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Castillo RG, Banerjee R, Allpress CJ, Rohde GT, Bill E, Que L, Lipscomb JD, DeBeer S. High-Energy-Resolution Fluorescence-Detected X-ray Absorption of the Q Intermediate of Soluble Methane Monooxygenase. J Am Chem Soc 2017; 139:18024-18033. [PMID: 29136468 PMCID: PMC5729100 DOI: 10.1021/jacs.7b09560] [Citation(s) in RCA: 85] [Impact Index Per Article: 12.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
Kα high-energy-resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) provides a powerful tool for overcoming the limitations of conventional XAS to identify the electronic structure and coordination environment of metalloprotein active sites. Herein, Fe Kα HERFD XAS is applied to the diiron active site of soluble methane monooxygenase (sMMO) and to a series of high-valent diiron model complexes, including diamond-core [FeIV2(μ-O)2(L)2](ClO4)4] (3) and open-core [(O═FeIV-O-FeIV(OH)(L)2](ClO4)3 (4) models (where, L = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine) (TPA*)). Pronounced differences in the HERFD XAS pre-edge energies and intensities are observed for the open versus closed Fe2O2 cores in the model compounds. These differences are reproduced by time-dependent density functional theory (TDDFT) calculations and allow for the pre-edge energies and intensity to be directly correlated with the local active site geometric and electronic structure. A comparison of the model complex HERFD XAS data to that of MMOHQ (the key intermediate in methane oxidation) is supportive of an open-core structure. Specifically, the large pre-edge area observed for MMOHQ may be rationalized by invoking an open-core structure with a terminal FeIV═O motif, though further modulations of the core structure due to the protein environment cannot be ruled out. The present study thus motivates the need for additional experimental and theoretical studies to unambiguously assess the active site conformation of MMOHQ.
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Affiliation(s)
- Rebeca G. Castillo
- Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34, D-45470 Mülheim an der Ruhr, Germany
| | - Rahul Banerjee
- Department of Biochemistry, Molecular Biology and Biophysics, 321 Church St. SE, Minneapolis, MN 55455
| | - Caleb J. Allpress
- Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455
| | - Gregory T. Rohde
- Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455
| | - Eckhard Bill
- Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34, D-45470 Mülheim an der Ruhr, Germany
| | - Lawrence Que
- Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455
| | - John D. Lipscomb
- Department of Biochemistry, Molecular Biology and Biophysics, 321 Church St. SE, Minneapolis, MN 55455
| | - Serena DeBeer
- Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34, D-45470 Mülheim an der Ruhr, Germany
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2
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Kerber WD, Perez KA, Ren C, Siegler MA. Speciation of Ferric Phenoxide Intermediates during the Reduction of Iron(III)−μ-Oxo Dimers by Hydroquinone. Inorg Chem 2014; 53:11507-16. [DOI: 10.1021/ic5014347] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- William D. Kerber
- Department of Chemistry, Bucknell University, Lewisburg, Pennsylvania 17837, United States
| | - Kaitlyn A. Perez
- Department of Chemistry, Bucknell University, Lewisburg, Pennsylvania 17837, United States
| | - Chuqiao Ren
- Department of Chemistry, Bucknell University, Lewisburg, Pennsylvania 17837, United States
| | - Maxime A. Siegler
- Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States
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Li Y, Myae Soe CM, Wilson JJ, Tuang SL, Apfel UP, Lippard SJ. Triptycene-based Bis(benzimidazole) Carboxylate-Bridged Biomimetic Diiron(II) Complexes. Eur J Inorg Chem 2013; 2013:2011-2019. [PMID: 23585728 PMCID: PMC3625018 DOI: 10.1002/ejic.201201387] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/16/2012] [Indexed: 11/06/2022]
Abstract
A triptycene-based bis(benzimidazole) ester ligand, L3, was designed to enhance the electron donating ability of the heterocyclic nitrogen atoms relative to those of the first generation bis(benzoxazole) analogs, L1 and L2. A convergent synthesis of L3 was designed and executed. Three-component titration experiments using UV-visible spectroscopy revealed that the desired diiron(II) complex could be obtained with a 1:2:1 ratio of L3:Fe(OTf)2(MeCN)2:external carboxylate reactants. X-ray crystallographic studies of two diiron complexes derived in this manner from L3 revealed their formulas to be [Fe2L3(μ-OH)(μ-O2CR)(OTf)2], where R = 2,6-bis(p-tolyl)benzoate (7) or triphenylacetate (8). The structures are similar to that of a diiron complex derived from L1, [Fe2L1(μ-OH)(μ-O2CArTol)(OTf)2] (9) with a notable difference being that, in 7 and 8, the geometry at iron more closely resembles square-pyramidal than trigonal-bipyramidal. Mössbauer spectroscopic analyses of 7 and 8 indicate the presence of high-spin diiron(II) cores. These results demonstrate the importance of substituting benzimidazole for benzoxazole for assembling biomimetic diiron complexes with syn disposition of two N-donor ligands, as found in O2-activating carboxylate-bridged diiron centers in biology.
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Affiliation(s)
- Yang Li
- Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, United States
| | - Chan Myae Myae Soe
- Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, United States
| | - Justin J. Wilson
- Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, United States
| | - Suan Lian Tuang
- Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, United States
| | - Ulf-Peter Apfel
- Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, United States
| | - Stephen J. Lippard
- Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, United States
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Valence-directed assembly and magnetic properties of two polynuclear pyrazine-2-amidoxime Fe complexes. INORG CHEM COMMUN 2012. [DOI: 10.1016/j.inoche.2012.05.036] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
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5
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An oxygenase that forms and deoxygenates toxic epoxide. Nature 2012; 483:359-62. [DOI: 10.1038/nature10862] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2011] [Accepted: 01/16/2012] [Indexed: 11/08/2022]
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6
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Xue G, Pokutsa A, Que L. Substrate-triggered activation of a synthetic [Fe2(μ-O)2] diamond core for C-H bond cleavage. J Am Chem Soc 2011; 133:16657-67. [PMID: 21899336 DOI: 10.1021/ja207131g] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
An [Fe(IV)(2)(μ-O)(2)] diamond core structure has been postulated for intermediate Q of soluble methane monooxygenase (sMMO-Q), the oxidant responsible for cleaving the strong C-H bond of methane and its hydroxylation. By extension, analogous species may be involved in the mechanisms of related diiron hydroxylases and desaturases. Because of the paucity of well-defined synthetic examples, there are few, if any, mechanistic studies on the oxidation of hydrocarbon substrates by complexes with high-valent [Fe(2)(μ-O)(2)] cores. We report here that water or alcohol substrates can activate synthetic [Fe(III)Fe(IV)(μ-O)(2)] complexes supported by tetradentate tris(pyridyl-2-methyl)amine ligands (1 and 2) by several orders of magnitude for C-H bond oxidation. On the basis of detailed kinetic studies, it is postulated that the activation results from Lewis base attack on the [Fe(III)Fe(IV)(μ-O)(2)] core, resulting in the formation of a more reactive species with a [X-Fe(III)-O-Fe(IV)═O] ring-opened structure (1-X, 2-X, X = OH(-) or OR(-)). Treatment of 2 with methoxide at -80 °C forms the 2-methoxide adduct in high yield, which is characterized by an S = 1/2 EPR signal indicative of an antiferromagnetically coupled [S = 5/2 Fe(III)/S = 2 Fe(IV)] pair. Even at this low temperature, the complex undergoes facile intramolecular C-H bond cleavage to generate formaldehyde, showing that the terminal high-spin Fe(IV)═O unit is capable of oxidizing a C-H bond as strong as 96 kcal mol(-1). This intramolecular oxidation of the methoxide ligand can in fact be competitive with intermolecular oxidation of triphenylmethane, which has a much weaker C-H bond (D(C-H) 81 kcal mol(-1)). The activation of the [Fe(III)Fe(IV)(μ-O)(2)] core is dramatically illustrated by the oxidation of 9,10-dihydroanthracene by 2-methoxide, which has a second-order rate constant that is 3.6 × 10(7)-fold larger than that for the parent diamond core complex 2. These observations provide strong support for the DFT-based notion that an S = 2 Fe(IV)═O unit is much more reactive at H-atom abstraction than its S = 1 counterpart and suggest that core isomerization could be a viable strategy for the [Fe(IV)(2)(μ-O)(2)] diamond core of sMMO-Q to selectively attack the strong C-H bond of methane in the presence of weaker C-H bonds of amino acid residues that define the diiron active site pocket.
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Affiliation(s)
- Genqiang Xue
- Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455, USA
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7
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Rather LJ, Weinert T, Demmer U, Bill E, Ismail W, Fuchs G, Ermler U. Structure and mechanism of the diiron benzoyl-coenzyme A epoxidase BoxB. J Biol Chem 2011; 286:29241-29248. [PMID: 21632537 DOI: 10.1074/jbc.m111.236893] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The coenzyme A (CoA)-dependent aerobic benzoate metabolic pathway uses an unprecedented chemical strategy to overcome the high aromatic resonance energy by forming the non-aromatic 2,3-epoxybenzoyl-CoA. The crucial dearomatizing reaction is catalyzed by three enzymes, BoxABC, where BoxA is an NADPH-dependent reductase, BoxB is a benzoyl-CoA 2,3-epoxidase, and BoxC is an epoxide ring hydrolase. We characterized the key enzyme BoxB from Azoarcus evansii by structural and Mössbauer spectroscopic methods as a new member of class I diiron enzymes. Several family members were structurally studied with respect to the diiron center architecture, but no structure of an intact diiron enzyme with its natural substrate has been reported. X-ray structures between 1.9 and 2.5 Å resolution were determined for BoxB in the diferric state and with bound substrate benzoyl-CoA in the reduced state. The substrate-bound reduced state is distinguished from the diferric state by increased iron-ligand distances and the absence of directly bridging groups between them. The position of benzoyl-CoA inside a 20 Å long channel and the position of the phenyl ring relative to the diiron center are accurately defined. The C2 and C3 atoms of the phenyl ring are closer to one of the irons. Therefore, one oxygen of activated O(2) must be ligated predominantly to this proximate iron to be in a geometrically suitable position to attack the phenyl ring. Consistent with the observed iron/phenyl geometry, BoxB stereoselectively should form the 2S,3R-epoxide. We postulate a reaction cycle that allows a charge delocalization because of the phenyl ring and the electron-withdrawing CoA thioester.
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Affiliation(s)
- Liv J Rather
- Mikrobiologie, Institut für Biologie II, Universität Freiburg, Schänzlestrasse 1, D-79104 Freiburg
| | - Tobias Weinert
- Max-Planck-Institut für Biophysik, Max-von-Laue-Strasse 3, D-60438 Frankfurt am Main, and
| | - Ulrike Demmer
- Max-Planck-Institut für Biophysik, Max-von-Laue-Strasse 3, D-60438 Frankfurt am Main, and
| | - Eckhard Bill
- Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany
| | - Wael Ismail
- Mikrobiologie, Institut für Biologie II, Universität Freiburg, Schänzlestrasse 1, D-79104 Freiburg
| | - Georg Fuchs
- Mikrobiologie, Institut für Biologie II, Universität Freiburg, Schänzlestrasse 1, D-79104 Freiburg
| | - Ulrich Ermler
- Max-Planck-Institut für Biophysik, Max-von-Laue-Strasse 3, D-60438 Frankfurt am Main, and.
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Tong JP, Shao F, Tao J, Huang RB, Zheng LS. Microwave-Assisted Synthesis of a Ferrimagnetic Dodecanuclear Iron(III) Complex with a Fe4(OH)4 Cubane Core. Inorg Chem 2011; 50:2067-9. [DOI: 10.1021/ic102411u] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Jia-Ping Tong
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China
| | - Feng Shao
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China
| | - Jun Tao
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China
| | - Rong-Bin Huang
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China
| | - Lan-Sun Zheng
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China
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Saito T, Kataoka Y, Nakanishi Y, Kitagawa Y, Kawakami T, Yamanaka S, Okumura M, Yamaguchi K. Theoretical studies on the electronic structure of the synthetic complex of soluble methanemonooxygenase intermediate Q. Supramol Chem 2010. [DOI: 10.1080/10610278.2010.510560] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
Affiliation(s)
- Toru Saito
- a Department of Chemistry , Graduate School of Science, Osaka University , 1-1 Machikaneyama, Toyonaka, Osaka, 560-0043, Japan
| | - Yusuke Kataoka
- a Department of Chemistry , Graduate School of Science, Osaka University , 1-1 Machikaneyama, Toyonaka, Osaka, 560-0043, Japan
| | - Yasuyuki Nakanishi
- a Department of Chemistry , Graduate School of Science, Osaka University , 1-1 Machikaneyama, Toyonaka, Osaka, 560-0043, Japan
| | - Yasutaka Kitagawa
- a Department of Chemistry , Graduate School of Science, Osaka University , 1-1 Machikaneyama, Toyonaka, Osaka, 560-0043, Japan
| | - Takashi Kawakami
- a Department of Chemistry , Graduate School of Science, Osaka University , 1-1 Machikaneyama, Toyonaka, Osaka, 560-0043, Japan
| | - Shusuke Yamanaka
- a Department of Chemistry , Graduate School of Science, Osaka University , 1-1 Machikaneyama, Toyonaka, Osaka, 560-0043, Japan
| | - Mitsutaka Okumura
- a Department of Chemistry , Graduate School of Science, Osaka University , 1-1 Machikaneyama, Toyonaka, Osaka, 560-0043, Japan
| | - Kizashi Yamaguchi
- a Department of Chemistry , Graduate School of Science, Osaka University , 1-1 Machikaneyama, Toyonaka, Osaka, 560-0043, Japan
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10
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Abstract
Coordination to transition-metal complexes changes both the thermodynamics and kinetics of oxygen reduction. Some of the intermediates (superoxo, hydroperoxo, and oxo species) are close analogues of organic oxygen-centered radicals and peroxides (ROO(*), ROOH, and RO(*)). Metal-based intermediates are typically less reactive, but more persistent, than organic radicals, which makes the two types of intermediates similarly effective in their reactions with various substrates. The self-exchange rate constant for hydrogen-atom transfer for the couples Cr(aq)OO(2+)/Cr(aq)OOH(2+) and L(1)(H(2)O)RhOO(2+)/L(1)(H(2)O)RhOOH(2+) was estimated to be 10(1+/-1) M(-1) s(-1). The use of this value in the simplified Marcus equation for the Cr(aq)O(2+)/Cr(aq)OOH(2+) cross reaction provided an upper limit k(CrO,CrOH) <or= 10((-2+/-1)) M(-1) s(-1) for Cr(aq)O(2+)/Cr(aq)OH(2+) self-exchange. Even though superoxo complexes react very slowly in bimolecular self-reactions, extremely fast cross reactions with organic counterparts, i.e., acylperoxyl radicals, have been observed. Many of the intermediates generated by the interaction of O(2) with reduced metal complexes can also be accessed by alternative routes, both thermal and photochemical.
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Affiliation(s)
- Andreja Bakac
- Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA
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11
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Million-fold activation of the [Fe(2)(micro-O)(2)] diamond core for C-H bond cleavage. Nat Chem 2010; 2:400-5. [PMID: 20414242 PMCID: PMC2859466 DOI: 10.1038/nchem.586] [Citation(s) in RCA: 183] [Impact Index Per Article: 13.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2009] [Accepted: 02/04/2010] [Indexed: 12/11/2022]
Abstract
In biological systems, the cleavage of strong C–H bonds is often carried out by iron centers – such as the methane monooxygenase in methane hydroxylation – through dioxygen activation mechanisms. High valent species with [Fe2(μ-O)2] diamond cores are thought to act as the oxidizing moieties, but the synthesis of complexes that cleave strong C–H bonds efficiently has remained a challenge. We report here the conversion of a synthetic complex with a valence-delocalized [Fe3.5(μ-O)2Fe3.5]3+ diamond core (1) into a complex with a valence-localized [HO-FeIII-O-FeIV=O]2+ open core (4), which cleaves C–H bonds over million-fold faster. This activity enhancement results from three factors: the formation of a terminal oxoiron(IV) moiety, the conversion of the low-spin (S = 1) FeIV=O center to a high-spin (S = 2) center, and the concentration of the oxidizing capability to the active terminal oxoiron(IV) moiety. This suggests that similar isomerization strategies might be employed by nonheme diiron enzymes.
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