KINETICS AND MECHANISM OF CHROMIUM(VI) REDUCTION TO CHROMIUM(III) BY L-CYSTEINE IN NEUTRAL AQUEOUS-SOLUTIONS

The reduction of chromate by a 50-250-fold excess of L-cysteine (pH = 7.0-7.7; self-buffered; [NaClO4] = 0-1 M; T = 288-308 K) was studied b y global analysis of kinetic data sets in coordinates of absorbance-wa velength (230-640 nm)-time. The observed changes were fitted by a sequ ence of three pseudo-first-order processes. The main Cr(III) product ( greater than or equal to 95%) was identified as (cis),S(trans)-bis(L-c ysteinato(2-))-chromate(III) on the basis of its UV-visible and CD spe ctra, The chemical natures of the intermediates and the reaction mecha nism were proposed on the basis of (i) observed rate constant dependen ces on the reaction conditions (k(1)(obs) = 0.19 + 35[RS(-)] s(-1); k( 2)(obs) = 30 [RSH](2)/(1 + 20[RSH]) s(-1); k(3)(obs) = 0.04 s(-1) at m u = 1 M and T = 298.1 K), where [RS(-)] and [RSH] are the concentratio ns of deprotonated and protonated forms of L-cysteine); (ii) estimated spectra of intermediates and their dependences on the reaction condit ions; and (iii) comparison of kinetic features of the studied reaction and the related processes (Cr(V) + L-cysteine; Cr(VI) + 2-mercaptoeth ylamine; Cr(VI) + 3-mercaptopropionic acid). The proposed mechanism in cludes the following: (i) formation of a Cr(VI) complex with two cyste ine ligands; (ii) its conversion to a precursor Cr(III) complex by seq uential one-electron reductions with three cysteine molecules; and (ii i) intramolecular rearrangement of the precursor Cr(III) complex leadi ng to the final product. Possible implications of the kinetic data to the studies of Cr(VI) genotoxicity mechanisms are discussed.