engleski

Handbook of Oxidants and Antioxidants in Exercise

The chapter comprises two main sections. In the first part some important general properties of free radicals are discussed. (i) A free radical is introduced as a species with an unpaired electron. (ii) As a consequence thereof, free radicals are usually highly reactive species. (iii) The knowledge of their chemical properties and absolute rate constants for their reactions is an indispensable requirement to assess any possible activity of free radicals. (iv) This information is best obtained by application of time-resolved detection techniques such as the radiation chemical method of pulse radiolysis and, complementary to this, laser flash photolysis. (v) Structural information on free radicals is mainly achieved by application of electron spin resonance (ESR); in biological systems spin trapping techniques are of particular value. (vi) Further useful information on the reactivity of free radicals may be obtained from redox potential measurements, and also be deduced from stable products of free radical reactions. In the second part the most important oxygen-centered and thiyl free radicals are discussed in details as there are: (1) the hydroxyl radicals ^.^OH, (2) peroxyl radicals ROO^.^, (3) alkoxyl radicals RO^.^, (4) phenoxyl radicals ArO^.^(5) semiquinone radicals ^-^O-Ar-O^.^, (6) superoxide O"2"^.-^, and (7) for comparison the redox-active species in disulfide / thiol systems. The examples chosen try to illustrate characteristic features of general interest such as: (a) acid-base equilibria (b) the change of redox properties within a free radical reaction mechanism, (c) the possible involvement of electronic resonance forms of free radicals, and (d) the consideration of free radical lifetimes with respect to possible reactions and migration of these reactive species in solution or biological systems.

free radicals, oxygen-centered radicals, rate constants, radicals, sulfur-centered radicals, time resolved pulse radiolysis

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