Formation of peroxynitrite from the oxidation of hydrogen peroxide by nitrosonium ion (NO+): A pulse radiolysis study

The rate constant of the reaction of N-.(3) With (NO)-N-. has been determined to be (4.4 +/- 0.5) x 10(9) M(-1) s(-1) using the pulse radiolysis technique at pH 7.6-8.3 and 21 degrees C. The reaction of N-.(3) with .NO takes place through an inner-sphere electron-transfer mechanism yielding N3NO as an intermediate, which subsequently decays to N2O and N-2. Peroxynitrite was formed when H2O2 was added to the N-.(3)/(NO)-N-. system at pH 5.8-8.3. The maximum yield of peroxynitrite, which was obtained at [H2O2] > 0.2 M, was similar to 34% of the initially produced N-.(3), indicating that N3NO does not react directly with H2O2 We conclude that, in the presence of high concentrations of H2O2, similar to 64% of the N3NO decomposes into N2O and Nz, whereas the remaining 34% yields NO+ or H2NO2+ which subsequently reacts with H2O2 to form peroxynitrite. The comparison of our kinetic results with those obtained previously in the H+/HNO2/H2O2 system shows that the nitrosating species in both systems differ. As H2NO2+ is the precursor of NO+ in the H+/HNO2/H2O2 system, we conclude that the reactive intermediate in our system is most probably NO+. From the dependence of the yield of peroxynitrite on [H2O2],, the ratio between the rate constants of the reactions of NO+ with H2O2 and H2O was determined to be 65 M(-1). These rate constants were estimated to be k(7) > 3 x 10(8) and k(-4)[H2O] > 4.6 x 10(6) s(-1), respectively.