Transmethylation by Cobalamins

Transmethylation by Cobalamins.— Two mechanisms of methyl transfer from cobalamins are found, a homolytic process with metals that have reduction potentials less than 0.50 V and an electrophilic (carbanion heterolytic) process for metals with potentials greater than 0.85 V. The PF /ii couple is 0.76 V and both oxidation states are required for methylation. A mechanism is proposed in which [Ptcy binds through the propionamide and acetamide side-chains of the corrin thereby promoting formation of the base on form and activating the Co— C bond. The methylation step involves [PtCIe] either in a redox switch where two electrons are abstracted from bound Pt followed by methyl transfer to the then bound Pt v or by direct electrophilic attack by [PtClel  

An electrophilic attack mechanism is proposed for the transmethylation reactions of organometallic complexes by methylcobalamin. Reaction rates show first-order dependences on methylcobalamin and the organometallic concentrations with second-order rate constants varying from 6.9 x 10 M s for MeHgOAc to 4.5 x 10 for MegSnOAc at 25 °C, pH 4.55, and ionic 

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