Formation of peroxynitrate from the reaction of peroxynitrite with CO₂: Evidence for carbonate radical production

The decomposition of peroxynitrite in the presence of bicarbonate and various substrates was studied using the stopped-flow technique. We have shown that peroxynitrite (ONOOH/ONOO^-) reacts with excess of CO₂ to form peroxynitrate (O₂NOOH/O₂NOO_-) in aerated solutions containing HCO₂^-, H₂O₂, CH₃OH, or (CH₃)₂CHOH. The yield of peroxynitrate increased with the increase in these substrate concentrations approaching 30-33% of added peroxynitrite. Competition kinetics study yields rate constants which are similar to those determined directly for the reactions of these substrates with the carbonate radical anion. We therefore suggest that the formation of peroxynitrate takes place via the following steps: (i) the rapid reaction of peroxynitrite with CO₂ to form ONOOCO₂^-; (ii) the homolytic cleavage of 30-33% of ONOOCO₂^- into °NO₂ and CO₃*^-; (iii) the reaction of CO₃*^- with the various substrates, yielding ultimately superoxide in aerated solutions; and (iv) the fast reaction of superoxide with °NO₂ to form O₂NOO^-.