Nitrosation by alkyl nitrites. Part 3. Reactions with cysteine in water in the pH range 6–13

Abstract

Cysteine reacts quite rapidly with isopentyl, isopropyl, and t-butyl nitrite in water at 25 °C in the pH range 6–13 to give the somewhat unstable S-nitrosocysteine. Rate measurements show clearly that reaction occurs exclusively via the RS^– species. Plots of the measured second-order rate constants against pH are in excellent agreement with the calculated curves, for all three alkyl nitrites, and the order of reactivity is (CH₃)₂CHCH₂CH₂ONO: (CH₃)₂CHONO:(CH₃)₃CONO 16:6.7:1. The methyl ester of cysteine behaves similarly whereas S-methylcysteine does not form the thionitrite. The results suggest that the reaction involves a direct electrophilic nitrosation by the alkyl nitrite itself at the sulphur site in RS^–. The conclusions are discussed in terms of steric effects and of the other known reactions of alkyl nitrites.