LINAC/LASER Determination of the Absolute Rate Constant for Thiyl and Hydroxyl Radical Reaction with Sulfhydryls in Aqueous Solution: Mercaptoethanol, Cysteamine, and N-Acetyl-l-cysteine

Abstract

The technique of combined pulse radiolysis/laser photolysis has been used to investigate the disulfide radical anion formation reactions for mercaptoethanol, cysteamine, and N-acetyl-l-cysteine over the pH range 7.0−13.0. The photolysis of the transient radical anions, formed by the one electron oxidation of the sulfhydryl, perturbs the disulfide radical anion/thiyl radical equilibrium, allowing rate constants for thiyl radical reaction with the parent sulfhydryl to be uniquely determined from the absorption bleach and subsequent recovery. These pH-dependent values were combined with measured disulfide radical anion equilibrium constants to calculate first-order disulfide radical anion dissociation rate constants. By computer modeling of established mechanisms for the disulfide radical anion growths, rate constants for the reaction of hydroxyl radicals with individual sulfhydryl species were calculated. These values are contrasted with previously reported values determined by using competition kinetics.