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 by global analysis of kinetic data sets in coordinates of absorbance-wavelength (230-640 nm)-time. The observed changes were fitted by a sequence of three pseudo-first-order processes. The main Cr(III) product (greater than or equal to 95%) was identified as N(cis),O(cis),S(trans)-bis(L-cysteinato(2-))-chromate(III) on the basis of its UV-visible and CD spectra, The chemical natures of the intermediates and the reaction mechanism were proposed on the basis of (i) observed rate constant dependences 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 mu = 1 M and T = 298.1 K), where [RS(-)] and [RSH] are the concentrations of deprotonated and protonated forms of L-cysteine); (ii) estimated spectra of intermediates and their dependences on the reaction conditions; and (iii) comparison of kinetic features of the studied reaction and the related processes (Cr(V) + L-cysteine; Cr(VI) + 2-mercaptoethylamine; Cr(VI) + 3-mercaptopropionic acid). The proposed mechanism includes the following: (i) formation of a Cr(VI) complex with two cysteine ligands; (ii) its conversion to a precursor Cr(III) complex by sequential one-electron reductions with three cysteine molecules; and (iii) intramolecular rearrangement of the precursor Cr(III) complex leading to the final product. Possible implications of the kinetic data to the studies of Cr(VI) genotoxicity mechanisms are discussed.