Submitted by Dr. Tamer Fouad, M.D.
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Free radicals are a chemical species that possess an
unpaired electron in the outer shell of the molecule. |
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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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