Superoxide
 Lewis structureof superoxide. The six outer-shell electrons of eachoxygenatom 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 | |
PubChemCID
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| UNII | |
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| Properties | |
| O−2 | |
| Molar mass | 31.998g·mol−1 |
| Conjugate acid | Hydroperoxyl |
Except where otherwise noted, data are given for materials in theirstandard state(at 25 °C [77 °F], 100 kPa).
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Inchemistry,asuperoxideis acompoundthat contains the superoxideion,which has the chemical formulaO−2.[1]The systematic name of the anion isdioxide(1−).Thereactive oxygen ionsuperoxide is particularly important as the product of the one-electronreductionofdioxygenO2,which occurs widely in nature.[2]Molecular oxygen(dioxygen) is adiradicalcontaining twounpaired electrons,and superoxide results from the addition of an electron which fills one of the twodegeneratemolecular orbitals,leaving a charged ionic species with a single unpaired electron and a net negative charge of −1. Both dioxygen and the superoxide anion arefree radicalsthat exhibitparamagnetism.[3]Superoxide was historically also known as "hyperoxide".[4]
Salts[edit]
Superoxide forms salts withalkali metalsandalkaline earth metals.The saltssodium superoxide(NaO2),potassium superoxide(KO2),rubidium superoxide(RbO2) andcaesium superoxide(CsO2) are prepared by the reaction ofO2with the respective alkali metal.[5][6]
The alkali salts ofO−2are orange-yellow in color and quite stable, if they are kept dry. Upon dissolution of these salts in water, however, the dissolvedO−2undergoesdisproportionation(dismutation) extremely rapidly (in apH-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 ofpotassium superoxideas an oxygen source inchemical oxygen generators,such as those used on theSpace Shuttleand onsubmarines.Superoxides are also used infirefighters'oxygen tanksto provide a readily available source of oxygen. In this process,O−2acts as aBrønsted base,initially forming thehydroperoxylradical (HO2).
The superoxide anion,O−2,and its protonated form,hydroperoxyl,are inequilibriumin anaqueous solution:[8]
- O−2+ H2O ⇌ HO2+ OH−
Given that the hydroperoxyl radical has apKaof around 4.8,[9]superoxide predominantly exists in the anionic form at neutral pH.
Potassium superoxide is soluble indimethyl sulfoxide(facilitated bycrown ethers) and is stable as long as protons are not available. Superoxide can also be generated inaproticsolvents bycyclic voltammetry.
Superoxide salts also decompose in the solid state, but this process requires heating:
- 2 NaO2→ Na2O2+ O2
Biology[edit]
Superoxide is common in biology, reflecting the pervasiveness of O2and 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 asreactive oxygen species.[3]It is generated by theimmune systemto kill invadingmicroorganisms.Inphagocytes,superoxide is produced in large quantities by theenzymeNADPH oxidasefor use in oxygen-dependent killing mechanisms of invading pathogens. Mutations in the gene coding for the NADPH oxidase cause an immunodeficiency syndrome calledchronic granulomatous disease,characterized by extreme susceptibility to infection, especiallycatalase-positiveorganisms. In turn, micro-organisms genetically engineered to lack the superoxide-scavenging enzymesuperoxide dismutase(SOD) losevirulence.Superoxide is also deleterious when produced as a byproduct ofmitochondrialrespiration(most notably byComplex IandComplex III), as well as several other enzymes, for examplexanthine oxidase,[11]which can catalyze the transfer of electrons directly to molecular oxygen under strongly reducing conditions.
Because superoxide is toxic at high concentrations, nearly all aerobic organisms express SOD. SOD efficiently catalyzes thedisproportionationof superoxide:
- 2 HO2→ O2+ H2O2
Other proteins that can be both oxidized and reduced by superoxide (such ashemoglobin) have weak SOD-like activity. Genetic inactivation ( "knockout") of SOD produces deleteriousphenotypesin organisms ranging from bacteria to mice and have provided important clues as to the mechanisms of toxicity of superoxide in vivo.
Yeastlacking 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.[11]Mice lacking cytosolic SOD (CuZnSOD) are viable but suffer from multiple pathologies, including reduced lifespan,liver cancer,muscle atrophy,cataracts,thymic involution, haemolytic anemia, and a very rapid age-dependent decline in female fertility.[11]
Superoxide may contribute to the pathogenesis of many diseases (the evidence is particularly strong forradiationpoisoning andhyperoxicinjury), and perhaps also toagingvia the oxidative damage that it inflicts on cells. While the action of superoxide in the pathogenesis of some conditions is strong (for instance, mice and rats overexpressing CuZnSOD or MnSOD are more resistant to strokes and heart attacks), the role of superoxide in aging must be regarded as unproven, for now. Inmodel organisms(yeast, the fruit fly Drosophila, and mice), geneticallyknocking outCuZnSOD shortens lifespan and accelerates certain features of aging: (cataracts,muscle atrophy,macular degeneration,andthymic involution). But the converse, increasing the levels of CuZnSOD, does not seem to consistently increase lifespan (except perhaps inDrosophila).[11]The most widely accepted view is that oxidative damage (resulting from multiple causes, including superoxide) is but one of several factors limiting lifespan.
The binding ofO2by reduced (Fe2+)hemeproteins involves formation of Fe(III) superoxide complex.[12]
Assay in biological systems[edit]
The assay of superoxide in biological systems is complicated by its short half-life.[13]One approach that has been used in quantitative assays converts superoxide tohydrogen peroxide,which is relatively stable. Hydrogen peroxide is then assayed by a fluorimetric method.[13]As a free radical, superoxide has a strongEPRsignal, and it is possible to detect superoxide directly using this method. For practical purposes, this can be achieved only in vitro under non-physiological conditions, such as high pH (which slows the spontaneous dismutation) with the enzymexanthine oxidase.Researchers have developed a series of tool compounds termed "spin traps"that can react with superoxide, forming a meta-stable radical (half-life1–15 minutes), which can be more readily detected by EPR. Superoxide spin-trapping was initially carried out withDMPO,but phosphorus derivatives with improved half-lives, such asDEPPMPOandDIPPMPO,have become more widely used.[citation needed]
Bonding and structure[edit]
Superoxides are compounds in which theoxidation numberof oxygen is −1⁄2.Whereas molecular oxygen (dioxygen) is adiradicalcontaining twounpaired electrons,the addition of a second electron fills one of its twodegeneratemolecular orbitals,leaving a charged ionic species with single unpaired electron and a net negative charge of −1. Both dioxygen and the superoxide anion arefree radicalsthat exhibitparamagnetism.
The derivatives of dioxygen have characteristic O–O distances that correlate with theorderof the O–O bond.
| Dioxygen compound | name | O–O distance (Å) | O–O bond order |
|---|---|---|---|
| O+2 | dioxygenylcation | 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[edit]
- 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[edit]
- ^Hayyan, M.; Hashim, M.A.; Al Nashef, I.M. (2016)."Superoxide Ion: Generation and Chemical Implications".Chem. Rev.116(5): 3029–3085.doi:10.1021/acs.chemrev.5b00407.PMID26875845.
- ^Sawyer, D. T.Superoxide Chemistry,McGraw-Hill,doi:10.1036/1097-8542.669650
- ^abValko, M.; Leibfritz, D.; Moncol, J.; Cronin, MTD.; Mazur, M.; Telser, J. (August 2007). "Free radicals and antioxidants in normal physiological functions and human disease".International Journal of Biochemistry & Cell Biology.39(1): 44–84.doi:10.1016/j.biocel.2006.07.001.PMID16978905.
- ^Hayyan, Maan; Hashim, Mohd Ali; Alnashef, Inas M. (2016)."Superoxide Ion: Generation and Chemical Implications".Chemical Reviews.116(5): 3029–3085.doi:10.1021/acs.chemrev.5b00407.PMID26875845.
- ^Holleman, A.F. (2001). Wiberg, Nils (ed.).Inorganic chemistry(1st English ed.). San Diego, CA & Berlin: Academic Press, W. de Gruyter.ISBN0-12-352651-5.
- ^Vernon Ballou, E.; C. Wood, Peter; A. Spitze, LeRoy; Wydeven, Theodore (1 July 1977). "The Preparation of Calcium Superoxide from Calcium Peroxide Diperoxyhydrate".Ind. Eng. Chem. Prod. Res. Dev.16(2): 180–186.doi:10.1021/i360062a015.
- ^Cotton, F. Albert;Wilkinson, Geoffrey(1988),Advanced Inorganic Chemistry(5th ed.), New York: Wiley-Interscience, p. 461,ISBN0-471-84997-9
- ^Bielski, Benon H. J.; Cabelli, Diane E.; Arudi, Ravindra L.; Ross, Alberta B. (1985)."Reactivity of HO2/O2−Radicals in Aqueous Solution ".J. Phys. Chem. Ref. Data.14(4): 1041–1091.Bibcode:1985JPCRD..14.1041B.doi:10.1063/1.555739.
- ^"HO•
2:the forgotten radical Abstract "(PDF).Archived fromthe original(PDF)on 2017-08-08. - ^Yang, Wen; Hekimi, Siegfried (2010)."A Mitochondrial Superoxide Signal Triggers Increased Longevity inCaenorhabditis elegans".PLOS Biology.8(12): e1000556.doi:10.1371/journal.pbio.1000556.PMID21151885.
- ^abcdMuller, F. L.; Lustgarten, M. S.; Jang, Y.; Richardson <first4=A.; Van Remmen, H. (2007). "Trends in oxidative aging theories".Free Radic. Biol. Med.43(4): 477–503.doi:10.1016/j.freeradbiomed.2007.03.034.PMID17640558.
{{cite journal}}:CS1 maint: numeric names: authors list (link) - ^ Yee, Gereon M.; Tolman, William B. (2015). "Chapter 5, Section 2.2.2Fe(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.doi:10.1007/978-3-319-12415-5_5.ISBN978-3-319-12414-8.PMID25707468.
- ^abRapoport, R.; Hanukoglu, I.; Sklan, D. (May 1994)."A fluorimetric assay for hydrogen peroxide, suitable for NAD(P)H-dependent superoxide generating redox systems".Anal Biochem.218(2): 309–13.doi:10.1006/abio.1994.1183.PMID8074285.S2CID40487242.
- ^Abrahams, S. C.; Kalnajs, J. (1955)."The Crystal Structure of α-Potassium Superoxide".Acta Crystallographica.8(8): 503–506.Bibcode:1955AcCry...8..503A.doi:10.1107/S0365110X55001540.