Effects of ionizing radiation on deoxyribonucleic acid and related systems. Part 1. The rôle of oxygen

Abstract

Exposure of aqueous solutions of calf-thymus DNA to ⁶⁰Co γ-rays at 77 K results in the formation of equal yields of G⁺ and T^– centres, detected by e.s.r. spectroscopy. In the presence of oxygen, O₂^– ions are also formed at the expense of T^–, the primary yield of G⁺ centres being unaffected. On annealing above 77 K the e.s.r. features for O₂^– ions were initially modified as a result of changes in solvation, and then, at ca. 193 K, they were lost irreversibly. A large growth in signals due to RO₂· radicals was observed simultaneously. These radicals are formed from T^– anions or the protonated form, TH, and probably also indirectly from G⁺ radicals. In parallel experiments, under comparable conditions, we have analysed for single and double stand breaks using plasmid DNA (pBR 322). Both single and double strand breaks persist under conditions in which G⁺ and T^– are the only detectable primary radical products. The presence of oxygen causes only a slight increase in damage to DNA.