Reduction of selenite by cysteine in ionic media

The reaction is due to the following interactionHSeO3-+H2Cys0⇄k-STkST[ST]→kSe(0)Productswhere 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 ask1=kST×1-k-STk-ST+kSe(0)×αHSeO3-×αH2Cys0×[Cys]Twhere the second-order rate constant (kST) was calculated as (2.0 ± 0.2) × 103 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 + kSe(0)) approximates 1 and may be neglected.k1=kST×αHSeO3-×αH2Cys0×[Cys]TEnvironmental significance of the results and the importance of abiotic vs biotic reactions are also briefly discussed.