OF THE FERROUS SITE IN RUBREDOXIN JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Gebhard, M. S., Koch, S. A., Millar, M., Devlin, F. J., Stephens, P. J., SOLOMON, E. I. 1991; 113 (5): 1640-1649

  View details for Web of Science ID A1991EZ47700030

• SPECTROSCOPIC AND CHEMICAL STUDIES OF THE LACCASE TRINUCLEAR COPPER ACTIVE-SITE - GEOMETRIC AND ELECTRONIC-STRUCTURE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Cole, J. L., Clark, P. A., SOLOMON, E. I. 1990; 112 (26): 9534-9548

  View details for Web of Science ID A1990EQ05100013

• STRUCTURE-FUNCTION CORRELATIONS IN COPPER CLUSTERS IN PROTEINS 27TH INTERNATIONAL CONF ON COORDINATION CHEMISTRY SOLOMON, E. I., Cole, J. L., Baldwin, M. J. INT UNION PURE APPLIED CHEMISTRY. 1990: 1063–66

  View details for Web of Science ID A1990DG15200013

• VARIABLE-ENERGY PHOTOELECTRON SPECTROSCOPIC COMPARISON OF THE BONDING IN FERRIC SULFIDE AND FERRIC-CHLORIDE - AN ALTERNATIVE DESCRIPTION OF THE NEAR-IR VISIBLE SPIN-FORBIDDEN TRANSITIONS IN HIGH-SPIN D5 COMPLEXES INORGANIC CHEMISTRY Butcher, K. D., Gebhard, M. S., SOLOMON, E. I. 1990; 29 (11): 2067-2074

  View details for Web of Science ID A1990DF86500008

• VARIABLE-PHOTON-ENERGY PHOTOELECTRON SPECTROSCOPIC STUDIES OF HIGH-SPIN-D6 TETRAHEDRAL FECL-4(2-) - ELECTRONIC RELAXATION EFFECTS ON IONIZATION INORGANIC CHEMISTRY Butcher, K. D., Didziulis, S. V., Briat, B., SOLOMON, E. I. 1990; 29 (9): 1626-1637

  View details for Web of Science ID A1990DB56000009

• REACTIVITY OF THE LACCASE TRINUCLEAR COPPER ACTIVE-SITE WITH DIOXYGEN - AN X-RAY ABSORPTION-EDGE STUDY JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Cole, J. L., TAN, G. O., Yang, E. K., Hodgson, K. O., SOLOMON, E. I. 1990; 112 (6): 2243-2249

  View details for Web of Science ID A1990CU14400025

• VARIABLE PHOTON ENERGY PHOTOELECTRON-SPECTROSCOPY ON FECL4- - AN UNUSUAL ELECTRONIC-STRUCTURE FOR HIGH-SPIN D5 COMPLEXES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Butcher, K. D., Didziulis, S. V., Briat, B., SOLOMON, E. I. 1990; 112 (6): 2231-2242

  View details for Web of Science ID A1990CU14400024

• SINGLE-CRYSTAL SPECTRAL STUDIES OF FE(S2,3,5,6-(ME)4C6H)4- - THE ELECTRONIC-STRUCTURE OF THE FERRIC TETRATHIOLATE ACTIVE-SITE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Gebhard, M. S., Deaton, J. C., Koch, S. A., Millar, M., SOLOMON, E. I. 1990; 112 (6): 2217-2231

  View details for Web of Science ID A1990CU14400023

• X-RAY ABSORPTION-EDGE SPECTROSCOPY OF LIGANDS BOUND TO OPEN-SHELL METAL-IONS - CHLORINE K-EDGE STUDIES OF COVALENCY IN CUCL42- JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hedman, B., Hodgson, K. O., SOLOMON, E. I. 1990; 112 (4): 1643-1645

  View details for Web of Science ID A1990CP25900062

• X-RAY ABSORPTION-EDGE AND EXAFS STUDY OF THE COPPER SITES IN ZNO METHANOL SYNTHESIS CATALYSTS JOURNAL OF THE AMERICAN CHEMICAL SOCIETY KAU, L. S., Hodgson, K. O., SOLOMON, E. I. 1989; 111 (18): 7103-7109

  View details for Web of Science ID A1989AN01700032

• CHEMISTRY OF COPPER OVERLAYERS ON ZINC-OXIDE SINGLE-CRYSTAL SURFACES - MODEL ACTIVE-SITES FOR CU/ZNO METHANOL SYNTHESIS CATALYSTS JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Didziulis, S. V., Butcher, K. D., Cohen, S. L., SOLOMON, E. I. 1989; 111 (18): 7110-7123

  View details for Web of Science ID A1989AN01700033

• DETAILED SPECTROSCOPIC ANALYSIS OF HALF-MET HEMOCYANINS - MIXED-VALENT CONTRIBUTIONS TO ELECTRONIC-PROPERTIES AND STRUCTURE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Westmoreland, T. D., WILCOX, D. E., Baldwin, M. J., MIMS, W. B., SOLOMON, E. I. 1989; 111 (16): 6106-6123

  View details for Web of Science ID A1989AK07700019

• SPECTROSCOPIC STUDIES OF THE CHARGE-TRANSFER AND VIBRATIONAL FEATURES OF BINUCLEAR COPPER(II) AZIDE COMPLEXES - COMPARISON TO THE COUPLED BINUCLEAR COPPER ACTIVE-SITE IN MET AZIDE HEMOCYANIN AND TYROSINASE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pate, J. E., ROSS, P. K., Thamann, T. J., Reed, C. A., Karlin, K. D., Sorrell, T. N., SOLOMON, E. I. 1989; 111 (14): 5198-5209

  View details for Web of Science ID A1989AE69200029

• SPECTROSCOPIC STUDIES OF THE COUPLED BINUCLEAR FERRIC ACTIVE-SITE IN METHEMERYTHRINS AND OXYHEMERYTHRIN - THE ELECTRONIC-STRUCTURE OF EACH IRON CENTER AND THE IRON-OXO AND IRON PEROXIDE BONDS JOURNAL OF THE AMERICAN CHEMICAL SOCIETY REEM, R. C., McCormick, J. M., Richardson, D. E., Devlin, F. J., Stephens, P. J., Musselman, R. L., SOLOMON, E. I. 1989; 111 (13): 4688-4704

  View details for Web of Science ID A1989AC96200024

• EDGE AND EXAFS STUDIES OF CU COORDINATION IN DEOXY HEMOCYANIN PHYSICA B-CONDENSED MATTER Tan, G., KAU, L. S., Hodgson, K. O., SOLOMON, E. I. 1989; 158 (1-3): 110-111

  View details for Web of Science ID A1989AE26500040

• LOW-ENERGY X-RAY ABSORPTION-EDGE SPECTROSCOPY - APPLICATIONS TO THE NITROGENASE COFACTOR AND ELECTRONIC-STRUCTURE OF S AND CL IN INORGANIC SOLIDS PHYSICA B-CONDENSED MATTER Hedman, B., Frank, P., GHELLER, S. F., Newton, W. E., SOLOMON, E. I., Hodgson, K. O. 1989; 158 (1-3): 71-73

  View details for Web of Science ID A1989AE26500025

• DETAILED SPECTRAL STUDIES OF COPPER-ACETATE - EXCITED-STATE INTERACTIONS IN COPPER DIMERS JOURNAL OF THE AMERICAN CHEMICAL SOCIETY ROSS, P. K., Allendorf, M. D., SOLOMON, E. I. 1989; 111 (11): 4009-4021

  View details for Web of Science ID A1989U736700038

• TRANSVERSE AND LONGITUDINAL ZEEMAN EFFECT ON [PPH4][FECL4] - ASSIGNMENT OF THE LIGAND-FIELD TRANSITIONS AND THE ORIGIN OF THE A-6(1) GROUND-STATE ZERO-FIELD SPLITTING INORGANIC CHEMISTRY Deaton, J. C., Gebhard, M. S., SOLOMON, E. I. 1989; 28 (5): 877-889

  View details for Web of Science ID A1989T670500016

• VARIABLE PHOTON ENERGY PHOTOELECTRON SPECTROSCOPIC STUDIES OF COVALENT BONDING IN 3D10 TRANSITION-METAL COMPOUNDS INORGANIC CHEMISTRY Didziulis, S. V., Cohen, S. L., Butcher, K. D., SOLOMON, E. I. 1988; 27 (13): 2238-2250

  View details for Web of Science ID A1988P033800008

• ELECTRONIC-STRUCTURE OF PLASTOCYANIN - EXCITED-STATE SPECTRAL FEATURES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Gewirth, A. A., SOLOMON, E. I. 1988; 110 (12): 3811-3819

  View details for Web of Science ID A1988N751500015

• VARIABLE PHOTON ENERGY PHOTOELECTRON SPECTROSCOPIC STUDIES OF COPPER CHLORIDES - AN EXPERIMENTAL PROBE OF METAL-LIGAND BONDING AND CHANGES IN ELECTRONIC-STRUCTURE ON IONIZATION JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Didziulis, S. V., Cohen, S. L., Gewirth, A. A., SOLOMON, E. I. 1988; 110 (1): 250-268

  View details for Web of Science ID A1988L760000041

• Coupled binuclear copper proteins: catalytic mechanisms and structure-reactivity correlations. Progress in clinical and biological research SOLOMON, E. I. 1988; 274: 309-329

  View details for PubMedID 3136462

• ELECTRON-PARAMAGNETIC RESONANCE STUDIES OF THE TUNGSTEN-CONTAINING FORMATE DEHYDROGENASE FROM CLOSTRIDIUM-THERMOACETICUM BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Deaton, J. C., SOLOMON, E. I., Watt, G. D., WETHERBEE, P. J., DURFOR, C. N. 1987; 149 (2): 424-430

  Abstract

  The redox centers in the tungsten-containing formate dehydrogenase from Clostridium thermoaceticum were examined by potentiometric titration and electron paramagnetic resonance spectroscopy. At low temperature two overlapping iron-sulfur signals which correlated with enzymatic activity were observed with formal potentials near -400 mV vs. SHE. Based on their temperature dependences, one signal is assigned to a reduced Fe2S2 cluster and one to a reduced Fe4S4 cluster. Quantitation of signal intensity suggests two Fe2S2 and two Fe4S4 clusters per formate dehydrogenase molecule. Another signal (g = 2.101, 1.980, 1.950) present in low concentrations at more negative potentials was observable up to 200 degrees K and is not attributed to any iron-sulfur cluster. The possible origin of this signal is analyzed using ligand field theory, and the redox behavior is considered with respect to possible ligation at the active site.

  View details for Web of Science ID A1987L261800016

  View details for PubMedID 2827642

• CHEMICAL AND SPECTROSCOPIC STUDIES OF THE COUPLED BINUCLEAR COPPER SITE IN TYPE-2 DEPLETED RHUS LACCASE - COMPARISON TO THE HEMOCYANINS AND TYROSINASE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY SPIRASOLOMON, D. J., SOLOMON, E. I. 1987; 109 (21): 6421-6432

  View details for Web of Science ID A1987K557100031

• X-RAY ABSORPTION-EDGE DETERMINATION OF THE OXIDATION-STATE AND COORDINATION-NUMBER OF COPPER - APPLICATION TO THE TYPE-3 SITE IN RHUS-VERNICIFERA LACCASE AND ITS REACTION WITH OXYGEN JOURNAL OF THE AMERICAN CHEMICAL SOCIETY KAU, L. S., SPIRASOLOMON, D. J., PENNERHAHN, J. E., Hodgson, K. O., SOLOMON, E. I. 1987; 109 (21): 6433-6442

  View details for Web of Science ID A1987K557100032

• VIBRATIONAL, ELECTRONIC, AND RESONANCE RAMAN SPECTRAL STUDIES OF [CU2(XYL-O-)O2]+, A COPPER(II) PEROXIDE MODEL COMPLEX OF OXYHEMOCYANIN JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pate, J. E., CRUSE, R. W., Karlin, K. D., SOLOMON, E. I. 1987; 109 (9): 2624-2630

  View details for Web of Science ID A1987H107700013

• SPECTROSCOPIC AND THEORETICAL-STUDIES OF THE UNUSUAL ELECTRON-PARAMAGNETIC-RES PARAMETERS OF DISTORTED TETRAHEDRAL CUPRIC SITES - CORRELATIONS TO X-RAY SPECTRAL FEATURES OF CORE LEVELS INORGANIC CHEMISTRY Gewirth, A. A., Cohen, S. L., Schugar, H. J., SOLOMON, E. I. 1987; 26 (7): 1133-1146

  View details for Web of Science ID A1987G750200032

• ALLOSTERIC INTERACTIONS IN SIPUNCULID AND BRACHIOPOD HEMERYTHRINS BIOCHEMISTRY Richardson, D. E., Emad, M., REEM, R. C., SOLOMON, E. I. 1987; 26 (4): 1003-1013

  Abstract

  Chemical and spectroscopic consequences of allosteric interactions for ligand binding to sipunculid (Phascolopsis gouldii) and brachiopod (Lingula reevii) hemerythrins (Hrs) have been investigated. Possible allosteric effectors for homotropic effects in sipunculid Hrs have been examined, but only reduction in ligand affinity is observed without cooperativity. In contrast to sipunculid Hr, L. reevii Hr binds O2 cooperatively in the pH range 7-8 and exhibits a Bohr effect. Spectroscopic comparisons of the sipunculid and brachiopod Hrs show no significant differences in the active site structures; therefore, modulation of oxygen affinity is attributable to effects linking the site to quaternary structural changes in the octamer. Oxygen equilibria can be fit with a conformational model incorporating a minimum of three states, tensed (T), relaxed (R), and an R-T hybrid. Resonance Raman spectra of L. reevii oxyHr show a shift in the peroxo stretching frequency when the pH is lowered from pH 7.7 (predominantly R oxyHr) to pH 6.3 (a mixture of R, T, and R-T hybrid), but P. gouldii Hr does not have a frequency shift under the same conditions. In contrast to hemoglobins, ligand binding to the deoxy and met forms is noncooperative for brachiopod (and sipunculid) Hrs. It is thus suggested that conformational changes in the protein are linked to the oxidation state change that accompanies oxygenation of the coupled binuclear iron site (deoxy [FeIIFeII]----oxy [FeIIIFeIII]). The total allosteric energy expended in oxygenation is about 1.4 kcal/mol, and such a shift is possible in the relaxed-tense conversion with relatively limited constraints of the iron coordination environment via the protein quaternary structure. The mechanism of cooperativity in the binuclear copper oxygen carrier hemocyanin is discussed in light of these results.

  View details for Web of Science ID A1987G208400005

  View details for PubMedID 3032242

• SPECTROSCOPIC STUDIES OF THE BINUCLEAR FERROUS ACTIVE-SITE OF DEOXYHEMERYTHRIN - COORDINATION-NUMBER AND PROBABLE BRIDGING LIGANDS FOR THE NATIVE AND LIGAND BOUND FORMS JOURNAL OF THE AMERICAN CHEMICAL SOCIETY REEM, R. C., SOLOMON, E. I. 1987; 109 (4): 1216-1226

  View details for Web of Science ID A1987G075700035

• POLARIZED, SINGLE-CRYSTAL, ELECTRONIC SPECTRAL STUDIES OF CU2CL62- - EXCITED-STATE EFFECTS OF THE BINUCLEAR INTERACTION INORGANIC CHEMISTRY DESJARDINS, S. R., WILCOX, D. E., Musselman, R. L., SOLOMON, E. I. 1987; 26 (2): 288-300

  View details for Web of Science ID A1987F878200017

• XANES EXAFS STUDY OF THE COPPER ACTIVE-SITE IN METHANOL SYNTHESIS CATALYST JOURNAL DE PHYSIQUE KAU, L. S., SOLOMON, E. I., Hodgson, K. O. 1986; 47 (C-8): 289-292

  View details for Web of Science ID A1986G515900055

• LOW-TEMPERATURE MAGNETIC CIRCULAR-DICHROISM STUDIES OF NATIVE LACCASE - CONFIRMATION OF A TRINUCLEAR COPPER ACTIVE-SITE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY SPIRASOLOMON, D. J., Allendorf, M. D., SOLOMON, E. I. 1986; 108 (17): 5318-5328

  View details for Web of Science ID A1986D723800043

• RESONANCE RAMAN STUDIES OF THE COUPLED BINUCLEAR COPPER ACTIVE-SITE IN MET AZIDE HEMOCYANIN SPECTROCHIMICA ACTA PART A-MOLECULAR AND BIOMOLECULAR SPECTROSCOPY Pate, J. E., Thamann, T. J., SOLOMON, E. I. 1986; 42 (2-3): 313-318

  View details for Web of Science ID A1986C301300029

• SPECTROSCOPIC STUDIES OF ACTIVE-SITES - BLUE COPPER AND ELECTRONIC STRUCTURAL ANALOGS ACS SYMPOSIUM SERIES SOLOMON, E. I., Gewirth, A. A., Cohen, S. L. 1986; 307: 236-266

  View details for Web of Science ID A1986C402100016

• ELECTRONIC-STRUCTURE AND BONDING OF THE BLUE COPPER SITE IN PLASTOCYANIN JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Penfield, K. W., Gewirth, A. A., SOLOMON, E. I. 1985; 107 (15): 4519-4529

  View details for Web of Science ID A1985AMS1800024

• LOW-TEMPERATURE MAGNETIC CIRCULAR-DICHROISM STUDIES OF NATIVE LACCASE - SPECTROSCOPIC EVIDENCE FOR EXOGENOUS LIGAND BRIDGING AT A TRINUCLEAR COPPER ACTIVE-SITE PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Allendorf, M. D., SPIRA, D. J., SOLOMON, E. I. 1985; 82 (10): 3063-3067

  Abstract

  The detailed nature of N-3 binding at the multi-copper active site in native laccase is investigated through a combination of low-temperature magnetic circular dichroism (LTMCD) and absorption spectroscopies. This combination of techniques allows charge-transfer spectral features associated with N-3 binding to the paramagnetic type 2 Cu(II) to be differentiated from those associated with binding to the antiferromagnetically coupled, and therefore diamagnetic, binuclear type 3 Cu(II) site. Earlier absorption titration studies have indicated that N-3 binds with two different binding constants, yielding a high-affinity and a low-affinity form. The studies presented here are interpreted as strong evidence that low-affinity N-3 bridges the paramagnetic type 2 and diamagnetic type 3 binuclear Cu(II) sites in fully oxidized laccase. This assignment is further supported by features in the MCD spectrum whose intensity correlates with an EPR signal associated with uncoupled type 3 Cu(II) sites. In these sites, N-3 has displaced the endogenous bridge, thereby rendering the site paramagnetic and detectable by both LTMCD and EPR spectroscopy. High-affinity N-3 is found to bind to the paramagnetic type 2 Cu(II) site in a limited fraction of the protein molecules that contains reduced type 3 sites. Finally, the possible role of this trinuclear (type 2-type 3) Cu(II) active site in enabling the irreversible reduction of dioxygen to water is considered.

  View details for Web of Science ID A1985AHX3700001

  View details for PubMedID 2987909

  View details for PubMedCentralID PMC397714

• SUBSTRATE-ANALOG BINDING TO THE COUPLED BINUCLEAR COPPER ACTIVE-SITE IN TYROSINASE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY WILCOX, D. E., Porras, A. G., Hwang, Y. T., Lerch, K., Winkler, M. E., SOLOMON, E. I. 1985; 107 (13): 4015-4027

  View details for Web of Science ID A1985ALC3500043

• ACTIVATION OF NIT-1 NITRATE REDUCTASE BY W-FORMATE DEHYDROGENASE BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Deaton, J. C., SOLOMON, E. I., DURFOR, C. N., WETHERBEE, P. J., Burgess, B. K., Jacobs, D. B. 1984; 121 (3): 1042-1047

  Abstract

  Formate dehydrogenase ( FDH ) from Clostridium thermoaceticum is a known tungsten enzyme. FDH was tested for the presence of nitrogenase-type cofactor and nitrate reductase-type cofactor by the Azotobacter vinelandii UW-45 and Neurospora crassa nit-1 reconstitution assays, respectively. Tungsten formate dehydrogenase (W- FDH ), containing only a small Mo impurity, activated the nit-1 nitrate reductase extracts when molybdate was also added, but not when tungstate was added. These results show W- FDH contains the cofactor common to all known Mo-enzymes except nitrogenase. The difference between the redox chemistries of W- FDH and W-substituted sulfite oxidase appears to relate to differences in tungsten ligation other than that donated by the cofactor or to variations in the protein environment surrounding the tungsten active site.

  View details for Web of Science ID A1984SY02000042

  View details for PubMedID 6234890

• EXTENDED X-RAY ABSORPTION FINE-STRUCTURE STUDY OF THE COUPLED BINUCLEAR COPPER ACTIVE-SITE OF TYROSINASE FROM NEUROSPORA-CRASSA BIOCHIMICA ET BIOPHYSICA ACTA WOOLERY, G. L., Powers, L., WINKLER, M., SOLOMON, E. I., Lerch, K., SPIRO, T. G. 1984; 788 (2): 155-161

  Abstract

  Cu K-edge X-ray absorption spectra have been recorded for the enzyme tyrosinase from Neurospora crassa, in its oxy, resting (met-aquo), and inhibitor-bound (met-mimosine) forms. The K-edges proper resemble those of oxy- and met-hemocyanin, and confirm the presence of CuII. The forbidden 1s----3d transition is noticeably stronger for the 1-mimosine-bound enzyme, implying some distortion of the tetragonal Cu coordination group on inhibitor binding. The extended fine structure (EXAFS) beyond the K-edge has been analyzed. The first shell scattering is consistent with the presence of two N- and two O-ligand atoms, at 2.0 and 1.9 A, for all three forms of the enzyme; there is no evidence for heavy atom (S) scattering in the first shell. As in analogous hemocyanin derivatives, the outer shell scattering contains contributions from distant atoms of imidazole ligands, as well as from an addition scattering atom, at 3.4-3.6 A. For oxy-tyrosinase the additional scatterer is unambiguously a heavy atom (Cu), although a larger Debye-Waller factor suggests a somewhat less rigid binuclear site than in oxy-hemocyanin.

  View details for Web of Science ID A1984TF86500001

  View details for PubMedID 6234942

• EXAFS STUDIES OF BINUCLEAR COPPER SITE OF OXYZIDOHEMOCYANIN, DEOXYZIDOHEMOCYANIN, METAQUOZIDOHEMOCYANIN, METFLUOROZIDOHEMOCYANIN, AND METAZIDOHEMOCYANIN FROM ARTHROPODS AND MOLLUSKS JOURNAL OF THE AMERICAN CHEMICAL SOCIETY WOOLERY, G. L., Powers, L., WINKLER, M., SOLOMON, E. I., SPIRO, T. G. 1984; 106 (1): 86-92

  View details for Web of Science ID A1984RZ11000019

• ON THE SPECTRAL FEATURES ASSOCIATED WITH PEROXIDE REACTIVITY OF THE COUPLED BINUCLEAR COPPER ACTIVE-SITE IN TYPE-2 DEPLETED AND NATIVE RHUS LACCASE BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS PENNERHAHN, J. E., Hedman, B., Hodgson, K. O., SPIRA, D. J., SOLOMON, E. I. 1984; 119 (2): 567-574

  Abstract

  We report herein an X-ray absorption spectroscopic (XAS) determination of the oxidation state of the copper sites in T2D and native Rhus laccase. The increase in intensity of the 330 nm absorption feature which results from peroxide titration of T2D laccase (T3: [Cu(I)Cu(I)], T1: [Cu(II)]) is found to correlate linearly with the percent of oxidation of the binuclear copper site (determined by XAS analysis). This indicates that peroxide oxidizes but does not bind to the T3 site. We have used this correlation to determine that native laccase, as isolated, contains approximately 25% reduced T3 sites and that all spectral changes observed upon peroxide addition to native laccase can be accounted for by oxidation of these reduced sites. The importance of this result to previous reports of peroxide binding at the laccase active site is discussed.

  View details for Web of Science ID A1984SK06000020

  View details for PubMedID 6231927

• RESONANT FLUORESCENCE STUDY OF THE EU-3+-SUBSTITUTED CA-2+ SITE IN BUSYCON HEMOCYANIN - STRUCTURAL COUPLING BETWEEN THE HETEROTROPIC ALLOSTERIC EFFECTOR AND THE COUPLED BINUCLEAR COPPER ACTIVE-SITE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hwang, Y. T., Andrews, L. J., SOLOMON, E. I. 1984; 106 (13): 3832-3838

  View details for Web of Science ID A1984SW83900021

• ELECTRON-PARAMAGNETIC-RES STUDIES OF THE ELECTRON-PARAMAGNETIC-RES NONDETECTABLE MET DERIVATIVE OF HEMOCYANIN - PERTURBATIONS AND DISPLACEMENT OF THE ENDOGENOUS BRIDGE IN THE COUPLED BINUCLEAR COPPER ACTIVE-SITE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY WILCOX, D. E., Long, J. R., SOLOMON, E. I. 1984; 106 (7): 2186-2194

  View details for Web of Science ID A1984SL47700046

• QUANTITATIVE CU(I) DETERMINATION USING X-RAY ABSORPTION-EDGE SPECTROSCOPY - OXIDATION OF THE REDUCED BINUCLEAR COPPER SITE IN TYPE-2 DEPLETED RHUS LACCASE BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Hahn, J. E., Co, M. S., SPIRA, D. J., Hodgson, K. O., SOLOMON, E. I. 1983; 112 (2): 737-745

  Abstract

  We report a procedure, through difference comparison of X-ray absorption edge spectra, for the quantitative determination of Cu(I) content in copper complexes of mixed oxidation state composition. This technique is tested on copper model systems and then used to quantitatively determine that untreated T2D Rhus laccase contains 70 +/- 15% Cu(I). Whereas excess ferricyanide is demonstrated not to alter the Cu(I) content of the untreated T2D, aqueous peroxide and nitrite at pH 6.0 are shown to oxidize the cuprous type 3 site and generate met T2D protein forms.

  View details for Web of Science ID A1983QN34500056

  View details for PubMedID 6221724

• HIGH-RESOLUTION ELECTRON-ENERGY LOSS VIBRATIONAL STUDIES OF CO COORDINATION TO THE (1010) SURFACE OF ZNO JOURNAL OF THE AMERICAN CHEMICAL SOCIETY DAMICO, K. L., McFeely, F. R., SOLOMON, E. I. 1983; 105 (21): 6380-6383

  View details for Web of Science ID A1983RN40600003

• ACTIVE-SITES IN COPPER PROTEINS AN ELECTRONIC-STRUCTURE OVERVIEW STRUCTURE AND BONDING SOLOMON, E. I., Penfield, K. W., WILCOX, D. E. 1983; 53: 2-57

  View details for Web of Science ID A1983QA20500001

• NITRITE REACTIVITY OF THE BINUCLEAR COPPER SITE IN T2D RHUS LACCASE - PREPARATION OF HALF MET-NO2- T2D LACCASE AND ITS CORRELATION TO HALF MET-NO2- HEMOCYANIN AND TYROSINASE BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS SPIRA, D. J., SOLOMON, E. I. 1983; 112 (2): 729-736

  Abstract

  Through chemistry directly comparable to that of the hemocyanins and tyrosinase, half met-NO2- T2D laccase derivatives have been prepared; this NO2- reactivity entails both two electron oxidation of the cuprous binuclear site in deoxy T2D laccase and one electron reduction of the coupled cupric site in the met derivative. However, the labile ligand substitution chemistry and lack of dimer formation in half met-NO2- T2D are in marked contrast to behavior of the simpler binuclear copper containing proteins under analagous conditions. This chemistry supports and extends our earlier studies on the ferrocyanide-generated half met T2D which first indicated an inability of exogenous ligands to bridge the binuclear copper site in laccase.

  View details for Web of Science ID A1983QN34500055

  View details for PubMedID 6303331

• EXAFS INVESTIGATION OF THE BINUCLEAR CUPRIC SITE IN MET T2D RHUS LACCASE AND ITS AZIDE BOUND DERIVATIVE BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS SPIRA, D. J., Co, M. S., SOLOMON, E. I., Hodgson, K. O. 1983; 112 (2): 746-753

  Abstract

  EXAFS analysis of met T2D Rhus laccase and its azide bound derivative indicates an average of 0.33 S at 2.09 A and 3-4 N (or O) atoms at 2.00 A per copper atom for the three copper centers. Using the plastocyanin Cu(II) EXAFS spectrum to model the type 1 site in laccase, a difference EXAFS spectrum for the type 3 site is generated; this spectrum enables assignment of the one S ligand in met T2D to the type 1 site and indicates no evidence of a detectable copper scatterer for the coupled binuclear copper site. Implications regarding type 3 optical features and related studies on the hemocyanins are also discussed.

  View details for Web of Science ID A1983QN34500057

  View details for PubMedID 6221725

• DETAILED ABSORPTION, REFLECTANCE, AND UV PHOTOELECTRON SPECTROSCOPIC AND THEORETICAL-STUDIES OF THE CHARGE-TRANSFER TRANSITIONS OF CUCL42- - CORRELATION OF THE SQUARE-PLANAR AND THE TETRAHEDRAL LIMITS JOURNAL OF THE AMERICAN CHEMICAL SOCIETY DESJARDINS, S. R., Penfield, K. W., Cohen, S. L., Musselman, R. L., SOLOMON, E. I. 1983; 105 (14): 4590-4603

  View details for Web of Science ID A1983QY50300014

• OBSERVATION OF AN ELECTRIC QUADRUPOLE TRANSITION IN THE X-RAY ABSORPTION-SPECTRUM OF A CU(II) COMPLEX CHEMICAL PHYSICS LETTERS Hahn, J. E., Scott, R. A., Hodgson, K. O., Doniach, S., DESJARDINS, S. R., SOLOMON, E. I. 1982; 88 (6): 595-598

  View details for Web of Science ID A1982NV69600016

• The nature of the binuclear copper site in Limulus and other hemocyanins. Progress in clinical and biological research SOLOMON, E. I., EICKMAN, N. C., HIMMELWRIGHT, R. S., Hwang, Y. T., Plon, S. E., WILCOX, D. E. 1982; 81: 189-230

  View details for PubMedID 6289350

• PREPARATION OF A SPECTRAL PROBE DERIVATIVE OF THE HEMOCYANIN BIO-POLYMER - EFFECTS OF ALLOSTERIC INTERACTIONS ON THE COUPLED BINUCLEAR COPPER ACTIVE-SITE PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA-BIOLOGICAL SCIENCES Hwang, Y. T., SOLOMON, E. I. 1982; 79 (8): 2564-2568

  Abstract

  A series of derivatives for both the arthropod and mollusc hemocyanin biopolymers has been prepared; the derivatives contain a small fraction of electron paramagnetic resonance-detectable half-met [Cu(II) CU(I)] sites dispersed among the non-detectable oxy binuclear copper active sites. Upon deoxygenation, large changes in the electron paramagnetic resonance signal of these half-met spectral probe derivatives are observed, which are further adjusted by the heterotropic effectors Ca2+ and H+. The active site structural changes indicated that these spectral changes as the hemocyanins go from a relaxed to a tensed quaternary structure are then discussed.

  View details for Web of Science ID A1982NL32300030

  View details for PubMedID 6283534

  View details for PubMedCentralID PMC346240

• PREPARATION AND CHARACTERIZATION OF A STABLE HALF MET DERIVATIVE OF TYPE-2 DEPLETED RHUS LACCASE - EXOGENOUS LIGAND-BINDING TO THE TYPE-3 SITE BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS SPIRA, D. J., Winkler, M. E., SOLOMON, E. I. 1982; 107 (2): 721-726

  View details for Web of Science ID A1982NY96700044

  View details for PubMedID 6289841

• ANGLE-RESOLVED ULTRAVIOLET PHOTO-ELECTRON SPECTROSCOPIC STUDIES OF CO BINDING TO 3 CHEMICALLY DIFFERENT SURFACES OF ZNO - CONFIRMATION OF STEP-BINDING SITES ON (0001BAR) JOURNAL OF THE AMERICAN CHEMICAL SOCIETY DAMICO, K. L., Trenary, M., Shinn, N. D., SOLOMON, E. I., McFeely, F. R. 1982; 104 (19): 5102-5105

  View details for Web of Science ID A1982PH54900017

• ANION BINDING TO OXIDIZED TYPE-2 DEPLETED AND NATIVE LACCASE - A SPECTROSCOPICALLY EFFECTIVE MODEL FOR EXOGENOUS LIGAND-BINDING TO THE TYPE-3 TYPE-2 ACTIVE-SITE BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Winkler, M. E., SPIRA, D. J., LUBIEN, C. D., Thamann, T. J., SOLOMON, E. I. 1982; 107 (2): 727-734

  View details for Web of Science ID A1982NY96700045

  View details for PubMedID 6289842

• AN ANGLE RESOLVED PHOTOEMISSION DETERMINATION OF THE COORDINATION OF CO ON THE ZNO(0001) SURFACE JOURNAL OF CHEMICAL PHYSICS MCCLELLAN, M. R., Trenary, M., Shinn, N. D., SAYERS, M. J., DAMICO, K. L., SOLOMON, E. I., McFeely, F. R. 1981; 74 (8): 4726-4731

  View details for Web of Science ID A1981LM74100058

• SPECTROSCOPIC STUDIES ON PLASTOCYANIN SINGLE-CRYSTALS - A DETAILED ELECTRONIC-STRUCTURE DETERMINATION OF THE BLUE COPPER ACTIVE-SITE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Penfield, K. W., Gay, R. R., HIMMELWRIGHT, R. S., EICKMAN, N. C., Norris, V. A., Freeman, H. C., SOLOMON, E. I. 1981; 103 (15): 4382-4388

  View details for Web of Science ID A1981LZ20400016

• CHEMICAL AND SPECTROSCOPIC STUDIES OF THE BINUCLEAR COPPER ACTIVE-SITE OF NEUROSPORA TYROSINASE - COMPARISION TO HEMOCYANINS JOURNAL OF THE AMERICAN CHEMICAL SOCIETY HIMMELWRIGHT, R. S., EICKMAN, N. C., LUBIEN, C. D., Lerch, K., SOLOMON, E. I. 1980; 102 (24): 7339-7344

  View details for Web of Science ID A1980KR73600031

• CHEMICAL AND SPECTROSCOPIC COMPARISON OF THE BINUCLEAR COPPER ACTIVE-SITE OF MOLLUSK AND ARTHROPOD HEMOCYANINS JOURNAL OF THE AMERICAN CHEMICAL SOCIETY HIMMELWRIGHT, R. S., EICKMAN, N. C., LUBIEN, C. D., SOLOMON, E. I. 1980; 102 (16): 5378-5388

  View details for Web of Science ID A1980KC18800044

• SPECTROSCOPIC STUDIES OF STELLACYANIN, PLASTOCYANIN, AND AZURIN - ELECTRONIC-STRUCTURE OF THE BLUE COPPER SITES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY SOLOMON, E. I., Hare, J. W., Dooley, D. M., Dawson, J. H., Stephens, P. J., Gray, H. B. 1980; 102 (1): 168-178

  View details for Web of Science ID A1980HZ89300029

• PHOTOELECTRON STUDY OF THE INTERACTION OF CO WITH ZNO JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Gay, R. R., Nodine, M. H., Henrich, V. E., Zeiger, H. J., SOLOMON, E. I. 1980; 102 (22): 6752-6761

  View details for Web of Science ID A1980KM24700014

• ANGLE-RESOLVED PHOTOEMISSION INVESTIGATION OF THE BONDING GEOMETRY OF CO TO ZNO (1010) CHEMICAL PHYSICS LETTERS SAYERS, M. J., MCCLELLEN, M. R., Gay, R. R., SOLOMON, E. I., McFeely, F. R. 1980; 75 (3): 575-578

  View details for Web of Science ID A1980KR08400039

• ULTRAVIOLET PHOTOEMISSION-STUDIES OF THE BONDING OF CO TO THE ZNO(1010) SURFACE AND ITS INTERACTION WITH ATOMIC-HYDROGEN JOURNAL OF VACUUM SCIENCE & TECHNOLOGY DAMICO, K. L., MCCLELLAN, M. R., SAYERS, M. J., Gay, R. R., McFeely, F. R., SOLOMON, E. I. 1980; 17 (5): 1080-1084

  View details for Web of Science ID A1980KG06300044

• SPECTROSCOPIC AND THEORETICAL-ANALYSIS OF THE INTENSE T-1(1U) ]-A-1(1G) TRANSITIONS IN MO(CO)6 AND W(CO)6 INORGANIC CHEMISTRY TROGLER, W. C., DESJARDINS, S. R., SOLOMON, E. I. 1979; 18 (8): 2131-2136

  View details for Web of Science ID A1979HE54200016

• COMPARISON OF HALF-MET AND MET APO HEMOCYANIN - LIGAND BRIDGING AT THE BINUCLEAR COPPER ACTIVE-SITE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY HIMMELWRIGHT, R. S., EICKMAN, N. C., SOLOMON, E. I. 1979; 101 (6): 1576-1586

  View details for Web of Science ID A1979GM80700034

• LIGAND DISPLACEMENT-REACTIONS OF OXYHEMOCYANIN - COMPARISON OF REACTIVITIES OF ARTHROPODS AND MOLLUSKS BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Hepp, A. F., HIMMELWRIGHT, R. S., EICKMAN, N. C., SOLOMON, E. I. 1979; 89 (4): 1050-1057

  View details for Web of Science ID A1979HK08300003

  View details for PubMedID 227375

• GEOMETRIC AND ELECTRONIC-STRUCTURE OF OXYHEMOCYANIN - SPECTRAL AND CHEMICAL CORRELATIONS TO MET APO, HALF MET, MET, AND DIMER ACTIVE-SITES PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA EICKMAN, N. C., HIMMELWRIGHT, R. S., SOLOMON, E. I. 1979; 76 (5): 2094-2098

  Abstract

  The chemical and spectral properties of a series of hemocyanin derivatives were systematically compared to provide insight into the geometric and electronic structure of the oxyhemocyanin active site. The binuclear copper site is characterized as two tetragonal Cu(II) atoms bridged by both an endogenous protein ligand and the exogenous ligand (i.e., peroxide), with the lack of an electron paramagnetic resonance signal being the result of antiferromagnetic exchange via the endogenous bridge. A transition dipole-vector coupling model is used to assign the unique absorption spectral properties of oxyhemocyanin: the bands at 570 and 486 nm are assigned as components of the peroxide pi v* to copper dx2-y2 charge transfer. The 345-nm band is one component of the pi sigma* leads to dx2-y2 charge transfer. The model also predicts an end-to-end bridging geometry for the peroxide in oxyhemocyanin.

  View details for Web of Science ID A1979GW30700002

  View details for PubMedID 287049

  View details for PubMedCentralID PMC383542

• REACTIONS AND INTERCONVERSION OF MET AND DIMER HEMOCYANIN BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS HIMMELWRIGHT, R. S., EICKMAN, N. C., SOLOMON, E. I. 1979; 86 (3): 628-634

  View details for Web of Science ID A1979GJ93400023

  View details for PubMedID 218579

• MAGNETIC-SUSCEPTIBILITY STUDIES OF LACCASE AND OXYHEMOCYANIN PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Dooley, D. M., Scott, R. A., ELLINGHAUS, J., SOLOMON, E. I., Gray, H. B. 1978; 75 (7): 3019-3022

  Abstract

  The magnetic susceptibility of Rhus vernicifera laccase has been remeasured over the temperature range 5-260 K. In contrast to our previous results [Solomon, E.I., Dooley, D. M., Wang R.-H., Gray, H.B., Cerdonio, M., Mogno, F. & Romani, G. L. (1975) J. Am. Chem. Soc. 98, 1029-1031] linear chi versus T-1 behavior was observed. The susceptibility of Limulus polyphemus oxyhemocyanin has also been measured in the range 5-260 K. Only weak paramagnetism, attributable to dissolved oxygen and a small amount of paramagnetic impurities, was observed. Analysis of the data establishes a lower limit of 550 cm-1 for J, consistent with our earlier work. The temperature dependence of the susceptibility of laccase is quantitatively accounted for by the presence of two paramagnetic copper ions (types 1 and 2) per enzyme molecule. Curie law behavior at low temperatures rules out significant interaction between the two coppper types, indicating that these redox centers are well separated (several angstroms) and are not connected by bridging ligands. Formulation of the type 3 site as binuclear Cu(II) requires J greater than or equal to 500 cm-1.

  View details for Web of Science ID A1978FJ88100003

  View details for PubMedID 98765

  View details for PubMedCentralID PMC392703

• PREPARATION AND CHARACTERIZATION OF MET APO HEMOCYANIN - SINGLE COPPER(II) ACTIVE-SITE BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS HIMMELWRIGHT, R. S., EICKMAN, N. C., SOLOMON, E. I. 1978; 81 (1): 243-247

  View details for Web of Science ID A1978ET25800035

• SPECTROSCOPIC STUDIES OF LIGAND PERTURBATION EFFECTS ON HALF OXIDIZED ACTIVE-SITE OF BUSYCON-CANALICULATUM HEMOCYANIN BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS HIMMELWRIGHT, R. S., EICKMAN, N. C., SOLOMON, E. I. 1978; 81 (1): 237-242

  View details for Web of Science ID A1978ET25800034

  View details for PubMedID 207273

• CHEMICAL AND SPECTROSCOPIC CONFORMATION OF AN EXOGENOUS LIGAND BRIDGE IN HALF MET HEMOCYANIN BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS HIMMELWRIGHT, R. S., EICKMAN, N. C., SOLOMON, E. I. 1978; 84 (2): 300-305

  View details for Web of Science ID A1978FS03100004

  View details for PubMedID 214069

• PRODUCTION OF HYDROGEN BY UV IRRADIATION OF MO2(SO4)44-IN AQUEOUS SULFURIC-ACID - ELECTRONIC ABSORPTION-SPECTRUM OF K3MO2(SO4)4.3.5H2O AT 15 K JOURNAL OF THE AMERICAN CHEMICAL SOCIETY ERWIN, D. K., GEOFFROY, G. L., Gray, H. B., HAMMOND, G. S., SOLOMON, E. I., TROGLER, W. C., ZAGARS, A. A. 1977; 99 (11): 3620-3621

  View details for Web of Science ID A1977DG50800017

• STUDIES OF POLARIZATION BEHAVIOR, TEMPERATURE-DEPENDENCE, AND VIBRONIC STRUCTURE OF 23000-CM-1 ABSORPTION SYSTEM IN ELECTRONIC-SPECTRA OF MO2(O2CCH3)4 AND RELATED COMPOUNDS - EMISSION-SPECTRUM OF MO2(O2CCF3)4 AT 1.3K INORGANIC CHEMISTRY TROGLER, W. C., SOLOMON, E. I., TRAJBERG, I. B., BALLHAUSEN, C. J., Gray, H. B. 1977; 16 (4): 828-836

  View details for Web of Science ID A1977DA25600021

• EMISSION-SPECTRA AND LIFETIMES OF RE2CL82-,RE2BR82-, AND MO2CL84- AT 1.3K UPON EXCITATION OF DELTA-]DELTA] TRANSITION INORGANIC CHEMISTRY TROGLER, W. C., SOLOMON, E. I., Gray, H. B. 1977; 16 (12): 3031-3033

  View details for Web of Science ID A1977EC51500009

• SUSCEPTIBILITY STUDIES OF LACCASE AND OXYHEMOCYANIN USING AN ULTRASENSITIVE MAGNETOMETER - ANTIFERROMAGNETIC BEHAVIOR OF TYPE-3 COPPER IN RHUS LACCASE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY SOLOMON, E. I., Dooley, D. M., Wang, R. H., Gray, H. B., Cerdonio, M., MOGNO, F., Romani, G. L. 1976; 98 (4): 1029-1031

  View details for Web of Science ID A1976BF76400035

• SPECTROSCOPIC STUDIES AND A STRUCTURAL MODEL FOR BLUE COPPER CENTERS IN PROTEINS PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA SOLOMON, E. I., Hare, J. W., Gray, H. B. 1976; 73 (5): 1389-1393

  Abstract

  Low temperature absorption, circular dichroism, and magnetic circular dichroism spectral studies of the blue copper proteins Rhus vernicifera stellacyanin, bean plastocyanin, and Pseudomonas aeruginosa azurin have been made. Low energy bands attributable to the d-d transitions 2B2 leads to 2E and 2B2 leads to 2B1 in a flattened tetrahedral (D 2d) copper-(II) center are observed in these proteins at about 5000 and 10,000 cm-1, respectively. The band positions accord well with ligand field calculations based on a tetrahedral structure that is distorted approximately 6 degrees toward a square plane. The ligands in this flattened tetrahedral coordination unit in bean plastocyanin are identified from various spectroscopic experiments as His-38, Cys-85, His-88, and a deprotonated peptide nitrogen (N) a few residues above His-38.

  View details for Web of Science ID A1976BS77400005

  View details for PubMedID 818636

  View details for PubMedCentralID PMC430299

• TEMPERATURE-DEPENDENCE OF MAGNETIC-SUSCEPTIBILITY OF COBALT(II) STELLACYANIN BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS SOLOMON, E. I., Wang, R. H., McMillin, D. R., Gray, H. B. 1976; 69 (4): 1039-1042

  View details for Web of Science ID A1976BN62800028

  View details for PubMedID 179538

• INFRARED SPECTRAL STUDIES OF METAL-BINDING EFFECTS ON SECONDARY STRUCTURE OF BEAN PLASTOCYANIN JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hare, J. W., SOLOMON, E. I., Gray, H. B. 1976; 98 (11): 3205-3209

  View details for Web of Science ID A1976BS45600025

  View details for PubMedID 1262643

• INFRARED AND VISIBLE CIRCULAR-DICHROISM AND MAGNETIC CIRCULAR-DICHROISM STUDIES ON COBALT(II)-SUBSTITUTED BLUE COPPER PROTEINS JOURNAL OF THE AMERICAN CHEMICAL SOCIETY SOLOMON, E. I., Rawlings, J., McMillin, D. R., Stephens, P. J., Gray, H. B. 1976; 98 (25): 8046-8048

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• SIMULTANEOUS PAIR ELECTRONIC-TRANSITIONS IN YB2O3 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Schugar, H. J., SOLOMON, E. I., CLEVELAND, W. L., Goodman, L. 1975; 97 (22): 6442-6450

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• IDENTIFICATION OF STRUCTURE OF T-31G(I)[-A-32G BAND IN NI(H2O)6++ COMPLEX MOLECULAR PHYSICS SOLOMON, E. I., BALLHAUSEN, C. J. 1975; 29 (1): 279-299

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