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Whole-Cell Mössbauer Studies

    In 1998 we found that purified FNR transcription factor, E. coli's oxygen sensor, contains a [4Fe-S] cluster which converts to a [2Fe-2S] cluster in the presence of O2. We wondered whether this conversion would also take place in whole cells. Using overexpressed FNR we were able to show that this process indeed  takes place in cells.1 Since then we have studied a variety of recombinant proteins in their cellular environent, e.g. ref 2. More recently we have studied yeast mitochondria, and found that one can obtained a rather detailed picture of  most iron environments in this organelle.3

(1)     Popescu, C.; Bates, D. M.; Beinert, H.; Münck, E., Kiley, P. J. “Mössbauer Spectroscopy as a Tool for the Study of Activation/Inactivation of the Transcription Regulator FNR in Whole Cells of E. coli”, Proc. Natl. Acad.    Sci. USA 1998, 95, 13431-13435.

(2)     Vrajmasu, V.; Bominaar, E. L.; Meyer, J.; Münck, E. “Mössbauer Study of Reduced Rubredoxin as Purified and in Whole Cells: Structural Correlation Analysis of Spin Hamiltonian Parameters” Inorg. Chem. 2002, 41,6358-6371.

(3)     Garber-Morales, J. Holmes-Hampton, G. P.; Miao, R.; Guo, Y.; Münck, E.; Lindahl. P. A. “Characterization of Iron in Mitochondria Isolated from Respiring and Fermenting Yeast” Biochemistry, 2010, 49, 5436-44.

 Iron Sulfur Clusters

     We have been fortunate to be involved in characterizing novel structures such as the nitrogenase M and P-centers,1 the coupled heme-[4Fe-4S] chromophore of E. coli. sulfite reductase,2 [3Fe-4S] clusters,3 the clusters of carbon monoxide dehydrogenase,4 the key FeIVFeIV intermediate (compound Q) in the catalytic cycle of methane monooxygenase,5 the H-cluster of [Fe]-hydrogenases,6 and the oxygen sensor of E. coli.7 For well established structures, we have been interested in attaining new oxidation states, e.g. the all-ferrous [4Fe-4S] cluster.8 In general terms, our interests are described in Ref 9. We continue to study a variety of iron-sulfur proteins.

        Our idea of the nitrogenase P-cluster shortly
before crystallographic refinement by Rees et al. (1992)

(1)      Surerus, K. K.; Hendrich, M. P.; Christie, P.; Rottgardt, D.; Orme-Johnson, W. H.; Münck, E. J. Am. Chem. Soc. 1992, 114, 8579-8590.
(2)      Bominaar, E. L.; Hu, Z.; Münck, E; Girerd, J.-J.; Borshch, S. J. Am. Chem. Soc. 1995, 117, 6976-89.
(3)      Emptage, M. H.; Kent, T. A.; Huynh, B. H.; Rawlings, J.; Orme-Johnson, W.H.; Münck, E. J. Biol. Chem. 1980, 255, 1793-1796.
(4)      Xia, J.; Hu, Z.; Popescu, C. V.; Lindahl, P. A.; Münck, E. J. Am. Chem. Soc . 1997, 119, 8301-12.
(5)      Shu, L.; Nesheim, J. C.; Kauffmann, K.; Münck, E.; Lipscomb, J. D.; Que Jr., L. Science 1997, 275, 515-518.
(6)      Popescu, C. V.; Münck, E. J. Am. Chem. Soc. 1999, 121, 7877-7884.
(7)      Popescu, C.; Bates, D. M.; Beinert, H.; Münck, E.; Kiley, P. J. Proc. Natl. Acad. Sci. USA 1998, 95, 13431-13435.
(8)      Yoo, S. J.; Angove H. C.; Burgess, B.K.; Hendrich, M.P.; Münck, E. J. Am. Chem. Soc. 1999, 121, 2534-2545.
(9)      Beinert, H.; Holm, R.H.; Münck, E. "Iron-Sulfur Clusters: Nature’s Modular Multipurpose Structures" Science 1997, 277, 653-659.
(10)    Chakrabati, M.; Deng, L.; Holm, R. H.; Munck, E.; Bominaar, E. L. Inorg. Chem. 2009, 48, 2735-2727 (Cover page Inorg. Chem. Apr. 6, 2009).



High-Valent complexes of Biological Relevance

    Together with the group of Lawrence Que, Jr., at the University of Minnesota, we are studying the electronic structures of high-valent iron complexes relevant to oxygen activation. Our joint projects have produced a larger number of interesting compounds. Among these is the first nonheme FeIV-oxo complex,1 [FeIV(O)(TMC)(NCMe)]2+. This complex has electronic spin S = 1. For nonheme proteins the combination of carboxylate, histidine, H2O, OH- ligands generally produces high-spin (S = 2) FeIV sites, an environment not easily produced in a synthetic complex.

    Collaborating with the groups of T. J. Collins here at CMU and L. Que, Jr. we reported in 2008 a comprehensive spectroscopic study of the first FeV=O complex, using the Collins TAML ligand (figure 2).2 In 2012 we followed with the second FeV=O complex.3 This complex was generated by reacting [FeIV(O)(TMC)(NCMe)]2+ at -44 °C with tert-butyl hydroperoxide in the presence of a strong base. The new species exhibits in the glass-forming 3:1 butyronitrile/MeCN solvent an S = 1/2 EPR signal with very narrow lines (4 gauss). The high-resolution allowed mapping of 14N, 57Fe and 17O hyperfine tensors by EPR. It required detailed DFT analysis (with choice of a suitable functional) to interpret the Mossbauer, EPR and resonance Raman data in a consistent way. The S = 1/2 species turned out to be an iron(V) complex having axial oxo and acetylimido ligands, namely [FeV(O)(TMC)(NC(O)CH3)]+. We are currently pursuing other systems for which FeV=O species are indicated, including complexes implicated in water oxidation.

    We have recently studied a group of high-valent diiron complexes based on the TPA ligand (traded by insiders as the Castro Brothers). The FeIVFeIV complex has two local S = 1 sites which are ferromagnetically coupled to yield an S = 2 system state.4 One site, Feb, has a terminal oxo group; Fea has a hydroxo ligand. (figure 3) Given that the Fe-O-Fe angle is 130°, the observation of ferromagnetic coupling was puzzling, but could be explained quite well after realizing that the two sites have different ligand fields that produce a crucial pair of orthogonal magnetic orbitals. Reduction by one electron renders Fea site high-spin FeIII. Concomitantly Feb undergoes a transition to high-spin (Sb=2) FeIV=O, a transition that is driven by superexchange interactions between Fea and Feb (By going high-spin, the FeIV=O site enables efficient antiferromagnetic pathways between the two Fe).5 For the same TPA ligand our collaborators have produced dinuclear complexes with open and closed cores, such as O=FeIV-O-FeIV-OH and FeIV(µ-O)2FeIV. Core opening increases H-bond cleaving reactivity roughly 1000-fold; the spin transition at the oxo site yields an additional 1000-fold increase (see Fig. 4 of ref 6).





(1)     Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.; Bukowsky, M. R.; Stubna, A.; Münck, E.; Nam, W.; Que Jr., L. Science 2003, 299, 1037-1039.

(2)     Tiago de Oliveira, F.; Chanda, A.; Banerjee, D.; Mondal, D.; Bominaar, E.; Münck, E.; Collins, T. C. Science 2006, 315, 835-838. (PMID: 17185561)

(3)     Van Heuvelen, K. M.; Fiedler, A. T.; Shan, X.;  De Hont, R. F.; Meier, K. K.; Bominaar, E. L.; Münck, E.;  Que, Jr., L.  Proc. Natl. Acad. Sci .2012, 109, 1193-38.

(4)     Martinho, M.; Xue, G.; Fiedler, A. T.; Que, Jr., L.; Bominaar, E. L.; Münck, E.  J. Am. Chem. Soc. 2009, 131, 5823-5830.

(5)     De Hont, R. F.; Xue, G.; Hendrich, M. P.; Que, L. Jr.; Bominaar, E. L.; Munck, E. Inorg. Chem. 2010, 49, 8310-8322.

(6)     Xue, G.; De Hont, R. F.; Münck, E.; Que, L. Jr. Nature Chem. 2010, 2, 400-405.



Dioxygenases and Monooxygenases    Together with the group of J. D. Lipscomb at the University of Minnesota we are studying various oxygen activating enzymes, among them the Fe2+ homoprotocatechuate 2,3 dioxygenase (2,3 HPCD) which cleaves the aromatic ring of homoprotocatechate (HPCA) adjacent to the vicinal hydroxyl groups. This enzyme represents a large group of dioxygenases involved in bacterial degradation pathways of natural and man-made compounds. In the native state of 2,3 HPCD the Fe2+ is coordinated by a 2-His-1-Glu facial triad; the three remaining coordination sites are occupied by water molecules which are displaced upon (bidentate) substrate and O2 binding. The figure summarizes current ideas of the mechanism. After oxygen binds, an electron is transferred through the iron to the oxygen, giving both substrates radical character (SQ· -FeII-O2·- in step D ). Recombination of the radicals would yield an alkylperoxo intermediate, step E.  A subsequent Criegee-type rearrangement would result in O-O bond cleaving to yield a lactone intermediate, with the second oxygen retained on the iron. Hydrolysis of the lactone by this oxygen would then yield the product.

    For the native enzyme and three mutants we have followed the catalytic reaction with rapid freeze-quench Mössbauer and EPR spectroscopy and characterized a variety of intermediates, among them an Fe3+-superoxo species and a semiquinone-Fe3+-hydroperoxide complex (1-3). For native HPCD and its mutants, E. Kovaleva and J. D. Lipscomb have obtained  > 50 high-resolution X-structures as well as a wealth of kinetic data. For a variety of states we have obtained well resolved Mössbauer spectra which, in conjunction with quantum chemical calculations, can be used to gain insight into the electronic structures of crucial intermediates. This will allow us to relate electronic and X-ray structure data along the reaction pathways and thus obtain crucial information about reactivity. The figure shows Mössbauer spectra of an E-S complex; red lines: spectral simulations).

    We are continuing ongoing studies aimed at characterizing  intermediates of the reaction cycle of methane monooxygenase, such as species Q, Q' and P*.

(1)     Mbughuni, M. M., Chakrabarti, M., Hayden, J. A., Bominaar, E. L., Hendrich, M. P., Münck, E., and Lipscomb, J. D. (2010) Trapping and spectroscopic characterization of an FeIII-superoxo intermediate from a nonheme mononuclear iron-containing enzyme. Proc. Natl. Acad. Sci. U. S. A. 107, 16788-16793.

(2)     Mbughuni, M. M., Chakrabarti, M., Hayden, J. A., Meier, K. K., Dalluge, J. J., Hendrich, M. P., Münck, E., and Lipscomb, J. D. (2011) Oxy-intermediates of homoprotocatechuate 2,3-dioxygenase: Facile electron transfer between substrates. Biochemistry 50, 10262-10274.

(3)     Mbughuni, M. M.; Meier, K. K.; Münck, E.; Lipscomb, J. D. "Substrate-Mediated Oxygen Activation by Homoprotocatechuate 2,3-Dioxygenase: Intermediates Formed by a Tyrosine 257 Variant" Biochemistry  2012,51, 8743-54.



Electronic Structure Analysis

     The spectroscopic studies of our group have often been complemented by DFT calculations. These computations provide theoretical estimates of experimentally determined spin-Hamiltonian parameters, such as zero-field splittings, exchange-coupling constants, 57Fe isomer shifts, quadrupole splittings, and magnetic hyperfine coupling constants, and give detailed insights into the dependency of these parameters on molecular geometry and electronic structure. These studies have clarified the origin of the unquenched orbital momentum in the diketiminate complexes of iron,1,2 the intrinsic mechanism for the distortion of the Fe(SR)4 center in rubredoxins,3 the influence of excited spin triplet states on the zero-field splitting in reduced form of these centers,4 and the oxidation state of the cofactor in nitrogenase.5

(1)    Andres, H.; Bominaar, E.L.; Smith, J.M.; Eckert, N.A.; Holland, P.L.; Münck, E. J. Am. Chem. Soc. 2002, 124, 3012-3025.
(2)    Stoian, S.A.; Yu, Y.; Smith, J.M.; Holland, P.L.; Bominaar, E.L.; Münck, E. Inorg. Chem. 2005, 44, 4915-4922.
(3)    Vrajmasu, V.V.; Münck, E.; Bominaar, E.L. Inorg. Chem. 2004, 43, 4862-4866; ibid. 4867-4879.
(4)    Vrajmasu, V.V.; Bominaar, E.L.; Meyer, J.; Münck, E. Inorg. Chem. 2002, 41, 6358-6371.
(5)    Vrajmasu, V.V.; Münck, E.; Bominaar, E.L. Inorg. Chem. 2003, 42, 5974-5988.




updated 11/2012