Thiamine Derivatives of Disulfide Type. IX. Exchange Reactions between Symmetrical Disulfide and L-Cysteine

Kinetic studies were made about the exchange reactions between dibenzyl disulfide (I) and cysteine, and between diphenyl disulfide (III) and cysteine, in order to determine the dependence on the reaction rates of the structures of disulfides and of thiols. It was demonstrated that the greater the pKa of the thiols or the smaller the pKa of the leaving molecule from disulfides, the greater are the rate constants. It was also shown that these exchange reactions stood in equilibria. Being compared this with the reactions between thiamine derivatives of disulfide type and thils where reverse reaction does not proceed, it may be concluded that the absence of reverse reaction is due to the thiamine's behavior in aqueous solution that the thioaniontype structure readily turn to thiazolium-type one below neutral pH.