Dynamics of interaction of vitamin C with some potent nitrovasodilators, S-nitroso-N-acetyl-d,l-penicillamine (SNAP) and S-nitrosocaptopril (SNOCap), in aqueous solution Original Research Article

The reductive decomposition of both SNAP and SNOCap by ascorbate in aqueous solution (in the presence of EDTA) was thoroughly investigated. Nitric oxide (NO) release from the reaction occurs in an ascorbate concentration and pH dependent manner. Rates and hence NO release increased drastically with increasing pH, signifying that the most highly ionized form of ascorbate is the more reactive species. The experiments were monitored spectrophotometrically, and second-order rate constants calculated at 37 °C for the reduction of SNAP are kb=9.81±1.39×10−3 M−1 s−1 and kc=662±38 M−1 s−1 and for SNOCap are kb=2.57±1.29×10−2 M−1 s−1 and kc=49.7±1.3 M−1 s−1. kb and kc are the second-order rate constants via the ascorbate monoanion (HA−) and dianion (A2−) pathways, respectively. Activation parameters were also calculated and are ΔHb‡=93±7 kJ mol−1, ΔSb‡=15±2 J K−1 mol−1 and ΔHc‡=51±5 kJ mol−1, ΔSc‡=−28±3 J K−1 mol−1 with respect to the reactions involving SNAP. Those for the reaction between SNOCap and ascorbate were calculated to be ΔHb‡=63±11 kJ mol−1, ΔSb‡=−71±20 J K−1 mol−1 and ΔHc‡=103±7 kJ mol−1, ΔSc‡=118±8 J K−1 mol−1. The effect of Cu2+/Cu+ ions on the reductive decompositions of these S-nitrosothiols was also investigated in absence of EDTA. SNOCap exhibits relatively high stability at near physiological conditions (37 °C and pH 7.55) even in the presence of micromolar concentrations of Cu2+, with decomposition rate constant being 0.011 M−1 s−1 in comparison to SNAP which is known to be more susceptible to catalytic decomposition by Cu2+ (second-order rate constant of 20 M−1 s−1 at pH 7.4 and 25 °C). It was also observed that the reductive decomposition of SNAP is not catalyzed by alkali metal ions, however, there was an increase in rate as the ionic strength increases from 0.2 to 0.5 mol dm−3 NaCl.