Direct Determination of the Number of Electrons Needed To Reduce Coenzyme F430 Pentamethyl Ester to the Ni(I) Species Exhibiting the Electron Paramagnetic Resonance and Ultraviolet−Visible Spectra Characteristic for the MCRred1State of Methyl-coenzyme M Reductase

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Abstract
The UV−visible and electron paramagnetic resonance (EPR) spectra of MCRred1, the catalytically active state of methyl-coenzyme M reductase, are almost identical to those observed when free coenzyme F430 or its pentamethyl ester (F430M) are reduced to the Ni(I) valence state. Investigations and proposals concerning the catalytic mechanism of MCR were therefore based on MCRred1 containing NiIF430 until, in a recent report, Tang et al. (J. Am. Chem. Soc. 2002, 124, 13242) interpreted their resonance Raman data and titration experiments as indicating that, in MCRred1, coenzyme F430 is not only reduced at the nickel center but at one of the CN double bonds of the hydrocorphinoid macrocycle as well. To resolve this contradiction, we have investigated the stoichiometry of the reduction of coenzyme F430 pentamethyl ester (F430M) by three independent methods. Spectroelectrochemistry showed clean reduction to a single product that exhibits the UV−vis spectrum typical for MCRred1. In three bulk electrolysis experiments, 0.96 ± 0.1 F/mol was required to generate the reduced species. Reduction with decamethylcobaltocene in tetrahydrofuran (THF) consumed 1 mol of (Cp*)2Co/mol of F430M, and the stoichiometry of the reoxidation of the reduced form with the two-electron oxidant methylene blue was 0.46 ± 0.05 mol of methylene blue/mol of reduced F430M. These experiments demonstrate that the reduction of coenzyme F430M to the species having almost identical UV−vis and EPR spectra as MCRred1 is a one-electron process and therefore inconsistent with a reduction of the macrocycle chromophore.

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