Radicals are molecules with a free (unpaired) electron that are highly reactive. Radicals are formed in all living organisms during oxidation reactions that occur as part of normal metabolism. Under certain circumstances such as environmental stress, wounding, and pathogen attack, the concentration of free radicals is increased beyond the normal levels. Radicals can do considerable damage to living organisms when left unchecked. This results in part from their reactivity, particularly towards DNA and membranes (lipids and proteins), and in part from the chain reactions they can initiate. Chain reactions occur when a radical reacts with another molecule, abstracts an electron, and thereby creates a new radical that can react with other molecules.
Reactive oxygen species (ROS) are molecules that contain an oxygen atom and that are highly reactive as a result of the presence of a free radical, or a configuration of the oxygen atom whereby there are more electrons than usual. Examples of the first class include the hydroxyl radical (OH), and the superoxide radical (O2' —), whereas the peroxide (O22—) and hypochlorite (ClO' —) ions belong to the second class. Hydrogen peroxide (H2O2) is also considered as a ROS because of its reactivity (Halliwell, 1991), as we have seen in Section 18.104.22.168. The hydroxyl radical is the most reactive. In fact, it is considered the most reactive radical known, with an ability to react with a very wide range of (bio-)molecules. It can be produced via the Fenton reaction, first described in 1894 (Figure 2-15).
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