A superoxide, also known by the obsolete name hyperoxide, is a compound that
contains the superoxide anion with the chemical formula O−
2. The systematic name of the anion is dioxide(1−). Superoxide anion is
particularly important as the product of the one-electron reduction of dioxygen
O2, which occurs widely in nature. Whereas molecular oxygen is a diradical
containing two unpaired electrons, 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 superoxide ion are free radicals that exhibit paramagnetism.
Properties Superoxides are compounds in which the
oxidation number of oxygen is −½. The O–O bond distance in O−
2 is 1.33 Å, vs. 1.21 Å in O2 and 1.49 Å in O2−
2. The salts CsO2, RbO2, KO2, and NaO2 are
prepared by the direct reaction of O2 with the respective alkali metal. The
overall trend corresponds to a reduction in the bond order from 2, to 1.5, to 1.
The alkali salts of O− 2 are orange-yellow in color and quite
stable, provided they are kept dry. Upon dissolution of these salts in water,
however, the dissolved O− 2 undergoes disproportionation extremely
rapidly: 4 O−
2 + 2 H2O → 3O2 + 4 OH− This reaction 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 and on submarines. Superoxides are also used in
firefighters' oxygen tanks in order to provide a readily available source of
oxygen. In this process O−
2 acts as a Brønsted base, initially forming the radical HO2·. But the pKa of
its conjugate acid, hydrogen superoxide, is 4.88 so that at neutral pH 7 all but
0.3% of superoxide is in the anionic form, O−
2. Potassium superoxide can be dissolved in
dimethyl sulfoxide and is stable as long as protons are not available. Superoxide
can also be generated in a-protic solvents by cyclic voltammetry.
Salts also decompose in the solid state, but this process requires heating:
2 NaO2 → Na2O2 + O2 The derivatives of dioxygen, O2, have
characteristic O–O distances that correlate with the bond order of the O–O
bond. Biology
Superoxide is biologically quite toxic and is deployed by the immune system to
kill invading microorganisms. In phagocytes, superoxide is produced in
large quantities by the enzyme NADPH oxidase 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-positive organisms. In turn, micro-organisms genetically
engineered to lack superoxide dismutase lose virulence. Superoxide is also
deleterious when produced as a byproduct of mitochondrial respiration, as well as
several other enzymes, for example xanthine oxidase.
Because superoxide is toxic, nearly all organisms living in the presence of
oxygen contain isoforms of the superoxide-scavenging enzyme superoxide
dismutase, or SOD. SOD is an extremely efficient enzyme; it catalyzes the
neutralization of superoxide nearly as fast as the two can diffuse together
spontaneously in solution. Other proteins that can be both oxidized and
reduced by superoxide have weak SOD-like activity. Genetic inactivation of SOD
produces deleterious phenotypes in organisms ranging from bacteria to mice
and have provided important clues as to the mechanisms of toxicity of superoxide
in vivo. Yeast 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 die
around 21 days after birth due to neurodegeneration, cardiomyopathy, and
lactic acidosis. Mice lacking cytosolic SOD 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, and perhaps also to aging via the oxidative
damage that it inflicts on cells. While the action of superoxide in the
pathogenesis of some conditions is strong, the role of superoxide in aging
must be regarded as unproven for now. In model organisms, genetically knocking
out CuZnSOD shortens lifespan and accelerates certain features of aging.
But the converse, increasing the levels of CuZnSOD, does not seem, to
consistently increase lifespan. The most widely accepted view is that oxidative
damage is but one of several factors limiting lifespan.
= Fe(III)-Superoxo Intermediates = The first step in the activation of O2
by iron-porphyrin complexes involves binding to the Fe(II) ion. Initial
attempts to prepare such 1:1 Fe-O2 adducts were met with difficulty due to
the propensity of these early models to undergo auto-oxidation via the
intermediacy ofdiiron intermediates. Building upon the early, now classic
work in this area, recent studies have provided new insights. The presence of
superoxide has been confirmed by various techniques, including L-edge X-ray
absorption spectroscopy and Mössbauer spectroscopy.
Assay in biological systems The assay of superoxide generated in
biological systems is a difficult task because of its high reactivity and short
half-life. One approach that has been used in quantitative assays converts
superoxide to hydrogen peroxide, 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 when its abundance is high enough. For practical purposes,
this can be achieved only in vitro under non-physiological conditions, such as
high pH with the enzyme xanthine oxidase. Researchers have developed a
series of tool compounds termed "spin traps"(see spin trapping) that can react
with superoxide, forming a meta-stable radical, which can be more readily
detected by EPR. Superoxide spin trapping was initially carried out with
DMPO, but, more recently, phosphorus derivatives with improved half-lives,
such as DEPPMPO and DIPPMPO, have become more widely used.
See also Oxygen, O2
Ozonide, O− Peroxide, O2−
Oxide, O2− Dioxygenyl, O+
Antimycin A - used in fishery management, this compound produces large
quantities of this free radical. Paraquat - used as an 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
External links http:pubs.acs.orgabsbk-2002-0818.ch014