Peroxynitrous acid--where is the hydroxyl radical?

Peroxynitrite is an inorganic toxin of physiological interest, formed from the diffusion-controlled reaction of superoxide and nitrogen monoxide with a rate constant of (1.6 +/- 0.3) x 10(10) M(-1) s(-1). On the basis of three experiments we conclude that homolysis of the O-O bond in peroxynitrous acid is unlikely: (1) the yield of nitrite from the decomposition of peroxynitrite shows a dependence on the peroxynitrite concentration and is lower than expected for homolysis; (2) the yield of [15N]nitrate from the reaction of [15N]nitrite with peroxynitrous acid predicted by homolysis does not correspond to that found experimentally, and (3) the reaction of peroxynitrous acid with monohydroascorbate does not yield ascorbyl radicals. Activation volumes determined from high-pressure kinetic studies are inconclusive.