Reduction of selenite by cysteine in ionic media

This paper focuses on the abiotic reduction of selenite (Se(IV)) by cysteine (Cys, NH 3 + CH(CH 2 SH)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 ° 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 HSeO 3 þ H 2 Cys 0 ! k ST k ST ½ ST ! k Se ð 0 Þ Products where the selenotrisulfide [ST] is the complex intermediate RS-Se-SR with R = NH 3 + CH(CH 2 )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 ¼ k ST 1 k ST k ST þ k Se ð 0 Þ a HSeO 3 a H 2 Cys 0 ½ Cys T where the second-order rate constant ( k ST ) was calculated as (2.0 ± 0.2) 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 ¼ k ST a HSeO 3 a H 2 Cys 0 ½ Cys T Environmental significance of the results and the importance of abiotic vs biotic reactions are also briefly discussed.