ESR studies of the effects of complexing agents on radiation damage in DNA.

The effects of a range of complexing agents on the radiation damage routes in frozen aqueous DNA have been analyzed by ESR techniques in a semi-quantitative way. Establishment of additive-free DNA standards for this study revealed an ESR pattern different from the four elementary shapes which contributed ∼ 13X of the total spin seen after annealing to 210 K. It is proposed that this represents a population of sugar radicals; hitherto unseen intermediates in additive-free systems between secondary base radicals and strand breaks. The intercalating aminoanthraquinones, mitoxantrone and ametantrone, were found to compete very efficiently with thymine for electron capture, and behaved as protecting agents. Concomitant strand break studies did not support this, and more extensive ESR studies suggested that the intercalator radical centre could also lead to strand breaks. It is therefore proposed that the mode of action of these drugs does not involve free radical processes in DNA. Use of these agents provided evidence that electron migra tion in DNA is much more extensive than previously thought. Copper ions were found to reduce the amounts of T/TH detected, with no effect on G . During this evaluation, evidence appeared for the existence of a site on DNA that could bind two Cu(II) ions so close together that they became ESR silent. Of special interest was the finding that this binding was preferred over that of single Cu(II) ions, and it is proposed that this might have a physiological significance. The dominance of thymine over cytosine as the site of electron capture is reinforced by the above studies, but it is suggested that it is the nature of the system under study that determines the ratio of T to C.