Functional modeling of cobalamine-independent methionine synthase with pyrazolylborate-zinc-thiolate complexes

A series of new pyrazolylborate-zinc-thiolate complexes Tp(Ph,Me)Zn-SR and Tp(Me,Me)Zn-SR, including two homocysteine derivatives, were prepared and structurally characterized. Their reactions with methyl iodide in nonpolar media resulted in the formation of the thioethers MeSR, including two methionine derivatives, and Tp(R',Me)Zn-I in all cases. Methylation of the thiolates could also be achieved with dimethyl sulfate and trimethylsulfonium iodide but not with trimethyl phosphate or N-methylpyridinium salts. The accumulated evidence indicates that the methylation occurs intramolecularly, i.e., at the zinc-bound thiolates: (i) The reactions occur readily in nonpolar media. (ii) Thiolate exchange at Tp(Ph,Me)Zn-SR with [PPN]SR' is slower than thiolate alkylation. (iii) The methylation of Tp(Ph,Me)Zn-SBn with MeI is a clean second-order reaction with k'' = 1.75 x 10(-2) M(-1) s(-1) at 300 K.