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Title: Reactions of the diiron(IV) intermediate Q in soluble methane monooxygenase with fluoromethanes

Abstract

Soluble methane monooxygenases utilize a carboxylate-bridged diiron center and dioxygen to convert methane to methanol. A diiron(IV) oxo intermediate Q is the active species for this process. Alternative substrates and theoretical studies can help elucidate the mechanism. Experimental results for reactions with derivatized methanes were previously modeled by a combination of quantum mechanical/molecular mechanical techniques and the model was extended to predict the relative reactivity of fluoromethane. We therefore studied reactions of Q with CF {sub n}H{sub 4-n} (n = 1-3) to test the prediction. The kinetics of single-turnover reactions of Q with these substrates were monitored by double-mixing stopped-flow optical spectroscopy. For fluoro- and difluoromethane, conversion to the alcohols occurred with second-order rate constants less than that of methane, the values being 28,700 (CH{sub 4}) > 25,000 (CFH{sub 3}) > 9300 (CF{sub 2}H{sub 2}) M{sup -1} s{sup -1}. KIE values for C-H versus C-D activation above the classical limit were observed, requiring modification of the theoretical predictions.

Authors:
 [1];  [2]
  1. Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139 (United States)
  2. Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139 (United States). E-mail: lippard@mit.edu
Publication Date:
OSTI Identifier:
20793196
Resource Type:
Journal Article
Resource Relation:
Journal Name: Biochemical and Biophysical Research Communications; Journal Volume: 338; Journal Issue: 1; Other Information: DOI: 10.1016/j.bbrc.2005.08.220; PII: S0006-291X(05)01946-7; Copyright (c) 2005 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; CARBOXYLATION; HYDROXYLASES; KINETICS; METHANE; METHANOL; OXIDATION; QUANTUM MECHANICS; SUBSTRATES; SULFUR IONS

Citation Formats

Beauvais, Laurance G., and Lippard, Stephen J.. Reactions of the diiron(IV) intermediate Q in soluble methane monooxygenase with fluoromethanes. United States: N. p., 2005. Web. doi:10.1016/J.BBRC.2005.0.
Beauvais, Laurance G., & Lippard, Stephen J.. Reactions of the diiron(IV) intermediate Q in soluble methane monooxygenase with fluoromethanes. United States. doi:10.1016/J.BBRC.2005.0.
Beauvais, Laurance G., and Lippard, Stephen J.. Fri . "Reactions of the diiron(IV) intermediate Q in soluble methane monooxygenase with fluoromethanes". United States. doi:10.1016/J.BBRC.2005.0.
@article{osti_20793196,
title = {Reactions of the diiron(IV) intermediate Q in soluble methane monooxygenase with fluoromethanes},
author = {Beauvais, Laurance G. and Lippard, Stephen J.},
abstractNote = {Soluble methane monooxygenases utilize a carboxylate-bridged diiron center and dioxygen to convert methane to methanol. A diiron(IV) oxo intermediate Q is the active species for this process. Alternative substrates and theoretical studies can help elucidate the mechanism. Experimental results for reactions with derivatized methanes were previously modeled by a combination of quantum mechanical/molecular mechanical techniques and the model was extended to predict the relative reactivity of fluoromethane. We therefore studied reactions of Q with CF {sub n}H{sub 4-n} (n = 1-3) to test the prediction. The kinetics of single-turnover reactions of Q with these substrates were monitored by double-mixing stopped-flow optical spectroscopy. For fluoro- and difluoromethane, conversion to the alcohols occurred with second-order rate constants less than that of methane, the values being 28,700 (CH{sub 4}) > 25,000 (CFH{sub 3}) > 9300 (CF{sub 2}H{sub 2}) M{sup -1} s{sup -1}. KIE values for C-H versus C-D activation above the classical limit were observed, requiring modification of the theoretical predictions.},
doi = {10.1016/J.BBRC.2005.0},
journal = {Biochemical and Biophysical Research Communications},
number = 1,
volume = 338,
place = {United States},
year = {Fri Dec 09 00:00:00 EST 2005},
month = {Fri Dec 09 00:00:00 EST 2005}
}
  • Intermediate Q, the methane-oxidizing species of soluble methane monooxygenase, is proposed to have an [Fe{sup IV}{sub 2}({mu}-O){sub 2}] diamond core. In an effort to obtain a synthetic precedent for such a core, bulk electrolysis at 900 mV (versus Fc{sup +/0}) has been performed in MeCN at -40{sup o}C on a valence-delocalized [Fe{sup III}Fe{sup IV}({mu}-O){sub 2}(L{sup b}){sub 2}]{sup 8+} complex (1b) (E{sub 1/2} = 760 mV versus Fc{sup +/0}). Oxidation of 1b results in the near-quantitative formation of a deep red complex, designated 2b, that exhibits a visible spectrum with {lambda}{sub max} at 485 nm (9,800 M{sup -1}{center_dot}cm{sup -1}) and 875more » nm (2,200 M{sup -1}{center_dot}cm{sup -1}). The 4.2 K Moessbauer spectrum of 2b exhibits a quadrupole doublet with {delta} = -0.04(1) mm{center_dot}s{sup -1} and {Delta}E{sub Q} = 2.09(2) mm{center_dot}s{sup -1}, parameters typical of an iron(IV) center. The Moessbauer patterns observed in strong applied fields show that 2b is an antiferromagnetically coupled diiron(IV) center. Resonance Raman studies reveal the diagnostic vibration mode of the [Fe{sub 2}({mu}-O){sub 2}] core at 674 cm{sup -1}, downshifting 30 cm{sup 01} upon {sup 18}O labeling. Extended x-ray absorption fine structure (EXAFS) analysis shows two O/N scatterers at 1.78 {angstrom} and an Fe scatterer at 2.73 {angstrom}. Based on the accumulated spectroscopic evidence, 2b thus can be formulated as [Fe{sup IV}{sub 2}({mu}-O){sub 2}(L{sup b}){sub 2}]{sup 4+}, the first synthetic complex with an [Fe{sup IV}{sub 2}({mu}-O){sub 2}] core. A comparison of 2b and its mononuclear analog [Fe{sup IV}(O)(L{sup b})(NCMe)]{sup 2+} (4b) reveals that 4b is 100-fold more reactive than 2b in oxidizing weak C-H bonds. This surprising observation may shed further light on how intermediate Q carries out the hydroxylation of methane.« less
  • No abstract prepared.
  • We present new optical and resonance Raman spectroscopic data that characterize the first intermediate as a diiron(III) peroxo species. The time-resolved optical and freeze-quench resonance Raman spectroscopic experiments strongly support assignment of the first intermediate, variously designated P or L, in the MMO hydroxylase reaction cycle as a symmetrical diiron(III) peroxo species. 32 refs., 2 figs.
  • The electronic structures of key species involved in methane hydroxylation performed by the
  • In their efforts to model high-valent intermediates in the oxygen activation cycles of nonheme diiron enzymes such as methane monooxygenase (MMOH-Q) and ribonucleotide reductase (RNR R2-X), the authors have synthesized and spectroscopically characterized a series of bis({mu}-oxo)diiron(III,IV) complexes, [Fe{sub 2}({mu}-O){sub 2}(L){sub 2}](ClO{sub 4}){sub 3}, where L is tris(2-pyridylmethyl)amine (TPA) or its ring-alkylated derivatives. They now report the crystal structure of [Fe{sub 2}({mu}-O){sub 2}(5-Et{sub 3}-TPA){sub 2}](ClO{sub 4}){sub 3} (2), the first example of a structurally characterized reactive iron(IV)-oxo species, which provides accurate metrical parameters for the diamond core structure proposed for this series of complexes. Complex 2 has Fe-{mu}-O distances ofmore » 1.805(3) {angstrom} and 1.860(3) {angstrom}, an Fe-Fe distance of 2.683(1) {angstrom}, and an Fe-{mu}-O-Fe angle of 94.1(1){degree}. The EXAFS spectrum of 2 can be fit well with a combination of four shells: 1 O at 1.82 {angstrom}, 2--3 N at 2.03 {angstrom}, 1 Fe at 2.66 {angstrom}, and 7 C at 2.87 {angstrom}. The distances obtained are in very good agreement with the crystal structure data for 2, though the coordination numbers for the first coordination sphere are underestimated. The EXAFS spectra of MMOH-Q and RNR R2-X contain features that match well with those of 2 (except for the multi-carbon shell at 2.87 {angstrom} arising from pyridyl carbons which are absent in the enzymes), suggesting that an Fe{sub 2}({mu}-O){sub 2} core may be a good candidate for the core structures of the enzyme intermediates. The implications of these studies are discussed.« less