Formation of Peroxynitrite from the Nitrosation of Hydrogen Peroxide by an Oxygenated Nitric Oxide Solution

Peroxynitrite is formed when ^.NO is added to oxygenated solutions of hydrogen-peroxide. The formation rates and the yields of peroxynitrite were determined using the stopped-flow technique at pH 7.5-11.7. The stoichiometry of this process has been determined, and is given by 4^.NO + O₂ + 2H₂O₂ → 2ONOO^- + 2NO₂^- + 4H⁺. Kinetic studies show that the rate law of the nitrosation process is given by -d[O₂]/dt = k₁[^.NO]²[O₂] with k₁ = (2.4 ± 0.3) × 10⁶ M^-2 s^-. The kinetic results are identical to those obtained for the autoxidation of ^.NO, indicating that the rate determining step of the nitrosation process is the formation of ONOONO (or ONONO₂ or O₂NNO₂), which is the precursor of ^.NO₂ and of N₂O₃. The stoichiometry of the nitrosation process suggests that ^.NO₂ and/or N₂O₃ are the reactive species. Competitive kinetic studies demonstrate that the yield of peroxynitrite is independent of ^.NO concentrations, indicating that the reactive intermediate is N₂O₃. The rate constant of the reaction of N₂O₃ with HO₂^- was determined to be (1.0 ± 0.6) × 10⁹ M^-1 s^-1, whereas that with H₂O₂ is at least 4 orders of magnitude lower. The nitrosation of H₂O₂ by ^.NO takes place only in the presence of oxygen, and under the conditions of this study, we found no evidence for the direct nitrosation of H₂O₂ by ^.NO.