The Pulse Radiolysis of Penicillamine and Penicillamine Disulfide in Aqueous Solution

Aqueous solutions of penicillamine (RSH) and penicillamine disulfide (RSSR) have been pulse irradiated at several pH's and the ensuing reactions studied. The rate constant for the reaction of solvated electrons with penicillamine disulfide decreases from 7.3 × 10−9 M−1 s−1 at pH 7 to 0.8 × 109 M−1 s−1 at pH 10. The first-order rate constant for the decomposition of the resulting radical anion, RSSR−, increases from 1.77 × 106 s−1 at pH 7 to 6.5 × 106 s−1 at pH 10. Both of these pH effects are attributed to deprotonation of the two amino groups of the disulfide which have microscopic pK values of 7.9 and 8.5. The equilibrium constant for the reaction[Formula: see text]is (2.31 ± 0.2) × 102 M−1 at pH 8 but increases to 3.04 × 102 M−1 at pH 7 and decreases to 0.96 × 102 M−1 at pH 9. Measured from the kinetics of the build-up of RSSR−, k−2 = 1.83 × 106 s−1 at pH 8.Decay of RSSR− in the presence of RSH follows second-order kinetics at high dose rates and first-order kinetics at low dose rates. At high dose rates two second-order reactions[Formula: see text]appear to be involved with rate constants of 1.7 × 109 M−1 s−1 and 6.4 × 109 M−1 s−1 respectively at pH 8, At low dose rates the decay appears to be due to a pseudo first-order reaction between RSSR− and H2O, k ~ 2 × 103 s−1, with valine (RH) and penicillamine trisulfide (RSSR) as principal products. These products are obtained in similar yields by gamma radiolysis of penicillamine solutions. At pH 5, the thiyl radical decays by second-order kinetics, 2k = (2.85 ± 0.15) × 109 M−1 s−1.Studies of solutions containing penicillamine and methanol showed that penicillamine repairs methanol radicals very efficiently by hydrogen transfer. k = (1.1 ± 0.1) × 108M−1 s−1. The implications of the results for chemical radioprotection are discussed.