All Publications

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


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

    View details for DOI 10.1021/jacs.0c09379

    View details for PubMedID 33315385

  • 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


    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

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


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

    View details for PubMedID 30429333

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


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

    View details for PubMedID 30362717

  • Kinetic and spectroscopic investigation of oxygen activation at a single iron center via Gibbs free energy coupling: Generation of an active alkane oxidation catalyst Cunningham, L., Babicz, J., Tucker, W., McCracken, J., Rybak-Akimova, E., Solomon, E., Caradonna, J. AMER CHEMICAL SOC. 2018
  • Correlating the structures and activities of the resting oxidized and native intermediate states of a small laccase by paramagnetic NMR JOURNAL OF INORGANIC BIOCHEMISTRY Machczynski, M. C., Babicz, J. T. 2016; 159: 62-69


    Multicopper oxidases (MCO) are the fastest and most efficient known catalysts of the oxygen-reduction reaction. When all four copper ions are oxidized during catalysis, the native intermediate state (NI) decays in seconds to the resting oxidized state (RO), which returns to the catalytic cycle via reduction, but at a much slower rate than NI. We report the long-lived (months at 4°C) NI state of the small laccase (SLAC) MCO and the subsequent characterization of both its RO and NI states by paramagnetic (1)H NMR. We find that the RO state of the trinuclear cluster (TNC) is best described as an isolated Type-3 dicopper site, antiferromagnetically coupled by a hydroxo group with -2J=500cm(-1). The NI state is more complicated; we develop a theoretical treatment for the case in which all three copper ions in the TNC are coupled, and find that the results are consistent with three coupling constants of -2J=300, 240, and 160cm(-1). These couplings result in a ground doublet state, a low-lying excited doublet state at 121cm(-1), and a quartet excited state at 411cm(-1), in good agreement with DFT models in which the Type-2 copper has a terminal hydroxo ligand.

    View details for DOI 10.1016/j.jinorgbio.2016.02.002

    View details for Web of Science ID 000378021000009

    View details for PubMedID 26918900