Reduction of selenite by cysteine in ionic media

This paper focuses on the abiotic reduction of selenite (Se(IV)) by cysteine (Cys, NH3+CH(CH2SH) COOH), which is a representative thiol produced by aquatic organism under oxidative stress. The rates of reduction of Se(IV) by cysteine were measured in deaerated NaCl solutions and natural waters as a function of pH (4.0-9.0), temperature (10-40 degrees C), and ionic strength (0.01-1.0 M). The rates showed a complex dependence on pH with similar values from pH 4.0-5.0, increasing values from pH 5.0-7.0 and then decreasing values at pH higher than 7.0. An apparent energy of activation obtained was 31 +/- 6 kJ mol(-1), which was independent of ionic strength. The reaction is due to the following interaction HSeO3- + H(2)Cys(0) k(ST)reversible arrow k(-ST)[ST] k(Se(0))-> Products where the selenotrisulfide [ST] is the complex intermediate RS-Se-SR with R = NH3+CH(CH2)COO-. The intermediate ST then decomposes in the final products Se(0) and cystine. The pseudo-first-order rate constant is written as k(1) = k(ST) x (1 - k(-ST)/k(-ST) + k(Se)(0)) x alpha(HSeO3) x alpha(H2Cys0) x [Cys](T) where the second-order rate constant (k(ST)) was calculated as (2.0 +/- 0.2) x 10(3) M-1 min(-1) and was compared with the second-order rate constants of other reductants (hydrogen sulfide, ascorbic acid). In neutrality and alkaline solution, [ST] undergoes rapid decomposition and the term (1 - k(-ST)/k(-ST) + k(Se(0))) approximates 1 and may be neglected. k(1) = k(ST) x alpha(-)(HSeO3) x alpha(H2Cys0) x [Cys](T) Environmental significance of the results and the importance of abiotic vs biotic reactions are also briefly discussed. (C) 2013 Elsevier Ltd. All rights reserved.