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chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, 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 the one-electron reduction of
dioxygen There are several known allotropes of oxygen. The most familiar is molecular oxygen (), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (). Others are: * Ato ...
, which occurs widely in nature. Molecular oxygen (dioxygen) is a diradical containing two
unpaired electron In chemistry, an unpaired electron is an electron that occupies an orbital of an atom singly, rather than as part of an electron pair. Each atomic orbital of an atom (specified by the three quantum numbers n, l and m) has a capacity to contai ...
s, 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 In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired electron, unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemical reaction, chemi ...
that exhibit
paramagnetism Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...
. Superoxide was historically also known as "hyperoxide".


Salts

Superoxide forms salts with
alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
s and
alkaline earth metal The alkaline earth metals are six chemical elements in group (periodic table), group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar p ...
s. The salts sodium superoxide (), potassium superoxide (), rubidium superoxide () and caesium superoxide () are prepared by the reaction of with the respective alkali metal. The alkali salts of are orange-yellow in color and quite stable, if they are kept dry. Upon dissolution of these salts in water, however, the dissolved undergoes disproportionation (dismutation) extremely rapidly (in a pH-dependent manner): : This reaction (with moisture and carbon dioxide in exhaled air) is the basis of the use of potassium superoxide as an oxygen source in chemical oxygen generators, such as those used on the
Space Shuttle The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...
and on
submarine A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or infor ...
s. Superoxides are also used in firefighters' oxygen tanks to provide a readily available source of oxygen. In this process, acts as a Brønsted base, initially forming the hydroperoxyl radical (). The superoxide anion, , and its protonated form, hydroperoxyl, are in equilibrium in an
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, also known as sodium chloride (NaCl), in water ...
: : Given that the hydroperoxyl radical has a p''K''a of around 4.8, superoxide predominantly exists in the anionic form at neutral pH. Potassium superoxide is soluble in
dimethyl sulfoxide Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula . This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is ...
(facilitated by crown ethers) and is stable as long as protons are not available. Superoxide can also be generated in aprotic solvents by cyclic voltammetry. Superoxide salts also decompose in the solid state, but this process requires heating: :


Biology

Superoxide is common in biology, reflecting the pervasiveness of O2 and its ease of reduction. Superoxide is implicated in a number of biological processes, some with negative connotations, and some with beneficial effects. Like hydroperoxyl, superoxide is classified as
reactive oxygen species In chemistry and biology, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl ...
. It is generated by the
immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
to kill invading
microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
s. In phagocytes, superoxide is produced in large quantities by the
enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
NADPH oxidase NADPH oxidase (nicotinamide adenine dinucleotide phosphate oxidase) is a membrane-bound enzyme complex that faces the extracellular space. It can be found in the plasma membrane as well as in the membranes of phagosomes used by neutrophil white ...
for use in oxygen-dependent killing mechanisms of invading pathogens. Mutations in the gene coding for the NADPH oxidase cause an immunodeficiency syndrome called chronic granulomatous disease, characterized by extreme susceptibility to infection, especially
catalase Catalase is a common enzyme found in nearly all living organisms exposed to oxygen (such as bacteria, plants, and animals) which catalyzes the decomposition of hydrogen peroxide to water and oxygen. It is a very important enzyme in protecting ...
- positive organisms. In turn, micro-organisms genetically engineered to lack the superoxide-scavenging enzyme superoxide dismutase (SOD) lose virulence. Superoxide is also deleterious when produced as a byproduct of
mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...
l respiration (most notably by Complex I and Complex III), as well as several other enzymes, for example xanthine oxidase, which can catalyze the transfer of electrons directly to molecular oxygen under strongly reducing conditions. Because superoxide is toxic at high concentrations, nearly all aerobic organisms express SOD. SOD efficiently catalyzes the disproportionation of superoxide: : Other proteins that can be both oxidized and reduced by superoxide (such as
hemoglobin Hemoglobin (haemoglobin, Hb or Hgb) is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. Hemoglobin ...
) have weak SOD-like activity. Genetic inactivation ("
knockout A knockout (abbreviated to KO or K.O.) is a fight-ending, winning criterion in several full-contact combat sports, such as boxing, kickboxing, Muay Thai, mixed martial arts, karate, some forms of taekwondo and other sports involving striking, ...
") of SOD produces deleterious
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...
s in organisms ranging from bacteria to mice and have provided important clues as to the mechanisms of toxicity of superoxide in vivo.
Yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom (biology), kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are est ...
lacking both mitochondrial and cytosolic SOD grow very poorly in air, but quite well under anaerobic conditions. Absence of cytosolic SOD causes a dramatic increase in mutagenesis and genomic instability. Mice lacking mitochondrial SOD (MnSOD) die around 21 days after birth due to neurodegeneration, cardiomyopathy, and lactic acidosis. Mice lacking cytosolic SOD (CuZnSOD) are viable but suffer from multiple pathologies, including reduced lifespan, liver cancer, muscle atrophy, cataracts, thymic involution, haemolytic anemia, and a very rapid age-dependent decline in female fertility. Superoxide may contribute to the pathogenesis of many diseases (the evidence is particularly strong for
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...
poisoning and hyperoxic injury), and perhaps also to
aging Ageing (or aging in American English) is the process of becoming Old age, older until death. The term refers mainly to humans, many other animals, and fungi; whereas for example, bacteria, perennial plants and some simple animals are potentiall ...
via the oxidative damage that it inflicts on cells. While the action of superoxide in the pathogenesis of some conditions is strong (for instance, mice and rats overexpressing CuZnSOD or MnSOD are more resistant to strokes and heart attacks), the role of superoxide in aging must be regarded as unproven, for now. In
model organism A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mo ...
s (yeast, the fruit fly Drosophila, and mice), genetically knocking out CuZnSOD shortens lifespan and accelerates certain features of aging: ( cataracts, muscle atrophy,
macular degeneration Macular degeneration, also known as age-related macular degeneration (AMD or ARMD), is a medical condition which may result in blurred vision, blurred or vision loss, no vision in the center of the visual field. Early on there are often no sym ...
, and
thymic involution Thymic involution is the shrinking (Involution (medicine), involution) of the thymus with age, resulting in changes in the architecture of the thymus and a decrease in tissue mass. Thymus involution is one of the major characteristics of verteb ...
). But the converse, increasing the levels of CuZnSOD, does not seem to consistently increase lifespan (except perhaps in ''
Drosophila ''Drosophila'' (), from Ancient Greek δρόσος (''drósos''), meaning "dew", and φίλος (''phílos''), meaning "loving", is a genus of fly, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or p ...
''). The most widely accepted view is that oxidative damage (resulting from multiple causes, including superoxide) is but one of several factors limiting lifespan. The binding of by reduced ()
heme Heme (American English), or haem (Commonwealth English, both pronounced /Help:IPA/English, hi:m/ ), is a ring-shaped iron-containing molecule that commonly serves as a Ligand (biochemistry), ligand of various proteins, more notably as a Prostheti ...
proteins involves formation of Fe(III) superoxide complex.


Assay in biological systems

The assay of superoxide in biological systems is complicated by its short half-life. One approach that has been used in quantitative assays converts superoxide to
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...
, which is relatively stable. Hydrogen peroxide is then assayed by a fluorimetric method. As a free radical, superoxide has a strong EPR signal, and it is possible to detect superoxide directly using this method. For practical purposes, this can be achieved only in vitro under non-physiological conditions, such as high pH (which slows the spontaneous dismutation) with the enzyme xanthine oxidase. Researchers have developed a series of tool compounds termed " spin traps" that can react with superoxide, forming a meta-stable radical (
half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...
1–15 minutes), which can be more readily detected by EPR. Superoxide spin-trapping was initially carried out with DMPO, but phosphorus derivatives with improved half-lives, such as DEPPMPO and DIPPMPO, have become more widely used.


Bonding and structure

Superoxides are compounds in which the oxidation number of oxygen is −. Whereas molecular oxygen (dioxygen) is a diradical containing two
unpaired electron In chemistry, an unpaired electron is an electron that occupies an orbital of an atom singly, rather than as part of an electron pair. Each atomic orbital of an atom (specified by the three quantum numbers n, l and m) has a capacity to contai ...
s, the addition of a second electron fills one of its two degenerate molecular orbitals, leaving a charged ionic species with single unpaired electron and a net negative charge of −1. Both dioxygen and the superoxide anion are
free radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired electron, unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemical reaction, chemi ...
that exhibit
paramagnetism Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...
. The derivatives of dioxygen have characteristic O–O distances that correlate with the order of the O–O bond.


See also

*
Oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
, * Ozonide, *
Peroxide In chemistry, peroxides are a group of Chemical compound, compounds with the structure , where the R's represent a radical (a portion of a complete molecule; not necessarily a free radical) and O's are single oxygen atoms. Oxygen atoms are joined ...
, *
Oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...
, *
Dioxygenyl The dioxygenyl ion, , has been studied in both the gas phase and in salts with anions that cannot be oxidized. The first synthesis was []. Rather than the triple bond of , the bond order is considered to be . Relative to most molecules, this ioni ...
, *Antimycin A – used in fishery management, this compound produces large quantities of this free radical. * Paraquat – used as a herbicide, this compound produces large quantities of this free radical. * Xanthine oxidase – This form of the enzyme xanthine dehydrogenase produces large amounts of superoxide.


References

{{oxygen compounds Anions Oxygen compounds Oxyanions Immune system Free radicals Reactive oxygen species