ON THE PH-DEPENDENT YIELD OF HYDROXYL RADICAL PRODUCTS FROM PEROXYNITRITE

Nitric oxide reacts rapidly with superoxide to give the strongly oxidi zing peroxynitrite anion (ONOO-), which undergoes spontaneous first-or der decomposition when protonated. The oxidative chemistry of peroxyni trite (ONOO-) is highly pH-dependent. At acidic pH, peroxynitrous acid (ONOOH) oxidizes dimethylsulfoxide to formaldehyde and 2,2'-azinobis- (3-ethyl-1,2-dihydrobenzothiazoline 6-sulfonate) (ABTS) to the greenis h-colored ABTS(+) radical cation. The product yield from dimethylsulfo xide and ARTS decreased at more alkaline pH with apparent pK(a)s of 7. 9 and 8.2, respectively. Decreasing yield with increasing pH could not be explained by the oxidation of either formaldehyde or ABTS(+) by pe roxynitrite. In the presence of 50 mM dimethylsulfoxide, nitrogen diox ide was formed in approximately equimolar amounts to the other reactio n product, formaldehyde. The yield of nitrogen dioxide also decreased with an apparent pK(a) of 8.0. We propose that the complex oxidative c hemistry of peroxynitrite is controlled by the pa-dependent isomerizat ion of the relatively stable cis-configuration (predominant at high pH ) to the trans-configuration. Trans-peroxynitrous acid can form a vibr ationally excited intermediate capable of reacting like hydroxyl radic al. The vibrationally excited intermediate can also directly rearrange to nitric acid, reducing the apparent hydroxyl radical yield to less than 30%. The loss of hydroxyl radical-like reactivity can be explaine d on the basis of ionization of trans-peroxynitrous acid to the Irans- anion, which in turn undergoes internal rearrangement to nitrate witho ut forming a strong oxidant. We propose that the pK(a) of 6.8 measured by absorbance at 302 nm in phosphate buffer corresponds to protonatio n of cis-peroxynitrite anion, whereas the loss of hydroxyl radical-lik e reactivity with a pK(a) of 8 corresponds to that of the trans-peroxy nitrite anion.