Hole Transfer and Differential Radical Recombination in X-Irradiated Doped Crystalline DNA Model Systems

Abstract

Radiation induced hole transfer and differential recombination of radicals at room temperature and lower temperatures (77 and 12 K) have been studied in crystals of cytosine·HCl doped with 5-methylcytosine·HCl (doping level 0.25−1.1 mol %). The main oxidation product stabilized in the doped crystals at room temperature is an allylic radical, called the 3αH radical, which is formed in the 5-methylcytosine dopant by net H-abstraction from the methyl group. This radical has previously been observed in various crystalline cytosine nucleosides and nucleotides shown to contain 5-methylated impurities, and it is of interest to investigate why this radical is formed in disproportionately large yields. Two effects are important in this respect. First, the 3αH radical in the present system is far less prone to recombination than the initially formed cytosine radicals, rendering the relative yield of this radical much greater than expected from the concentration of the dopant in the crystals. Second, as 5-methylcytosine has a lower ionization potential than cytosine, the 3αH radical may in addition be formed by hole transfer from oxidized cytosine to 5-methylcytosine followed by deprotonation at the methyl group. A simple model is presented which isolates the effect of such hole transfer on the relative radical yields from the effect of differential recombination. On the basis of the experimental data, and according to this model, the 3αH radical most probably is formed by fast hole transfer and radical trapping upon irradiation at room temperature. At lower irradiation temperatures the model predicts that the 3αH radical is not the dominant oxidation radical in crystalline 5-methylcytosine.