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Hydroperoxyl radical ball.png
Preferred IUPAC name
Systematic IUPAC name
Other names
Peroxyl radical, hydrogen superoxide
3D model (JSmol)
PubChem CID
  • InChI=1S/HO2/c1-2/h1H Yes check.svgY
  • [O]O
Molar mass 33.006 g·mol−1
Acidity (pKa)4.88 [1]
Basicity (pKb)9.12 (basicity of superoxide ion)
Conjugate base Superoxide anion
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

The hydroperoxyl radical , also known as the hydrogen superoxide, is the protonated form of superoxide with the chemical formula HO2. This species plays an important role in the atmosphere and as a reactive oxygen species in cell biology. [2]


Structure and reactions

The molecule has a bent structure. [3]

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

+ H2O HO2 + OH

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. [4]

It oxidizes nitric oxide to nitrogen dioxide: [2]

NO + HO2 → NO2 + HO

Reactive oxygen species in biology

Together with its conjugate base superoxide, hydroperoxyl is an important reactive oxygen species. Unlike O
, 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.

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 carbonium monoxide and of hydrocarbons by the hydroxyl radical. [5]

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.

Related Research Articles

Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricants, to prevent oxidation, and to foods to prevent spoilage, in particular the rancidification of oils and fats. In cells, antioxidants such as glutathione, mycothiol or bacillithiol, and enzyme systems like superoxide dismutase, can prevent damage from oxidative stress.

<span class="mw-page-title-main">Superoxide dismutase</span> Class of enzymes

Superoxide dismutase (SOD, EC is an enzyme that alternately catalyzes the dismutation (or partitioning) of the superoxide (O
) radical into ordinary molecular oxygen (O2) and hydrogen peroxide (H
). Superoxide is produced as a by-product of oxygen metabolism and, if not regulated, causes many types of cell damage. Hydrogen peroxide is also damaging and is degraded by other enzymes such as catalase. Thus, SOD is an important antioxidant defense in nearly all living cells exposed to oxygen. One exception is Lactobacillus plantarum and related lactobacilli, which use a different mechanism to prevent damage from reactive O

In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula O−2. The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature. Molecular oxygen (dioxygen) is a diradical containing two unpaired electrons, and superoxide results from the addition of an electron which fills one of the two degenerate molecular orbitals, leaving a charged ionic species with a single unpaired electron and a net negative charge of −1. Both dioxygen and the superoxide anion are free radicals that exhibit paramagnetism. Superoxide was historically also known as "hyperoxide".

<span class="mw-page-title-main">Reactive oxygen species</span> Highly reactive molecules formed from diatomic oxygen (O₂)

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.

<span class="mw-page-title-main">Hydroxyl radical</span> Neutral form of the hydroxide ion (OH−)

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 decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. It is also important in the field of radiation chemistry, since it leads to the formation of hydrogen peroxide and oxygen, which can enhance corrosion and SCC in coolant systems subjected to radioactive environments.

<span class="mw-page-title-main">Lipid peroxidation</span>

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 chain reaction mechanism. It most often affects polyunsaturated fatty acids, because they contain multiple double bonds in between which lie methylene bridges (-CH2-) that possess especially reactive hydrogen atoms. As with any radical reaction, the reaction consists of three major steps: initiation, propagation, and termination. The chemical products of this oxidation are known as lipid peroxides or lipid oxidation products (LOPs).

Respiratory burst is the rapid release of the reactive oxygen species (ROS), superoxide anion and hydrogen peroxide, from different cell types.

<span class="mw-page-title-main">Oxidative stress</span> Free radical toxicity

Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal redox state of cells can cause toxic effects through the production of peroxides and free radicals that damage all components of the cell, including proteins, lipids, and DNA. Oxidative stress from oxidative metabolism causes base damage, as well as strand breaks in DNA. Base damage is mostly indirect and caused by the reactive oxygen species generated, e.g., O2 (superoxide radical), OH (hydroxyl radical) and H2O2 (hydrogen peroxide). Further, some reactive oxidative species act as cellular messengers in redox signaling. Thus, oxidative stress can cause disruptions in normal mechanisms of cellular signaling.

<span class="mw-page-title-main">Nitroxyl</span> Chemical compound

Nitroxyl or azanone is the chemical compound HNO. It is well known in the gas phase. Nitroxyl can be formed as a short-lived intermediate in the solution phase. The conjugate base, NO, nitroxide anion, is the reduced form of nitric oxide (NO) and is isoelectronic with dioxygen. The bond dissociation energy of H−NO is 49.5 kcal/mol (207 kJ/mol), which is unusually weak for a bond to the hydrogen atom.

The Haber–Weiss reaction generates •OH (hydroxyl radicals) from H2O2 (hydrogen peroxide) and superoxide (•O2) catalyzed by iron ions. It was first proposed by Fritz Haber and his student Joseph Joshua Weiss in 1932.

<span class="mw-page-title-main">Reactive nitrogen species</span>

Reactive nitrogen species (RNS) are a family of antimicrobial molecules derived from nitric oxide (•NO) and superoxide (O2•−) produced via the enzymatic activity of inducible nitric oxide synthase 2 (NOS2) and NADPH oxidase respectively. NOS2 is expressed primarily in macrophages after induction by cytokines and microbial products, notably interferon-gamma (IFN-γ) and lipopolysaccharide (LPS).

Dioxygen plays an important role in the energy metabolism of living organisms. Free oxygen is produced in the biosphere through photolysis of water during photosynthesis in cyanobacteria, green algae, and plants. During oxidative phosphorylation in cellular respiration, oxygen is reduced to water, thus closing the biological water-oxygen redox cycle.

<span class="mw-page-title-main">Radical (chemistry)</span> Atom, molecule, or ion that has an unpaired valence electron; typically highly reactive

In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes.

Sulfanyl (HS), also known as the mercapto radical, hydrosulfide radical, or hydridosulfur, is a simple radical molecule consisting of one hydrogen and one sulfur atom. The radical appears in metabolism in organisms as H2S is detoxified. Sulfanyl is one of the top three sulfur-containing gasses in gas giants such as Jupiter and is very likely to be found in brown dwarfs and cool stars. It was originally discovered by Margaret N. Lewis and John U. White at the University of California in 1939. They observed molecular absorption bands around 325 nm belonging to the system designated by 2Σ+2Πi. They generated the radical by means of a radio frequency discharge in hydrogen sulfide. HS is formed during the degradation of hydrogen sulfide in the atmosphere of the Earth. This may be a deliberate action to destroy odours or a natural phenomenon.

All living cells produce reactive oxygen species (ROS) as a byproduct of metabolism. ROS are reduced oxygen intermediates that include the superoxide radical (O2) and the hydroxyl radical (OH•), as well as the non-radical species hydrogen peroxide (H2O2). These ROS are important in the normal functioning of cells, playing a role in signal transduction and the expression of transcription factors. However, when present in excess, ROS can cause damage to proteins, lipids and DNA by reacting with these biomolecules to modify or destroy their intended function. As an example, the occurrence of ROS have been linked to the aging process in humans, as well as several other diseases including Alzheimer's, rheumatoid arthritis, Parkinson's, and some cancers. Their potential for damage also makes reactive oxygen species useful in direct protection from invading pathogens, as a defense response to physical injury, and as a mechanism for stopping the spread of bacteria and viruses by inducing programmed cell death.

<span class="mw-page-title-main">Nickel superoxide dismutase</span>

Nickel superoxide dismutase (Ni-SOD) is a metalloenzyme that, like the other superoxide dismutases, protects cells from oxidative damage by catalyzing the disproportionation of the cytotoxic superoxide radical to hydrogen peroxide and molecular oxygen. Superoxide is a reactive oxygen species that is produced in large amounts during photosynthesis and aerobic cellular respiration. The equation for the disproportionation of superoxide is shown below:

<span class="mw-page-title-main">Dioxidanylium</span> Ion

Dioxidanylium, which is protonated molecular oxygen, or just protonated oxygen, is an ion with formula HO+
. It is formed when hydrogen containing substances combust, and exists in the ionosphere, and in plasmas that contain oxygen and hydrogen. Oxidation by O2 in superacids could be by way of the production of protonated molecular oxygen.

<span class="mw-page-title-main">Atheronals</span> Chemical compound

Atheronals are biologically relevant oxysterols formed in the reaction of cholesterol with ozone. Atheronal A is the major product of ozonolysis which is 3β-hydroxy-5-oxo-5,6-secocholestan-6-al. Atheronal B is formed by the intramolecular aldol reaction of atheronal A, which is 3β-hydroxy-5β-hydroxy-B-norcholestane-6β-carboxaldehyde.

Hydrogen chalcogenides are binary compounds of hydrogen with chalcogen atoms. Water, the first chemical compound in this series, contains one oxygen atom and two hydrogen atoms, and is the most common compound on the Earth's surface.

<span class="mw-page-title-main">Iron superoxide dismutase</span> Enzyme that catalyses reduction of superoxides

Iron superoxide dismutase (FeSOD) is a metalloenzyme that belongs to the superoxide dismutases family of enzymes. Like other superoxide dismutases, it catalyses the dismutation of superoxides into diatomic oxygen and hydrogen peroxide. Found primarily in prokaryotes such as Escherichia coli and present in all strict anaerobes, examples of FeSOD have also been isolated from eukaryotes, such as Vigna unguiculata.


  1. Bielski, Benon H. J.; Cabelli, Diane E.; Arudi, Ravindra L.; Ross, Alberta B. (1985). "Reactivity of HO2/O
    Radicals in Aqueous Solution"
    . J. Phys. Chem. Ref. Data. 14 (4): 1041–1091. Bibcode:1985JPCRD..14.1041B. doi:10.1063/1.555739.
  2. 1 2 Heard, Dwayne E.; Pilling, Michael J. (2003). "Measurement of OH and HO2 in the Troposphere". Chemical Reviews. 103 (12): 5163–5198. doi:10.1021/cr020522s. PMID   14664647.
  3. Liskow, Dean H.; Schaefer, Henry F. III; Bender, Charles F. (1971). "Geometry and electronic structureof the hydroperoxyl Radical". Journal of the American Chemical Society. 93 (25): 6734–7. doi:10.1021/ja00754a003.
  4. De Grey, Aubrey D. N. J. (2002). "HO2·: The Forgotten Radical". DNA and Cell Biology. 21 (4): 251–257. doi:10.1089/104454902753759672. PMID   12042065.
  5. "Hydroperoxyl radical". Glossary of Meteorology. American Meteorological Society. 25 April 2012. Retrieved 22 August 2013.