Transient Radicals of DNA Bases by Pulse Radiolysis. Effects of Cysteine and Cysteamine as Radioprotectors

Transient radicals of nucleic acid components formed by${\rm e}_{{\rm aq}}^{-}$and OH reaction during pulse radiolysis have been observed using the electron paramagnetic resonance (EPR) technique. The effect of cysteamine and cysteine on the decay properties of the radical has been investigated. For the natural bases, the predominant mechanism of protection seems to be a competition reaction for the OH rather than a hydrogen transfer from the thiol to the organic radical; however, hydrogen transfer does occur for dihydrothymine. Rate constants are given for these reactions and for reactions with OH and O2.