In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of the one-electron reduction of dioxygen , which occurs widely in nature. Molecular oxygen (dioxygen) is a diradical containing two unpaired electrons, and superoxide results from the addition of an electron which fills one of the two degenerate molecular orbitals, leaving a charged ionic species with a single unpaired electron and a net negative charge of −1. Both dioxygen and the superoxide anion are free radicals that exhibit paramagnetism.

Salts

Superoxide forms salts with alkali metals and alkaline earth metals. The salts sodium superoxide (), potassium superoxide (), rubidium superoxide () and caesium superoxide () are prepared by the reaction of with the respective alkali metal.

The alkali salts of are orange-yellow in color and quite stable, if kept dry. In water, the dissolved disproportionates extremely rapidly (written here for a basic solution):

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This reaction (with moisture and carbon dioxide in exhaled air) underlies the use of potassium superoxide as an oxygen source in chemical oxygen generators, as on the Space Shuttle and submarines, and in firefighters' oxygen tanks.

More generally, the superoxide anion, , is a weak Brønsted base. Its protonated form, hydroperoxyl (), has pK<sub>a</sub> around 4.8, and superoxide anion predominates at neutral pH:

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Hydroperoxyl is a strong oxidant, but superoxide is a strong nucleophile and reductant. Disproportionation to oxygen and peroxide occurs whenever the two coexist.

Potassium superoxide is soluble in dimethyl sulfoxide (facilitated by crown ethers) and is stable as long as protons are not available. Superoxide can also be generated in aprotic solvents by cyclic voltammetry.

Superoxide salts also decompose in the solid state, but this process requires heating:

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Biology

Superoxide is common in biology, reflecting the pervasiveness of O<sub>2</sub> and its ease of reduction. Superoxide is implicated in a number of biological processes, some with negative connotations, and some with beneficial effects.

Like hydroperoxyl, superoxide is classified as reactive oxygen species. It is generated by the immune system to kill invading microorganisms. In phagocytes, superoxide is produced in large quantities by the enzyme NADPH oxidase for use in oxygen-dependent killing mechanisms of invading pathogens. Mutations in the gene coding for the NADPH oxidase cause an immunodeficiency syndrome called chronic granulomatous disease, characterized by extreme susceptibility to infection, especially catalase-positive organisms. In turn, micro-organisms genetically engineered to lack the superoxide-scavenging enzyme superoxide dismutase (SOD) lose virulence. Superoxide is also deleterious when produced as a byproduct of mitochondrial respiration (most notably by Complex I and Complex III), as well as several other enzymes, for example xanthine oxidase, which can catalyze the transfer of electrons directly to molecular oxygen under strongly reducing conditions. Superoxide has been proposed to mediate long-distance electron transport between cytochrome c and Complex III through the aqueous solution, which suggests a role for mitochondrial regulation of reactive oxygen species.

Because superoxide is toxic at high concentrations, nearly all aerobic organisms express SOD. SOD efficiently catalyzes the disproportionation of superoxide:

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Other proteins that can be both oxidized and reduced by superoxide (such as hemoglobin) have weak SOD-like activity. Genetic inactivation ("knockout") of SOD produces deleterious phenotypes in organisms ranging from bacteria to mice and have provided important clues as to the mechanisms of toxicity of superoxide in vivo.

Yeast lacking both mitochondrial and cytosolic SOD grow very poorly in air, but quite well under anaerobic conditions. Absence of cytosolic SOD causes a dramatic increase in mutagenesis and genomic instability. Mice lacking mitochondrial SOD (MnSOD) die around 21 days after birth due to neurodegeneration, cardiomyopathy, and lactic acidosis.

Assay in biological systems

The assay of superoxide in biological systems is complicated by its short half-life. One approach that has been used in quantitative assays converts superoxide to hydrogen peroxide, which is relatively stable. Hydrogen peroxide is then assayed by a fluorimetric method.

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See also

  • Oxygen,
  • Ozonide,
  • Peroxide,
  • Oxide,
  • Dioxygenyl,
  • Antimycin A – used in fishery management, this compound produces large quantities of this free radical.
  • Paraquat – used as a herbicide, this compound produces large quantities of this free radical.
  • Xanthine oxidase – This form of the enzyme xanthine dehydrogenase produces large amounts of superoxide.

References