Reactive sulfur species: Kinetics and mechanism of the hydrolysis of cysteine thiosulfinate ester

The kinetics and mechanisms of the hydrolysis of cysteine thiosulfinate ester (CyS(=O)SCyx-, x = 0-2) have been investigated by stopped-flow spectrophotometry between pH 6 and pH 14. The rate-limiting reaction of hydroxide is observed for pH < 13. More complicated kinetics are observed above pH 13, where the hydrolysis of CyS(=O)SCy2(-1) can be fast relative to subsequent reactions. The eventual products of hydrolysis are a 1:1 molar ratio of cystine (CySSCy) and cysteine sufinic acid (CySO2H) under all reaction conditions. The rate of hydrolysis is dependent upon the proton state of CyS(=O)SCyx-. Furthermore, cysteine thiosulfonate ester (CyS(=O)2SCY) was observed as an intermediate during the hydrolysis of CyS(=O)SCyx- at lower pH. CyS(=O)(2)SCy eventually hydrolyzes to give stoichiometric amounts of CySSCy and CySO2H. However, CySO2H is observed under some conditions for which hydrolysis Of CyS(=O)2SCY is relatively slow, thus suggesting multiple hydrolysis pathways for CyS(=O)SCyx-. The mechanism up to the rate-limiting step is proposed to be as follows: CyS(=O)SCyo T (2-), = 7.92; H+ + CyS(=O)SCy-, pK(a3) = 7.32; CyS(=O)SCy- = H + CyS(=O)SCy2- pK(a4) CyS(=O)SCyo + OH- -> products, P(o)k(o) = (5.0 0.01) x 103 M-1 s(-1); CyS(=O)SCy- + OH- products, P1K1 = 60 18 M-1 s(-1); and CYS(=O)SCy2- + OH- -> products, P(2)k(2) = 0.36 0.01 M-1 s(-1), where P,, is a constant (1 :! P, :5 3) that accounts for the partitioning between the possible hydrolysis pathways and the stoichiometries of their net reactions.