Free Radicals as Reagents for Electron Transfer Processes in Proteins

Abstract

Free radicals produced from amino acids and polypeptide side chains were originally investigated in order to resolve the molecular basis of light or radiation induced damage. However, as discussed below they have also turned out to be effective tools for studying electron transfer processes within polypeptide matrices and yielded important insights into the mechanism of these reactions. Production and study of free radicals in proteins by pulse radiolysis was first introduced in the sixties. A major method developed for the purpose of investigating radiation chemistry, found a broad range of important applications in different fields of chemistry and biochemistry reaching far beyond the subject to which it was first applied (Adams, Fielden and Michael, 1975). The method is based on the excitation and ionization of solvent molecules by short pulses of high energy electrons. Introducing radiation (e.g. 5–10 MeV) into dilute aqueous solutions of a given solute causes primary changes in the solvent. Thus, water molecules undergo conversion into OH radicals, hydrated electrons and to a lesser extent H atoms, H₂ and H₂O₂ are also produced (the yields are usually presented as G values, i.e. number of chemical species produced per 100 eV of absorbed energy: e ⁻_aq = 2.9; OH = 2.8; H = 0.55; H₂O₂ = 0.75 H₂ = 0.45). The hydrated electrons and OH radicals present thermodynamic extremes of reducing and oxidizing potentials, respectively. Hence, they provide the possibility to initiate a wide range of electron transfer processes. However, their extreme reactivity is leading to non selective reactions. Hence, they are usually converted to less reactive ones, by protocols devised by radiation chemists.