Transfer of nitric oxide from nitrovasodilators to free thiols-Evidence of two distinct stages

Two distinct stages, which can be monitored spectrophotometrically, have been observed for the first time in multiple reactions between thiol, such as l-cysteine and some well-known nitrovasodilators, namely S-nitroso-N-acetyl-d,l-penicillamine (SNAP), S-nitrosoglutathione (GSNO), and S-nitrosocaptopril (SNOCap) in aqueous solution (in the presence of EDTA). The first part of the reaction occurs at stopped-flow time scale (∼10−2 s−1) in one single step and has been found to be transnitrosation; followed by a slow decomposition of the products of the transnitrosation reaction with the formation of a variety of nitrogen products. Reactivity with regard to the first stage occurs in the order GSNO > SNAP > SNOCap. The second stage shows first-order rate constants of the order 10−4 s−1, although some degree of complexity exists in elucidating an accurate rate equation with which a comparative study could be done.