In the body, it is known that enzymes can convert oxygen to the extremely reactive hydroxyl radical (HO^.). This radical can react with most of the chemicals within the body (e.g. proteins, fats, DNA) causing damage, which interferes with the proper functioning of cells. The formation of hydroxyl radicals has been linked to many important diseases as well as the ageing process. We also know that excessive exposure to environmental pollution, cigarette smoke or ultraviolet light can cause the body to produce harmful radicals.
 

Antioxidants 
Antioxidants are molecules that scavenge for harmful oxygen-centred radicals (HO^., RO^.). They react with the harmful radicals before they get a chance to damage the body. Naturally occurring antioxidants include vitamins C and E, which are found in many plants (including citrus fruits and vegetables), nuts and seeds. Indeed, vitamin E is often present in margarine and this prevents radical reactions leading to deterioration (or rancidity). Antioxidants have been proposed to play an important role in preventing cancer, heart disease, immune-deficiency diseases and ageing. However, the jury is still out as to which groups, if any, benefit from taking antioxidant supplements.
 

Radicals working in harmony 
As vitamin E is fat soluble it can protect the cell membrane from radical damage. Laboratory experiments suggest that this process involves the water soluble vitamin C. After donating a hydrogen atom to a reactive oxygen-centred radical, the vitamin E radical can be converted back to vitamin E by reaction with vitamin C.

VITAMIN C, (ascorbic acid), is one of the most important vitamins found in citrus juices, including orange juice. This antioxidant is necessary for tissue growth and repair as well as for the metabolism of other vitamins and amino acids.
Sources of Vitamin C

VITAMIN Ealso acts as an antioxidant in the cell membrane. This detoxifies free radicals (which are formed as by-products of metabolism), preventing damage to cell membranes, thereby reducing the risk of diseases including arthritis and cancer. Vitamin E also prevents the oxidation of CHOLESTEROL; a process that initiates the buildup of arterial plaque (atherogenesis) which can lead, ultimately, to serious heart disease.
Sources of Vitamin E

Excessive exposure to environmental pollution (e.g. exhaust fumes), UV  light or cigarette smoke, can cause the body to produce harmful radicals. It has been estimated for example, that 10¹⁵ radicals are present in ONE PUFF of cigarette smoke. These radicals include nitrogen monoxide (^.NO) and nitrogen dioxide (^.NO₂), which are known to react with many biological molecules within the lungs.

The destruction of the ozone layer by chlorofluorocarbons (CFCs) or Freons is also known to involve a radical reaction. We know that CFCs travel to the stratosphere where they absorb ultraviolet radiation and breakdown to form chlorine atoms. The chlorine atoms then react with ozone in the stratosphere to produce oxygen. In the process, the protective stratospheric ozone layer, which acts a shield for the earth against harmful ultraviolet radiation from the sun is destroyed. This is a very efficient process, it has been estimated that the formation of one chlorine atom can lead to the destruction of around 100,000 ozone molecules!

A. F. Parsons, 2000, An Introduction to Free Radical Chemistry, Blackwell Science, Oxford.

Polymers: D. M. Haddleton, S. Perrier, and S. A. F. Bon, Macromolecules, 2000, 33, 8246; K. A. Davis, K. Matyjaszewski, Macromolecules, 2000, 33, 4039.

Natural Product Synthesis: J. S. Bryans, J. M. Large and A. F. Parsons,. J. Chem. Soc., Perkin Trans. 1, 1999, 2897

Biological: J. M. C. Gutteridge and B. Halliwell, 1994, Antioxidants in Nutrition, Health and Disease, Oxford University Press, New York.

Further reading:
Z. B. Alfes, 1999, General Aspects of the Chemistry of Radicals, Wiley, New York .
N. L. Bauld, 1997, Radicals, Ion Radicals and Triplets, Wiley-VCH, New York.
D. P. Curran, N. A. Porter and B. Giese, 1996, Stereochemistry of Radical Reactions. Concepts, Guidelines and Synthetic Applications, VCH, Winheim.
H. Fischer (Ed.), 1983, Radical Reaction Rates in Liquids, Landolt-Bornstein, New Series, Vol. 13, Springer Verlag, Berlin.
J. Fossey, D. Lefort and J. Sorba, 1995, Free Radicals in Organic Synthesis, Wiley, New York.
B. Giese, 1986, Radicals in Organic Synthesis: Formation of Carbon-Carbon Bonds, Pergamon, Oxford.
J. K. Kocki, 1973, Free Radicals, Vols 1 and 2, Wiley, New York.
C. J. Moody and G. H. Whitman, 1992, Reactive Intermediates, Oxford University Press, Oxford.
W. B. Motherwell and D. Crich, 1992, Free Radical Chain Reactions in Organic Synthesis, Academic Press, London.
D. C. Nonhebel, J. M. Tedder and J. C. Walton, 1979, Radicals, Cambridge University Press Cambridge.
M. J. Perkins, 1994, Radical Chemistry, Ellis Horwood, New York.
M. J. Perkins, 2000, Radical Chemistry - The Fundamentals, Oxford University Press, New York.
C. Wentrup, 1984, Reactive Molecules, Wiley, New York.
 

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Last Updated: 6th January 2006. These pages are maintained by Izzi Montgomery.
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