The hydroperoxyl radical, also known as the hydrogen superoxide, is the

protonated In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid, i ...

form of

superoxide 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 th ...

with the chemical formula HO₂. This species plays an important role in the atmosphere and as a reactive oxygen species in cell biology.

Structure and reactions

The molecule has a bent structure. The superoxide anion, , and the hydroperoxyl radical exist in equilibrium in

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be ...

: : + H₂O HO₂ + OH⁻ The p''K''_a 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 Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its ch ...

to nitrogen dioxide: :NO + HO₂ → NO₂ + HO

Reactive oxygen species in biology

Together with its

conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...

, hydroperoxyl is an important reactive oxygen species. Unlike , 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 Lipid peroxidation is the chain of reactions of oxidative degradation of lipids. It is the process in which free radicals "steal" electrons from the lipids in cell membranes, resulting in cell damage. This process proceeds by a free radical chai ...

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 p''K''_a, 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.

References

{{Hydrides by group Free radicals Hydrogen compounds Reactive oxygen species Superoxides