Free Radical Formation in Deoxyguanosine-5′-Monophosphate γ-Irradiated in Frozen Solution. A Computer-Assisted Analysis of Temperature-Dependent ESR Spectra

Deoxyguanosine-5′-monophosphate (dGMP) was γ-irradiated at 77 K in frozen aqueous solution and then annealed in a stepwise fashion up to the melting point. During this process, the primary radicals formed in dGMP at 77 K are progressively converted into secondary radical species. This is observed as changes in the spectrum intensity and conformation. Computer-assisted analysis of these temperature-dependent spectra permitted us to identify the transient radical species involved and to draw up single-radical concentration kinetics vs temperature. The radiation chemical behavior of dGMP was found to be quite similar to that of dAMP, investigated previously. In both these purine derivatives, radical anions are converted into radicals of H-addition to C-8, and radical cations are converted into radicals of OH-addition to the same position. In dGMP, however, the cationic channel is only induced under certain experimental conditions (alkaline pH, presence of electron scavengers). At neutral pH,${\rm G}^{+}$ radicals are quite stable and finally become deactivated without being converted into secondary GOH radicals. Specific deuterium substitution at carbon C-8, and irradiation in H₂ O or in D₂ O, confirmed that both H⁺ and OH^- attachments do occur at C-8, and that both the H⁺ and OH^- groups come from the aqueous medium.