ABSTRACT
A. DEFINITION AND HISTORY Radicals are chemical fragments which contain an unpaired electron.
Radicals are always searching for another electron with which they can form a pair. Since in most chemical environments many electrons are available, radicals are expected to be highly reactive. Some radicals like the hydroxyl radical react with most molecules at close to diffusion controlled rates (1010M-1s-1). This means that the @OH reacts at almost every collision. There are however some not-able exceptions to this rule of high reactivity. The triphenylmethyl radical is highly stable (it can be kept in a test tube). This radical is stabilized by resonance, i.e., the unpairedelectron is not localized at a specific atom, but is spread over a large molecular domain. Due to its stability the triphenylmethyl radical was the first radical to be discovered by Gomberg [2] without the use of today’s sophisticated analytical instrumentation. The history of the early years of free radicals has been reviewed [23]. Other radicals of relatively low reactivity are oxygen, superoxide radical anion and nitric oxide. We shall discuss these important species later on. Although most radicals are highly reactive they do not (fortunately) react with water at room temperature. This stability of water towards radicals is due to the high H-OH bond dissociation energy (119 Kcal/mole). This bond can only be broken by high energy radiation (g radiation or X-rays). The radiation chemistry of water and aqueous solutions [24,25] has provided a clean source of hydroxyl radicals and has provided the scientific basis for the whole area of radical biology and medicine.
