Kinetics and mechanism of S-nitrosation of some thiol-containing amino acids and other thiols

Abstract

Rate constants for S-nitrosation of cysteine, cysteine methyl ester, N-acetylcysteine, penicillamine, N-acetylpenicillamine, glutathione, thioglycolic acid, and mercaptosuccinic acid have been determined in water at 25 °C. All are very reactive and show acid and nucleophilic (Cl^–, Br^–, and SCN^–) catalysis. The similarity of the rate constants for reaction via H₂NO₂⁺ and also for reaction via ClNO for the more reactive thiols suggests that these reactions are encounter-controlled. The limiting rate constant for ClNO reaction (ca. 1 × 10⁷ dm³ s^–1) is very similar to that for nitrosation of a range of aliphatic amines. The high reactivity of N-acetyl derivatives and also of glutathione can be explained in terms of an internal stabilisation of the developing positive charge on sulphur by the oxygen atom of the carbonyl group, which involves a six-membered-ring structure. For the more reactive thiols at higher [RSH] the rate of formation of ClNO, BrNO, or ONSCN tends to become rate-limiting. The derived rate constants for XNO formation and also for XNO hydrolysis agree reasonably, and also give values of the equilibrium constants for XNO formation which agree with the literature values which were measured directly.