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Peroxynitrite is the anion with the formula ONOO⁻. It is an unstable "valence isomer" of nitrate, NO₃^-, which has the same formula but a different structure. Although peroxynitrous acid is highly reactive, its conjugate base peroxynitrite is stable in basic solution.^[1] It is prepared by the reaction of hydrogen peroxide with nitrite:

H₂O₂ + NO₂^- → ONOO^- + H₂O

Peroxynitrite is an oxidant and nitrating agent. Because of its oxidizing properties, peroxynitrite can damage a wide array of molecules in cells, including DNA and proteins. Formation of peroxynitrite in vivo has been ascribed to the reaction of the free radical superoxide with nitric oxide^[2]:

O₂^- + NO → ONO₂^-

In the laboratory, a solution of peroxynitrite can be prepared by treating acidified hydrogen peroxide with a solution of sodium nitrite, followed by rapid addition of NaOH. Its concentration is indicated by the absorbance at 302 nm (pH 12, λ₃₀₂ = 1670 M^-1cm^-1).^[3]

ONOO⁻ reacts nucleophilically with carbon dioxide. In vivo, the concentration of carbon dioxide is about 1 mM, and its reaction with ONOO⁻ occurs quickly. Thus, under physiological conditions, the reaction of ONOO⁻ with carbon dioxide to form nitrosoperoxycarbonate (ONOOCO₂⁻) is by far the predominant pathway for ONOO⁻. ONOOCO₂⁻ homolyzes to form carbonate radical and nitrogen dioxide, again 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.

The conjugate acid of ONOO⁻ is peroxynitrous acid (ONOOH) (or phonetically "HOONO," pronounced "hoo-noh"), which has a pK_a of ~6.8. HOONO can homolyze to form nitrogen dioxide and hydroxyl radical as a pair of caged radicals. Approximately 66% of the time, these two radicals undergo electron transfer to form nitronium ion and hydroxide. The other 33% of the time, the two species escape the solvent cage as free radicals.

OH^. + NO₂ ← HOONO → OH^- + NO₂⁺

HOONO is thought to be an important in atmospheric chemistry.

1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. ^ Pacher, P.; Beckman, J. S.; Liaudet, L.; “Nitric Oxide and Peroxynitrite: in Health and disease” Physiological Reviews 2007, volume 87(1), page 315-424. PMID 17237348
3. ^ Beckman, J. S.; Koppenol, W. H. “Nitric Oxide, Superoxide, and Peroxynitrite: the Good, the Bad, and Ugly” American Journal of Physiology- Cell Physiology 1996, volume 271, page C1424-C1437.