Product formation and kinetic simulations in the pH range 1–14 account for a free-radical mechanism of peroxynitrite decomposition

The yields of nitrate and nitrite from decomposition of peroxynitrite in phosphate buffer at 37 °C were determined in the pH range 1–14. The NO2−/NO3− yields showed a stepwise variation with pH, with inflection points at approximately pH 3.1, 5.8, 6.8, 8.0, and 11.9. Nitrite formation increased strongly above pH 7 at the expense of nitrate, but above pH 12 nitrate again became the major product (80% at pH 14). At this pH, the Arrhenius parameters were Ea=24.1±0.2 kcal mol−1 and A=(4.9±1.3)×1012 s−1. The yields of NO2−, NO3−, and O2 measured at pH 5.8, 7.4, and 8.5 as a function of the initial peroxynitrite concentration (50–1000 μM) were linear only at pH 5.8. In the presence of carbon dioxide, oxygen production at pH 7.5 and pH 10 was found to be linear on the CO2 concentration. The experimental observations were satisfactorily reproduced by kinetic simulations including principal component analyses. These data strongly suggest that the chemistry of peroxynitrite is exclusively mediated by NO2 and HO radicals in the absence, and by NO2 and CO3− radicals in the presence of CO2.