S-nitrosothiols : novel decomposition pathways including reactions with sulfur and nitrogen nucleophiles
Spectrophotometric (including stopped-flow) techniques were used to examine the kinetics of NO-group transfer reactions (transnitrosation) between S-nitrosothiols (RSNO) and a wide range of sulfur/nitrogen nucleophiles in aqueous solution. A metal-ion chelator was added in all experiments to prevent RSNO decay and NO liberation catalysed by copper ions. In most cases reaction was envisaged as rate- determining attack of the nucleophile at the nitrogen atom of the -SNO moiety, and hence S-nitrosothiols essentially acted as electrophilic nitrosating agents. Sulfite, thiosulfate, thiourea, thiocyanate and thiomethoxide, were sufficiently nucleophilic to induce nitrosothiol decomposition at physiological pH. Reaction with sulfide (pH > 7.4) afforded the orange-yellow anion, SSNO, and embodies a potential quantitative test for RSNOs. S-Nitrosopenicillamine was reactive enough to allow a thorough investigation into its reaction at basic pH with primary, secondary (creating carcinogenic N-nitrosamines), and tertiary amines, as well as ambident (e.g. thiomorpholine) and alpha nucleophiles (e.g. azide ion). Parallels could be made with analogous studies using other nitroso compounds such as MNTS. The generality of the reaction of a S-nitrosothiol with a large excess of the corresponding or a different thiol was also assessed. Ammonia and not nitric oxide was confirmed as the primary nitrogenous product of this highly complicated process. Mechanistic details of the copper(I) catalysed decomposition of some novel S- nitroso derivatives (e.g. a synthesised S-nitroso-1 -thiosugar) are reported. The two- stage degradation pathway involved an initial Cu(^+) promoted component that halted at incomplete conversion, and was accompanied by a large thermal reaction. An explanation of this unique pattern has been offered in terms of the generation of a disulfide-Cu(^2+) complex, in which copper is/is not accessible to reduction.