Peroxynitrite

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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)].
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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].
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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)].
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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.

The structure of the peroxynitrite anion

Reactions of peroxynitrite leading to either apoptotic or necrotic cell death

Peroxynitrite (sometimes called peroxonitrite) is the anion with the formula ONOO⁻. It is an unstable structural 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][2] 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 the free radical nitric oxide^[3][4]:

·O₂⁻ + ·NO → ONO₂⁻

The resultant pairing of these two free radicals results in peroxynitrite, a molecule that is itself not a free radical, but that 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, ε₃₀₂ = 1670 M⁻¹ cm⁻¹).^[5]

Contents 1 As a nucleophile 2 Peroxynitrous acid 3 See also 4 References

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

Main article: Peroxynitrous acid

See also

• Nitrotyrosine
• Reactive nitrogen species

References

1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. ^ W. H. Koppenol
3. ^ Pacher, P.; Beckman, J. S.; Liaudet, L.; “Nitric Oxide and Peroxynitrite: in Health and disease” Physiological Reviews 2007, volume 87(1), page 315-424. doi:10.1152/physrev.00029.2006 PMID 17237348
4. ^ Csaba Szabó, Harry Ischiropoulos and Rafael Radi; Peroxynitrite: biochemistry,pathophysiology and development of therapeutics Nature Reviews Drug Discovery; 6, 662-680 (August 2007)
5. ^ 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.

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