Free radicals, oxidants, and antioxidants.

Free radicals are species with one or more unpaired electrons. The unpaired electron results in a species that is often highly reactive. Free radicals have a wide range of reactions; two broad classes of reactions are electron transfer and addition reactions resulting in covalent bond formation. Free radicals can be classified as reducing (donating an electron to an acceptor) or oxidizing (accepting an electron from a donor). Because of the wide range of reactivities of radicals, there is a thermodynamic hierarchy or pecking order for electron transfer reactions (Buettner, '93). Table 1 is arranged with the most oxidizing radicals at the top and most reducing at the bottom. This pecking order predicts the flow of electrons. This thermodynamic hierarchy helps to predict which species might react with each other. In general, the radical species higher in the pecking order steal electrons from the reduced species lower in the pecking order. The pecking order predicts that peroxyl radicals will react with vitamin E (tocopherol):