A nitrogen oxyanion containing an OO peroxo bond that is a structural isomer of the nitrate ion. These species are generally distinguished as ONOO− and NO3−, respectively. Other names for peroxynitrite include pernitrite and peroxonitrite; the systematic name recommended by the International Union of Pure and Applied Chemistry (IUPAC) is oxoperoxonitrate (1−). Energy calculations indicate that there are two stable conformations of ONOO−, for which all of the atoms lie in a plane with the peroxo OO and N&dbnd;O bonds forming dihedral angles of approximately 0° (cis isomer) or 180° (trans isomer) [notation (1)].
1
See also Chemical bonding.
Peroxynitrite is formed in nitrate salts or nitrate-containing solutions when exposed to ionizing radiation or ultraviolet light. Solutions can also be prepared by a variety of chemical reactions, including the reaction of hydrogen peroxide with nitrous acid (2); reaction of the hydroperoxide anion with organic and inorganic nitrosating agents (3); reaction of ozone with the azide ion (4); or, apparently, reaction of O2 with compounds capable of generating the nitroxyl anion (NO−) [5].
2 
3 
4 
5 
These preparations invariably contain unreacted materials or decomposition products, particularly nitrite ion, which can significantly modulate the peroxynitrite chemical reactivity. Peroxynitrite is also formed in radical-radical coupling reactions, notably superoxide (·O2−) with nitric oxide (·NO) [reaction (6)], and hydroxyl radical with nitrogen dioxide [reaction (7)].
6 
7 
See also Superoxide chemistry.
Peroxynitrite has been isolated as the tetramethylammonium salt by carrying out reaction (6) in liquid ammonia. Formation of peroxynitrite in both solids and solutions is indicated by the appearance of yellow coloration, which is due to tailing of intense near-ultraviolet absorption bands into the visible region.
Peroxynitrite is a powerful oxidant that has been shown to react with a wide variety of inorganic and organic reductants. Interest in these reactions has been greatly stimulated by recognition that ·NO and ·O2− radicals are generated in the bloodstream, neuronal tissues, and phagocytic cells of animals in sufficient quantities to form peroxynitrite [reaction (6)]. Correspondingly, major roles for this powerful oxidant have been proposed both in diseases and tissue damage associated with oxidative stress, and in natural cellular defense mechanisms against microbial infection. See also Bioinorganic chemistry.