Hydroperoxyl
Encyclopedia
The hydroperoxyl radical
, also known as the perhydroxyl radical, is the protonated form of superoxide
with the chemical formula
H
O
2.
in aqueous solution
:
The protonation/deprotonation equilibrium exhibits a pKa
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 oxididant 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 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.
in the stratosphere, and it is formed as a result of the oxidation of hydrocarbon
s in the troposphere
.
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
, also known as the perhydroxyl radical, is the protonated form of superoxide
Superoxide
A superoxide, also known by the obsolete name hyperoxide, is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide. It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature...
with the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
2.
Reactivity
The superoxide anion, O2−, and the hydroperoxyl radical are in equilibriumChemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
in aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
:
- O2− + H2O HO2 + OH−
The protonation/deprotonation equilibrium exhibits a pKa
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
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 oxididant in a number of biologically important reactions, such as the abstraction of hydrogen atoms from tocopherol
Tocopherol
Tocopherols are a class of chemical compounds of which many have vitamin E activity. It is a series of organic compounds consisting of various methylated phenols...
and polyunstaturated fatty acids in the lipid bilayer
Lipid bilayer
The lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...
. As such, it may be an important initiator of lipid peroxidation
Lipid peroxidation
Lipid peroxidation refers to the 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 chain reaction mechanism...
.
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.
Effect on environment
Hydroperoxyl is responsible for the destruction of ozoneOzone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
in the stratosphere, and it is formed as a result of the oxidation of hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s in the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
.