Hydroperoxyl

The hydroperoxyl radical, also known as the perhydroxyl radical, is the protonated form of superoxide with the chemical formula HO₂.

Formation

Hydroperoxyl is formed through the transfer of a hydrogen atom to molecular oxygen, an oxygen atom to a hydroxyl radical or a proton to a superoxide anion.

Reactivity

The superoxide anion, O₂⁻, and the hydroperoxyl radical are in equilibrium in aqueous solution:

The protonation/deprotonation equilibrium exhibits a pK_a of 4.88; consequently, about 0.3% of any superoxide present in the cytosol of a typical cell is in the protonated form.

Unlike O₂⁻, which predominantly acts as a reductant, 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.

Because dielectric constant has a strong effect on pK_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.