Direct rate constant measurement of radical disulphide anion formation for cysteine and cysteamine in aqueous solution

The techniques of pulse radiolysis, laser photolysis and absorption spectroscopy have been used to directly determine rate constants for radical disulphide anion formation for cysteine and cysteamine in aqueous solution. The measured values for cysteine, over the pH range 7–12, allowed calculation of individual rate constants for the constituent reactions RS. + RSH → RSSR−. + H+ and RS. + RS− → RSSR−. as (3.39 ± 0.31) × 108 and (1.21 ± 0.04) × 109 dm3 mol−1 s−1, respectively. Analogous values for cysteamine were also determined by this technique as (3.06 ± 0.16) × 108 and (3.65 ± 0.07) × 109 dm3 mol−1 s−1.