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Free Radicals, Types, Sources and Damaging Reactions

Submitted by Dr. Tamer Fouad, M.D.  


Free radicals are a chemical species that possess an unpaired electron in the outer shell of the molecule.


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Oxidative mechanisms in carcinogenesis

Structure of free radicals

Fig. 1: The structure of free radicals (Reiter & Jo Robenson 1995)



The chemistry of free radical generation

Free radicals can be generated both in-vivo and in-vitro by one of the following mechanisms:

1.     Homolytic cleavage of a covalent bond, in which a normal molecule fragments in two, each fragment retaining one of the paired electrons.  Homolytic cleavage occurs less commonly in biological systems, as it requires high-energy input from ultra-violet light, heat or ionising radiation.

2.     Loss of a single electron from a normal molecule.

3.     Addition of an electron to a normal molecule.

A fundamental fact about free radicals is that the unpaired electrons in their outer shells do not affect the charge on the resultant molecule. Free radicals can be negatively charged, positively charged or electrically neutral (Cheeseman and Slater, 1993). This is because charge is concerned with the number of negatively charged electrons in relation to the positively charged protons whereas free radicals are related only to the spatial arrangement of the outer electron. The unpaired electron may have been gained on top of a neutral molecule making it negative; alternatively, it may have resulted from the loss of an electron from the same molecule resulting in a positive charge. Likewise, if the original molecule were not neutral to begin with the addition or removal of an unpaired electron would result in a neutral charge.

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