Kinetic study of the reaction between cystine and sulfide in alkaline solutions

Abstract

The reaction between cystine (CySSCy) and hydrogen sulfide ion (HS−) in alkaline solutions has been studied by amino acid analysis and uv–visible spectroscopy. The reaction occurs in two reversible steps to form cysteine (CySH), S-thiocysteine (CySS−), and disulfide (S22−), as represented by the reactions [Formula: see text] and [Formula: see text]. The equilibrium and rate constants were pH dependent due to the presence of various charge types of reactants and products. The rate constants at 25 °C, pH 10.0, and μ = 0.17 M were determined to be: k1 = 3.7 ± 0.4 M−1 min−1, k−1 = 5.5 ± 0.6 M−1 min−1, k2 = 6.1 ± 0.5 M−1 min−1, and k−2 = 122 ± 20 M−1 min−1. When the rate constant k1 is expressed as k1 = A exp (−Ea/RT), values of A = (4.7 ± 0.3) × 1011 M−1 min−1 and Ea = 15.8 ± 0.9 kcal mol−1 were obtained. The ionic strength and pH dependence of k1 were also studied.