Electron paramagnetic resonance studies of the effects of 1 : 1 electrolytes on the action of ionizing radiation on aqueous DNA

Abstract

Exposure of dilute aqueous solutions of DNA to ⁶⁰Co γ-rays at 77 K gives EPR spectra characteristic of ˙OH radicals in ice, and DNA radicals thought to be mainly G˙⁺ radical cations and a mixture of C˙^– and T˙^– radical anions. These DNA radicals are probably stabilized by rapid gain or loss of protons. The ˙OH radicals form H₂O₂ in the ice-phase at ca. 130 K. The T˙^– centres are irreversibly converted to ˙TH radicals by protonation at C6 on annealing to ca. 200 K, but no other intermediate radical centres are clearly defined. All radical centres are lost prior to complete melting.

On the addition of LiCl or NaCl in the 0–1.0 mol dm^–3 region, there is a two-fold increase in the yields of DNA radicals, this being mainly in the anionic centres. Trapped Cl₂˙^– ions grow in with increasing concentration. On annealing, ˙TH radicals are formed, but, especially for LiCl systems, these are lost increasingly rapidly as the LiCl concentration increases. Despite the increases in target volume, very little change in the yields of strand-breaks is induced by these salts. Sodium bromide gives a smaller initial increase and more rapid loss of DNA radicals on annealing. The yields of ˙TH are considerably reduced indicating protection despite the increased target volume. This is reflected in a reduction in the yields of strand-breaks.

The tetroxy salt sodium perchlorate also gives rise to an increase of initial DNA radical concentration. In contrast to the halide salts, however, this is assigned to an increase in [G˙⁺] with little or no effect upon the concentration of anionic centres. Strand-break studies show a marked protection of DNA by the perchlorate ion.

These results are discussed in terms of the increase in effective target volume and the reactivities of the salt radicals Cl₂˙^–, Br₂˙^– and O˙^–, all of which are detected by EPR spectroscopy.