free radical

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Any molecule or atom which possesses one unpaired electron. There are some molecules which contain more than one unpaired electron (for example, oxygen); they normally are not considered as free radicals. Free radicals can be chemically very reactive (for example, the methyl radical) or they can be very stable entities (for example, nitric oxide).

Free radicals can be grouped into three major classes: atoms (for example, H, F, and Cl), inorganic radicals (for example, OH, CN, NO₂, and ClO₃), and organic radicals (for example, CH₃, CH₃CH₂, and C₆H₆⁻). Such radicals are of great importance since they often appear as intermediates in thermal and photochemical reactions. Radicals are also known to initiate and propagate polymerization and combustion reactions.

In general, free radicals are formed by the rupture of a bond in a stable molecule with the production of two fragments, each with an unpaired electron. The resulting free radicals may participate in further reactions or may combine to reform the original compound.

There are many ways in which radicals can be generated—among these are thermal decomposition, electric discharge photochemical reactions, electrolysis at an electrode such as mercury or platinum, rapid mixing of two reactants, and gamma- or x-ray irradiation.

A free radical is any chemical species which contains one or more unpaired electrons and which is capable of an independent existence. An unpaired electron is one that can alone occupy an atomic or molecular orbital, conventionally denoted by a superscript dot: X^•.

The biologically important free radicals are the oxygen species, superoxide O^•₂^-, the hydroxyl radical OH^•, and the reactive nitrogen species NO^•; each may play a significant physiological or pathophysiological role in the body.

See ageing; nitric oxide; oxygen.

Highly reactive molecules with an unpaired electron. See antioxidant nutrients.

Free radicals are unstable, chemically incomplete substances that ‘steal’ electrons from other molecules. They are highly reactive, damaging chemicals in the body such as enzymes, making them less effective. Free radicals occur naturally as products of oxidation and are formed in the body during respiration and other chemical processes. Exposure to pollution, cigarette smoke, and strong sunlight can increase the formation of free radicals. Once in the body, free radicals can damage tissues and delicate cell membranes. They can also damage DNA, disrupting our store of inherited information; this may lead to the initiation of certain cancers. Medical scientists believe that free radicals also contribute to at least fifty other major diseases including atherosclerosis, heart disease, rheumatoid arthritis, and lung disease. They may even accelerate the ageing process. Fortunately, our bodies have a good defence system to deal with free radicals. The system is based on chemicals, such as the enzyme sodium dismutase, which can donate electrons to the free radicals, quelling their hyper-reactivity. Chemicals in food, called antioxidants, also disarm free radicals. These antioxidants include beta-carotene, and vitamins A, C, and E.

We use much more oxygen during intense physical activity than when we are inactive. Consequently, we are likely to produce many more free radicals that could harm us. It is thought, however, that regular aerobic exercise may stimulate the formation of more chemicals with antioxidant properties to protect our bodies from free radical damage.

A compound with an unpaired electron or proton. It is unstable and reacts readily with other molecules.

A chemical group that has unshared electrons available for a reaction. Free radicals can damage the integrity of DNA and have been implicated as a cause of cancers. Antioxidants, such as vitamin C and vitamin E, neutralize free radicals.