Reactivity of sulfur nucleophiles towards S-nitrosothiols

Abstract

Rate constants have been measured for the reactions of a range of S-nitrosothiols with the following sulfur-centred nucleophiles: sulfite ion, thiourea, thiocyanate ion, thiosulfate ion, thiomethoxide ion and sulfide ion. Many of the reactions were very fast and were followed in a stopped-flow spectrophotometer. For the sulfite reaction the reactive species over the pH range 4–8 was shown to be exclusively SO₃²⁻. For two RSNO species the reactivity sequence was established as: SO₃²⁻ > MeS⁻ > S₂O₃²⁻ ≫ SC(NH₂)₂ > SCN⁻. The reaction with sulfide ion was also rapid and generated a fairly stable yellow species (λ_max 410 nm), which was probably the nitrosodisulfide ion ONSS⁻, but the absorbance–time data were too complex for a simple kinetic analysis. This reaction could have some potential as an analytical procedure for the determination of RSNO species. The kinetic results are discussed in terms of the factors affecting nucleophilicity and are compared with the corresponding reactions of other nitrosating species.