The Complex Chemistry of Peroxynitrite Decomposition: New Insights1

Abstract

The yield of hydroxyl radicals produced in the decomposition of peroxynitrous acid (HOONO) at room temperature in deoxygenated and bicarbonate free water at pH ∼6.8 has been determined to be roughly 10%. This value rests on a detailed study of the decomposition of peroxynitrous acid in the presence of dimethyl sulfoxide with stopped-flow kinetics and product analyses by unequivocal methods. The HO•/DMSO reaction is known to yield methane sulfinic acid (MSA) and CH3• radicals with 91% efficiency. MSA was quantified by 1H NMR and its measured yield was corrected to allow for its extensive further oxidation to methane sulfonic acid. Methyl radicals were quantified by trapping with a water-soluble, stable nitroxide. At low peroxynitrite concentrations these two techniques gave HO• yields of ca. 8% and ca. 13%, respectively. We conclude that in water the main (ca. 90%) decomposition pathway for peroxynitrite involves a rearrangement to nitric acid via an in-cage collapse of the singlet HO•/•NO2 radical pair which may, in part, be preceded by electron transfer to form an HO-/+NO2 intimate ion pair. We emphasize that, in contrast to many earlier reports, a distinct pathway to hydroxyl radicals is present, which implies that a significant portion of the oxidative nature of peroxynitrite can stem from hydroxyl radical-induced chemistry.