radical (rad´i-kal)

1. In chemistry, a group of elements or atoms usually passing intact from one compound to another, but usually incapable of prolonged existence in a free state (e.g., methyl, CH3); in chemical formulas, a r. is often distinguished by being enclosed in parentheses or brackets. 2. Thorough or extensive; relating or directed to the extirpation of the root or cause of a morbid process; e.g., a r. operation. 3. Denoting treatment by extreme, drastic, or innovative measures, as opposed to conservative. 4. free r [L. radix (radic-), root]
acid r. a r. formed from an acid by loss of one or more hydrogen ions; e.g., SO4-, NO3-.
color r. chromophore
free r. a radical in its (usually transient) uncombined state; an atom or atom group carrying an unpaired electron and no charge; e.g., hydroxyl and methyl
Free r.'s may be involved as short-lived, highly active intermediates in various reactions in living tissue, notably in photosynthesis. The free radical nitric oxide, NO., plays an important role in vasodilation.radical (4); It has been theorized that these also act in human tissues to promote heart disease, cancer, Alzheimer's disease, Parkinson's disease, and rheumatoid arthritis. Free radicals may be introduced to the body (through smoking, inhaling environmental pollutants, or exposure to UV radiation), and also occur naturally within the body as a result of metabolic process. They interact readily with nearby molecules, and may thereby cause cellular damage (including to genetic material). Free radicals may be involved in atherosclerosis, promoting the formation of arterial plaque. Natural enzymes such as superoxide dismutase and peroxidase are thought to counteract free radicals, and there is evidence that vitamins C, E, and beta-carotene also exert an antioxidant effect. Perhaps because they contain large amounts of these antioxidant substances, diets high in whole grains and fresh fruit (see low-fat diets) help lower the risk not only of heart disease, but possibly also of cancer.
oxygen derived free r.'s an atom or atom group having an unpaired electron on an oxygen atom, typically derived from molecular oxygen. For example, one-electron reduction of O2 produces the superoxide radical, O2·; other examples include the hydroperoxyl radical (HOO·), the hydroxyl radical (HO·), and nitric oxide (NO·).


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