Peroxynitrite
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Peroxynitrite (ONOO−) is a strong oxidation and nitration agent, which damages DNA, proteins and other cellular structures. Formation of peroxynitrite in vivo has been ascribed to the reaction of superoxide with nitric oxide.
The conjugate acid of ONOO− is peroxynitrous acid (ONOOH) (or phonetically "HOONO," pronounced "hoo-noh"), which has a pKa of ~6.8. HONOO can homolyze to form nitrogen dioxide and hydroxyl radical as a pair of caged radicals. Approximately 66% of the time, these two radicals recombine to form nitronium ion and hydroxide. The other 33% of the time, these two radicals escape the solvent cage and become free radicals. HOONO is thought to be an intermediate in important reaction pathways relating to atmospheric chemistry.
ONOO− can react nucleophilically with carbon dioxide. In vivo, the concentration of carbon dioxide is about 1 mM, and the reaction of ONOO− with carbon dioxide occurs very quickly. Thus, under physiological conditions, the reaction of ONOO− with carbon dioxide to form nitrosoperoxycarbonate (ONOOCO2−) is by far the predominant pathway for ONOO− reaction. ONOOCO2− homolyzes to form carbonate radical and nitrogen dioxide as a pair of caged radicals. Approximately 66% of the time, these two radicals recombine to form carbon dioxide and nitrate. The other 33% of the time, these two radicals escape the solvent cage and become free radicals. It is these radicals (carbonate radical and nitrogen dioxide) that are believed to cause peroxynitite-related cellular damage.
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