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, ...

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 t ...

with the

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbol ...

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: : + 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 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 In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. The reduction of molecular oxygen () p ...

. 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 Tocopherols (; TCP) are a class of organic chemical compounds (more precisely, various methylated phenols), many of which have vitamin E activity. Because the vitamin activity was first identified in 1936 from a dietary fertility factor in rat ...

and polyunstaturated fatty acids in the

lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vir ...

. 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 Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

. It is also present in the troposphere, where it is essentially a byproduct of the oxidation of carbon monoxide and of

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

s by the

hydroxyl radical The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals are produced from the ...

. 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