Thiolysis of Nim-2,4-dinitrophenyl-histidine peptides with 2-mercaptoethanol. An explannation for the pH optimum

The thiolysis of Nim-2,4-dinitrophenyl-histidine peptides with 2-mercaptoethanol demonstrates an optimal rate between pH 8.5 and 9.0. Base lability of both reactants and product was investigated as a possible cause for the pH optimum. N-2,4-Dinitrophenylimidazole, a model for Nim-2,4-dinitrophenylhistidine peptides, 2-mercaptoethanol and the product of thiolysis, 2,4-dinitrophenyl-S-mercaptoethan-2-ol, were subjected to aqueous basic conditions (pH 8.5 to 12.0). The reactions were followed spectrophotometrically, and products were identified by comparison of their ultra-violet spectra with commercially available or synthetic compounds. Mechanisms and rate constants for base hydrolysis of 2,4-dinitrophenyl-thio ethers are presented. Although N-2,4-dinitrophenylimidazole and 2,4-dinitrophenyl-S-mercaptoethan-2-oldohydrolyze, it is the oxidation of 2-mercaptoethanol to 2-mercaptoethanol disulfide which results in the decreasing rate of thiolysis above pH 9.0.