Formation of peroxynitrite from S-nitrosothiols and hydrogen peroxide

S-Nitrosocysteine reacts with hydrogen peroxide in aqueous buffer in the presence of EDTA to form the peroxynitrite anion, Spectral measurements show that at high pH (13.1) the peroxynitrite formed is relatively stable whereas at low pH (7.4) its decomposition rate is such that no peroxynitrite is observable. At an intermediate pH (11.6) it is possible to see both its formation and decay, Measured rate constants increase with the pH of the medium suggesting that reaction occurs by nucleophilic attack by the hydroperoxide ion at the nitroso nitrogen atom. By judicious choice of wavelength for rate measurements it is possible to obtain (in the pH range 10-13), both the second order rate constant for peroxynitrite formation and the first order rate constant for peroxynitrite decomposition. The latter is in reasonable agreement with the literature value. Other S-nitrosothiols behave similarly.