Mechanism of the Nitrosation of Thiols and Amines by Oxygenated •NO Solutions: the Nature of the Nitrosating Intermediates

Abstract

The nitrosation of various thiols and morpholine by oxygenated •NO solutions at physiological pH was investigated. The formation rates and the yields of the nitroso compounds were determined using the stopped-flow technique. The stoichiometry of this process has been determined, and is given by 4•NO + O2 + 2RSH/2RR‘NH → 2RSNO/2RR’NNO + 2NO2- + 2H+. Kinetic studies show that the rate law is −d[O2]/dt = k1[•NO]2[O2] with k1 = (2.54 ± 0.26) × 106 M-2 s-1 and −d[•NO]/dt = 4k1[•NO]2[O2] with 4k1 = (1.17 ± 0.12) × 107 M-2 s-1, independent of the kind of substrate present. The kinetic results are identical to those obtained for the autoxidation of •NO, indicating that the rate of the autoxidation of •NO is unaffected by the presence of thiols and amines. The nitrosation by •NO takes place only in the presence of oxygen, and therefore the rate of the formation of S-nitrosothiols from thiols and oxygenated •NO solution is relatively slow in biological systems. Under physiological conditions where [•NO] < 1 μM and ...