Reaction of peroxynitrite with carbon dioxide: intermediates and determination of the yield of CO₃ ^•– and NO₂ ^•

Abstract.

CO₂ catalyses the isomerization of the biological toxin ONOO^– to NO₃ ^– via an intermediate, presumably ONOOCO₂ ^–, which has an absorption maximum near 650 nm. The reflection spectrum of solid NMe₄ ⁺ONOO^– exposed to CO₂ shows a similar band near 650 nm; this absorption decays over minutes. Stopped-flow experiments in which CO₂ solutions were mixed with alkaline ONOO^– solutions indicate the formation of at least one intermediate. The initial absorption at 302 nm is less than that of ONOO^–, which indicates that reactions take place within the mixing time, and this absorption is dependent (but not linearly) on the ONOO^– and CO₂ concentrations. We found that reaction of peroxynitrite with carbon dioxide forms some trioxocarbonate(•1–) (CO₃ ^•–) and nitrogen dioxide (NO₂ ^•) radicals via homolysis of the O-O bond in ONOOCO₂ ^–. We determined the extent of radical formation by mixing peroxynitrite, carbon dioxide and nitrogen monoxide. The later reacts with CO₃ ^•– and NO₂ ^• radicals to form, effectively, three NO₂ ^– per homolysis; ONOOCO₂ ^– that does not undergo homolysis yields NO₃ ^– and CO₂. Based on the NO₃ ^– and NO₂ ^– analyses, the extent of conversion to NO₃ ^– is 96±1% and that of homolysis is 3±1%, respectively, significantly less than that reported in the literature.