The decomposition of peroxynitrite does not yield nitroxyl anion and singlet oxygen

In a recent article [Khan, A. U., Kovacic, D., Kolbanovsky, A., Desai, M., Frenkel, K. & Geacintov, N. E. (2000) Proc. Natl. Acad. Sci. USA 97, 2984-2989], the authors claimed that ONOO(-), after protonation to ONOOH, decomposes into (1)HNO and (1)O(2) according to a spin-conserved unimolecular mechanism. This claim was based partially on their observation that nitrosylhemoglobin is formed via the reaction of peroxynitrite with methemoglobin at neutral pH. However, thermochemical considerations show that the yields of (1)O(2) and (1)HNO are about 23 orders of magnitude lower than those of ( small middle dot)NO(2) and ( small middle dot)OH, which are formed via the homolysis of ONOOH. We also show that methemoglobin does not form with peroxynitrite any spectrally detectable product, but with contaminations of nitrite and H(2)O(2) present in the peroxynitrite sample. Thus, there is no need to modify the present view of the mechanism of ONOOH decomposition, according to which initial homolysis into a radical pair, [ONO( small middle dot) ( small middle dot)OH](cage), is followed by the diffusion of about 30% of the radicals out of the cage, while the rest recombines to nitric acid in the solvent cage.