INSENSITIVITY OF THE RATE OF DECOMPOSITION OF PEROXYNITRITE TO CHANGES IN VISCOSITY - EVIDENCE AGAINST FREE-RADICAL FORMATION

Peroxynitrite is a versatile and important biological oxidant that is produced from the reaction of nitric oxide and superoxide radicals. Tw o mechanisms have been proposed to rationalize oxidation reactions of peroxynitrite. One assumes that HO-ONO can homolize to form the hydrox yl radical and nitrogen dioxide, and that the hydroxyl radical is the proximate oxidant in peroxynitrite systems. The second argues this hom olysis is too slow to occur at ordinary temperatures and suggests an e xcited species, HOONO, is the proximate oxidant. If the radical mecha nism is correct, then peroxynitrite should disappear more slowly in so lvents of higher viscosity. This is true because for free radical init iators undergoing single-bond homolysis: (1) cage return is substantia l and more of the cages would return to re-form HO-ONO as the viscosit y of the medium increases; and (2) diffusion from the radical cage com petes effectively with other cage processes. We have studied the disap pearance of peroxynitrite at pH 5 and 7 in buffers with and without di oxane (as a control) or up to 30 wt % of the poly(ethylene glycol) (PE G) polymers, PEG 3350 and PEG 8000. These polyethers produce substanti al changes in viscosity, raising the viscosity from about 0.89 to abou t 17 mPa . s. The rate constant for diffusion should decrease by about 10- to 20-fold as the viscosity increases in this interval, and the r ate of diffusion from the solvent cage would be predicted to vary acco rdingly. However, at pH 5, where most of HOONO is undissociated, no ch ange in the rate of disappearance of peroxynitrite is observed with in creasing viscosity. At pH 7, a small increase in the observed rate con stant is found, but it is likely due to the greater concentration of t he undissociated HOONO in the ether-containing solvents resulting from a pK(a) shift. Thus, we conclude that the viscosity test does not sup port a free radical mechanism for the unimolecular decomposition of pe roxynitrite.