21] Electron and hydrogen atom transfer reactions: Determination of free radical redox potentials by pulse radiolysis

Abstract

Pulse radiolysis technique is proving to be, particularly useful for the kinetic and thermodynamic study of free-radical reactions in solution. A large variety of free-radical reactions relevant to biochemistry, biology, and medicine can be observed directly. Electron-transfer cascades, catalytic action of glutathione and related thiols in linking hydrogen transfer and electron-transfer reactions, and protection of vitamin E by vitamin C are just three examples. In the study of hydrogen atom and electron-transfer reactions, most of the experiments to date, particularly those linked to biology, have been undertaken in systems in which water is the principal component. The choice and concentrations of the selective free-radical scavengers to be used depend on the type of reaction to be studied and on the relative rates of reaction of the compounds of interest compared to those for the other solutes under study. In principle, in any multisolute system the proportion of a particular radical reacting with a particular component will depend on the rate constants of the individual reactions and the respective solute concentrations. Typical scavengers used in the studies of electron- and hydrogen-transfer reactions together with the absolute rate constants of the initial interactions, that must be used when designing the composition of solutions to be irradiated, are available.