Catalysis by zinc ion in the reactions of carcinogenic chromium(VI) with thiols

Abstract

The reactions of Cr(VI) with nine biological and nonbiological thiols (RSH) in aqueous acetate buffer (pH 5.60) have been followed by an spectrophotometric method, and a relatively stable intermediate (the thioester RSCrO 3 −) has been observed. The rate constants corresponding to the formation ( k 1), decomposition ( k −1), and redox transformation ( k 2) of that intermediate have been obtained in both the absence and presence of zinc ion. This ion acts as a true catalyst for both the formation and decomposition of the observed intermediate. The equilibrium constants for the formation of RSCrO 3 − from Cr(VI) and RSH for the different thiols have been determined. A mechanism is proposed for the formation of RSCrO 3 − in the presence of catalyst according to which the rate-determining step would be the bimolecular reaction between a zinc–thiolate complex and an acetyl–chromate ester, involving an interchange of ligands.