Peroxynitrite Is the Major Species Formed from Different Flux Ratios of Co-generated Nitric Oxide and Superoxide DIRECT REACTION WITH BORONATE-BASED FLUORESCENT PROBE

There is much interest in the nitration and oxidation reaction mechanisms initiated by superoxide radical anion (O(2)radical anion) and nitric oxide ((NO)-N-center dot). It is well known that O(2)radical anion and (NO)-N-center dot rapidly react to form a potent oxidant, peroxynitrite anion (ONOO-). However, indirect measurements with the existing probes (e.g. dihydrorhodamine) previously revealed a bell-shaped response to co-generated (NO)-N-center dot and O(2)radical anion fluxes, with the maximal yield of the oxidation or nitration product occurring at a 1:1 ratio. These results raised doubts on the formation of ONOO- per se at various fluxes of (NO)-N-center dot and O(2)radical anion. Using a novel fluorogenic probe, coumarin-7-boronic acid, that reacts stoichiometrically and rapidly with ONOO- (k = 1.1 x 10(6) M(-1)s(-1)), we report that ONOO- formation increased linearly and began to plateau after reaching a 1:1 ratio of co-generated (NO)-N-center dot and O(2)radical anion fluxes. We conclude that ONOO- is formed as the primary intermediate during the reaction between (NO)-N-center dot and O(2)radical anion co-generated at different fluxes.