Superoxide
Lewis structure of superoxide. The six outer-shell electrons of each oxygen atom are shown in black; one electron pair is shared (middle); the unpaired electron is shown in the upper-left; and the additional electron conferring a negative charge is shown in red.
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Names | |
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IUPAC name
Superoxide
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Systematic IUPAC name
Dioxidan-2-idylide | |
Other names
Hyperoxide, Dioxide(1−)
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Identifiers | |
3D model (
JSmol ) |
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ChEBI | |
ChemSpider | |
487 | |
KEGG | |
PubChem CID
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UNII | |
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Properties | |
O−2 | |
Molar mass | 31.998 g·mol−1 |
Conjugate acid
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Hydroperoxyl |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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In
Salts
Superoxide forms salts with alkali metals and alkaline earth metals. The salts sodium superoxide (NaO2), potassium superoxide (KO2), rubidium superoxide (RbO2) and caesium superoxide (CsO2) are prepared by the reaction of O2 with the respective alkali metal.[5][6]
The alkali salts of O−2 are orange-yellow in color and quite stable, if they are kept dry. Upon dissolution of these salts in water, however, the dissolved O−2 undergoes disproportionation (dismutation) extremely rapidly (in a pH-dependent manner):[7]
- 4 O−2 + 2 H2O → 3 O2 + 4 OH−
This reaction (with moisture and carbon dioxide in exhaled air) is the basis of the use of
The superoxide anion, O−2, and its protonated form, hydroperoxyl, are in equilibrium in an aqueous solution:[8]
- O−2 + H2O ⇌ HO2 + OH−
Given that the hydroperoxyl radical has a
Potassium superoxide is soluble in
Superoxide salts also decompose in the solid state, but this process requires heating:
- 2 NaO2 → Na2O2 + O2
Biology
Superoxide is common in biology, reflecting the pervasiveness of O2 and its ease of reduction. Superoxide is implicated in a number of biological processes, some with negative connotations, and some with beneficial effects.[10]
Like hydroperoxyl, superoxide is classified as
Because superoxide is toxic at high concentrations, nearly all aerobic organisms express SOD. SOD efficiently catalyzes the disproportionation of superoxide:
- 2 HO2 → O2 + H2O2
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.
Superoxide may contribute to the pathogenesis of many diseases (the evidence is particularly strong for
The binding of O2 by reduced (Fe2+) heme proteins involves formation of Fe(III) superoxide complex.[12]
Assay in biological systems
The assay of superoxide in biological systems is complicated by its short half-life.
Bonding and structure
Superoxides are compounds in which the
The derivatives of dioxygen have characteristic O–O distances that correlate with the order of the O–O bond.
Dioxygen compound | name | O–O distance (Å) | O–O bond order |
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O+2 | dioxygenyl cation | 1.12 | 2.5 |
O2 | dioxygen | 1.21 | 2 |
O−2 | superoxide | 1.28 | 1.5[14] |
O2−2 | peroxide | 1.49 | 1 |
See also
- Oxygen, O2
- Ozonide, O−3
- Peroxide, O2−2
- Oxide, O2−
- Dioxygenyl, O+2
- 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
- PMID 26875845.
- ^ PMID 16978905.
- PMID 26875845.
- ISBN 0-12-352651-5.
- .
- ISBN 0-471-84997-9
- doi:10.1063/1.555739.
- ^ "HO•
2: the forgotten radical Abstract" (PDF). Archived from the original (PDF) on 2017-08-08. - PMID 21151885.
- ^ PMID 17640558.)
{{cite journal}}
: CS1 maint: numeric names: authors list (link - ^
Yee, Gereon M.; Tolman, William B. (2015). "Chapter 5, Section 2.2.2 Fe(III)-Superoxo Intermediates". In Kroneck, Peter M.H.; Sosa Torres, Martha E. (eds.). Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases. Metal Ions in Life Sciences. Vol. 15. Springer. pp. 141–144. PMID 25707468.
- ^ S2CID 40487242.
- .