Hydroperoxyl

Structure and reactions

The molecule has a bent structure.

The superoxide anion, , and the hydroperoxyl radical exist in equilibrium in aqueous solution: : + + 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: : + → +

Reactive oxygen species in biology

Together with its conjugate base superoxide, hydroperoxyl is an important reactive oxygen species. Unlike , which has reducing properties, can act as an oxidant in a number of biologically important reactions, such as the abstraction of hydrogen atoms from tocopherol and polyunsaturated fatty acids in the lipid bilayer. As such, it may be an important initiator of lipid peroxidation.

Importance for atmospheric chemistry

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 . In the troposphere, the hydroperoxyl radical is a major byproduct of the oxidation of carbon monoxide and of hydrocarbons by the hydroxyl radical.

Gaseous hydroperoxyl is involved in reactions that influence both stratospheric and tropospheric ozone. In the stratosphere and in low-NOx regions of the troposphere, the hydroperoxyl radical can react directly with ozone, forming OH• and contributing to ozone depletion: + O3 → OH• + 2O2In these low-NOx regions of the troposphere, the hydroperoxyl radical also reacts with itself to form hydrogen peroxide. This process serves as a major source of hydrogen peroxide in the troposphere: + → H2O2 + O2Conversely, in high-NOx regions of the troposphere, the hydroperoxyl radical reacts with NO to produce OH• and : + NO → OH• + Subsequent photolysis of the resulting leads to ozone formation.

References

References

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2. (2003). "Measurement of OH and HO₂ in the Troposphere". Chemical Reviews.
3. (1971). "Geometry and electronic structureof the hydroperoxyl Radical". Journal of the American Chemical Society.
4. De Grey, Aubrey D. N. J.. (2002). "HO₂·: The Forgotten Radical". DNA and Cell Biology.
5. (25 April 2012). "Hydroperoxyl radical". American Meteorological Society.
6. (2000). "Chemistry of the upper and lower atmosphere: theory, experiments, and applications". Academic Press.