Reaction Mechanism of Selenate(IV) Ions with Several Thiols in Aqueous Media

In several enzymatic systems, selenium (an essential element commonly incorporated as sodium selenite to the human diet) is bonded through a mercapto group of a cysteine residue. The reaction of SeIV (as selenite) with different thiols in aqueous media was studied to increase the information about the mechanism of the enzyme reaction. The influence of the reagents' concentration on the reaction rate and temporal spectral changes was analyzed. A complex with a UV maximum between 270 and 290 nm was formed, which quickly decayed when the thiol concentration was increased. The proposed reaction mechanism includes a fast SeIV–thiol association step followed by two parallel steps (one is first‐order in the complex and the other one is globally second‐order). Kinetics and equilibrium constants were obtained, and the rate dependence of the thiol oxidation on the SeIV concentration was modelled for all thiols tested. Very good agreement between the experimental and calculated curves was observed.