Coenzyme F430 from Methanogenic Bacteria: Mechanistic Studies on the Reductive Cleavage of Sulfonium Ions Catalyzed by F430 Pentamethyl Ester

Article abstract of DOI:10.1002/hlca.19920750504

Mechanistic questions regarding the reductive cleavage of sulfonium ions by the Ni^I form of coenzyme F430 pentamethylester (F430M) were adressed in a series of kinetic studies and isotope labeling experiments.In neat DMF, methane formation from Dialkyl(methyl)sulfonium ions consistently showed a delay time of ca. 1 h.In the presence of excess propanethiol, no delay was observed and methane formation followed pseudo-first-order kinetics with a logarithmic dependence of the initial rate on the concentration of propanethiol.From the temperature dependence of the reaction rate, an estimate for the activation parameters ΔH(excit.) = 49 kJ mol^-1 and (apparent) ΔS(excit.) = -114 J K^-1 mol^-1 was derived.The observation of deuterium incorporation into methane from (CH3)2CHOD, but not from (CH3)2CDOH, indicates that the fourth H-entity is introduced into CH4 as a proton, and that free CH3 radicals are not involved.In contrast to the reaction with the homogeneous one-electron reductant sodium naphthalide, the F430M-catalyzed reduction of mixed dialkyl(methyl)sulfonium ions showed a pronounced selectivity for the cleavage of Me-S over that of alkyl-S (alkyl <> Me) bonds.Mechanisms that are consistent with these results, as well as possible explanations for the time delay and the apparent highly negative entropy of activation, are discussed.