Peroxynitrite

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. It is prepared by the reaction of hydrogen peroxide with nitrite:
 * H2O2 +  NO2-  &rarr;  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 the free radical nitric oxide :
 * O2- +  NO  &rarr;  ONO2-

The resultant paring of these two free radicals results in peroxynitrite, a molecule which is itself not a free radical, but which is a powerful oxidant.

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, &lambda;302 = 1670 M-1cm-1).

As a nucleophile
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 peroxynitrite-related cellular damage.

Peroxynitrous acid
The conjugate acid of ONOO− is peroxynitrous acid (ONOOH) (or phonetically "HOONO," pronounced "hoo-noh"), which has a pKa 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  &larr;  HOONO  &rarr;  OH-  +  NO2+

HOONO is thought to be important in atmospheric chemistry.