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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

Types of free radicals in the body

The most important free radicals in the body are the radical derivatives of oxygen better known as reactive oxygen species (Cheeseman and Slater, 1993). These include oxygen in its triplet state (3O2) or singlet state (1O2), superoxide anion (O2?), hydroxyl radical (?OH), nitric oxide (NO?), peroxynitrite (ONOO), hypochlorous acid (HOCl), hydrogen peroxide (H2O2) alkoxyl radical (LO?), and the peroxyl radical (LO?2). Others are carbon-centered free radical (CCI3?) that arises from the attack of an oxidizing radical on an organic molecule. Hydrogen centered radicals result from attack of the H atom (H?). Another form is the sulfur-centered radical produced in the oxidation of glutathione resulting in the thiyl radical (R-S?). A nitrogen-centered radical also exists for example the phenyl diazine radical.

Table 1: Biologically significant free radicals.

Reactive Oxygen Species


Superoxide radical


Hydroxyl radical


Peroxyl radical


Hydrogen peroxide


Singlet oxygen


Nitric oxide




Hypochlorous acid


Fig. 2: Molecular orbital configuration of oxygen. Ground-state oxygen O2 has two unpaired electrons in the outer (p?2p) orbitals and is therefore a free radical. Singlet oxygen has two paired electrons in this orbit and is not a free radical. The superoxide ion (O2?) has an unpaired electron and so is a free radical, but the peroxide ion (O22) has paired electrons and so is not a free radical (Ward and Peters 1995).

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