Effects of ionising radiation on deoxyribonucleic acid. Part 3. The effect of iodoacetamide

Abstract

Exposure of frozen aqueous solutions of calf thymus DNA to ⁶⁰Co γ-rays at 77 K gave equal yields of electron-gain centres localised on thymine (T^–) and electron-loss centres localised on guanine (G⁺) as judged by e.s.r. spectroscopy. In the presence of iodoacetamide (ICH₂CONH₂) electron capture by the additive to form ˙CH₂CONH₂ and I^– occurred in competition with electron capture by thymine. The yield of G⁺ remained unaffected as estimated from e.s.r. spectra using computer simulation. On annealing, the e.s.r. features for ˙CH₂CONH₂ were lost at ca. 190 K, with the simultaneous appearance of signals that were assigned to DNA radicals. Analysis for chemical damage using plasmid DNA (pBR 322), irradiated under similar conditions, revealed an increase in the number of single strand-breaks and approximately no change in the number of double strand-breaks arising from the presence of iodoacetamide. Various interpretations of the e.s.r. spectra of the secondary DNA radicals are offered, the most probable being that at least one of these species is formed by hydrogen-atom abstraction from the sugar moiety. We suggest that the increase in single strand-breaks occurs because, on warming, the trapped ˙CH₂CONH₂ radicals react with DNA to give sugar radicals, at least one of which leads to strand breaks. The observation that the sugar radicals are lost at temperatures below those at which the DNA radicals detected by e.s.r. normally decay, offers an explanation of why sugar radical intermediates have not been detected in our previous studies.