Methyl transfer from methylcobalamin to thiols. A reinvestigation

The methyl transfer from methylcobalamin to thiols has been reinvestigated. By use of methylcobalamin selectively enriched with 13C in the methyl moiety, the methyl transfer to thiols was followed by 13C NMR. The methyl transfer occurs in aqueous mildly alkaline (pH 8-12) solution, even in the complete absence of oxygen. 31P NMR and EPR studies demonstrate that cob(II)alamin is the final corrinoid product. However, the pH dependence of the methyl-transfer reaction from methylcobalamin to beta-mercaptoethanol is consistent only with a nucleophilic displacement of the methyl group by a thiolate anion, resulting in the heterolytic cleavage of the carbon-cobalt bond. Difference visible spectroscopic measurements of the reaction mixture suggest that cob(I)alamin is formed as an intermediate.