A synthetic precedent for the [FeIV2(μ-O)2] diamond core proposed for methane monooxygenase intermediate Q
- Genqiang Xue*,
- Dong Wang*,
- Raymond De Hont†,
- Adam T. Fiedler*,
- Xiaopeng Shan*,
- Eckard Münck†,‡, and
- Lawrence Que, Jr.*,‡
- *Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455; and
- †Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213
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Edited by Edward I. Solomon, Stanford University, Stanford, CA, and approved November 1, 2007 (received for review September 7, 2007)
Abstract
Intermediate Q, the methane-oxidizing species of soluble methane monooxygenase, is proposed to have an [FeIV
2(μ-O)2] diamond core. In an effort to obtain a synthetic precedent for such a core, bulk electrolysis at 900 mV (versus Fc+/0) has been performed in MeCN at −40°C on a valence-delocalized [FeIIIFeIV(μ-O)2(Lb)2]3+ complex (1b) (E
1/2 = 760 mV versus Fc+/0). Oxidation of 1b results in the near-quantitative formation of a deep red complex, designated 2b, that exhibits a visible
spectrum with λmax at 485 nm (9,800 M−1·cm−1) and 875 nm (2,200 M−1·cm−1). The 4.2 K Mössbauer spectrum of 2b exhibits a quadrupole doublet with δ = −0.04(1) mm·s−1 and ΔE
Q = 2.09(2) mm·s−1, parameters typical of an iron(IV) center. The Mössbauer 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 [Fe2(μ-O)2] core at 674 cm−1, downshifting 30 cm−1 upon 18O labeling. Extended x-ray absorption fine structure (EXAFS) analysis shows two O/N scatterers at 1.78 Å and an Fe scatterer
at 2.73 Å. Based on the accumulated spectroscopic evidence, 2b thus can be formulated as [FeIV
2(μ-O)2(Lb)2]4+, the first synthetic complex with an [FeIV
2(μ-O)2] core. A comparison of 2b and its mononuclear analog [FeIV(O)(Lb)(NCMe)]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.
Footnotes
- ‡To whom correspondence may be addressed. E-mail: que{at}chem.umn.edu or emunck{at}cmu.edu
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Author contributions: G.X., D.W., E.M., and L.Q. designed research; G.X., D.W., R.D.H., and A.T.F. performed research; G.X. contributed new reagents/analytic tools; G.X., D.W., R.D.H., A.T.F., X.S., and E.M. analyzed data; and G.X., D.W., R.D.H., A.T.F., E.M., and L.Q. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission.
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See Commentary on page 20641.
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This article contains supporting information online at www.pnas.org/cgi/content/full/0708516105/DC1.
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↵ § The previously reported reversible redox wave at 560 mV versus Fc+/0 (ref. 21) was irreproducible and probably artifactual.
- © 2007 by The National Academy of Sciences of the USA
