Theoretical study of the reduction mechanism of sulfoxides by thiols

Theoretical computations have been carried out to investigate the reaction mechanism of the sulfoxide reduction by thiols in solution. This reaction is a suitable model for enzymatic processes involving methionine sulfoxide reductases (Msrs). Recent investigations on the Msr mechanism have clearly shown that a sulfenic acid intermediate is formed on the catalytic cysteine of the active site concomitantly to the methionine product. In contrast, experimental studies for the reaction of a number of thiols and sulfoxides in solution did not observe sulfenic acid formation. Only, a disulfide was identified as the final product of the process. The present study has been carried out at the MP2/6-311+G(3d2f,2df,2p)//B3LYP/6-311G(d,p) level of theory. The solvent effect in DMSO has been incorporated using a discrete-continuum model. The calculations provide a basic mechanistic framework that allows discussion on the apparent discrepancy existing between experimental data in solution and in the enzymes. They show that, in the early steps of the process in solution, a sulfurane intermediate is formed the rate of which is limiting. Then, a proton transfer from a second thiol molecule to the sulfurane leads to the formation of either a sulfenic acid or a disulfide though the latter is much more stable than the former. If a sulfenic acid is formed in solution, it should react with a thiol molecule making its experimental detection difficult or even unfeasible.