Nitric oxide reacts rapidly with superoxide to give the strongly oxidizing peroxynitrite anion (ONOO-), which undergoes spontaneous first-order decomposition when protonated. The oxidative chemistry of peroxynitrite (ONOO-) is highly pH-dependent. At acidic pH, peroxynitrous acid (ONOOH) oxidizes dimethylsulfoxide to formaldehyde and 2,2'-azino-bis-(3-ethyl-1,2- dihydrobenzothiazoline 6-sulfonate) (ABTS) to the greenish-colored ABTS+ radical cation. The product yield from dimethylsulfoxide and ABTS 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 peroxynitrite. In the presence of 50 mM dimethylsulfoxide, nitrogen dioxide was formed in approximately equimolar amounts to the other reaction product, formaldehyde. The yield of nitrogen dioxide also decreased with an apparent pK(a) of 8.0. We propose that the complex oxidative chemistry of peroxynitrite is controlled by the pH-dependent isomerization of the relatively stable cis-configuration (predominant at high pH) to the trans-configuration. Trans-peroxynitrous acid can form a vibrationally excited intermediate capable of reacting like hydroxyl radical. 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 explained on the basis of ionization of trans-peroxynitrous acid to the trans-anion, which in turn undergoes internal rearrangement to nitrate without forming a strong oxidant.(ABSTRACT TRUNCATED AT 250 WORDS)