Britt Hedman

All Publications

• Heterologous synthesis of the complex homometallic cores of nitrogenase P- and M-clusters in Escherichia coli. Proceedings of the National Academy of Sciences of the United States of America Quechol, R., Solomon, J. B., Liu, Y. A., Lee, C. C., Jasniewski, A. J., Górecki, K., Oyala, P., Hedman, B., Hodgson, K. O., Ribbe, M. W., Hu, Y. 2023; 120 (44): e2314788120

  Abstract

  Nitrogenase is an active target of heterologous expression because of its importance for areas related to agronomy, energy, and environment. One major hurdle for expressing an active Mo-nitrogenase in Escherichia coli is to generate the complex metalloclusters (P- and M-clusters) within this enzyme, which involves some highly unique bioinorganic chemistry/metalloenzyme biochemistry that is not generally dealt with in the heterologous expression of proteins via synthetic biology; in particular, the heterologous synthesis of the homometallic P-cluster ([Fe8S7]) and M-cluster core (or L-cluster; [Fe8S9C]) on their respective protein scaffolds, which represents two crucial checkpoints along the biosynthetic pathway of a complete nitrogenase, has yet to be demonstrated by biochemical and spectroscopic analyses of purified metalloproteins. Here, we report the heterologous formation of a P-cluster-containing NifDK protein upon coexpression of Azotobacter vinelandii nifD, nifK, nifH, nifM, and nifZ genes, and that of an L-cluster-containing NifB protein upon coexpression of Methanosarcina acetivorans nifB, nifS, and nifU genes alongside the A. vinelandii fdxN gene, in E. coli. Our metal content, activity, EPR, and XAS/EXAFS data provide conclusive evidence for the successful synthesis of P- and L-clusters in a nondiazotrophic host, thereby highlighting the effectiveness of our metallocentric, divide-and-conquer approach that individually tackles the key events of nitrogenase biosynthesis prior to piecing them together into a complete pathway for the heterologous expression of nitrogenase. As such, this work paves the way for the transgenic expression of an active nitrogenase while providing an effective tool for further tackling the biosynthetic mechanism of this important metalloenzyme.

  View details for DOI 10.1073/pnas.2314788120

  View details for PubMedID 37871225

• 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

• 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

• 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

• Incorporation of an Asymmetric Mo-Fe-S Cluster as an Artificial Cofactor into Nitrogenase. Chembiochem : a European journal of chemical biology Tanifuji, K., Jasniewski, A., Lee, C. C., Solomon, J., Nagasawa, T., Ohki, Y., Tatsumi, K., Hedman, B., Hodgson, K., Hu, Y., Ribbe, M. W. 2022

  Abstract

  Nitrogenase employs a sophisticated electron transfer system and a Mo-Fe-S-C cofactor, designated the M-cluster [(cit)MoFe7 S9C]), to reduce atmospheric N2 to bioaccessible NH3. Previously, we have shown that the cofactor-free form of nitrogenase can be repurposed as a protein scaffold for the incorporation of a synthetic Fe-S cluster [Fe6 S9(SEt)2]4-. Here, we demonstrate the utility of an asymmetric Mo-Fe-S cluster [Cp*MoFe5S9(SH)]3- as an alternative artificial cofactor upon incorporation into the cofactor-free nitrogenase scaffold. The resultant semi-artificial enzyme catalytically reduces C2H2 to C2H4, and CN- into short-chain hydrocarbons, yet it is clearly distinct in activity from its [Fe6S9(SEt)2]4--reconstituted counterpart, pointing to the possibility to employ molecular design and cluster synthesis strategies to further develop semi-artificial or artificial systems with desired catalytic activities.

  View details for DOI 10.1002/cbic.202200384

  View details for PubMedID 35925843

• Radical SAM-dependent formation of a nitrogenase cofactor core on NifB. Journal of inorganic biochemistry Liu, Y. A., Quechol, R., Solomon, J. B., Lee, C. C., Ribbe, M. W., Hu, Y., Hedman, B., Hodgson, K. O. 2022; 233: 111837

  Abstract

  Nitrogenase is a versatile metalloenzyme that reduces N2, CO and CO2 at its cofactor site. Designated the M-cluster, this complex cofactor has a composition of [(R-homocitrate)MoFe7S9C], and it is assembled through the generation of a unique [Fe8S9C] core prior to the insertion of Mo and homocitrate. NifB is a radical S-adenosyl-L-methionine (SAM) enzyme that is essential for nitrogenase cofactor assembly. This review focuses on the recent work that sheds light on the role of NifB in the formation of the [Fe8S9C] core of the nitrogenase cofactor, highlighting the structure, function and mechanism of this unique radical SAM methyltransferase.

  View details for DOI 10.1016/j.jinorgbio.2022.111837

  View details for PubMedID 35550498

• Characterization of a Nitrogenase Iron Protein Substituted with a Synthetic [Fe4Se4] Cluster. Angewandte Chemie (International ed. in English) Solomon, J., Tanifuji, K., Lee, C. C., Jasniewski, A., Hedman, B., Hodgson, K., Hu, Y., Ribbe, M. 2022

  Abstract

  The Fe protein of nitrogenase plays multiple roles in substrate reduction and cluster maturation via its redox active [Fe4S4] cluster. Here we report the synthesis and characterization of a water-soluble [Fe4Se4] cluster that is used to substitute the [Fe4S4] cluster of the Azotobacter vinelandii Fe protein (AvNifH). Biochemical, EPR and XAS/EXAFS analyses demonstrate the ability of the [Fe4Se4] cluster to adopt the super-reduced, all-ferrous state upon its incorporation into AvNifH. Moreover, these studies reveal that the [Fe4Se4] cluster in AvNifH already assumes a partial all-ferrous state ([Fe4Se4]0) in the presence of dithionite, where its [Fe4S4] counterpart in AvNifH exists solely in the reduced state ([Fe4S4]1+). Such a discrepancy in the redox properties of the AvNifH-associated [Fe4Se4] and [Fe4S4] clusters can be used to distinguish the differential redox requirements for the substrate reduction and cluster maturation of nitrogenase, pointing to the utility of chalcogen-substituted FeS clusters in future mechanistic studies of nitrogenase catalysis and assembly.

  View details for DOI 10.1002/anie.202202271

  View details for PubMedID 35218104

• 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

• Tracing the incorporation of the "ninth sulfur" into the nitrogenase cofactor precursor with selenite and tellurite. Nature chemistry Tanifuji, K., Jasniewski, A. J., Villarreal, D., Stiebritz, M. T., Lee, C. C., Wilcoxen, J., Okhi, Y., Chatterjee, R., Bogacz, I., Yano, J., Kern, J., Hedman, B., Hodgson, K. O., Britt, R. D., Hu, Y., Ribbe, M. W. 2021

  Abstract

  Molybdenum nitrogenase catalyses the reduction of N2 to NH3 at its cofactor, an [(R-homocitrate)MoFe7S9C] cluster synthesized via the formation of a [Fe8S9C] L-cluster prior to the insertion of molybdenum and homocitrate. We have previously identified a [Fe8S8C] L*-cluster, which is homologous to the core structure of the L-cluster but lacks the 'ninth sulfur' in the belt region. However, direct evidence and mechanistic details of the L*- to L-cluster conversion upon 'ninth sulfur' insertion remain elusive. Here we trace the 'ninth sulfur' insertion using SeO32- and TeO32- as 'labelled' SO32-. Biochemical, electron paramagnetic resonance and X-ray absorption spectroscopy/extended X-ray absorption fine structure studies suggest a role of the 'ninth sulfur' in cluster transfer during cofactor biosynthesis while revealing the incorporation of Se2-- and Te2--like species into the L-cluster. Density functional theory calculations further point to a plausible mechanism involving in situ reduction of SO32- to S2-, thereby suggesting the utility of this reaction to label the catalytically important belt region for mechanistic investigations of nitrogenase.

  View details for DOI 10.1038/s41557-021-00799-8

  View details for PubMedID 34635813

• 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

• 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

• 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

• 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

• 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

• Biological sulfur in the blood cells of Ascidia ceratodes: XAS spectroscopy and a cellular-enzymatic hypothesis for vanadium reduction in the ascidians. Journal of inorganic biochemistry Frank, P. n., Carlson, R. M., Carlson, E. J., Hedman, B. n., Hodgson, K. O. 2020; 205: 110991

  Abstract

  Two samples of living blood cells and of cleared blood plasma from the Phlebobranch tunicate Ascidia ceratodes from Bodega Bay, California, and one of fresh Henze solution from A. ceratodes of Monterey Bay, California, have been examined using sulfur K-edge x-ray absorption spectroscopy (XAS). Biological sulfur included sulfate esters, sulfate and bisulfate ions, benzothiazole, thianthrene, epi-sulfide, thiol and disulfide. Glutathione dominated reduced sulfur, from which an average intracellular Voltage of -0.21 V was calculated. Sulfate-bisulfate ratios yielded blood cell pH values of 2.0 and 2.8. Total blood cell [sulfur] was 373±9 mM or 296±73 mM from BaSO4 gravimetry. Two plasma samples (pH 6.9 or 7.0; [S] = 33±6 mM or 26±4 mM) were dominated by sulfate and disulfide. Fresh Henze solution evidenced a sulfur inventory similar to blood cells, with calculated pH = 2.7. A V(III)-sulfonate fraction varied systematically with intracellular pH across six independent blood cell samples, implying a vanadium mobilization pathway. Bodega Bay and Monterey Bay A. ceratodes appear to maintain alternative suites of low-valent sulfur. The significance of the vanabins to vanadium metabolism is critically examined in terms of known protein - V(IV) biochemistry. Finally, a detailed hypothesis for the reduction of [VO4]3- to V(III) in ascidians is introduced. A vanadium oxido-reductase is proposed to span the signet ring membrane and to release V(III) into the inner acidic vacuole. The V(V) to V(III) reduction is predicted require an inner-sphere mechanism, a thiol reductant, 7-coordinate V(III), a biologically accessible Voltage, and proton-facilitated release of V(III).

  View details for DOI 10.1016/j.jinorgbio.2019.110991

  View details for PubMedID 31945647

• 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

• 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

• Synchrotron X-radiolysis of L-cysteine at the sulfur K-edge: Sulfurous products, experimental surprises, and dioxygen as an oxidoreductant JOURNAL OF CHEMICAL PHYSICS Frank, P., Sarangi, R., Hedman, B., Hodgson, K. O. 2019; 150 (10)

  View details for DOI 10.1063/1.5079419

  View details for Web of Science ID 000461371300031

• 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

• Spectroscopic Characterization of an Eight-Iron Nitrogenase Cofactor Precursor that Lacks the "9th Sulfur". Angewandte Chemie (International ed. in English) Jasniewski, A. J., Wilcoxen, J. n., Tanifuji, K. n., Hedman, B. n., Hodgson, K. O., Britt, R. D., Hu, Y. n., Ribbe, M. W. 2019

  Abstract

  Nitrogenases catalyze the reduction of N2 to NH4+ at its cofactor site. Designated the M-cluster, this [MoFe7 S9 C(R-homocitrate)] cofactor is synthesized via the transformation of a [Fe4 S4 ] cluster pair into an [Fe8 S9 C] precursor (designated the L-cluster) prior to insertion of Mo and homocitrate. We report the characterization of an eight-iron cofactor precursor (designated the L*-cluster), which is proposed to have the composition [Fe8 S8 C] and lack the "9th sulfur" in the belt region of the L-cluster. Our X-ray absorption and electron spin echo envelope modulation (ESEEM) analyses strongly suggest that the L*-cluster represents a structural homologue to the l-cluster except for the missing belt sulfur. The absence of a belt sulfur from the L*-cluster may prove beneficial for labeling the catalytically important belt region, which could in turn facilitate investigations into the reaction mechanism of nitrogenases.

  View details for DOI 10.1002/anie.201907593

  View details for PubMedID 31411369

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• Spin-Polarization-Induced Preedge Transitions in the Sulfur K-Edge XAS Spectra of Open-Shell Transition-Metal Sulfates: Spectroscopic Validation of s-Bond Electron Transfer. Inorganic chemistry Frank, P., Szilagyi, R. K., Gramlich, V., Hsu, H., Hedman, B., Hodgson, K. O. 2017; 56 (3): 1080-1093

  Abstract

  Sulfur K-edge X-ray absorption spectroscopy (XAS) spectra of the monodentate sulfate complexes [M(II)(itao)(SO4)(H2O)0,1] (M = Co, Ni, Cu) and [Cu(Me6tren)(SO4)] exhibit well-defined preedge transitions at 2479.4, 2479.9, 2478.4, and 2477.7 eV, respectively, despite having no direct metal-sulfur bond, while the XAS preedge of [Zn(itao)(SO4)] is featureless. The sulfur K-edge XAS of [Cu(itao)(SO4)] but not of [Cu(Me6tren)(SO4)] uniquely exhibits a weak transition at 2472.1 eV, an extraordinary 8.7 eV below the first inflection of the rising K-edge. Preedge transitions also appear in the sulfur K-edge XAS of crystalline [M(II)(SO4)(H2O)] (M = Fe, Co, Ni, and Cu, but not Zn) and in sulfates of higher-valent early transition metals. Ground-state density functional theory (DFT) and time-dependent DFT (TDDFT) calculations show that charge transfer from coordinated sulfate to paramagnetic late transition metals produces spin polarization that differentially mixes the spin-up (α) and spin-down (β) spin orbitals of the sulfate ligand, inducing negative spin density at the sulfate sulfur. Ground-state DFT calculations show that sulfur 3p character then mixes into metal 4s and 4p valence orbitals and various combinations of ligand antibonding orbitals, producing measurable sulfur XAS transitions. TDDFT calculations confirm the presence of XAS preedge features 0.5-2 eV below the rising sulfur K-edge energy. The 2472.1 eV feature arises when orbitals at lower energy than the frontier occupied orbitals with S 3p character mix with the copper(II) electron hole. Transmission of spin polarization and thus of radical character through several bonds between the sulfur and electron hole provides a new mechanism for the counterintuitive appearance of preedge transitions in the XAS spectra of transition-metal oxoanion ligands in the absence of any direct metal-absorber bond. The 2472.1 eV transition is evidence for further radicalization from copper(II), which extends across a hydrogen-bond bridge between sulfate and the itao ligand and involves orbitals at energies below the frontier set. This electronic structure feature provides a direct spectroscopic confirmation of the through-bond electron-transfer mechanism of redox-active metalloproteins.

  View details for DOI 10.1021/acs.inorgchem.6b00991

  View details for PubMedID 28068071

• 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

• 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

• Assembly scaffold NifEN: A structural and functional homolog of the nitrogenase catalytic component. Proceedings of the National Academy of Sciences of the United States of America Fay, A. W., Blank, M. A., Rebelein, J. G., Lee, C. C., Ribbe, M. W., Hedman, B., Hodgson, K. O., Hu, Y. 2016; 113 (34): 9504-9508

  Abstract

  NifEN is a biosynthetic scaffold for the cofactor of Mo-nitrogenase (designated the M-cluster). Previous studies have revealed the sequence and structural homology between NifEN and NifDK, the catalytic component of nitrogenase. However, direct proof for the functional homology between the two proteins has remained elusive. Here we show that, upon maturation of a cofactor precursor (designated the L-cluster) on NifEN, the cluster species extracted from NifEN is spectroscopically equivalent and functionally interchangeable with the native M-cluster extracted from NifDK. Both extracted clusters display nearly indistinguishable EPR features, X-ray absorption spectroscopy/extended X-ray absorption fine structure (XAS/EXAFS) spectra and reconstitution activities, firmly establishing the M-cluster-bound NifEN (designated NifEN(M)) as the only protein other than NifDK to house the unique nitrogenase cofactor. Iron chelation experiments demonstrate a relocation of the cluster from the surface to its binding site within NifEN(M) upon maturation, which parallels the insertion of M-cluster into an analogous binding site in NifDK, whereas metal analyses suggest an asymmetric conformation of NifEN(M) with an M-cluster in one αβ-half and an empty cluster-binding site in the other αβ-half, which led to the proposal of a stepwise assembly mechanism of the M-cluster in the two αβ-dimers of NifEN. Perhaps most importantly, NifEN(M) displays comparable ATP-independent substrate-reducing profiles to those of NifDK, which establishes the M-cluster-bound αβ-dimer of NifEN(M) as a structural and functional mimic of one catalytic αβ-half of NifDK while suggesting the potential of this protein as a useful tool for further investigations of the mechanistic details of nitrogenase.

  View details for DOI 10.1073/pnas.1609574113

  View details for PubMedID 27506795

  View details for PubMedCentralID PMC5003292

• Solvation structure of the halides from x-ray absorption spectroscopy. journal of chemical physics Antalek, M., Pace, E., Hedman, B., Hodgson, K. O., Chillemi, G., Benfatto, M., Sarangi, R., Frank, P. 2016; 145 (4): 044318-?

  Abstract

  Three-dimensional models for the aqueous solvation structures of chloride, bromide, and iodide are reported. K-edge extended X-ray absorption fine structure (EXAFS) and Minuit X-ray absorption near edge (MXAN) analyses found well-defined single shell solvation spheres for bromide and iodide. However, dissolved chloride proved structurally distinct, with two solvation shells needed to explain its strikingly different X-ray absorption near edge structure (XANES) spectrum. Final solvation models were as follows: iodide, 8 water molecules at 3.60 ± 0.13 Å and bromide, 8 water molecules at 3.40 ± 0.14 Å, while chloride solvation included 7 water molecules at 3.15 ± 0.10 Å, and a second shell of 7 water molecules at 4.14 ± 0.30 Å. Each of the three derived solvation shells is approximately uniformly disposed about the halides, with no global asymmetry. Time-dependent density functional theory calculations simulating the chloride XANES spectra following from alternative solvation spheres revealed surprising sensitivity of the electronic state to 6-, 7-, or 8-coordination, implying a strongly bounded phase space for the correct structure during an MXAN fit. MXAN analysis further showed that the asymmetric solvation predicted from molecular dynamics simulations using halide polarization can play no significant part in bulk solvation. Classical molecular dynamics used to explore chloride solvation found a 7-water solvation shell at 3.12 (-0.04/+0.3) Å, supporting the experimental result. These experiments provide the first fully three-dimensional structures presenting to atomic resolution the aqueous solvation spheres of the larger halide ions.

  View details for DOI 10.1063/1.4959589

  View details for PubMedID 27475372

  View details for PubMedCentralID PMC4967075

• 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

• Uncoupling binding of substrate CO from turnover by vanadium nitrogenase PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Lee, C. C., Fay, A. W., Weng, T., Krest, C. M., Hedman, B., Hodgson, K. O., Hu, Y., Ribbe, M. W. 2015; 112 (45): 13845-13849

  Abstract

  Biocatalysis by nitrogenase, particularly the reduction of N2 and CO by this enzyme, has tremendous significance in environment- and energy-related areas. Elucidation of the detailed mechanism of nitrogenase has been hampered by the inability to trap substrates or intermediates in a well-defined state. Here, we report the capture of substrate CO on the resting-state vanadium-nitrogenase in a catalytically competent conformation. The close resemblance of this active CO-bound conformation to the recently described structure of CO-inhibited molybdenum-nitrogenase points to the mechanistic relevance of sulfur displacement to the activation of iron sites in the cofactor for CO binding. Moreover, the ability of vanadium-nitrogenase to bind substrate in the resting-state uncouples substrate binding from subsequent turnover, providing a platform for generation of defined intermediate(s) of both CO and N2 reduction.

  View details for DOI 10.1073/pnas.1519696112

  View details for Web of Science ID 000364470300049

  View details for PubMedID 26515097

  View details for PubMedCentralID PMC4653198

• A high-resolution XAS study of aqueous Cu(II) in liquid and frozen solutions: Pyramidal, polymorphic, and non-centrosymmetric. journal of chemical physics Frank, P., Benfatto, M., Qayyam, M., Hedman, B., Hodgson, K. O. 2015; 142 (8): 084310-?

  Abstract

  High-resolution EXAFS (k = 18 Å(-1)) and MXAN XAS analyses show that axially elongated square pyramidal [Cu(H2O)5](2+) dominates the structure of Cu(II) in aqueous solution, rather than 6-coordinate JT-octahedral [Cu(H2O)6](2+). Freezing produced a shoulder at 8989.6 eV on the rising XAS edge and an altered EXAFS spectrum, while 1s → 3d transitions remained invariant in energy position and intensity. Core square pyramidal [Cu(H2O)5](2+) also dominates frozen solution. Solvation shells were found at ∼3.6 Å (EXAFS) or ∼3.8 Å (MXAN) in both liquid and frozen phases. However, MXAN analysis revealed that about half the time in liquid solution, [Cu(H2O)5](2+) associates with an axially non-bonding 2.9 Å water molecule. This distant water apparently organizes the solvation shell. When the 2.9 Å water molecule is absent, the second shell is undetectable to MXAN. The two structural arrangements may represent energetic minima of fluxional dissolved aqueous [Cu(H2O)5](2+). The 2.9 Å trans-axial water resolves an apparent conflict of the [Cu(H2O)5](2+) core model with a dissociational exchange mechanism. In frozen solution, [Cu(H2O)5](2+) is associated with either a 3.0 Å axial non-bonded water molecule or an axial ClO4 (-) at 3.2 Å. Both structures are again of approximately equal presence. When the axial ClO4 (-) is present, Cu(II) is ∼0.5 Å above the mean O4 plane. This study establishes [Cu(H2O)5](2+) as the dominant core structure for Cu(II) in water solution, and is the first to both empirically resolve multiple extended solution structures for fluxional [Cu(H2O)5](2+) and to provide direct evidence for second shell dynamics.

  View details for DOI 10.1063/1.4908266

  View details for PubMedID 25725734

  View details for PubMedCentralID PMC4349298

• 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

• Goniometer-based femtosecond crystallography with X-ray free electron lasers PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Cohen, A. E., Soltis, S. M., Gonzalez, A., Aguila, L., Alonso-Mori, R., Barnes, C. O., Baxter, E. L., Brehmer, W., Brewster, A. S., Brunger, A. T., Calero, G., Chang, J. F., Chollet, M., Ehrensberger, P., Eriksson, T. L., Feng, Y., Hattne, J., Hedman, B., Hollenbeck, M., Holton, J. M., Keable, S., Kobilka, B. K., Kovaleva, E. G., Kruse, A. C., Lemke, H. T., Lin, G., Lyubimov, A. Y., Manglik, A., Mathews, I. I., McPhillips, S. E., Nelson, S., Peters, J. W., Sauter, N. K., Smith, C. A., Song, J., Stevenson, H. P., Tsai, Y., Uervirojnangkoorn, M., Vinetsky, V., Wakatsuki, S., Weis, W. I., Zadvornyy, O. A., Zeldin, O. B., Zhu, D., Hodgson, K. O. 2014; 111 (48): 17122-17127

  Abstract

  The emerging method of femtosecond crystallography (FX) may extend the diffraction resolution accessible from small radiation-sensitive crystals and provides a means to determine catalytically accurate structures of acutely radiation-sensitive metalloenzymes. Automated goniometer-based instrumentation developed for use at the Linac Coherent Light Source enabled efficient and flexible FX experiments to be performed on a variety of sample types. In the case of rod-shaped Cpl hydrogenase crystals, only five crystals and about 30 min of beam time were used to obtain the 125 still diffraction patterns used to produce a 1.6-Å resolution electron density map. For smaller crystals, high-density grids were used to increase sample throughput; 930 myoglobin crystals mounted at random orientation inside 32 grids were exposed, demonstrating the utility of this approach. Screening results from cryocooled crystals of β2-adrenoreceptor and an RNA polymerase II complex indicate the potential to extend the diffraction resolution obtainable from very radiation-sensitive samples beyond that possible with undulator-based synchrotron sources.

  View details for DOI 10.1073/pnas.1418733111

  View details for Web of Science ID 000345920800042

  View details for PubMedID 25362050

  View details for PubMedCentralID PMC4260607

• 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

• 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

• Biosynthesis of Nitrogenase Metalloclusters CHEMICAL REVIEWS Ribbe, M. W., Hu, Y., Hodgson, K. O., Hedman, B. 2014; 114 (8): 4063-4080

  View details for DOI 10.1021/cr400463x

  View details for Web of Science ID 000335086300003

  View details for PubMedID 24328215

  View details for PubMedCentralID PMC3999185

• XAS spectroscopy, sulfur, and the brew within blood cells from Ascidia ceratodes. Journal of inorganic biochemistry Frank, P., Hedman, B., Hodgson, K. O. 2014; 131: 99-108

  Abstract

  We report the first use of K-edge X-ray absorption spectroscopy (XAS) as a direct spectroscopic probe of pH and cytosolic emf within living cells. A new accuracy metric of model-based fits to K-edge spectra is further developed. Sulfur functional groups in three collections of living blood cells and one sample of cleared blood plasma from the tunicate Ascidia ceratodes were speciated using K-edge XAS. Cysteine and cystine, the preferred thiol-disulfide model, averaged about 12% of total sulfur. Sulfate monoesters and cyclic diesters unexpectedly constituted 36% of blood cell sulfur. Soluble sulfate averaged about 25% across the three blood cell samples, while the ratio of SO4(2-) to HSO4(-) implied average signet ring vacuolar pH values of 0.85, 1.4, or 3.1. Intracellular (VSO4)(+) was unobserved, while [V(RSO3)n]((3-n)+) was detected in the two lowest pH blood cell samples. About 5% of sulfur was distributed as mono- or dibenzothiophene or ethylene-epi-sulfide, or as a thiadiazole reminiscent of the polycarpathiamines. Blood plasma was dominated by sulfate (83%), but with 15% of an alkylsulfate ester and about 2% of low-valent sulfur. Gravimetric analysis of soluble sulfate yielded average concentrations of blood cell sulfur. Average [cysteine] and [cystine] (ranging ~10-30mM and ~20-90mM, respectively) implied blood-cell cytosolic emf values of approximately -0.20V. High cellular [cysteine] is consistent with the proposed model for enzymatic reduction of vanadate by endogenous thiol, wherein the trajectory of metal site-symmetry is controlled and directed through to a thermodynamically favored 7-coordinate V(III) product.

  View details for DOI 10.1016/j.jinorgbio.2013.11.004

  View details for PubMedID 24333825

• 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

• 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

• X-ray absorption spectroscopic investigation of the electronic structure differences in solution and crystalline oxyhemoglobin PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Wilson, S. A., Green, E., Mathews, I. I., Benfatto, M., Hodgson, K. O., Hedman, B., Sarangi, R. 2013; 110 (41): 16333-16338

  Abstract

  Hemoglobin (Hb) is the heme-containing O2 transport protein essential for life in all vertebrates. The resting high-spin (S = 2) ferrous form, deoxy-Hb, combines with triplet O2, forming diamagnetic (S = 0) oxy-Hb. Understanding this electronic structure is the key first step in understanding transition metal-O2 interaction. However, despite intense spectroscopic and theoretical studies, the electronic structure description of oxy-Hb remains elusive, with at least three different descriptions proposed by Pauling, Weiss, and McClure-Goddard, based on theory, spectroscopy, and crystallography. Here, a combination of X-ray absorption spectroscopy and extended X-ray absorption fine structure, supported by density functional theory calculations, help resolve this debate. X-ray absorption spectroscopy data on solution and crystalline oxy-Hb indicate both geometric and electronic structure differences suggesting that two of the previous descriptions are correct for the Fe-O2 center in oxy-Hb. These results support the multiconfigurational nature of the ground state developed by theoretical results. Additionally, it is shown here that small differences in hydrogen bonding and solvation effects can tune the ground state, tipping it into one of the two probable configurations. These data underscore the importance of solution spectroscopy and show that the electronic structure in the crystalline form may not always reflect the true ground-state description in solution.

  View details for DOI 10.1073/pnas.1315734110

  View details for Web of Science ID 000325395600026

  View details for PubMedID 24062465

  View details for PubMedCentralID PMC3799373

• 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

• 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

• AutoDrug: fully automated macromolecular crystallography workflows for fragment-based drug discovery. Acta crystallographica. Section D, Biological crystallography Tsai, Y., McPhillips, S. E., González, A., McPhillips, T. M., Zinn, D., Cohen, A. E., Feese, M. D., Bushnell, D., Tiefenbrunn, T., Stout, C. D., Ludaescher, B., Hedman, B., Hodgson, K. O., Soltis, S. M. 2013; 69: 796-803

  Abstract

  AutoDrug is software based upon the scientific workflow paradigm that integrates the Stanford Synchrotron Radiation Lightsource macromolecular crystallography beamlines and third-party processing software to automate the crystallography steps of the fragment-based drug-discovery process. AutoDrug screens a cassette of fragment-soaked crystals, selects crystals for data collection based on screening results and user-specified criteria and determines optimal data-collection strategies. It then collects and processes diffraction data, performs molecular replacement using provided models and detects electron density that is likely to arise from bound fragments. All processes are fully automated, i.e. are performed without user interaction or supervision. Samples can be screened in groups corresponding to particular proteins, crystal forms and/or soaking conditions. A single AutoDrug run is only limited by the capacity of the sample-storage dewar at the beamline: currently 288 samples. AutoDrug was developed in conjunction with RestFlow, a new scientific workflow-automation framework. RestFlow simplifies the design of AutoDrug by managing the flow of data and the organization of results and by orchestrating the execution of computational pipeline steps. It also simplifies the execution and interaction of third-party programs and the beamline-control system. Modeling AutoDrug as a scientific workflow enables multiple variants that meet the requirements of different user groups to be developed and supported. A workflow tailored to mimic the crystallography stages comprising the drug-discovery pipeline of CoCrystal Discovery Inc. has been deployed and successfully demonstrated. This workflow was run once on the same 96 samples that the group had examined manually and the workflow cycled successfully through all of the samples, collected data from the same samples that were selected manually and located the same peaks of unmodeled density in the resulting difference Fourier maps.

  View details for DOI 10.1107/S0907444913001984

  View details for PubMedID 23633588

  View details for PubMedCentralID PMC3640469

• 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

• Geometric and electronic structures of the His-Fe(IV)=O and His-Fe(IV)-Tyr hemes of MauG JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY Jensen, L. M., Meharenna, Y. T., Davidson, V. L., Poulos, T. L., Hedman, B., Wilmot, C. M., Sarangi, R. 2012; 17 (8): 1241-1255

  Abstract

  Biosynthesis of the tryptophan tryptophylquinone (TTQ) cofactor activates the enzyme methylamine dehydrogenase. The diheme enzyme MauG catalyzes O-atom insertion and cross-linking of two Trp residues to complete TTQ synthesis. Solution optical and Mössbauer spectroscopic studies have indicated that the reactive form of MauG during turnover is an unusual bisFe(IV) intermediate, which has been formulated as a His-ligated ferryl heme [Fe(IV)=O] (heme A), and an Fe(IV) heme with an atypical His/Tyr ligation (heme B). In this study, Fe K-edge X-ray absorption spectroscopy and extended X-ray absorption fine structure studies have been combined with density functional theory (DFT) and time-dependent DFT methods to solve the geometric and electronic structures of each heme site in the MauG bisFe(IV) redox state. The ferryl heme site (heme A) is compared with the well-characterized compound I intermediate of cytochrome c peroxidase. Heme B is unprecedented in biology, and is shown to have a six-coordinate, S = 1 environment, with a short (1.85-Å) Fe-O(Tyr) bond. Experimentally calibrated DFT calculations are used to reveal a strong covalent interaction between the Fe and the O(Tyr) ligand of heme B in the high-valence form. A large change in the Fe-O(Tyr) bond distance on going from Fe(II) (2.02 Å) to Fe(III) (1.89 Å) to Fe(IV) (1.85 Å) signifies increasing localization of spin density on the tyrosinate ligand upon sequential oxidation of heme B to Fe(IV). As such, O(Tyr) plays an active role in attaining and stabilizing the MauG bisFe(IV) redox state.

  View details for DOI 10.1007/s00775-012-0939-3

  View details for Web of Science ID 000311669200010

  View details for PubMedID 23053529

  View details for PubMedCentralID PMC3508316

• The x-ray absorption spectroscopy model of solvation about sulfur in aqueous L-cysteine JOURNAL OF CHEMICAL PHYSICS Sarangi, R., Frank, P., Benfatto, M., Morante, S., Minicozzi, V., Hedman, B., Hodgson, K. O. 2012; 137 (20)

  Abstract

  The environment of sulfur in dissolved aqueous L-cysteine has been examined using K-edge x-ray absorption spectroscopy (XAS), extended continuum multiple scattering (ECMS) theory, and density functional theory (DFT). For the first time, bound-state and continuum transitions representing the entire XAS spectrum of L-cysteine sulfur are accurately reproduced by theory. Sulfur K-edge absorption features at 2473.3 eV and 2474.2 eV represent transitions to LUMOs that are mixtures of S-C and S-H σ∗ orbitals significantly delocalized over the entire L-cysteine molecule. Continuum features at 2479, 2489, and 2530 eV were successfully reproduced using extended continuum theory. The full L-cysteine sulfur K-edge XAS spectrum could not be reproduced without addition of a water-sulfur hydrogen bond. Density functional theory analysis shows that although the Cys(H)S⋯H-OH hydrogen bond is weak (∼2 kcal) the atomic charge on sulfur is significantly affected by this water. MXAN analysis of hydrogen-bonding structures for L-cysteine and water yielded a best fit model featuring a tandem of two water molecules, 2.9 Å and 5.8 Å from sulfur. The model included a S(cys)⋯H-O(w1)H hydrogen-bond of 2.19 Å and of 2.16 Å for H(2)O(w1)⋯H-O(w2)H. One hydrogen-bonding water-sulfur interaction alone was insufficient to fully describe the continuum XAS spectrum. However, density functional theoretical results are convincing that the water-sulfur interaction is weak and should be only transient in water solution. The durable water-sulfur hydrogen bond in aqueous L-cysteine reported here therefore represents a break with theoretical studies indicating its absence. Reconciling the apparent disparity between theory and result remains the continuing challenge.

  View details for DOI 10.1063/1.4767350

  View details for Web of Science ID 000312252100067

  View details for PubMedID 23206038

  View details for PubMedCentralID PMC3526153

• X-ray-induced photo-chemistry and X-ray absorption spectroscopy of biological samples JOURNAL OF SYNCHROTRON RADIATION George, G. N., Pickering, I. J., Pushie, M. J., Nienaber, K., Hackett, M. J., Ascone, I., Hedman, B., Hodgson, K. O., Aitken, J. B., Levina, A., Glover, C., Lay, P. A. 2012; 19: 875-886

  Abstract

  As synchrotron light sources and optics deliver greater photon flux on samples, X-ray-induced photo-chemistry is increasingly encountered in X-ray absorption spectroscopy (XAS) experiments. The resulting problems are particularly pronounced for biological XAS experiments. This is because biological samples are very often quite dilute and therefore require signal averaging to achieve adequate signal-to-noise ratios, with correspondingly greater exposures to the X-ray beam. This paper reviews the origins of photo-reduction and photo-oxidation, the impact that they can have on active site structure, and the methods that can be used to provide relief from X-ray-induced photo-chemical artifacts.

  View details for DOI 10.1107/S090904951203943X

  View details for Web of Science ID 000310151000005

  View details for PubMedID 23093745

  View details for PubMedCentralID PMC3480274

• 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

• (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

• Revisiting the Polyoxometalate-Based Late-Transition-Metal-Oxo Complexes: The "Oxo Wall" Stands INORGANIC CHEMISTRY O'Halloran, K. P., Zhao, C., Ando, N. S., Schultz, A. J., Koetzle, T. F., Piccoli, P. M., Hedman, B., Hodgson, K. O., Bobyr, E., Kirk, M. L., Knottenbelt, S., Depperman, E. C., Stein, B., Anderson, T. M., Cao, R., Geletii, Y. V., Hardcastle, K. I., Musaev, D. G., Neiwert, W. A., Fang, X., Morokuma, K., Wu, S., Koegerler, P., Hill, C. L. 2012; 51 (13): 7025-7031

  Abstract

  Terminal oxo complexes of the late transition metals Pt, Pd, and Au have been reported by us in Science and Journal of the American Chemical Society. Despite thoroughness in characterizing these complexes (multiple independent structural methods and up to 17 analytical methods in one case), we have continued to study these structures. Initial work on these systems was motivated by structural data from X-ray crystallography and neutron diffraction and (17)O and (31)P NMR signatures which all indicated differences from all previously published compounds. With significant new data, we now revisit these studies. New X-ray crystal structures of previously reported complexes K(14)[P(2)W(19)O(69)(OH(2))] and "K(10)Na(3)[Pd(IV)(O)(OH)WO(OH(2))(PW(9)O(34))(2)]" and a closer examination of these structures are provided. Also presented are the (17)O NMR spectrum of an (17)O-enriched sample of [PW(11)O(39)](7-) and a careful combined (31)P NMR-titration study of the previously reported "K(7)H(2)[Au(O)(OH(2))P(2)W(20)O(70)(OH(2))(2)]." These and considerable other data collectively indicate that previously assigned terminal Pt-oxo and Au-oxo complexes are in fact cocrystals of the all-tungsten structural analogues with noble metal cations, while the Pd-oxo complex is a disordered Pd(II)-substituted polyoxometalate. The neutron diffraction data have been re-analyzed, and new refinements are fully consistent with the all-tungsten formulations of the Pt-oxo and Au-oxo polyoxometalate species.

  View details for DOI 10.1021/ic2008914

  View details for Web of Science ID 000305853600009

  View details for PubMedID 22694272

• 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

• 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

• 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

• The X-ray Absorption Spectroscopic Model of the Copper(II) Imidazole Complex Ion in Liquid Aqueous Solution: A Strongly Solvated Square Pyramid INORGANIC CHEMISTRY Frank, P., Benfatto, M., Hedman, B., Hodgson, K. O. 2012; 51 (4): 2086-2096

  Abstract

  Cu K-edge extended X-ray absorption fine structure (EXAFS) and Minuit X-ray absorption near-edge structure (MXAN) analyses were combined to evaluate the structure of the copper(II) imidazole complex ion in liquid aqueous solution. Both methods converged to the same square-pyramidal inner coordination sphere [Cu(Im)(4)L(ax)](2+) (L(ax) indeterminate) with four equatorial nitrogen atoms at EXAFS, 2.02 ± 0.01 Å, and MXAN, 1.99 ± 0.03 Å. A short-axial N/O scatterer (L(ax)) was found at 2.12 ± 0.02 Å (EXAFS) or 2.14 ± 0.06 Å (MXAN). A second but very weak axial Cu-N/O interaction was found at 2.9 ± 0.1 Å (EXAFS) or 3.0 ± 0.1 Å (MXAN). In the MXAN fits, only a square-pyramidal structural model successfully reproduced the doubled maximum of the rising K-edge X-ray absorption spectrum, specifically excluding an octahedral model. Both EXAFS and MXAN also found eight outlying oxygen scatterers at 4.2 ± 0.3 Å that contributed significant intensity over the entire spectral energy range. Two prominent rising K-edge shoulders at 8987.1 and 8990.5 eV were found to reflect multiple scattering from the 3.0 Å axial scatterer and the imidazole rings, respectively. In the MXAN fits, the imidazole rings took in-plane rotationally staggered positions about copper. The combined (EXAFS and MXAN) model for the unconstrained cupric imidazole complex ion in liquid aqueous solution is an axially elongated square-pyramidal core, with a weak nonbonded interaction at the second axial coordination position and a solvation shell of eight nearest-neighbor water molecules. This core square-pyramidal motif has persisted through [Cu(H(2)O)(5)](2+), [Cu(NH(3))(4)(NH(3),H(2)O)](2+), (1, 2) and now [Cu(Im)(4)L(ax))](2+) and appears to be the geometry preferred by unconstrained aqueous-phase copper(II) complex ions.

  View details for DOI 10.1021/ic2017819

  View details for Web of Science ID 000300466300017

  View details for PubMedID 22316238

  View details for PubMedCentralID PMC3328689

• High-Resolution Analysis of Zn2+ Coordination in the Alkaline Phosphatase Superfamily by EXAFS and X-ray Crystallography JOURNAL OF MOLECULAR BIOLOGY Bobyr, E., Lassila, J. K., Wiersma-Koch, H. I., Fenn, T. D., Lee, J. J., Nikolic-Hughes, I., Hodgson, K. O., Rees, D. C., Hedman, B., Herschlag, D. 2012; 415 (1): 102-117

  Abstract

  Comparisons among evolutionarily related enzymes offer opportunities to reveal how structural differences produce different catalytic activities. Two structurally related enzymes, Escherichia coli alkaline phosphatase (AP) and Xanthomonas axonopodis nucleotide pyrophosphatase/phosphodiesterase (NPP), have nearly identical binuclear Zn(2+) catalytic centers but show tremendous differential specificity for hydrolysis of phosphate monoesters or phosphate diesters. To determine if there are differences in Zn(2+) coordination in the two enzymes that might contribute to catalytic specificity, we analyzed both x-ray absorption spectroscopic and x-ray crystallographic data. We report a 1.29-Å crystal structure of AP with bound phosphate, allowing evaluation of interactions at the AP metal site with high resolution. To make systematic comparisons between AP and NPP, we measured zinc extended x-ray absorption fine structure for AP and NPP in the free-enzyme forms, with AMP and inorganic phosphate ground-state analogs and with vanadate transition-state analogs. These studies yielded average zinc-ligand distances in AP and NPP free-enzyme forms and ground-state analog forms that were identical within error, suggesting little difference in metal ion coordination among these forms. Upon binding of vanadate to both enzymes, small increases in average metal-ligand distances were observed, consistent with an increased coordination number. Slightly longer increases were observed in NPP relative to AP, which could arise from subtle rearrangements of the active site or differences in the geometry of the bound vanadyl species. Overall, the results suggest that the binuclear Zn(2+) catalytic site remains very similar between AP and NPP during the course of a reaction cycle.

  View details for DOI 10.1016/j.jmb.2011.10.040

  View details for PubMedID 22056344

• 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

• 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

• 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

• Structural Models of the [Fe4S4] Clusters of Homologous Nitrogenase Fe Proteins INORGANIC CHEMISTRY Blank, M. A., Lee, C. C., Hu, Y., Hodgson, K. O., Hedman, B., Ribbe, M. W. 2011; 50 (15): 7123-7128

  Abstract

  The iron (Fe) proteins of molybdenum (Mo)-, vanadium (V)-, and iron (Fe)-only nitrogenases are encoded by nifH, vnfH, and anfH, respectively. While the nifH-encoded Fe protein has been extensively studied over recent years, information regarding the properties of the vnfH- and anfH-encoded Fe proteins has remained scarce. Here, we present a combined biochemical, electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) analysis of the [Fe(4)S(4)] clusters of NifH, VnfH, and AnfH of Azotobacter vinelandii . Our data show that all three Fe proteins contain [Fe(4)S(4)] clusters of very similar spectroscopic and geometric structural properties, although NifH differs more from VnfH and AnfH with regard to the electronic structure. These observations have an interesting impact on the theory of the plausible sequence of evolution of nitrogenase Fe proteins. More importantly, the results presented herein provide a platform for future investigations of the differential activities of the three Fe proteins in nitrogenase biosynthesis and catalysis.

  View details for DOI 10.1021/ic200636k

  View details for Web of Science ID 000293036000036

  View details for PubMedID 21718019

  View details for PubMedCentralID PMC3143205

• Structural Analysis of a Ni-Methyl Species in Methyl-Coenzyme M Reductase from Methanothermobacter marburgensis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Cedervall, P. E., Dey, M., Li, X., Sarangi, R., Hedman, B., Ragsdale, S. W., Wilmot, C. M. 2011; 133 (15): 5626-5628

  Abstract

  We present the 1.2 Å resolution X-ray crystal structure of a Ni-methyl species that is a proposed catalytic intermediate in methyl-coenzyme M reductase (MCR), the enzyme that catalyzes the biological formation of methane. The methyl group is situated 2.1 Å proximal of the Ni atom of the MCR coenzyme F(430). A rearrangement of the substrate channel has been posited to bring together substrate species, but Ni(III)-methyl formation alone does not lead to any observable structural changes in the channel.

  View details for DOI 10.1021/ja110492p

  View details for Web of Science ID 000290358200002

  View details for PubMedID 21438550

  View details for PubMedCentralID PMC3086036

• 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

• 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

• Spectroscopic Characterization of the Isolated Iron-Molybdenum Cofactor (FeMoco) Precursor from the Protein NifEN ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Fay, A. W., Blank, M. A., Lee, C. C., Hu, Y., Hodgson, K. O., Hedman, B., Ribbe, M. W. 2011; 50 (34): 7787-7790

  View details for DOI 10.1002/anie.201102724

  View details for Web of Science ID 000294175700008

  View details for PubMedID 21726031

  View details for PubMedCentralID PMC3395727

• 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

• 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

• 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

• Characterization of Isolated Nitrogenase FeVco JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Fay, A. W., Blank, M. A., Lee, C. C., Hu, Y., Hodgson, K. O., Hedman, B., Ribbe, M. W. 2010; 132 (36): 12612-12618

  Abstract

  The cofactors of the Mo- and V-nitrogenases (i.e., FeMoco and FeVco) are homologous metal centers with distinct catalytic properties. So far, there has been only one report on the isolation of FeVco from Azotobacter chroococcum. However, this isolated FeVco species did not carry the full substrate-reducing capacity, as it is unable to restore the N(2)-reducing ability of the cofactor-deficient MoFe protein. Here, we report the isolation and characterization of a fully active species of FeVco from A. vinelandii. Our metal and activity analyses show that FeVco has been extracted intact, carrying with it the characteristic capacity to reduce C(2)H(2) to C(2)H(6) and, perhaps even more importantly, the ability to reduce N(2) to NH(3). Moreover, our EPR and XAS/EXAFS investigations indicate that FeVco is similar to, yet distinct from FeMoco in electronic properties and structural topology, which could account for the differences in the reactivity of the two cofactors. The outcome of this study not only permits the proposal of the first EXAFS-based structural model of the isolated FeVco but also lays a foundation for future catalytic and structural investigations of this unique metallocluster.

  View details for DOI 10.1021/ja1019657

  View details for Web of Science ID 000282074200023

  View details for PubMedID 20718463

  View details for PubMedCentralID PMC2940275

• 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

• 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

• 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

• Formation of a homocitrate-free iron-molybdenum cluster on NifEN: Implications for the role of homocitrate in nitrogenase assembly DALTON TRANSACTIONS Fay, A. W., Blank, M. A., Yoshizawa, J. M., Lee, C. C., Wiig, J. A., Hu, Y., Hodgson, K. O., Hedman, B., Ribbe, M. W. 2010; 39 (12): 3124-3130

  Abstract

  Molybdenum (Mo)-dependent nitrogenase is a complex metalloprotein that catalyzes the biological reduction of dinitrogen (N(2)) to ammonia (NH(3)) at the molybdenum-iron cofactor (FeMoco) site of its molybdenum-iron (MoFe) protein component. Here we report the formation of a homocitrate-free, iron-molybdenum ("FeMo") cluster on the biosynthetic scaffold of FeMoco, NifEN. Such a NifEN-associated "FeMo" cluster exhibits EPR features similar to those of the NifEN-associated, fully-complemented "FeMoco", which originate from the presence of Mo in both cluster species; however, "FeMo" cluster and "FeMoco" display different temperature-dependent changes in the line shape and the signal intensity of their respective EPR features, which reflect the impact of homocitrate on the redox properties of these clusters. XAS/EXAFS analysis reveals that the Mo centers in both "FeMo" cluster and "FeMoco" are present in a similar coordination environment, although Mo in "FeMo" cluster is more loosely coordinated as compared to that in "FeMoco" with respect to the Mo-O distances in the cluster, likely due to the absence of homocitrate that normally serves as an additional ligand for the Mo in the cluster. Subsequent biochemical investigation of the "FeMo" cluster not only facilitates the determination of the sequence of events in the mobilization of Mo and homocitrate during FeMoco maturation, but also permits the examination of the role of homocitrate in the transfer of FeMoco between NifEN and MoFe protein. Combined outcome of these studies establishes a platform for future structural analysis of the interactions between NifEN and MoFe protein, which will provide useful insights into the mechanism of cluster transfer between the two proteins.

  View details for DOI 10.1039/c000264j

  View details for Web of Science ID 000275378500022

  View details for PubMedID 20221547

  View details for PubMedCentralID PMC3027845

• Stepwise formation of P-cluster in nitrogenase MoFe protein PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Lee, C. C., Blank, M. A., Fay, A. W., Yoshizawa, J. M., Hu, Y., Hodgson, K. O., Hedman, B., Ribbe, M. W. 2009; 106 (44): 18474-18478

  Abstract

  The P-cluster of nitrogenase is one of the most complex biological metallocenters known to date. Despite the recent advances in the chemical synthesis of P-cluster topologs, the biosynthetic mechanism of P-cluster has not been well defined. Here, we present a combined biochemical, electron paramagnetic resonance, and X-ray absorption spectroscopy/extended X-ray absorption fine-structure investigation of the maturation process of P-clusters in DeltanifH molybdenum-iron (MoFe) protein. Our data indicate that the previously identified, [Fe(4)S(4)]-like cluster pairs in DeltanifH MoFe protein are indeed the precursors to P-clusters, which can be reductively coupled into the mature [Fe(8)S(7)] structures in the presence of Fe protein, MgATP, and dithionite. Moreover, our observation of a biphasic maturation pattern of P-clusters in DeltanifH MoFe protein provides dynamic proof for the previously hypothesized, stepwise assembly mechanism of the two P-clusters in the alpha(2)beta(2)-tetrameric MoFe protein, i.e., one P-cluster is formed in one alphabeta dimer before the other in the second alphabeta dimer.

  View details for DOI 10.1073/pnas.0909149106

  View details for Web of Science ID 000271429800014

  View details for PubMedID 19828444

  View details for PubMedCentralID PMC2774011

• Optimization of FeMoco Maturation on NifEN JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yoshizawa, J. M., Blank, M. A., Fay, A. W., Lee, C. C., Wiig, J. A., Hu, Y., Hodgson, K. O., Hedman, B., Ribbe, M. W. 2009; 131 (26): 9321-9325

  Abstract

  Mo-nitrogenase catalyzes the reduction of dinitrogen to ammonia at the cofactor (i.e., FeMoco) site of its MoFe protein component. Biosynthesis of FeMoco involves NifEN, a scaffold protein that hosts the maturation of a precursor to a mature FeMoco before it is delivered to the target location in the MoFe protein. Previously, we have shown that the NifEN-bound precursor could be converted in vitro to a fully complemented "FeMoco" in the presence of 2 mM dithionite. However, such a conversion was incomplete, and Mo was only loosely associated with the NifEN-bound "FeMoco". Here we report the optimized maturation of the NifEN-associated precursor in 20 mM dithionite. Activity analyses show that upon the optimal conversion of precursor to "FeMoco", NifEN is capable of activating a FeMoco-deficient form of MoFe protein to the same extent as the isolated FeMoco. Furthermore, EPR and XAS/EXAFS analyses reveal the presence of a tightly organized Mo site in NifEN-bound "FeMoco", which allows the observation of a FeMoco-like S = 3/2 EPR signal and the modeling of a NifEN-bound "FeMoco" that adopts a conformation very similar to that of the MoFe protein-associated FeMoco. The sensitivity of FeMoco maturation to dithionite concentration suggests an essential role of redox chemistry in this process, and the optimal potential of dithionite solution could serve as a guideline for future identification of in vivo electron donors for FeMoco maturation.

  View details for DOI 10.1021/ja9035225

  View details for Web of Science ID 000267633300046

  View details for PubMedID 19514721

  View details for PubMedCentralID PMC2716083

• 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

• 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

• 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

• 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

• The XAS Model of Dissolved Cu(II) and Its Significance to Biological Electron Transfer 14th International Conference on X-Ray Absorption Fine Structure (XAFS14) Frank, P., Benfatto, M., Hedman, B., Hodgson, K. O. IOP PUBLISHING LTD. 2009

  View details for DOI 10.1088/1742-6596/190/1/012059

  View details for Web of Science ID 000275152100060

• 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

• 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

• 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

• Solution [Cu(amm)](2+) is a strongly solvated square pyramid: A full account of the copper K-edge XAS spectrum within single-electron theory INORGANIC CHEMISTRY Frank, P., Benfatto, M., Hedman, B., Hodgson, K. O. 2008; 47 (10): 4126-4139

  Abstract

  The solution structure of Cu(II) in 4 M aqueous ammonia, [Cu(amm)](2+), was assessed using copper K-edge extended X-ray absorption fine structure (EXAFS) and Minuit XANes (MXAN) analyses. Tested structures included trigonal planar, planar and D2d -tetragonal, regular and distorted square pyramids, trigonal bipyramids, and Jahn-Teller distorted octahedra. Each approach converged to the same axially elongated square pyramid, 4 x Cu-Neq=2.00+/-0.02 A and 1 x Cu-Nax=2.16+/-0.02 A (EXAFS) or 2.20+/-0.07 A (MXAN), with strongly localized solvation shells. In the MXAN model, four equatorial ammonias averaged 13 degrees below the Cu(II) xy-plane, which was 0.45+/-0.1 A above the mean N4 plane. When the axial ligand equilibrium partial occupancies of about 0.65 ammonia and 0.35 water were included, EXAFS modeling found Cu-Lax distances of 2.16 and 2.31 A, respectively, reproducing the distances found in the crystal structures of [Cu(NH3)5](2+) and [Cu(NH3)4(H2O)](2+). A transverse axially localized solvent molecule was found at 2.8 A (EXAFS) or 3.1 A (MXAN). Six second-shell solvent molecules were also found at about 3.4+/-0.01 (EXAFS) or 3.8+/-0.2 A (MXAN). The structure of Cu(II) in 4 M pH 10 aqueous NH 3 may be notationally described as {[Cu(NH 3)4.62(H2O)0.38](solv)}(2+).6solv, solv=H2O, NH 3. The prominent shoulder and duplexed maximum of the rising K-edge XAS of [Cu(amm)](2+) primarily reflect the durable and well-organized solvation shells, not found around [Cu(H2O)5](2+), rather than two-electron shakedown transitions. Not accounting for solvent scattering thus may confound XAS-based estimates of metal-ligand covalency. [Cu(amm)](2+) continues the dissymmetry previously found for the solution structure of [Cu(H2O)5](2+), again contradicting the rack-bonding theory of blue copper proteins.

  View details for DOI 10.1021/ic7021243

  View details for PubMedID 18426203

• 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

• Iron complexes of dendrimer-appended carboxylates for activating dioxygen and oxidizing hydrocarbons JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Zhao, M., Helms, B., Slonkina, E., Friedle, S., Lee, D., DuBois, J., Hedman, B., Hodgson, K. O., Frechet, J. M., Lippard, S. J. 2008; 130 (13): 4352-4363

  Abstract

  The active sites of metalloenzymes are often deeply buried inside a hydrophobic protein sheath, which protects them from undesirable hydrolysis and polymerization reactions, allowing them to achieve their normal functions. In order to mimic the hydrophobic environment of the active sites in bacterial monooxygenases, diiron(II) compounds of the general formula [Fe2([G-3]COO)4(4-RPy)2] were prepared, where [G-3]COO- is a third-generation dendrimer-appended terphenyl carboxylate ligand and 4-RPy is a pyridine derivative. The dendrimer environment provides excellent protection for the diiron center, reducing its reactivity toward dioxygen by about 300-fold compared with analogous complexes of terphenyl carboxylate ([G-1]COO-) ligands. An FeIIFeIII intermediate was characterized by electronic, electron paramagnetic resonance, Mössbauer, and X-ray absorption spectroscopic analyses following the oxygenation of [Fe2([G-3]COO)4(4-PPy)2], where 4-PPy is 4-pyrrolidinopyridine. The results are consistent with the formation of a superoxo species. This diiron compound, in the presence of dioxygen, can oxidize external substrates.

  View details for DOI 10.1021/ja076817a

  View details for Web of Science ID 000254549000036

  View details for PubMedID 18331028

  View details for PubMedCentralID PMC2396528

• The uptake and fate of vanadyl ion in ascidian blood cells and a detailed hypothesis for the mechanism and location of biological vanadium reduction. A visible and X-ray absorption spectroscopic study JOURNAL OF INORGANIC BIOCHEMISTRY Frank, P., Carlson, E. J., Carlson, R. M., Hedman, B., Hodgson, K. O. 2008; 102 (4): 809-823

  Abstract

  Vanadium K-edge X-ray absorption spectroscopy (XAS) has been used to track the uptake and fate of VO(2+) ion in blood cells from Ascidia ceratodes, following exposure to dithiothreitol (DTT) or to DTT plus VO(2+). The full range of endogenous vanadium was queried by fitting the XAS of blood cells with the XAS spectra of model vanadium complexes. In cells exposed only to DTT, approximately 0.4% of a new V(III) species was found in a site similar to Na[V(edta)(H(2)O)]. With exposure to DTT and VO(2+), average intracellular [VO(aq)](2+) increased from 3% to 5%, and 6% of a new complexed form of vanadyl ion appeared evidencing a ligand array similar to [VO(edta)](2-). At the same time, the relative ratio of blood cell [V(H(2)O)(6)](3+) increased at the expense of [V(H(2)O)(5)(SO(4))](+) in a manner consistent with a significant increase in endogenous acidity. In new UV/Visible experiments, VO(2+) could be reduced to 7-coordinate [V(nta)(H(2)O)(3)] or [V(nta)(ida)](2-) with cysteine methyl ester in pH 6.5 solution. Ascorbate reduced [VO(edta)](2-) to 7-coordinate [V(edta)(H(2)O)](-), while [VO(trdta)](2-) was unreactive. These results corroborate the finding that the reductive EMF of VO(2+) is increased by the availability of a 7-coordinate V(III) product. Finally, a new and complete hypothesis is proposed for an ascidian vanadate reductase. The structure of the enzyme active site, the vanadate-vanadyl-vanadic reduction mechanism, the cellular locale, and elements of the regulatory machinery governing the biological reduction of vanadate and vanadyl ion by ascidians are all predicted. Together these constitute the new field of vanadium redox enzymology.

  View details for DOI 10.1016/j.jinorgbio.2007.12.001

  View details for Web of Science ID 000255131500024

  View details for PubMedID 18234345

  View details for PubMedCentralID PMC2945689

• 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

• When identical functional groups are not identical: A DFT study of the effects of molecular environment on sulfur K-edge X-ray absorption spectra INORGANICA CHIMICA ACTA Sarangi, R., Frank, P., Hodgson, K. O., Hedman, B. 2008; 361 (4): 956-964

  View details for DOI 10.1016/j.ica.2007.05.047

  View details for Web of Science ID 000254858100017

• 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

• 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

• LATE TRANSITION METAL-OXO COMPOUNDS AND OPEN-FRAMEWORK MATERIALS THAT CATALYZE AEROBIC OXIDATIONS ADVANCES IN INORGANIC CHEMISTRY, VOL 60 Cao, R., Han, J. W., Anderson, T. M., Hillesheim, D. A., Hardcastle, K. I., Slonkina, E., Hedman, B. B., Hodgson, K. O., Kirk, M. L., Musaev, D. G., Morokuma, K., Geletii, Y. V., Hill, C. L. 2008; 60: 245-272

  View details for DOI 10.1016/S0898-8838(08)00006-8

  View details for Web of Science ID 000260085300006

• 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

• 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

• 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

• Photoreduction of the active site of the metalloprotein putidaredoxin by synchrotron radiation ACTA CRYSTALLOGRAPHICA SECTION D-BIOLOGICAL CRYSTALLOGRAPHY Corbett, M. C., Latimer, M. J., Poulos, T. L., Sevrioukova, I. F., Hodgson, K. O., Hedman, B. 2007; 63: 951-960

  Abstract

  X-ray damage to protein crystals is often assessed on the basis of the degradation of diffraction intensity, yet this measure is not sensitive to the rapid changes that occur at photosensitive groups such as the active sites of metalloproteins. Here, X-ray absorption spectroscopy is used to study the X-ray dose-dependent photoreduction of crystals of the [Fe(2)S(2)]-containing metalloprotein putidaredoxin. A dramatic decrease in the rate of photoreduction is observed in crystals cryocooled with liquid helium at 40 K compared with those cooled with liquid nitrogen at 110 K. Whereas structural changes consistent with cluster reduction occur in the active site of the crystal measured at 110 K, no such changes occur in the crystal measured at 40 K, even after an eightfold increase in dose. When the structural results from extended X-ray absorption fine-structure measurements are compared with those obtained by crystallography on this and similar proteins, it is apparent that X-ray-induced photoreduction has had an impact on the crystallographic data and subsequent structure solutions. These results strongly indicate the importance of using liquid-helium-based cooling for metalloprotein crystallography in order to avoid the subtle yet important changes that can take place at the metalloprotein active sites when liquid-nitrogen-based cooling is used. The study also illustrates the need for direct measurement of the redox states of the metals, through X-ray absorption spectroscopy, simultaneously with the crystallographic measurements.

  View details for DOI 10.1107/S0907444907035160

  View details for Web of Science ID 000249167300003

  View details for PubMedID 17704563

• 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

• 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

• Conformational differences between Azotobacter vinelandii nitrogenase MoFe proteins as studied by small-angle X-ray scattering BIOCHEMISTRY Corbett, M. C., Hu, Y., Fay, A. W., Tsuruta, H., Ribbe, M. W., Hodgson, K. O., Hedman, B. 2007; 46 (27): 8066-8074

  Abstract

  The nitrogenase MoFe protein is a heterotetramer containing two unique high-nuclearity metalloclusters, FeMoco and the P-cluster. FeMoco is assembled outside the MoFe protein, whereas the P-cluster is assembled directly on the MoFe protein polypeptides. MoFe proteins isolated from different genetic backgrounds have been analyzed using biochemical and spectroscopic techniques in attempting to elucidate the pathway of P-cluster biosynthesis. The DeltanifH MoFe protein is less stable than other MoFe proteins and has been shown by extended X-ray absorption fine structure studies to contain a variant P-cluster that most likely exists as two separate [Fe4S4]-like clusters instead of the subunit-bridging [Fe8S7] cluster found in the wild-type and DeltanifB forms of the MoFe protein [Corbett, M. C., et al. (2004) J. Biol. Chem. 279, 28276-28282]. Here, a combination of small-angle X-ray scattering and Fe chelation studies is used to show that there is a correlation between the state of the P-cluster and the conformation of the MoFe protein. The DeltanifH MoFe protein is found to be larger than the wild-type or DeltanifB MoFe proteins, an increase in size that can be modeled well by an opening of the subunit interface consistent with P-cluster fragmentation and solvent exposure. Importantly, this opening would allow for the insertion of P-cluster precursors into a region of the MoFe protein that is buried in the wild-type conformation. Thus, DeltanifH MoFe protein could represent an early intermediate in MoFe protein biosynthesis where the P-cluster precursors have been inserted, but P-cluster condensation and tetramer stabilization have yet to occur.

  View details for DOI 10.1021/bi7005064

  View details for Web of Science ID 000247677700014

  View details for PubMedID 17567155

• 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

• 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

• 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

• Toward the Biological Reduction Mechanism of Vanadyl Ion in the Blood Cells of Vanadium-Sequestering Tunicates 5th International Symposium on Chemistry and Biological Chemistry of Vanadium Frank, P., Carlson, E. J., Carlson, R. M., Hedman, B., Hodgson, K. O. AMER CHEMICAL SOC. 2007: 281–295

  View details for Web of Science ID 000269059400021

• PySpline: A modern, cross-platform program for the processing of raw averaged XAS edge and EXAFS data 13th International Conference on X-Ray Absorption Fine Structure (XAFS13) Tenderholt, A., Hedman, B., Hodgson, K. O. AMER INST PHYSICS. 2007: 105–107

  View details for Web of Science ID 000245805900022

• Dioxygen reactivity of a copper(I) complex with a N3S thioether chelate; Peroxo-dicopper(II) formation including sulfur-ligation INORGANIC CHEMISTRY Hatcher, L. Q., Lee, D., Vance, M. A., Milligan, A. E., Sarangi, R., Hodgson, K. O., Hedman, B., Solomon, E. I., Karlin, K. D. 2006; 45 (25): 10055-10057

  Abstract

  Employing a tetradentate N3S(thioether) ligand, LN3S, dioxygen reactivity of a copper(I) complex, [(LN3S)CuI]+ (1) was examined. In CH2Cl2, acetone (at -80 degrees C), or 2-methyltetrahydrofuran (at -128 degrees C), 1 reacts with O2 producing the end-on bound peroxodicopper(II) complex [{(LN3S)CuII}2(mu-1,2-O2(2-))]2+ (2), the first reported copper-dioxygen adduct with sulfur (thioether) ligation. Its absorption spectrum contains an additional low-energy feature (but not a Cu-S CT band) compared to the previously well-characterized N4 ligand complex, [{(TMPA)CuII}2(mu-1,2-O2(2-))]2+ (3) (TMPA = tris(2-pyridylmethyl)amine). Resonance Raman spectroscopy confirms the peroxo formulation {nu(O-O) = 817 cm-1 (16-18O2 Delta = 46 cm-1) and nu(Cu-O) = 545 cm-1 (16-18O2 Delta = 26 cm-1), in close analogy to that known for 3 {nu(O-O) = 827 cm-1 and nu(Cu-O) = 561 cm-1}. Direct evidence for thioether ligation comes from EXAFS spectroscopy {Cu K-edge; Cu-S = 2.4 A}.

  View details for DOI 10.1021/ic061813c

  View details for Web of Science ID 000242479700022

  View details for PubMedID 17140210

• A systematic resolution of sulfur in reticulated vitreous carbon using X-ray absorption spectroscopy INORGANIC CHEMISTRY Frank, P., George, S. D., Anxolabehere-Mallart, E., Hedman, B., Hodgson, K. O. 2006; 45 (24): 9864-9876

  Abstract

  Sulfur K-edge X-ray absorption spectroscopy (XAS) was used to characterize the approximately 0.1% sulfur found both in native reticulated vitreous carbon (RVC) foam and in RVC oxidatively modified using 0.2 M KMnO4 in 2 M H2SO4. Sulfur valences and functional groups were assessed using K-edge XAS spectral curve-fitting and employing explicit sulfur compounds as models. For native RVC, these were episulfide (approximately 3%), thianthrene (approximately 9%), disulfide (approximately 10%), sulfenate ester (approximately 12%), benzothiophene (approximately 24%), N,N'-thiobisphthalimide (approximately 30%), alkyl sulfonate (approximately 1.2%), alkyl sulfate monoester (approximately 6%), and sulfate dianion (approximately 6%). Permanganate oxidation of RVC diminished sulfenic sulfur to approximately 9%, thianthrenic sulfur to approximately 7%, and sulfate dianion to approximately 1% but increased sulfate monoester to approximately 12%, and newly produced sulfone (approximately 2%) and sulfate diester (approximately 5%). A simple thermodynamic model was derived that allows proportionate functional group comparisons despite differing (approximately +/-15%) total sulfur contents between RVC batches. The limits of accuracy in the XAS curve-fitting analysis are discussed in terms of microenvironments and extended structures in RVC carbon that cannot be exactly modeled by small molecules. Sulfate esters cover approximately 0.15% of the RVC surface, increasing to approximately 0.51% following permanganate/sulfuric acid treatment. The detection of episulfide directly corroborates a proposed mechanism for the migration of elemental sulfur through carbon.

  View details for DOI 10.1021/ic0610637

  View details for PubMedID 17112284

• Nitrogenase Fe protein: A molybdate/homocitrate insertase PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Hu, Y., Corbettt, M. C., Fay, A. W., Webber, J. A., Hodgson, K. O., Hedman, B., Ribbe, M. W. 2006; 103 (46): 17125-17130

  Abstract

  The Fe protein is indispensable for nitrogenase catalysis and biosynthesis. However, its function in iron-molybdenum cofactor (FeMoco) biosynthesis has not been clearly defined. Here we show that the Fe protein can act as a Mo/homocitrate insertase that mobilizes Mo/homocitrate for the maturation of FeMoco precursor on NifEN. Further, we establish that Mo/homocitrate mobilization by the Fe protein likely involves hydrolysis of MgATP and protein-protein interaction between the Fe protein and NifEN. Our findings not only clarify the role of the Fe protein in FeMoco assembly and assign another function to this multitask enzyme but also provide useful insights into a mechanism of metal trafficking required for the assembly of complex metalloproteins such as nitrogenase.

  View details for DOI 10.1073/pnas.0602651103

  View details for Web of Science ID 000242249400013

  View details for PubMedID 17062756

  View details for PubMedCentralID PMC1859896

• FeMo cofactor maturation on NifEN PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Hu, Y., Corbett, M. C., Fay, A. W., Webber, J. A., Hodgson, K. O., Hedman, B., Ribbe, M. W. 2006; 103 (46): 17119-17124

  Abstract

  FeMo cofactor (FeMoco) biosynthesis is one of the most complicated processes in metalloprotein biochemistry. Here we show that Mo and homocitrate are incorporated into the Fe/S core of the FeMoco precursor while it is bound to NifEN and that the resulting fully complemented, FeMoco-like cluster is transformed into a mature FeMoco upon transfer from NifEN to MoFe protein through direct protein-protein interaction. Our findings not only clarify the process of FeMoco maturation, but also provide useful insights into the other facets of nitrogenase chemistry.

  View details for DOI 10.1073/pnas.0602647103

  View details for Web of Science ID 000242249400012

  View details for PubMedID 17050696

  View details for PubMedCentralID PMC1859895

• 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

• 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

• Reversible O-O bond cleavage in copper-dioxygen isomers: Impact of anion basicity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ottenwaelder, X., Rudd, D. J., Corbett, M. C., Hodgson, K. O., Hedman, B., Stack, T. D. 2006; 128 (29): 9268-9269

  Abstract

  Low-temperature oxygenation of copper(I) complexes of N,N,N',N'-tetraethylpropane-1,3-diamine yields solutions containing both mu-eta2:eta2-peroxodicopper(II) (P) and bis(mu-oxo)dicopper(III) (O) valence isomers. The P/O equilibrium position depends on the nature of the counteranion; P is favored with more basic anions. Titration and EXAFS experiments as well as DFT calculations suggest that axial donation from a sulfonate anion to the copper centers imparts an electronic/electrostatic bias toward the P isomer.

  View details for DOI 10.1021/ja061132g

  View details for Web of Science ID 000239120700005

  View details for PubMedID 16848427

  View details for PubMedCentralID PMC2526018

• 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

• A new structural motif for biological iron: Iron K-edge XAS reveals a [Fe-4-mu-(OR)(5)(OR)(9-10)] cluster in the ascidian Perophora annectens INORGANIC CHEMISTRY Frank, P., DeTomaso, A., Hedman, B., Hodgson, K. O. 2006; 45 (10): 3920-3931

  Abstract

  The Phlebobranch ascidian Perophora annectens surprisingly exhibited a biological Fe/V ratio of approximately 15:1 on multichannel X-ray fluorescence analysis of two independent collections of organisms. Iron K-edge X-ray absorption spectroscopy (XAS) indicated a single form of iron. The XAS K-edge of the first collection of blood cells was shifted approximately +1 eV relative to that of the second, indicating redox activity with average iron oxidation states of 2.67+ and 2.60+. The first-derivative iron XAS K-edge features at 7120.5, 7124, and 7128 eV resembled the XAS of magnetite but not of ferritin or of dissolved Fe(II) or Fe(III). Pseudo-Voigt fits to blood-cell iron K-edge XAS spectra yielded 12.4 integrated units of preedge intensity, indicating a noncentrosymmetric environment. The non-phase-corrected extended X-ray absorption fine structure (EXAFS) Fourier transform spectrum showed a first-shell O/N peak at 1.55 angstroms and an intense Fe-Fe feature at 2.65 angstroms. Fits to the EXAFS required a split first shell with two O at 1.93 angstroms and three O at 2.07 angstroms, consistent with terminal and bridging alkoxide ligands, respectively. More distant shells included three C at 2.87 angstroms, two Fe at 3.08 angstroms, three O at 3.29 angstroms, and one Fe at 3.8 angstroms. Structural models consistent with these findings include a [Fe4(OR)13](2-/3-) broken-edged Fe4O5 cuboid or a [Fe4(OR)14](3-/4-) "Jacob's ladder" with three edge-fused Fe2(OR)2 rhombs. Either of these models represents an entirely new structural motif for biological iron. Vanadium domination of blood-cell metals cannot be a defining trait of Phlebobranch tunicates so long as P. annectens is included among them.

  View details for DOI 10.1021/ic051445x

  View details for Web of Science ID 000237593100018

  View details for PubMedID 16676950

• 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

• 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

• Structural insights into a protein-bound iron-molybdenum cofactor precursor PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Corbett, M. C., Hu, Y. L., Fay, A. W., Ribbe, M. W., Hedman, B., Hodgson, K. O. 2006; 103 (5): 1238-1243

  Abstract

  The iron-molybdenum cofactor (FeMoco) of the nitrogenase MoFe protein is a highly complex metallocluster that provides the catalytically essential site for biological nitrogen fixation. FeMoco is assembled outside the MoFe protein in a stepwise process requiring several components, including NifB-co, an iron- and sulfur-containing FeMoco precursor, and NifEN, an intermediary assembly protein on which NifB-co is presumably converted to FeMoco. Through the comparison of Azotobacter vinelandii strains expressing the NifEN protein in the presence or absence of the nifB gene, the structure of a NifEN-bound FeMoco precursor has been analyzed by x-ray absorption spectroscopy. The results provide physical evidence to support a mechanism for FeMoco biosynthesis. The NifEN-bound precursor is found to be a molybdenum-free analog of FeMoco and not one of the more commonly suggested cluster types based on a standard [4Fe-4S] architecture. A facile scheme by which FeMoco and alternative, non-molybdenum-containing nitrogenase cofactors are constructed from this common precursor is presented that has important implications for the biosynthesis and biomimetic chemical synthesis of FeMoco.

  View details for DOI 10.1073/pnas.0507853103

  View details for Web of Science ID 000235094300017

  View details for PubMedID 16423898

  View details for PubMedCentralID PMC1360540

• 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

• 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

• 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

• Nitrogenase reactivity with P-cluster variants PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Hu, Y. L., Corbett, M. C., Fay, A. W., Webber, J. A., Hedman, B., Hodgson, K. O., Ribbe, M. W. 2005; 102 (39): 13825-13830

  Abstract

  Nitrogenase is a multicomponent metalloenzyme that catalyzes the conversion of atmospheric dinitrogen to ammonia. For decades, it has been generally believed that the [8Fe-7S] P-cluster of nitrogenase component 1 is indispensable for nitrogenase activity. In this study, we identified two catalytically active P-cluster variants by activity assays, metal analysis, and EPR spectroscopic studies. Further, we showed that both P-cluster variants resemble [4Fe-4S]-like centers based on x-ray absorption spectroscopic experiments. We believe that our findings challenge the dogma that the standard P-cluster is the only cluster species capable of supporting substrate reduction at the FeMo cofactor and provide important insights into the general mechanism of nitrogenase catalysis and assembly.

  View details for DOI 10.1073/pnas.0506967102

  View details for Web of Science ID 000232231900023

  View details for PubMedID 16166259

  View details for PubMedCentralID PMC1236593

• 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

• 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

• X-ray, absorption spectroscopic study of the reduced hydroxylases of methane monooxygenase and toluene/o-xylene monooxygenase: Differences in active site structure and effects of the coupling proteins MMOB and ToMOD INORGANIC CHEMISTRY Rudd, D. J., Sazinsky, M. H., LIPPARD, S. J., Hedman, B., Hodgson, K. O. 2005; 44 (13): 4546-4554

  Abstract

  The diiron active sites of the reduced hydroxylases from methane monooxygenase (MMOH(red)) and toluene/o-xylene monooxygenase (ToMOH(red)) have been investigated by X-ray absorption spectroscopy (XAS). Results of Fe K-edge and extended X-ray absorption fine structure analysis reveal subtle differences between the hydroxylases that may be correlated to access of the active site. XAS data were also recorded for each hydroxylase in the presence of its respective coupling protein. MMOB affects the outer-shell scattering contributions in the diiron site of MMOH(red), whereas ToMOD exerts its main effect on the first-shell ligation of ToMOH(red); it also causes a slight decrease in the Fe-Fe separation. These results provide an initial step toward delineating the differences in structure and reactivity in bacterial multicomponent monooxygenase proteins.

  View details for DOI 10.1021/ic048794w

  View details for PubMedID 15962961

• 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

• 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

• X-ray absorption spectroscopic investigation of the spin-transition character in a series of single-site perturbed iron(II) complexes INORGANIC CHEMISTRY Rudd, D. J., Goldsmith, C. R., Cole, A. P., Stack, D. P., Hodgson, K. O., Hedman, B. 2005; 44 (5): 1221-1229

  Abstract

  Select ferrous spin-transition complexes with the pentadentate ligand 2,6-bis(bis(2-pyridyl)methoxymethane)pyridine (PY5) were examined using variable-temperature solution solid-state magnetic susceptibility, crystallography, X-ray absorption spectroscopy (XAS), and UV/vis absorption spectroscopy. Altering the single exogeneous ligand, X, of [Fe(PY5)(X)]n)+ is sufficient to change the spin-state of the complexes. When X is the weak-field ligand Cl-, the resultant Fe complex is high-spin from 4 to 300 K, whereas the stronger-field ligand MeCN generates a low-spin complex over this temperature range. With intermediate-strength exogenous ligands (X = N3-, MeOH), the complexes undergo a spin-transition. [Fe(PY5)(N3)]+, as a crystalline solid, transitions gradually from a high-spin to a low-spin complex as the temperature is decreased, as evidenced by X-ray crystallography and solid-state magnetic susceptibility measurements. The spin-transition is also evident from changes in the pre-edge and EXAFS regions of the XAS Fe K-edge spectra on a ground crystalline sample. The spin-transition observed with [Fe(PY5)(MeOH)]2+ appears abrupt by solid-state magnetic susceptibility measurements, but gradual by XAS analysis, differences attributed to sample preparation. This research highlights the strengths of XAS in determining the electronic and geometric structure of such spin-transition complexes and underscores the importance of identical sample preparation in the investigation of these physical properties.

  View details for DOI 10.1021/ic0487651

  View details for Web of Science ID 000227346300021

  View details for PubMedID 15732962

• Mo K- and L-edge X-ray absorption spectroscopic study of the ADP.AlF4--stabilized nitrogenase complex: comparison with MoFe protein in solution and single crystal. Journal of synchrotron radiation Corbett, M. C., Tezcan, F. A., Einsle, O., Walton, M. Y., Rees, D. C., Latimer, M. J., Hedman, B., Hodgson, K. O. 2005; 12: 28-34

  Abstract

  The utility of using X-ray absorption spectroscopy (XAS) to study metalloproteins and, specifically, the enzyme complex nitrogenase, is highlighted by this study comparing both the structural and Mo-localized electronic features of the iron-molybdenum cofactor (FeMoco) in isolated MoFe protein and in the ADP.AlF4--stabilized complex of the MoFe protein with the Fe protein. No major differences are found at Mo between the two protein forms. The excellent quality of the data at both the Mo K and L edges will provide a baseline for analysis of other intermediates in the nitrogenase cycle. A new capability to delineate various contributions in the resting state of FeMoco is being pursued through polarized single-crystal XAS. The initial results point to the feasibility of using this technique for the analysis of scattering from the as yet unidentified atom at the center of FeMoco.

  View details for PubMedID 15616362

• 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

• MoK- and L-edge X-ray absorption spectroscopic study of the ADP center dot AIF(4)(-)-stabilized nitrogenase complex: comparison with MoFe protein in solution and single crystal JOURNAL OF SYNCHROTRON RADIATION Corbett, M. C., Tezcan, F. A., Einsle, O., Walton, M. Y., Rees, D. C., Latimer, M. J., Hedman, B., Hodgson, K. O. 2005; 12: 28-34

  Abstract

  The utility of using X-ray absorption spectroscopy (XAS) to study metalloproteins and, specifically, the enzyme complex nitrogenase, is highlighted by this study comparing both the structural and Mo-localized electronic features of the iron-molybdenum cofactor (FeMoco) in isolated MoFe protein and in the ADP.AlF4--stabilized complex of the MoFe protein with the Fe protein. No major differences are found at Mo between the two protein forms. The excellent quality of the data at both the Mo K and L edges will provide a baseline for analysis of other intermediates in the nitrogenase cycle. A new capability to delineate various contributions in the resting state of FeMoco is being pursued through polarized single-crystal XAS. The initial results point to the feasibility of using this technique for the analysis of scattering from the as yet unidentified atom at the center of FeMoco.

  View details for DOI 10.1107/S0909049504027827

  View details for Web of Science ID 000225960900007

• Integrated instrumentation for combined polarized single-crystal XAS and diffraction data acquisition for biological applications JOURNAL OF SYNCHROTRON RADIATION Latimer, M. J., Ito, K., McPhillips, S. E., Hedman, B. 2005; 12: 23-27

  Abstract

  Single-crystal X-ray absorption spectroscopy (XAS) instrumentation, allowing sequential integrated XAS and crystallographic data acquisition during the same experiment and on the same beamline, has been developed for SSRL beamline 9-3, a wiggler side station dedicated to general user biological XAS. The implementation includes a Huber kappa goniometer, Canberra 30-element Ge detector for XAS data collection, open-flow LHe and LN2 crystal coolers, a microscope for crystal alignment in the beam, and a MarCCD crystallography detector. The kappa goniometer allows a large accessible angular range with an open geometry, affording access to detectors and open stream coolers, as well as future instrumentation. Applicable standard hardware on SSRL crystallography beamlines has been incorporated, with crystallographic data collection controlled via the Blu-Ice software developed by the SSRL SMB macromolecular crystallography group. XAS data collection is handled through the SSRL standard XAS-Collect software. Initial diffraction and XAS data from single crystals using an open-flow cryostat are presented. The instrument will be available to general users after the SPEAR3 upgrade in 2004, and future expansion for use in high-throughput structural genomics XAS is proposed.

  View details for DOI 10.1107/S0909049504027839

  View details for Web of Science ID 000225960900006

  View details for PubMedID 15616361

• Ligand K-edge X-ray absorption spectroscopy: covalency of ligand-metal bonds COORDINATION CHEMISTRY REVIEWS Solomon, E. I., Hedman, B., Hodgson, K. O., Dey, A., Szilagyi, R. K. 2005; 249 (1-2): 97-129

  View details for DOI 10.1016/j.ccr.2004.03.020

  View details for Web of Science ID 000226477400005

• 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

• Dioxygen activation at a single copper site: Structure, bonding, and mechanism of formation of 1 : 1 Cu-O-2 adducts JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Aboelella, N. W., Kryatov, S. V., Gherman, B. F., Brennessel, W. W., Young, V. G., Sarangi, R., Rybak-Akimova, E. V., Hodgson, K. O., Hedman, B., SOLOMON, E. I., Cramer, C. J., Tolman, W. B. 2004; 126 (51): 16896-16911

  Abstract

  To evaluate the fundamental process of O(2) activation at a single copper site that occurs in biological and catalytic systems, a detailed study of O(2) binding to Cu(I) complexes of beta-diketiminate ligands L (L(1) = backbone Me; L(2) = backbone tBu) by X-ray crystallography, X-ray absorption spectroscopy (XAS), cryogenic stopped-flow kinetics, and theoretical calculations was performed. Using synchrotron radiation, an X-ray diffraction data set for L(2)CuO(2) was acquired, which led to structural parameters in close agreement to theoretical predictions. Significant Cu(III)-peroxo character for the complex was corroborated by XAS. On the basis of stopped-flow kinetics data and theoretical calculations for the oxygenation of L(1)Cu(RCN) (R = alkyl, aryl) in THF and THF/RCN mixtures between 193 and 233 K, a dual pathway mechanism is proposed involving (a) rate-determining solvolysis of RCN by THF followed by rapid oxygenation of L(1)Cu(THF) and (b) direct, bimolecular oxygenation of L(1)Cu(RCN) via an associative process.

  View details for DOI 10.1021/ja045678j

  View details for Web of Science ID 000225910400048

  View details for PubMedID 15612729

• High covalence in CuSO4 and the radicalization of sulfate: An X-ray absorption and density functional study INORGANIC CHEMISTRY Szilagyi, R. K., Frank, P., George, S. D., Hedman, B., Hodgson, K. O. 2004; 43 (26): 8318-8329

  Abstract

  Sulfur K-edge X-ray absorption spectroscopy (XAS) of anhydrous CuSO(4) reveals a well-resolved preedge transition feature at 2478.8 eV that has no counterpart in the XAS spectra of anhydrous ZnSO(4) or copper sulfate pentahydrate. Similar but weaker preedge features occur in the sulfur K-edge XAS spectra of [Cu(itao)SO(4)] (2478.4 eV) and [Cu[(CH(3))(6)tren]SO(4)] (2477.7 eV). Preedge features in the XAS spectra of transition metal ligands are generally attributed to covalent delocalization of a metal d-orbital hole into a ligand-based orbital. Copper L-edge XAS of CuSO(4) revealed that 56% of the Cu(II) 3d hole is delocalized onto the sulfate ligand. Hybrid density functional calculations on the two most realistic models of the covalent delocalization pathways in CuSO(4) indicate about 50% electron delocalization onto the sulfate oxygen-based 2p orbitals; however, at most 14% of that can be found on sulfate sulfur. Both experimental and computational results indicated that the high covalence of anhydrous CuSO(4) has made sulfate more like the radical monoanion, inducing an extensive mixing and redistribution of sulfur 3p-based unoccupied orbitals to lower energy in comparison to sulfate in ZnSO(4). It is this redistribution, rather than a direct covalent interaction between Cu(II) and sulfur, that is the origin of the observed sulfur XAS preedge feature. From pseudo-Voigt fits to the CuSO(4) sulfur K-edge XAS spectrum, a ground-state 3p character of 6% was quantified for the orbital contributing to the preedge transition, in reasonable agreement with the DFT calculation. Similar XAS fits indicated 2% sulfur 3p character for the preedge transition orbitals in [Cu(itao)SO(4)] and [Cu[(CH(3))(6)tren]SO(4)]. The covalent radicalization of ligands similar to sulfate, with consequent energy redistribution of the virtual orbitals, represents a new mechanism for the induction of ligand preedge XAS features. The high covalence of the Cu sites in CuSO(4) was found to be similar to that of Cu sites in oxidized cupredoxins, including its anistropic nature, and can serve as the simplest inorganic examples of intramolecular electron-transfer processes.

  View details for DOI 10.1021/ic030311l

  View details for Web of Science ID 000225906700024

  View details for PubMedID 15606178

• Determination by X-ray absorption spectroscopy of the Fe-Fe separation in the oxidized form of the hydroxylase of methane monooxygenase alone and in the presence of MMOD INORGANIC CHEMISTRY Rudd, D. J., Sazinsky, M. H., Merkx, M., LIPPARD, S. J., Hedman, B., Hodgson, K. O. 2004; 43 (15): 4579-4589

  Abstract

  The diiron active site in the hydroxylase of Methylococcus capsulatus (Bath) methane monooxygenase (MMOH) has been studied in the oxidized form by X-ray absorption spectroscopy (XAS). Previous investigations by XAS and X-ray crystallography have identified two different distances (3.0 and 3.4 angstroms) between the two Fe atoms in the dinuclear site. The present study has employed a systematic extended X-ray absorption fine structure (EXAFS) fitting methodology, utilizing known and simulated active site and relevant model structures, to determine unambiguously the Fe-Fe separation in the oxidized form of MMOH. Consistent and unique fits were only possible for an Fe-Fe distance of 3.0 angstroms. This methodology was then applied to study potential changes in the active site local structure in the presence of MMOD, a protein of unknown function in multicomponent MMO. Fe K-edge and EXAFS analyses revealed negligible changes in the diiron site electronic and geometric structure upon addition of MMOD to oxidized MMOH.

  View details for DOI 10.1021/ic049716b

  View details for Web of Science ID 000222846300015

  View details for PubMedID 15257585

• 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

• Comparison of iron-molybdenum cofactor-deficient nitrogenase MoFe proteins by X-ray absorption spectroscopy - Implications for P-cluster biosynthesis JOURNAL OF BIOLOGICAL CHEMISTRY Corbett, M. C., Hu, Y. L., Naderi, F., Ribbe, M. W., Hedman, B., Hodgson, K. O. 2004; 279 (27): 28276-28282

  Abstract

  Nitrogenase, the enzyme system responsible for biological nitrogen fixation, is believed to utilize two unique metalloclusters in catalysis. There is considerable interest in understanding how these metalloclusters are assembled in vivo. It has been presumed that immature iron-molybdenum cofactor-deficient nitrogenase MoFe proteins contain the P-cluster, although no biosynthetic pathway for the assembly of this complex cluster has been identified as yet. Through the comparison by iron K-edge x-ray absorption edge and extended fine structure analyses of cofactor-deficient MoFe proteins resulting from nifH and nifB deletion strains of Azotobacter vinelandii, a novel [Fe-S] cluster is identified in the DeltanifH MoFe protein. The iron-iron scattering displayed by the DeltanifH MoFe protein is more similar to that of a standard [Fe(4)S(4)]-containing protein than that of the DeltanifB MoFe protein, which is shown to contain a "normal" P-cluster. The iron-sulfur scattering of the DeltanifH MoFe protein, however, indicates differences in its cluster from an [Fe(4)S(4)](Cys)(4) site that may be consistent with the presence of either oxygenic or nitrogenic ligation. Based on these results, models for the [Fe-S] center in the DeltanifH MoFe protein are constructed, the most likely of which consist of two separate [Fe(4)S(4)] sites, each with some non-cysteinyl coordination. This type of model suggests that the P-cluster is formed by the condensation of two [Fe(4)S(4)] fragments, possibly concomitant with Fe protein (NifH)-induced conformational change.

  View details for DOI 10.1074/jbc.M403156200

  View details for Web of Science ID 000222265400058

  View details for PubMedID 15102840

• 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

• 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

• 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

• 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

• X-ray absorption spectroscopy of a structural analogue of the oxidized active sites in the sulfite oxidase enzyme family and related molybdenum(V) complexes INORGANIC CHEMISTRY Jalilehvand, F., Lim, B. S., HOLM, R. H., Hedman, B., Hodgson, K. O. 2003; 42 (18): 5531-5536

  Abstract

  X-ray absorption spectroscopy (XAS) (edge and extended X-ray absorption fine structure (EXAFS)) has been applied to the characterization of three molybdenum(V,VI) monodithiolene complexes with unidentate coligands, [MoO(SC(6)H(2)-2,4,6-Pr(i)()(3))(2)(bdt)](-) (1), [MoOCl(SC(6)H(2)-2,4,6-Pr(i)(3))(bdt)](-) (2), and [MoO(2)(SC(6)H(2)-2,4,6-Pr(i)(3))(bdt)](-) (3) (bdt = benzene-1,2-dithiolate). These complexes are related to the active site in the xanthine oxidase and sulfite oxidase families and, as in the enzyme sites, bind monodentate thiolate. By comparison to the data of crystalline oxidized chicken sulfite oxidase, it is shown that complex 3, whose thiolate simulates binding by the highly conserved cysteine, is an accurate structural analogue of the oxidized site of this enzyme. Normalized edge spectra, EXAFS data, Fourier transforms, and GNXAS-based fit results are presented. As in earlier studies, this provides characterization of new analogue complexes by XAS to facilitate identification of related sites in proteins.

  View details for DOI 10.1021/ic030039f

  View details for Web of Science ID 000185219500019

  View details for PubMedID 12950200

• 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

• 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

• 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

• Spectroscopic comparison of the five-coordinate [Cu(SMeIm)(HB(3,5-iPr(2)pz)(3))] with the four-coordinate [Cu(SCPh3)(HB(3,5-iPr(2)pz)(3))]: effect of coordination number increase on a blue cop per type site INORGANICA CHIMICA ACTA Basumallick, L., George, S. D., Randall, D. W., Hedman, B., Hodgson, K. O., Fujisawa, K., Solomon, E. I. 2002; 337: 357-365

  View details for Web of Science ID 000178473200037

• 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

• Structural and spectroscopic studies of valence-delocalized diiron(III) complexes supported by carboxylate-only bridging ligands INORGANIC CHEMISTRY Lee, D., Dubois, J. L., Pierce, B., Hedman, B., Hodgson, K. O., Hendrich, M. P., Lippard, S. J. 2002; 41 (12): 3172-3182

  Abstract

  The synthesis, molecular structures, and spectroscopic properties of a series of valence-delocalized diiron(II,III) complexes are described. One-electron oxidation of diiron(II) tetracarboxylate complexes afforded the compounds [Fe(2)(mu-O(2)CAr(Tol))(4)L(2)]X, where L = 4-(t)BuC(5)H(4)N (1b), C(5)H(5)N (2b), and THF (3b); X = PF(6)(-) (1b and 3b) and OTf(-) (2b). In 1b-3b, four mu-1,3 carboxylate ligands span relatively short Fe...Fe distances of 2.6633(11)-2.713(3) A. Intense (epsilon = 2700-3200 M(-1) cm(-1)) intervalence charge transfer bands were observed at 620-670 nm. EPR spectroscopy confirmed the S = (9)/(2) ground spin state of 1b-3b, the valence-delocalized nature of which was probed by X-ray absorption spectroscopy. The electron delocalization between paramagnetic metal centers is described by double exchange, which, for the first time, is observed in diiron clusters having no single-atom bridging ligand(s).

  View details for DOI 10.1021/ic011050n

  View details for Web of Science ID 000176219400015

  View details for PubMedID 12054996

• 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

• 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

• Structural characterization of metallopeptides designed as scaffolds for the stabilization of nickel(II)-Fe4S4 bridged assemblies by X-ray absorption spectroscopy JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Musgrave, K. B., Laplaza, C. E., HOLM, R. H., Hedman, B., Hodgson, K. O. 2002; 124 (12): 3083-3092

  Abstract

  In earlier work, de novo designed peptides with a helix-loop-helix motif and 63 residues have been synthesized as potential scaffolds for stabilization of the [Ni(II)-X-Fe(4)S(4)] bridged assembly that is the spectroscopically deduced structure of the A-Cluster in clostridial carbon monoxide dehydrogenase. The 63mers contain a consensus tricysteinyl ferredoxin domain in the loop for binding an Fe(4)S(4) cluster and Cys and His residues proximate to the loop for binding Ni(II), with one Cys residue designed as the bridge X. The metallopeptides HC(4)H(2)-[Fe(4)S(4)]-Ni and HC(5)H-[Fe(4)S(4)]-M, containing three His and one Cys residue for Ni(II) coordination and two His and two Cys residues for binding M = Ni(II) and Co(II), have been examined by Fe-, Ni-, and Co-K edge spectroscopy and EXAFS. All peptides bind an [Fe(4)S(4)](2+) cubane-type cluster. Interpretation of the Ni and Co data is complicated by the presence of a minority population of six-coordinate species with low Z ligands, designated for simplicity as [M(OH(2))(6)](2+). Best fits of the data were obtained with ca. 20% [M(OH(2))(6)](2+) and ca. 80% M(II) with mixed N/S coordination. The collective XAS results for HC(4)H(2)-[Fe(4)S(4)]-Ni and HC(5)H-[Fe(4)S(4)]-M demonstrate the presence of an Fe(4)S(4) cluster and support the existence of the distorted square-planar coordination units [Ni(II)(S.Cys)(N.His)(3)] and [Ni(II)(S.Cys)(2)(N.His)(2)] in the HC(4)H(2) and HC(5)H metallopeptides, respectively. In the HC(5)H metallopeptide, tetrahedral [Co(II)(S.Cys)(2)(N.His)(2)] is present. We conclude that the designed scaffolded binding sites, including Ni-(mu(2)-S.Cys)-Fe bridges, have been achieved. This is the first XAS study of a de novo designed metallopeptide intended to stabilize a bridged biological assembly, and one of a few XAS analyses of metal derivatives of designed peptides. The scaffolding concept should be extendable to other bridged metal assemblies.

  View details for DOI 10.1021/ja011861q

  View details for PubMedID 11902899

• 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

• Aryl C-H activation by Cu-II to form an organometallic Aryl-Cu-III species: A novel twist on copper disproportionation ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Ribas, X., JACKSON, D. A., Donnadieu, B., Mahia, J., Parella, T., Xifra, R., Hedman, B., Hodgson, K. O., Llobet, A., Stack, T. D. 2002; 41 (16): 2991-2994

  View details for Web of Science ID 000177597700023

  View details for PubMedID 12203435

• 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

• Synthesis and spectroscopic studies of non-heme diiron(III) species with a terminal hydroperoxide ligand: Models for hemerythrin INORGANIC CHEMISTRY Mizoguchi, T. J., Kuzelka, J., Spingler, B., Dubois, J. L., Davydov, R. M., Hedman, B., Hodgson, K. O., LIPPARD, S. J. 2001; 40 (18): 4662-4673

  Abstract

  Two compounds, [Fe2(mu-OH)(mu-Ph4DBA)(TMEDA)2(OTf)] (4) and [Fe2(mu-OH)(mu-Ph4DBA)(DPE)2(OTf)] (7), where Ph4DBA(2-) is the dinucleating bis(carboxylate) ligand dibenzofuran-4,6-bis(diphenylacetate), have been prepared as synthetic models for the dioxygen-binding non-heme diiron protein hemerythrin (Hr). X-ray crystallography reveals that, in the solid state, these compounds contain the asymmetric coordination environment found at the diiron center in the reduced form of the protein, deoxyHr. Mössbauer spectra of the models (4, delta = 1.21(2), DeltaE(Q) = 2.87(2) mm s(-1); 7, delta(av) = 1.23(1), DeltaE(Qav) = 2.79(1) mm s(-1)) and deoxyHr (delta = 1.19, DeltaE(Q) = 2.81 mm s(-1)) are also in good agreement. Oxygenation of the diiron(II) complexes dissolved in CH2Cl2 containing 3 equiv of N-MeIm (4) or neat EtCN (7) at -78 degrees C affords a red-orange solution with optical bands at 336 nm (7300 M(-1) cm(-1)) and 470 nm (2600 M(-1) cm(-1)) for 4 and at 334 nm (6400 M(-1) cm(-1)) and 484 nm (2350 M(-1) cm(-1)) for 7. These spectra are remarkably similar to that of oxyHr, 330 nm (6800 M(-1) cm(-1)) and 500 nm (2200 M(-1) cm(-1)). The electron paramagnetic resonance (EPR) spectrum of the cryoreduced, mixed-valence dioxygen adduct of 7 displays properties consistent with a (mu-oxo)diiron(II,III) core. An investigation of 7 and its dioxygen-bound adduct by extended X-ray absorption fine structure (EXAFS) spectroscopy indicates that the oxidized species contains a (mu-oxo)diiron(III) core with iron-ligand distances in agreement with those expected for oxide, carboxylate, and amine/hydroperoxide donor atoms. The analogous cobalt complex [Co2(mu-OH)(mu-Ph4DBA)(TMEDA)2(OTf)] (6) was synthesized and structurally characterized, but it was unreactive toward dioxygen.

  View details for Web of Science ID 000170642600025

  View details for PubMedID 11511213

• IR and EXAFS spectroscopic studies of glyphosate protonation and copper(II) complexes of glyphosate in aqueous solution INORGANIC CHEMISTRY Sheals, J., Persson, P., Hedman, B. 2001; 40 (17): 4302-4309

  Abstract

  The varying degrees of protonation of N-(phosphonomethyl)glycine (PMG, glyphosate) were investigated with infrared (IR) spectroscopy and ab initio frequency calculations. The zwitterionic nature of PMG in solution was confirmed, and intramolecular hydrogen bonding was identified. Successive protonation of the PMG molecule follows the order amine, phosphonate, carboxylate. Intramolecular hydrogen bonding is indicated to exist at all stages of protonation: between both RCO(2-) and RNH(2)(+) and RPO(3)(2-) and RNH(2+) in HL(2)(-) (where L represents the ligand PMG); between RCO(2)(-) and RNH(2)(+) in H(2)L(-); predominantly between RPO(3)(2-) and RNH(2)(+) in H(3)L. There are strong indications that the zwitterion is intact throughout the pH range investigated. Results from IR and extended X-ray absorption fine structure (EXAFS) spectroscopies provide new evidence for structures of N-(phosphonomethyl)glycinecopper(II) complexes. The structures of 1:1 complexes, CuL(-) and CuHL, are essentially the same, differing only in protonation of the phosphonate group. Copper(II) lies at the center of a Jahn-Teller distorted octahedron with all three donor groups (amine, carboxylate, phosphonate) of PMG chelating with copper(II) to form two five-membered chelate rings oriented in the equatorial plane. EXAFS indicates that oxygen (most likely a water molecule) is a fourth ligand, which would thus occupy the fourth corner in the equatorial plane of the elongated octahedron. CuL(2)(4-) most probably forms an isomeric mixture in solution, and there are indications that this mixture is dominated by complexes where two PMG ligands are bound to copper(II) via equatorial and axial positions, with both phosphonate and carboxylate donor groups responsible for chelation at axial positions.

  View details for DOI 10.1021/ic000849g

  View details for Web of Science ID 000170381600027

  View details for PubMedID 11487336

• 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

• 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

• 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

• A Short Copper-Copper Distance in a (μ-1,2-Peroxo)dicopper(II) Complex Having a 1,8-Naphthyridine Unit as an Additional Bridge. Angewandte Chemie (International ed. in English) He, C., DuBois, J. L., Hedman, B., Hodgson, K. O., Lippard, S. J. 2001; 40 (8): 1484-1487

  Abstract

  A copper-copper separation of about 2.84 Å is determined by extended X-ray absorption fine structure studies for the (μ-1,2-peroxo)dicopper(II) species 1, which has only a 1,8-naphthyridine unit as an additional bridge. Complex 1 was prepared by the reaction of O2 with a dicopper(I) complex formed from BPMAN.

  View details for DOI 10.1002/1521-3773(20010417)40:8<1484::AID-ANIE1484>3.0.CO;2-Z

  View details for PubMedID 29712360

• 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

• A short copper-copper distance in a (mu-1,2-peroxo)dicopper(II) complex having a 1,8-naphthyridine unit as an additional bridge ANGEWANDTE CHEMIE-INTERNATIONAL EDITION He, C., Dubois, J. L., Hedman, B., Hodgson, K. O., LIPPARD, S. J. 2001; 40 (8): 1484-?

  Abstract

  A copper-copper separation of about 2.84 Å is determined by extended X-ray absorption fine structure studies for the (μ-1,2-peroxo)dicopper(II) species 1, which has only a 1,8-naphthyridine unit as an additional bridge. Complex 1 was prepared by the reaction of O2 with a dicopper(I) complex formed from BPMAN.

  View details for Web of Science ID 000168195500024

• 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

• 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

• Spectroscopic and electronic structural studies of blue copper model complexes. 2. Comparison of three- and four-coordinate Cu(II)-thiolate complexes and fungal laccase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Randall, D. W., George, S. D., Holland, P. L., Hedman, B., Hodgson, K. O., Tolman, W. B., Solomon, E. I. 2000; 122 (47): 11632-11648

  View details for DOI 10.1021/ja001592o

  View details for Web of Science ID 000165696800008

• X-ray absorption edge and EXAFS studies of the blue copper site in stellacyanin: Effects of axial amide coordination JOURNAL OF PHYSICAL CHEMISTRY B DeBeer, S., Randall, D. W., Nersissian, A. M., Valentine, J. S., Hedman, B., Hodgson, K. O., Solomon, E. I. 2000; 104 (46): 10814-10819

  View details for DOI 10.1021/jp001334d

  View details for Web of Science ID 000165647300012

• X-ray spectroscopy of enzyme active site analogues and related molecules: Bis(dithiolene)molybdenum(IV) and -tungsten(IV,VI) complexes with variant terminal ligands INORGANIC CHEMISTRY Musgrave, K. B., Lim, B. S., Sung, K. M., HOLM, R. H., Hedman, B., Hodgson, K. O. 2000; 39 (23): 5238-5247

  Abstract

  The X-ray absorption spectra at the molybdenum and selenium K-edges and the tungsten L2,3-edges are acquired for a set of 14 Mo(IV) and W(IV,VI) bis(dithiolene) complexes related to the active sites of molybdo- and tungstoenzymes. The set includes square pyramidal [MoIVL(S2C2Me2)2]- (L = O2-, R3SiO-, RO-, RS-, RSe-) and [WIV(OR)(S2C2Me2)2]-, distorted trigonal prismatic [MoIV(CO)(SeR)(S2C2Me2)2]- and [WIV(CO)L(S2C2Me2)2]- (L = RS-, RSe-), and distorted octahedral [WVIO(OR)(S2C2Me2)2]-. The dithiolene simulates the pterin-dithiolene cofactor ligand, and L represents a protein ligand. Bond lengths are determined by EXAFS analysis using the GNXAS protocol. Normalized edge spectra, non-phase-shift-corrected Fourier transforms, and EXAFS data and fits are presented. Bond lengths determined by EXAFS and X-ray crystallography agree to < or = 0.02 A as do the M-Se distances determined by both metal and selenium EXAFS. The complexes [MoIV(QR)(S2C2Me2)2]- simulate protein ligation by the DMSO reductase family of enzymes, including DMSO reductase itself (Q = O), dissimilatory nitrate reductase (Q = S), and formate dehydrogenase (Q = Se). Edge shifts of these complexes correlate with the ligand electronegativities. Terminal ligand binding is clearly distinguished in the presence of four Mo-S(dithiolene) interactions. Similarly, five-coordinate [ML(S2C2Me2)2]- and six-coordinate [M(CO)L(S2C2Me2)2]- are distinguishable by edge and EXAFS spectra. This study expands a previous XAS investigation of bis(dithiolene)metal(IV,V,VI) complexes (Musgrave, K. B.; Donahue, J. P.; Lorber, C.; Holm, R. H.; Hedman, B.; Hodgson, K. O. J. Am. Chem. Soc. 1999, 121, 10297) by including a larger inventory of molecules with variant physiologically relevant terminal ligation. The previous and present XAS results should prove useful in characterizing and refining metric features and structures of enzyme sites.

  View details for Web of Science ID 000165386300011

  View details for PubMedID 11154582

• 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

• Metallocyclopeptide complexes with M-II(S center dot Cys)(4) chromophores INORGANIC CHEMISTRY Nivorozhkin, A. L., Segal, B. M., Musgrave, K. B., Kates, S. A., Hedman, B., Hodgson, K. O., HOLM, R. H. 2000; 39 (11): 2306-2313

  Abstract

  The tetracysteinyl peptide cyclo[Lys1,12](Gln-Cys-Gly-Val-Cys-Gly-Lys-Cys-Ile-Ala-Cys-Lys) ([symbol: see text] L(Cys.SH)4) was synthesized by solid-phase methods using an Fmoc/t-Bu/allyl strategy on a PAL-PEG-PS support. The formation of the 1:1 complexes with M = Fe2+, Co2+, and Ni2+ was observed by spectrophotometric monitoring of reactions in aqueous solution at pH 7.5. Size exclusion chromatography indicated that the peptide is a monomer and the complexes are dimers [M2([symbol: see text]L(Cys.S)4)2] in aqueous buffer at pH 7.5. Cobalt and nickel K-edge X-ray absorption data and EXAFS analysis of [Co2([symbol: see text] L(Cys.S)4)2] and [Ni2([symbol: see text] L(Cys.S)4)2] as lyophilized solids are reported. Derived bond distances are Co-S = 2.30 A and Ni-S = 2.21 A. From the collective results provided by absorption spectra, K-edges, EXAFS, and bond length comparisons with known structures, it is shown that [Fe2([symbol: see text] L(Cys.S)4)2] and [Co2([symbol: see text] L(Cys.S)4)2] possess distorted tetrahedral structures and [Ni2([symbol: see text] L(Cys.S)4)2] has distorted square planar stereochemistry. The Co(II) chromophore is particularly distinctive of the assigned structure, displaying three components of the parent tetrahedral ligand field transition 4A2-->4T1(P) (610, 685, 740 nm). The observed structures conform to the intrinsic stereochemical preferences of the metal ions. Structures for the binuclear complexes are suggested. These are the first characterized metal complexes of a cysteinyl cyclopeptide and among the few well-documented complexes of synthetic cyclopeptides. This study is a desirable first step in the design of cyclic peptides for the binding of mononuclear and polynuclear metal centers.

  View details for Web of Science ID 000087386600012

  View details for PubMedID 12526490

• 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

• An X-ray spectroscopic investigation of bis(dithiolene)molybdenum(IV,V,VI) and -tungsten(IV,V,VI) complexes: Symmetrized structural representations of the active sites of molybdoenzymes in the DMSO reductase family and of tungstoenzymes in the AOR and F(M)DH families JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Musgrave, K. B., Donahue, J. P., Lorber, C., HOLM, R. H., Hedman, B., Hodgson, K. O. 1999; 121 (44): 10297-10307

  View details for Web of Science ID 000083719800006

• 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

• The unperturberd oxo-sulfido functional group cis-Mo(VI)OS related to that in the xanthine oxidase family of molybdoenzyme: Synthesis, structural characterization, and reactivity aspects INORGANIC CHEMISTRY Thapper, A., Donahue, J. P., Musgrave, K. B., Willer, M. W., Nordlander, E., Hedman, B., Hodgson, K. O., HOLM, R. H. 1999; 38 (18): 4104-4114

  View details for Web of Science ID 000082525000023

• 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

• Exogenous substrate reactivity with a [Cu(III)(2)O-2](2+) core: Structural implications JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mahadevan, V., Dubois, J. L., Hedman, B., Hodgson, K. O., Stack, T. D. 1999; 121 (23): 5583-5584

  View details for Web of Science ID 000081006000031

• Large area silicon drift detectors for X-rays - New results. IEEE TRANSACTIONS ON NUCLEAR SCIENCE Iwanczyk, J. S., Tull, C. R., Segal, J. D., Kenney, C. J., Bradley, J., Hedman, B., Hodgson, K. O. 1999; 46 (3): 284-288

  View details for Web of Science ID 000081469800031

• 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

• A study of solid [{Cu(MePY2)}(2)O-2](2+) using resonance Raman and X-ray absorption spectroscopies: An intermediate Cu2O2 core structure or a solid solution? JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pidcock, E., DeBeer, S., Obias, H. V., Hedman, B., Hodgson, K. O., Karlin, K. D., Solomon, E. I. 1999; 121 (9): 1870-1878

  View details for Web of Science ID 000079143700013

• 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

• Sulfur allocation and vanadium-sulfate interactions in whole blood cells from the tunicate Ascidia ceratodes, investigated using X-ray absorption spectroscopy INORGANIC CHEMISTRY Frank, P., Hedman, B., Hodgson, K. O. 1999; 38 (2): 260-270

  View details for Web of Science ID 000078290900010

• 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

• A new structure for controlling dark current due to surface generation in drift detectors NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT Segal, J. D., Patt, B. E., Iwanczyk, J. S., Vilkelis, G., Plummer, J. D., Hedman, B., Hodgson, K. O. 1998; 414 (2-3): 307-316

  View details for Web of Science ID 000076090100019

• An edge with XAS NATURE STRUCTURAL BIOLOGY George, G. N., Hedman, B., Hodgson, K. O. 1998; 5: 645-647

  View details for Web of Science ID 000075169700011

  View details for PubMedID 9699615

• EXAFS studies on the P-N and P-OX states of the P-clusters in nitrogenase JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY Musgrave, K. B., Liu, H. I., Ma, L., Burgess, B. K., WATT, G., Hedman, B., Hodgson, K. O. 1998; 3 (4): 344-352

  View details for Web of Science ID 000075592900002

• 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

• All-ferrous titanium(III) citrate reduced Fe protein of nitrogenase: An XAS study of electronic and metrical structure JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Musgrave, K. B., Angove, H. C., Burgess, B. K., Hedman, B., Hodgson, K. O. 1998; 120 (21): 5325-5326

  View details for Web of Science ID 000074039100027

• Catalytic galactose oxidase models: Biomimetic Cu(II)-phenoxyl-radical reactivity SCIENCE Wang, Y. D., Dubois, J. L., Hedman, B., Hodgson, K. O., Stack, T. D. 1998; 279 (5350): 537-540

  Abstract

  Biomimetic functional models of the mononuclear copper enzyme galactose oxidase are presented that catalytically oxidize benzylic and allylic alcohols to aldehydes with O2 under mild conditions. The mechanistic fidelity between the models and the natural system is pronounced. Modest structural mimicry proves sufficient to transfer an unusual ligand-based radical mechanism, previously unprecedented outside the protein matrix, to a simple chemical system.

  View details for Web of Science ID 000071616000038

  View details for PubMedID 9438841

• 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

• 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

• 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

• Multiple-edge XAS studies of cyanide-bridged iron-copper molecular assemblies relevant to cyanide-inhibited heme-copper oxidases using four-body multiple-scattering analysis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Zhang, H. H., Filipponi, A., DICICCO, A., Scott, M. J., HOLM, R. H., Hedman, B., Hodgson, K. O. 1997; 119 (10): 2470-2478

  View details for Web of Science ID A1997WM81400016

• 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

• X-ray absorption and EPR studies on the copper ions associated with the particulate methane monooxygenase from Methylococcus capsulatus (Bath). Cu(I) ions and their implications JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Nguyen, H. H., Nakagawa, K. H., Hedman, B., Elliott, S. J., Lidstrom, M. E., Hodgson, K. O., Chan, S. I. 1996; 118 (50): 12766-12776

  View details for Web of Science ID A1996VY90000034

• Simulation and modelling of a new silicon X-ray drift detector design for synchrotron radiation applications 9th International Workshop on Room Temperature Semiconductor X-Ray and Gamma-Ray Detectors, Associated Electronics and Applications Iwanczyk, J. S., Patt, B. E., Segal, J., Plummer, J., Vilkelis, G., Hedman, B., Hodgson, K. O., Cox, A. D., Rehn, L., Metz, J. ELSEVIER SCIENCE BV. 1996: 288–94

  View details for Web of Science ID A1996VW52700065

• 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

• Multiple-Edge XAS Studies of Synthetic Iron-Copper Bridged Molecular Assemblies Relevant to Cytochrome c Oxidase. Structure Determination Using Multiple-Scattering Analysis with Statistical Evaluation of Errors. Inorganic chemistry Zhang, H. H., Filipponi, A., Di Cicco, A., Lee, S. C., Scott, M. J., Holm, R. H., Hedman, B., Hodgson, K. O. 1996; 35 (17): 4819-4828

  Abstract

  An X-ray absorption spectroscopy study has been carried out at the Fe and Cu K-edges for two bridged molecular assemblies, both of which contain an Fe-X-Cu (X = O(2)(-), OH(-)) bridge unit, some of whose features are relevant to the binuclear site of cytochrome c oxidase. The two complexes [(OEP)Fe-O-Cu(Me(6)tren)](1+) and [(OEP)Fe-(OH)-Cu(Me(5)tren)(OClO(3))](1+) have similar structural fragments around the metal centers except that they differ significantly in the bridge structure (the former contains a linear oxo bridge while the latter has a bent hydroxo bridge). We report a comparative study of these complexes using multiple-scattering (MS) EXAFS analysis and the program package GNXAS. It is found that there is a dramatic increase in the amplitude of the Fe-X-Cu MS pathway as the bridge unit approaches linearity. Full EXAFS MS analysis enables accurate quantitation of bridge metrical details and geometry for both complexes. These studies were done with an expanded version of GNXAS, which allows for simultaneous multiple-edge fitting. Such multiple-edge analysis (using both Fe and Cu edge data) allows common pathways (in this case involving the Fe-X-Cu bridge) to be constrained to be the same, thus improving the observation/variable ratio and enhancing sensitivity for determination of the bridge structure. The accuracy of the structural determination for the bridge units is evaluated by a statistical analysis methodology in which correlations among fitting parameters are identified and contour plots are used to determine random error. The overall error in the EXAFS structural determination is found by establishing the variance with the crystallographically determined values: for the EXAFS-determined parameters at distances below 4 Å, distances and angles deviated on average from crystallographic values by 0.014 Å and 1.5 degrees, respectively. It is also established that structural features in the Fe absorption preedge are diagnostic of oxo vs hydroxo ligation. The relevance of this study to the structural definition of binuclear bridged sites in cytochrome c oxidase and other metalloenzymes is considered.

  View details for PubMedID 11666681

• 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

• The biochemistry of the particulate methane monooxygenase Nguyen, H. H., Zhu, M., Elliott, S. J., Nakagawa, K. H., Hedman, B., Costello, A. M., Peeples, T. L., Wilkinson, B., Morimoto, H., Williams, P. G., Floss, H. G., Lidstrom, M. E., Hodgson, K. O., Chan, S. I., Lidstrom, M. E., Tabita, F. R. KLUWER ACADEMIC PUBL. 1996: 150–58

  View details for Web of Science ID A1996BF34V00021

• THE MULTIELEMENT MERCURIC IODIDE DETECTOR ARRAY WITH COMPUTER-CONTROLLED MINIATURIZED ELECTRONICS FOR EXAFS IEEE TRANSACTIONS ON NUCLEAR SCIENCE Patt, B. E., Iwanczyk, J. S., SZCZEBIOT, R., MACULEWICZ, G., Wang, M., Wang, Y. J., Hedman, B., Hodgson, K. O., Cox, A. D. 1995; 42 (4): 558-564

  View details for Web of Science ID A1995RP81900066

• X-RAY-ABSORPTION SPECTROSCOPIC STUDIES OF THE DIIRON CENTER IN METHANE MONOOXYGENASE IN THE PRESENCE OF SUBSTRATE AND THE COUPLING PROTEIN OF THE ENZYME-SYSTEM INORGANIC CHEMISTRY DeWitt, J. G., Rosenzweig, A. C., Salifoglou, A., Hedman, B., LIPPARD, S. J., Hodgson, K. O. 1995; 34 (10): 2505-2515

  View details for Web of Science ID A1995QX70600007

• X-RAY-ABSORPTION NEAR-EDGE STUDIES OF CYTOCHROME P-450-CAM, CHLOROPEROXIDASE, AND MYOGLOBIN - DIRECT EVIDENCE FOR THE ELECTRON RELEASING CHARACTER OF A CYSTEINE THIOLATE PROXIMAL LIGAND JOURNAL OF BIOLOGICAL CHEMISTRY Liu, H. I., Sono, M., Kadkhodayan, S., Hager, L. P., Hedman, B., Hodgson, K. O., Dawson, J. H. 1995; 270 (18): 10544-10550

  Abstract

  The low spin ferric and low and high spin ferrous forms of myoglobin, bacterial cytochrome P-450-CAM, and chloroperoxidase have been examined by Fe-K x-ray absorption edge spectroscopy. The positions of the absorption edge and the shapes of preedge and edge regions of imidazole adducts of ferric P-450-CAM and chloroperoxidase are essentially the same when compared with thiolate-ligated ferric myoglobin. As these three protein derivatives all have six-coordinate, low spin, ferric hemes with axial imidazole and thiolate ligands, the superposition of x-ray absorption edge spectral properties demonstrates that the protein environment does not effect the spectra, provided one compares heme iron centers with identical coordination numbers, spin and oxidation states, and ligand sets. In contrast, a 0.96 eV difference is observed in the energy of the absorption edge for imidazole- and thiolate-ligated ferric myoglobin with the latter shifted to lower energy as observed for ferrous myoglobin states. Similarly, in the low spin ferric-imidazole and ferrous-CO states, the energies of the absorption edge for chloroperoxidase and P-450-CAM are shifted in the direction of the ferrous state (to lower energy) when compared with those for analogous myoglobin derivatives. In the deoxyferrous high spin state, comparison of the edge spectra of chloroperoxidase with analogous data for cytochrome P-450-CAM suggests that the electron density at the iron is similar for these two protein states. The shifts observed in the energies of the x-ray absorption edge for the thiolate-ligated states of these proteins relative to derivatives lacking a thiolate ligand provide a direct measure of the electron releasing character of a thiolate axial ligand. These results therefore support the suggested role of the cysteinate proximal ligand of P-450 as a strong internal electron donor to promote O-O bond cleavage in the putative ferric-peroxide intermediate to generate the proposed ferryl-oxo "active oxygen" state of the reaction cycle.

  View details for Web of Science ID A1995QW60100030

  View details for PubMedID 7737989

• 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

• 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

• USING GNXAS, A MULTIPLE-SCATTERING EXAFS ANALYSIS, FOR DETERMINATION OF THE FE-N-O ANGLE IN (FENO)(7) COMPLEXES 8th International Conference on X-Ray Absorption Fine Structure (XAFS VIII) Westre, T. E., DICICCO, A., Filipponi, A., Natoli, C. R., Hedman, B., SOLOMON, E. I., Hodgson, K. O. ELSEVIER SCIENCE BV. 1995: 137–39

  View details for Web of Science ID A1995QP96100050

• 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

• 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

• OXYGEN-BRIDGED IRON-COPPER ASSEMBLIES PERTINENT TO HEME-COPPER OXIDASES - SYNTHESIS AND STRUCTURE OF AN [FE-III-(OH)-CU-II] BRIDGE AND EXAFS MULTIPLE-SCATTERING EFFECTS OF LINEAR OXO AND NONLINEAR HYDROXO BRIDGES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Scott, M. J., Zhang, H. H., Lee, S. C., Hedman, B., Hodgson, K. O., HOLM, R. H. 1995; 117 (1): 568-569

  View details for Web of Science ID A1995QC06100080

• 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

• INTERACTION OF VANADIUM AND SULFATE IN BLOOD-CELLS FROM THE TUNICATE ASCIDIA-CERATODES - OBSERVATIONS USING X-RAY-ABSORPTION EDGE STRUCTURE AND EPR SPECTROSCOPIES INORGANIC CHEMISTRY Frank, P., Hedman, B., Carlson, R. M., Hodgson, K. O. 1994; 33 (17): 3794-3803

  View details for Web of Science ID A1994PC83400024

• 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

• LARGE-SCALE ISOLATION AND CHARACTERIZATION OF THE MOLYBDENUM-IRON CLUSTER FROM NITROGENASE JOURNAL OF BIOLOGICAL CHEMISTRY Ma, L., Gavini, N., Liu, H. I., Hedman, B., Hodgson, K. O., Burgess, B. K. 1994; 269 (27): 18007-18015

  Abstract

  Here we report the large scale isolation and characterization of a species, designated MoFe cluster, that exhibits an S = 3/2 EPR signal, and the comparison of this entity to isolated FeMo cofactor in N-methylformamide and to the active site of the enzyme nitrogenase. MoFe cluster is isolated from purified nitrogenase by extraction into acidic methyl ethyl ketone and it is stable in that solvent in the absence of thiols. As initially isolated, MoFe cluster solutions exhibit an S = 1/2 EPR signal that arises from an oxidized species that can be reduced by dithionite or thiols to an EPR silent state and then to a state that exhibits an S = 3/2 EPR signal. The S = 3/2 signal is as sharp as the signal exhibited by the protein and much sharper than the signal exhibited by isolated FeMo cofactor. Circular dichroism experiments indicate that unlike the last two species, MoFe cluster does not contain the endogenous ligand R-homocitrate and thus, the sharpness of the S = 3/2 signal is an intrinsic property of the metal center and does not depend upon specific interactions with this organic ligand or with the protein. Metal analyses indicate that the metal core responsible for the S = 3/2 signal contains 6 Fe atoms per molybdenum. X-ray absorption spectroscopy experiments show that although the molybdenum atom in MoFe cluster retains its pseudo-octahedral geometry, its first coordination shell has one less iron atom than that of FeMo cofactor and there has been a significant change in the long range order of the cluster.

  View details for Web of Science ID A1994NV42200041

  View details for PubMedID 8027059

• SULFUR AND CHLORINE K-EDGE X-RAY-ABSORPTION SPECTROSCOPIC STUDIES OF PHOTOGRAPHIC MATERIALS JOURNAL OF THE AMERICAN CHEMICAL SOCIETY SMITH, T. A., DeWitt, J. G., Hedman, B., Hodgson, K. O. 1994; 116 (9): 3836-3847

  View details for Web of Science ID A1994NJ94600020

• EXAFS STUDIES OF FEMO-COFACTOR AND MOFE PROTEIN - DIRECT EVIDENCE FOR THE LONG-RANGE MO-FE-FE INTERACTION AND CYANIDE BINDING TO THE MO IN FEMO-COFACTOR JOURNAL OF THE AMERICAN CHEMICAL SOCIETY LIU, H. B., Filipponi, A., Gavini, N., Burgess, B. K., Hedman, B., DICICCO, A., Natoli, C. R., Hodgson, K. O. 1994; 116 (6): 2418-2423

  View details for Web of Science ID A1994NE02600022

• SELENOL BINDS TO IRON IN NITROGENASE IRON-MOLYBDENUM COFACTOR - AN EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE STUDY PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Conradson, S. D., Burgess, B. K., Newton, W. E., DICICCO, A., Filipponi, A., Wu, Z. Y., Natoli, C. R., Hedman, B., Hodgson, K. O. 1994; 91 (4): 1290-1293

  Abstract

  The biological N2-fixation reaction is catalyzed by the enzyme nitrogenase. The metal cluster active site of this enzyme, the iron-molybdenum cofactor (FeMoco), can be studied either while bound within the MoFe protein component of nitrogenase or after it has been extracted into N-methylformamide. The two species are similar but not identical. For example, the addition of thiophenol or selenophenol to isolated FeMoco causes its rather broad S = 3/2 electron paramagnetic resonance signal to sharpen and more closely approach the signal exhibited by protein-bound FeMoco. The nature of this thiol/selenol binding site has been investigated by using Se-K edge extended x-ray absorption fine structure (EXAFS) to study selenophenol ligated to FeMoco, and the results are reported here. EXAFS data analysis at the ligand Se-K edge was performed with a set of software, GNXAS, that provides for direct calculation of the theoretical EXAFS signals and least-squares fits to the experimental data. Data analysis results show definitively that the selenol (and by inference thiol) binds to Fe at a distance of 2.4 A. In contrast, unacceptable fits are obtained with either Mo or S as the liganded atom (instead of Fe). These results provide quantitative details about an exchangeable thiol/selenol binding site on FeMoco in its isolated, solution state and establish an Fe atom as the site of this reaction. Furthermore, the utility of ligand-based EXAFS as a probe of coordination in polynuclear metal clusters is demonstrated.

  View details for Web of Science ID A1994MX21000021

  View details for PubMedID 8108404

• HIGH-RESOLUTION XANES STUDIES ON VANADIUM-CONTAINING HALOPEROXIDASE - PH-DEPENDENCE AND SUBSTRATE-BINDING FEBS LETTERS KUSTHARDT, U., Hedman, B., Hodgson, K. O., Hahn, R., Vilter, H. 1993; 329 (1-2): 5-8

  Abstract

  High-resolution X-ray absorption vanadium K-edge spectra were recorded for samples of vanadium-containing bromoperoxidase from the brown alga, Ascophyllum nodosum, at pH 9, 7, 5 and 4, as well as for enzyme samples containing the substrates, hydrogen peroxide and bromide. The well-resolved features of the XANES spectra are discussed. The pH-dependence of the structure of the active site has been studied revealing no significant change of the absorption features. We were able to detect an interaction of H2O2 with the vanadium site of the bromoperoxidase using XAS spectroscopy, whereas addition of bromide causes no energy shift of the XANES spectrum. The possible role of vanadium during the enzymatic reaction is discussed on the basis of our results.

  View details for Web of Science ID A1993LT84100002

  View details for PubMedID 8354407

• HETEROMETAL CUBOIDAL CLUSTERS MFE4S6(PET3)4CL (M = V, MO) - SYNTHESIS, STRUCTURAL-ANALYSIS BY CRYSTALLOGRAPHY AND EXAFS, AND RELEVANCE TO THE CORE STRUCTURE OF THE IRON MOLYBDENUM COFACTOR OF NITROGENASE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Nordlander, E., Lee, S. C., Cen, W., Wu, Z. Y., Natoli, C. R., DICICCO, A., Filipponi, A., Hedman, B., Hodgson, K. O., HOLM, R. H. 1993; 115 (13): 5549-5558

  View details for Web of Science ID A1993LT17200024

• 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

• 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

• 20-ELEMENT HGI2 ENERGY DISPERSIVE-X-RAY ARRAY DETECTOR SYSTEM 1991 NUCLEAR SCIENCE SYMP AND MEDICAL IMAGING CONF ( NSS / MIUC ) Iwanczyk, J. S., DORRI, N., Wang, M., SZCZEBIOT, R. W., DABROWSKI, A. J., Hedman, B., Hodgson, K. O., Patt, B. E. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. 1992: 1275–80

  View details for Web of Science ID A1992JZ68400015

• ON THE STRUCTURE OF THE NICKEL IRON SULFUR CENTER OF THE CARBON-MONOXIDE DEHYDROGENASE FROM RHODOSPIRILLUM-RUBRUM - AN X-RAY ABSORPTION-SPECTROSCOPY STUDY PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA TAN, G. O., Ensign, S. A., Ciurli, S., Scott, M. J., Hedman, B., HOLM, R. H., Ludden, P. W., KORSUN, Z. R., Stephens, P. J., Hodgson, K. O. 1992; 89 (10): 4427-4431

  Abstract

  The nickel/iron/sulfur center of the carbon monoxide dehydrogenase (carbon monoxide:(acceptor)oxidoreductase; EC 1.2.99.2) enzyme from Rhodospirillum rubrum (Rr-CODH) was studied by x-ray absorption spectroscopy at the Ni K edge. Extended x-ray absorption fine structure data show that the first Ni coordination shell consists of 2 S atoms at 2.23 A and 2-3 N/O atoms at 1.87 A. The edge structure indicates a distorted tetrahedral or five-coordinate Ni environment in both oxidized and reduced Rr-CODH. By comparing second-shell extended x-ray absorption fine structure data of Rr-CODH to that of (Et4N)3[NiFe3S4(SEt)4], a cubane-type cluster, it was clearly established that Ni in the Rr-CODH center is not involved in the core of a NiFe3S4 cubane cluster. One model consistent with the results is a mononuclear Ni2+ site, bridged by S-Cys or sulfide to one or both of the Fe4S4 clusters of the enzyme, with the remaining coordination sites occupied by additional S-Cys or N/O-liganding amino acid residues.

  View details for Web of Science ID A1992HU97700043

  View details for PubMedID 1584775

  View details for PubMedCentralID PMC49095

• X-RAY ABSORPTION, MOSSBAUER, AND EPR STUDIES OF THE DINUCLEAR IRON CENTER IN THE HYDROXYLASE COMPONENT OF METHANE MONOOXYGENASE JOURNAL OF THE AMERICAN CHEMICAL SOCIETY DeWitt, J. G., BENTSEN, J. G., Rosenzweig, A. C., Hedman, B., Green, J., Pilkington, S., Papaefthymiou, G. C., Dalton, H., Hodgson, K. O., LIPPARD, S. J. 1991; 113 (24): 9219-9235

  View details for Web of Science ID A1991GQ93100031

• STRUCTURES OF SOLVATED CATIONS OF PALLADIUM(II) AND PLATINUM(II) IN DIMETHYL-SULFOXIDE, ACETONITRILE AND AQUEOUS-SOLUTION STUDIED BY EXAFS AND LAXS ACTA CHEMICA SCANDINAVICA HELLQUIST, B., BENGTSSON, L. A., HOLMBERG, B., HEDMAN, B., PERSSON, ELDING, L. I. 1991; 45 (5): 449–55

  View details for DOI 10.3891/acta.chem.scand.45-0449

  View details for Web of Science ID A1991FZ08900003

• EVIDENCE FROM EXAFS FOR A COPPER CLUSTER IN THE METALLOREGULATORY PROTEIN CUP2 FROM YEAST JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Nakagawa, K. H., Inouye, C., Hedman, B., Karin, M., Tullius, T. D., Hodgson, K. O. 1991; 113 (9): 3621-3623

  View details for Web of Science ID A1991FJ12100075

• INCLUSION OF A SMALL MOLECULE IN A BIG CAGE - PREPARATION AND STRUCTURE OF CATENA-[CATENA-(ALPHA,OMEGA-DIAMINOOCTANE)CADMIUM-MU-TETRACYANONICKELATE]-TOLUENE(1/1) INORGANIC CHEMISTRY Hasegawa, T., Franzen, S., Lambright, D., Oh, D. H., Balasubramanian, S., Hedman, B., Hodgson, K. O. 1991; 30 (7): 1441-1444

  View details for Web of Science ID A1991FE86600005

• STRUCTURE OF BIS(N-METHYLIMIDAZOLE)COPPER(I) TETRAFLUOROBORATE ACTA CRYSTALLOGRAPHICA SECTION C-CRYSTAL STRUCTURE COMMUNICATIONS TAN, G. O., Hodgson, K. O., Hedman, B., Clark, G. R., Garrity, M. L., Sorrell, T. N. 1990; 46: 1773-1775

  View details for Web of Science ID A1990EG32300008

• STRUCTURE OF ZN(CYCLAM)(CLO4)2 ACTA CRYSTALLOGRAPHICA SECTION C-CRYSTAL STRUCTURE COMMUNICATIONS Tyson, T. A., Hodgson, K. O., Hedman, B., Clark, G. R. 1990; 46: 1638-1640

  View details for Web of Science ID A1990EB00400021

• THE HGI2 ENERGY DISPERSIVE-X-RAY ARRAY DETECTORS AND MINIATURIZED PROCESSING ELECTRONICS PROJECT 1989 NUCLEAR SCIENCE SYMP Iwanczyk, J. S., DORRI, N., Wang, M., Szawlowski, M., Patt, B. E., Warburton, W. K., Hedman, B., Hodgson, K. O. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. 1990: 198–202

  View details for Web of Science ID A1990DF86900030

• 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

• CYANIDE AND METHYLISOCYANIDE BINDING TO THE ISOLATED IRON-MOLYBDENUM COFACTOR OF NITROGENASE JOURNAL OF BIOLOGICAL CHEMISTRY Conradson, S. D., Burgess, B. K., VAUGHN, S. A., Roe, A. L., Hedman, B., Hodgson, K. O., HOLM, R. H. 1989; 264 (27): 15967-15974

  Abstract

  19F NMR and x-ray absorption experiments have been performed with both the isolated FeMo cofactor and the MoFe protein of nitrogenase in search of direct evidence for substrate or inhibitor binding. Using 19F NMR as a probe and p-CF3C6H4S- as the receptor ligand, the data show that the nitrogenase inhibitors CN- and CH3NC bind to the isolated FeMo cofactor-RFS- complex in N-methylformamide with a finite formation constant. Their binding increases the electronic relaxation time of the complex and increases the life-time of the FeMo cofactor-p-CF3C6H4S- bond, Parallel molybdenum K edge and extended x-ray absorption fine structure experiments show that CH3NC does not bind to molybdenum. Although CO and N3- both relieve CN- and CH3NC inhibition of electron flow through nitrogenase, unlike the latter, they do not appear to bind to isolated FeMo cofactor. In experiments with the dithionite-reduced MoFe protein, we did not detect any changes in the molybdenum K edge or extended x-ray absorption fine structure spectra upon addition of CO, N2, C2H2, NaCN, CH3NC, or azide demonstrating that either these substrates and inhibitors do not bind to molybdenum or that the FeMo cofactor site of nitrogenase is inaccessible to substrate binding except under turnover conditions.

  View details for Web of Science ID A1989AQ95000040

  View details for PubMedID 2777773

• X-RAY ABSORPTION SPECTROSCOPIC STUDIES OF RUTHENIUM OCTAETHYLPORPHYRIN DIMERS JOURNAL OF THE CHEMICAL SOCIETY-CHEMICAL COMMUNICATIONS Asahina, H., ZISK, M. B., Hedman, B., McDevitt, J. T., Collman, J. P., Hodgson, K. O. 1989: 1360-1362

  View details for Web of Science ID A1989AT87500035

• ADVANCES IN MERCURIC IODIDE ENERGY DISPERSIVE-X-RAY ARRAY DETECTORS AND ASSOCIATED MINIATURIZED PROCESSING ELECTRONICS REVIEW OF SCIENTIFIC INSTRUMENTS Iwanczyk, J. S., DORRI, N., Wang, M., Szawlowski, M., Warburton, W. K., Hedman, B., Hodgson, K. O. 1989; 60 (7): 1561-1567

  View details for Web of Science ID A1989AG22100043

• X-RAY ABSORPTION-SPECTROSCOPY OF PROTEIN-A OF METHANE MONOOXYGENASE PHYSICA B-CONDENSED MATTER DeWitt, J., Hedman, B., Ericson, A., Hodgson, K. O., BENTSEN, J., Beer, R., LIPPARD, S. J., Green, J., Dalton, H. 1989; 158 (1-3): 97-98

  View details for Web of Science ID A1989AE26500034

• A STUDY OF THE ELECTRONIC-STRUCTURE OF S2O62-BY POLARIZED K-EDGE X-RAY ABSORPTION-SPECTROSCOPY PHYSICA B-CONDENSED MATTER Tyson, T. A., Roe, A. L., Hedman, B., Frank, P., Hodgson, K. O. 1989; 158 (1-3): 398-399

  View details for Web of Science ID A1989AE26500145

• 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

• ABINITIO EXAFS AND MULTIPLE-SCATTERING ANALYSIS OF SF6 PHYSICA B-CONDENSED MATTER Tyson, T. A., Benfatto, M., Natoli, C. R., Hedman, B., Hodgson, K. O. 1989; 158 (1-3): 425-427

  View details for Web of Science ID A1989AE26500157

• XAS STUDIES OF RUTHENIUM OCTAETHYLPORPHYRIN DIMERS PHYSICA B-CONDENSED MATTER Asahina, H., ZISK, M. B., Hedman, B., McDevitt, J. T., Collman, J. P., Hodgson, K. O. 1989; 158 (1-3): 217-218

  View details for Web of Science ID A1989AE26500082

• POLARIZED EXPERIMENTAL AND THEORETICAL K-EDGE X-RAY ABSORPTION STUDIES OF (SO4)-O-2-, CLO3(-), (S2O3)-O-2-, AND (S2O6)-O2- PHYSICAL REVIEW B Tyson, T. A., Roe, A. L., Frank, P., Hodgson, K. O., Hedman, B. 1989; 39 (10): 6305-6315

  View details for Web of Science ID A1989T985400001

• PHASE DETERMINATION BY MULTIPLE-WAVELENGTH X-RAY-DIFFRACTION - CRYSTAL-STRUCTURE OF A BASIC BLUE COPPER PROTEIN FROM CUCUMBERS SCIENCE Guss, J. M., Merritt, E. A., Phizackerley, R. P., Hedman, B., Murata, M., Hodgson, K. O., Freeman, H. C. 1988; 241 (4867): 806-811

  Abstract

  A novel x-ray diffraction technique, multiple-wavelength anomalous dispersion (MAD) phasing, has been applied to the de novo determination of an unknown protein structure, that of the "blue" copper protein isolated from cucumber seedlings. This method makes use of crystallographic phases determined from measurements made at several wavelengths and has recently been made technically feasible through the use of intense, polychromatic synchrotron radiation together with accurate data collection from multiwire electronic area detectors. In contrast with all of the conventional methods of solving protein structures, which require either multiple isomorphous derivatives or coordinates of a similar structure for molecular replacement, this technique allows direct solution of the classical "phase problem" in x-ray crystallography. MAD phase assignment should be particularly useful for determining structures of small to medium-sized metalloproteins for which isomorphous derivatives are difficult or impossible to make. The structure of this particular protein provides new insights into the spectroscopic and redox properties of blue copper proteins, an important class of metalloproteins widely distributed in nature.

  View details for Web of Science ID A1988P616200021

  View details for PubMedID 3406739

• NEW STRUCTURAL INSIGHTS INTO THE IRON MOLYBDENUM COFACTOR FROM AZOTOBACTER-VINELANDII NITROGENASE THROUGH SULFUR-K AND MOLYBDENUM-L X-RAY ABSORPTION-EDGE STUDIES JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hedman, B., Frank, P., GHELLER, S. F., Roe, A. L., Newton, W. E., Hodgson, K. O. 1988; 110 (12): 3798-3805

  View details for Web of Science ID A1988N751500013

• THE HGI2 ARRAY DETECTOR DEVELOPMENT PROJECT NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT Iwanczyk, J. S., Warburton, W. K., Hedman, B., Hodgson, K. O., Beyerle, A. 1988; 266 (1-3): 619-627

  View details for Web of Science ID A1988M829100108

• STRUCTURAL CHARACTERIZATION BY EXAFS SPECTROSCOPY OF THE BINUCLEAR IRON CENTER IN PROTEIN-A OF METHANE MONOOXYGENASE FROM METHYLOCOCCUS-CAPSULATUS (BATH) JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ericson, A., Hedman, B., Hodgson, K. O., Green, J., Dalton, H., BENTSEN, J. G., BEER, R. H., LIPPARD, S. J. 1988; 110 (7): 2330-2332

  View details for Web of Science ID A1988M741500070

• DEVELOPMENT OF MERCURIC IODIDE ENERGY DISPERSIVE-X-RAY ARRAY DETECTORS IEEE TRANSACTIONS ON NUCLEAR SCIENCE Iwanczyk, J. S., Warburton, W. K., DABROWSKI, A. J., Hedman, B., Hodgson, K. O., Patt, B. E. 1988; 35 (1): 93-97

  View details for Web of Science ID A1988N248100017

• A LARGE RESERVOIR OF SULFATE AND SULFONATE RESIDES WITHIN PLASMA-CELLS FROM ASCIDIA-CERATODES, REVEALED BY X-RAY ABSORPTION NEAR-EDGE STRUCTURE SPECTROSCOPY BIOCHEMISTRY Frank, P., Hedman, B., Carlson, R. M., Tyson, T. A., Roe, A. L., Hodgson, K. O. 1987; 26 (16): 4975-4979

  Abstract

  The study of sulfur within the plasma cells of Ascidia ceratodes [Carlson, R. M. K. (1975) Proc. Natl. Acad. Sci. U.S.A. 72, 2217-2221; Frank, P., Carlson, R. M. K., & Hodgson, K. O. (1986) Inorg. Chem. 25, 470-478; Hedman, B., Frank, P., Penner-Hahn, J. E., Roe, A. L., Hodgson, K. O., Carlson, R. M. K., Brown, G., Cerino, J., Hettel, R., Troxel, T., Winick, H., & Yang, J. (1986) Nucl. Instrum. Methods Phys. Res., Sect. A 246, 797-800] has been extended with X-ray absorption near-edge structure (XANES) spectroscopy. An intense absorption feature at 2482.4 eV and a second feature at 2473.7 eV indicate a large endogenous sulfate concentration, as well as smaller though significant amounts of thiol or thioether sulfur, respectively. A strong shoulder was observed at 2481.7 eV on the low-energy side of the sulfate absorption edge, deriving from a novel type of sulfur having a slightly lower oxidation state than sulfate sulfur. The line width of the primary transition on the sulfur edge of a vanadium (III) sulfate solution was found to be broadened relative to that of sodium sulfate, possibly deriving from the formation of the VSO4+ complex ion [Britton, H. T. S., & Welford, G. (1940) J. Chem. Soc., 761-764; Duffy, J. A., & Macdonald, W. J. D. (1970) J. Chem. Soc., 977-980; Kimura, T., Morinaga, M., & Nakano, J. (1972) Nippon Kagaku Zaishi, 664-667]. Similar broadening appears to characterize the oxidized sulfur types in vanadocytes. A very good linear correlation between oxidation state and peak position (in electronvolts) was found for a series of related sulfur compounds. This correlation was used to determine a 5+ oxidation state for the additional sulfur type at 2481.7 eV. (ABSTRACT TRUNCATED AT 250 WORDS)

  View details for Web of Science ID A1987J557100014

  View details for PubMedID 3663639

• EXAFS STUDIES OF BINUCLEAR IRON COMPLEXES AS MODELS FOR HEMERYTHRIN AND RELATED PROTEINS INORGANIC CHEMISTRY Hedman, B., Co, M. S., Armstrong, W. H., Hodgson, K. O., LIPPARD, S. J. 1986; 25 (20): 3708-3711

  View details for Web of Science ID A1986E224700040

• ELICITATION OF THIOMOLYBDATES FROM THE IRON-MOLYBDENUM COFACTOR OF NITROGENASE - COMPARISON WITH SYNTHETIC FE-MO-S COMPLEXES EUROPEAN JOURNAL OF BIOCHEMISTRY Newton, W. E., GHELLER, S. F., Hedman, B., Hodgson, K. O., LOUGH, S. M., McDonald, J. W. 1986; 159 (1): 111-115

  Abstract

  Aerial oxidation of the iron-molybdenum cofactor (FeMoco) of Azotobacter vinelandii nitrogenase has been shown to yield either the tetrathiomolybdate ion ([MoS4]2-) or the oxotrithiomolybdate ion ([MoOS3]2-), depending on the reaction conditions. Thus, when N-methylformamide (NMF) solutions of FeMoco either were titrated with measured aliquots of air or were diluted with air-saturated NMF, [MoOS3]2- was found to be the predominant product while dilution of NMF solutions of FeMoco with air-saturated methanol produced [MoS4]2- almost exclusively. Similar aerial oxidation of solutions of chemically synthesized Fe-Mo-S clusters showed that significant information about the molybdenum environment in these species could be deduced from the nature of the elicited thiomolybdates. The differences in decomposition products as a function of solvent are postulated to be due to the loss through precipitation of the reducing agent sodium dithionite on addition of methanol but not NMF. These overall decomposition results are discussed in the context of recent X-ray absorption spectroscopic data which suggest the presence of an 'MoS3' core in FeMoco. A possible mechanism whereby [MoS4]2- might be rapidly formed from this core is presented.

  View details for Web of Science ID A1986D928400015

  View details for PubMedID 3462002

• POLARIZED X-RAY ABSORPTION NEAR-EDGE STRUCTURE OF HIGHLY OXIDIZED CHROMIUM PORPHYRINS INORGANIC CHEMISTRY PENNERHAHN, J. E., Benfatto, M., Hedman, B., Takahashi, T., Doniach, S., Groves, J. T., Hodgson, K. O. 1986; 25 (13): 2255-2259

  View details for Web of Science ID A1986C875900032

• SULFUR K-EDGE X-RAY ABSORPTION STUDIES USING THE 54-POLE WIGGLER AT SSRL IN UNDULATOR MODE NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT Hedman, B., Frank, P., PENNERHAHN, J. E., Roe, A. L., Hodgson, K. O., Carlson, R. M., Brown, G., CERINO, J., HETTEL, R., TROXEL, T., Winick, H., Yang, J. 1986; 246 (1-3): 797-800

  View details for Web of Science ID A1986C799900165

• DEVELOPMENT OF MERCURIC IODIDE DETECTORS FOR XAS AND XRD MEASUREMENTS NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT Warburton, W. K., Iwanczyk, J. S., DABROWSKI, A. J., Hedman, B., PENNERHAHN, J. E., Roe, A. L., Hodgson, K. O., Beyerle, A. 1986; 246 (1-3): 558-560

  View details for Web of Science ID A1986C799900121

• IRON MOLYBDENUM COFACTOR OF AZOTOBACTER-VINELANDII NITROGENASE - OXIDATION REDUCTION PROPERTIES AND STRUCTURAL INSIGHTS POLYHEDRON Newton, W. E., Schultz, F. A., GHELLER, S. F., Lough, S., McDonald, J. W., Conradson, S. D., Hedman, B., Hodgson, K. O. 1986; 5 (1-2): 567-572

  View details for Web of Science ID A1986A651900089

• PROTEIN MICROCRYSTAL DIFFRACTION AND THE EFFECTS OF RADIATION-DAMAGE WITH ULTRA-HIGH-FLUX SYNCHROTRON RADIATION PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Hedman, B., Hodgson, K. O., Helliwell, J. R., Liddington, R., Papiz, M. Z. 1985; 82 (22): 7604-7607

  Abstract

  By using ultra-high-flux synchrotron x-radiation from a wiggler source, good Laue diffraction data have been obtained from protein microcrystals of size 30 X 35 X 10 microns3, mounted wet in glass capillaries. At the flux level of 10(13)-10(14) photons per sec/mm2, the radiation damage is still low enough to allow a large survey of reciprocal space for a microcrystal and a complete survey for a normal-sized protein crystal. The development of sources for ultra-high-intensity synchrotron radiation is thus an important improvement in the technique for determination of structure through protein crystallography as well as in other cases where crystal size is often a limiting factor.

  View details for Web of Science ID A1985AVA0100027

  View details for PubMedID 2415965

  View details for PubMedCentralID PMC391381

• MULTICOMPONENT POLYANIONS .36. HYDROLYSIS AND REDOX EQUILIBRIA OF THE H+-HVO42- SYSTEM IN 0.6 M NA(CL) - A COMPLEMENTARY POTENTIOMETRIC AND V-51 NMR-STUDY AT LOW VANADIUM CONCENTRATIONS IN ACID-SOLUTION ACTA CHEMICA SCANDINAVICA SERIES A-PHYSICAL AND INORGANIC CHEMISTRY PETTERSSON, L., HEDMAN, B., NENNER, A. M., ANDERSSON 1985; 39 (7): 499–506

  View details for DOI 10.3891/acta.chem.scand.39a-0499

  View details for Web of Science ID A1985AUG4400006

• 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

• THE STRUCTURE OF AMMONIUM HYDROGEN OXYDIACETATE ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE HERBERTSSON, H., HEDMAN, B. 1982; 38 (JAN): 320–22

  View details for DOI 10.1107/S0567740882002787

  View details for Web of Science ID A1982NF01400086

• MULTICOMPONENT POLYANIONS .31. THE STRUCTURE OF TETRAGUANIDINIUM DODECAMOLYBDOGERMANATE, [C(NH2)3]4MO12GEO40 ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE STRANDBERG, R., HEDMAN, B. 1982; 38 (MAR): 773–78

  View details for DOI 10.1107/S0567740882004063

  View details for Web of Science ID A1982NH02900018

• MULTICOMPONENT POLYANIONS .27. CRYSTAL-STRUCTURE AND ELECTRON-SPIN RESONANCE-SPECTRUM OF K6(V2,MO10)VO40-13H2O, A NEW ONE-ELECTRON HETEROPOLY BLUE ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE BJORNBERG, A., HEDMAN, B. 1980; 36 (MAY): 1018–22

  View details for DOI 10.1107/S0567740880005201

  View details for Web of Science ID A1980JZ21000004

• MULTICOMPONENT POLYANIONS .29. THE STRUCTURE OF TRISODIUM BIS(TETRAMETHYLAMMONIUM) MONOHYDROGENHEXAMOLYBDODIARSENATE HEPTAHYDRATE ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE HEDMAN, B. 1980; 36 (OCT): 2241–46

  View details for DOI 10.1107/S0567740880008503

  View details for Web of Science ID A1980KQ01000014

• MULTICOMPONENT POLYANIONS .19. MOLECULAR AND CRYSTAL-STRUCTURE OF NA5HMO5P2O23(H2O)11, A SUPERSTRUCTURE WITH SODIUM-COORDINATED "MONOHYDROGENPENTAMOLYBDODIPHOSPHATE ANIONS ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE HEDMAN, B., STRANDBERG, R. 1979; 35 (FEB): 278–84

  View details for DOI 10.1107/S0567740879003356

  View details for Web of Science ID A1979GL05800003

• MULTICOMPONENT POLYANIONS .18. NEUTRON-DIFFRACTION STUDY OF NA3MO9PO31(OH2)3.12-13H2O, A COMPOUND CONTAINING 9-MOLYBDOMONOPHOSPHATE ANIONS WITH MOLYBDENUM-COORDINATED WATER-MOLECULES ACTA CHEMICA SCANDINAVICA SERIES A-PHYSICAL AND INORGANIC CHEMISTRY HEDMAN, B. 1978; 32 (5): 439–46

  View details for DOI 10.3891/acta.chem.scand.32a-0439

  View details for Web of Science ID A1978FS30700010

• CRYSTAL-STRUCTURE OF NAVO3.1.89H2O ACTA CHEMICA SCANDINAVICA SERIES A-PHYSICAL AND INORGANIC CHEMISTRY BJORNBERG, A., HEDMAN, B. 1977; 31 (7): 579–84

  View details for DOI 10.3891/acta.chem.scand.31a-0579

  View details for Web of Science ID A1977DZ37900011

• MULTICOMPONENT POLYANIONS .16. MOLECULAR AND CRYSTAL-STRUCTURE OF NA6MO5P2O23(H2O)14, A COMPOUND CONTAINING SODIUM-COORDINATED PENTAMOLYBDODIPHOSPHATE ANIONS ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE HEDMAN, B. 1977; 33 (OCT): 3083–90

  View details for DOI 10.1107/S0567740877010309

  View details for Web of Science ID A1977DZ24700021

• MULTICOMPONENT POLYANIONS .15. MOLECULAR AND CRYSTAL-STRUCTURE OF NA[MO2O5(O3(OH)C6H8(OH)2)].2H2O, A PROTONIZED MANNITOLATODIMOLYBDATE COMPLEX ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE HEDMAN, B. 1977; 33 (OCT): 3077–83

  View details for DOI 10.1107/S0567740877010292

  View details for Web of Science ID A1977DZ24700020

• MULTICOMPONENT POLYANIONS .6. MOLECULAR AND CRYSTAL-STRUCTURE OF NA4H2MO5P2O23(H2O)10, A COMPOUND CONTAINING SODIUM-COORDINATED DIHYDROGENPENTAMOLYBDODIPHOSPHATE ANIONS ACTA CHEMICA SCANDINAVICA HEDMAN, B. 1973; 27 (9): 3335–54

  View details for DOI 10.3891/acta.chem.scand.27-3335

  View details for Web of Science ID A1973S157300023

• MULTICOMPONENT POLYANIONS .7. MOLECULAR AND CRYSTAL-STRUCTURE OF NA6MO7O24(H2O)14, A COMPOUND CONTAINING SODIUM-COORDINATED HEPTAMOLYBDATE ANIONS ACTA CHEMICA SCANDINAVICA SJOBOM, K., HEDMAN, B. 1973; 27 (10): 3673–91

  View details for DOI 10.3891/acta.chem.scand.27-3673

  View details for Web of Science ID A1973S472200009