The hydroperoxyl radical, also known as the perhydroxyl radical, is the protonated form of superoxide with the chemical formula HO2. This species plays an important role in the atmosphere and, as a reactive oxygen species, in cell biology.
|Preferred IUPAC name
|Systematic IUPAC name
3D model (JSmol)
|Molar mass||33.006 g·mol−1|
|Basicity (pKb)||9.12 (basicity of superoxide ion)|
|Conjugate base||Superoxide anion|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Structure and reactions
2 + H2O ⇌ HO2 + OH−
The pKa of HO2 is 4.88. Therefore, about 0.3% of any superoxide present in the cytosol of a typical cell is in the protonated form.
It oxidizes nitric oxide to nitrogen dioxide:
- NO + HO2 → NO2 + HO
Reactive oxygen species in biology
Together with its conjugate base superoxide, hydroperoxyl is an important reactive oxygen species. Unlike O−
2, which has reducing properties, HO2 can act as an oxidant in a number of biologically important reactions, such as the abstraction of hydrogen atoms from tocopherol and polyunstaturated fatty acids in the lipid bilayer. As such, it may be an important initiator of lipid peroxidation.
Importance for atmospheric chemistry
Gaseous hydroperoxyl is involved in reaction cycles that destroy stratospheric ozone. It is also present in the troposphere, where it is essentially a byproduct of the oxidation of carbon monoxide and of hydrocarbons by the hydroxyl radical.
Because dielectric constant has a strong effect on pKa, and the dielectric constant of air is quite low, superoxide produced (photochemically) in the atmosphere is almost exclusively present as HO2. As HO2 is quite reactive, it acts as a "cleanser" of the atmosphere by degrading certain organic pollutants. As such, the chemistry of HO2 is of considerable geochemical importance.
- "Reactivity of HO2/O−
2 Radicals in Aqueous Solution". J. Phys. Chem. Ref. Data. 14 (4): 1041–1091. 1985.
- Heard, Dwayne E.; Pilling, Michael J. (2003). "Measurement of OH and HO2 in the Troposphere". Chemical Reviews. 103: 5163–5198. doi:10.1021/cr020522s.CS1 maint: uses authors parameter (link)
- Liskow, Dean H.; Schaefer, Henry F., III; Bender, Charles F. (1971). "Geometry and electronic structureof the hydroperoxyl Radical". Journal of the American Chemical Society. 93: 6734–7. doi:10.1021/ja00754a003.CS1 maint: uses authors parameter (link)
- De Grey, Aubrey D. N. J. (2002). "HO2·: The Forgotten Radical". DNA and Cell Biology. 21: 251–257. doi:10.1089/104454902753759672.CS1 maint: uses authors parameter (link)
- "Hydroperoxyl radical". Glossary of Meteorology. American Meteorological Society. 25 April 2012. Retrieved 22 August 2013.