Kinetics and mechanism of thionitrite formation. Mercapto-carboxylic acid: a new range of efficient nitrous acid scavengers

Abstract

Rate constants have been measured by stopped-flow spectrophotometry, for the nitrosation of 3-mercaptopropanoic acid (MPA), mercaptosuccinic acid (MSA), and cysteine, in aqueous acid solution at 25 °C. For each substrate the rate equation, rate =k[HNO₂][H⁺][RSH], was established with k values, 4 764, 1 334, and 443 I² mol^–2 s^–1 for MPA, MSA, and cysteine, respectively. Each reaction was catalysed by added nucleophiles (Br^– and SCN^–) and the second-order rate constants k₂ for attack by NOBr and NOSCN were deduced from the variation of the measured rate constants with [Br^–] and [SCN^–]. For NOBr, k₂ value were in the region of 1 × 10⁵ l mol^–1 s^–1 and for NOSCN ca. 1 × 10⁴ I mol^–1 s^–1. These values are much smaller than those for aniline derivatives (which in some cases approach the diffusion-controlled limit), but since no protonation of the thiol group occurs, the overall reactivity of these mercapto-carboxylic acids generally exceeds that of e.g. aniline. All three are significantly more reactive than other conventional ‘nitrite traps’ such as hydrazoic acid, sulphamic acid, sulphanilamide, etc. This was also established indirectly by a previously established kinetic method based on the denitrosation of a nitrosamine in the presence of ‘nitrite traps’ and varying concentrations of the product secondary amine.