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.