Dealkylation of Quaternary Ammonium Salts by Thiolate Anions: A Model of the Cobalamin-independent Methionine Synthase Reaction.

Article abstract of DOI:10.1016/S0040-4020(01)85267-4

The reactions of thiolate ions derived from thiophenol and homocysteine with substituted quaternary ammonium salts result in alkyl transfer from nitrogen to sulfur.A radical mechanism for this transalkylation, accounts for the reactivity pattern of the substrate salts.In a model study of the cobalamin-independent methionine synthase reaction, 5,5,6,7-tetramethyl-5,6,7,8-tetrahydropteridinium salt (25), which can be considered as a model for the natural coenzyme 5-CH3H4-folate (1), was allowed to react with the thiolate of homocysteine, whereupon the formation of methionine was observed in good yield.These results suggest that in the enzymatic process the N(5)-CH3 bond may be activated for the methyl transfer step, by coordination of the N(5) with an electrophile or a proton at the active site.