The role of hydration in the distribution of free radical trapping in directly ionized DNA.

The purpose of this study was to elucidate the role of hydration (Gamma) in the distribution of free radical trapping in directly ionized DNA. Solid-state films of pUC18 (2686 bp) plasmids were hydrated to Gamma in the range 2.5 < or = Gamma < or = 22.5 mol water/mol nucleotide. Free radical yields, G(Sigmafr), measured by EPR at 4 K are seen to increase from 0.28 +/- 0.01 micromol/J at Gamma = 2.5 to 0.63 +/- 0.01 micromol/J at Gamma= 22.5, respectively. Based on a semi-empirical model of the free radical trapping events that follow the initial ionizations of the DNA components, we conclude that two-thirds of the holes formed on the inner solvation shell (Gamma < 10) transfer to the sugar-phosphate backbone. Likewise, of the holes produced by direct ionization of the sugar-phosphate, about one-third are trapped by deprotonation as neutral sugar-phosphate radical species, while the remaining two-thirds are found to transfer to the bases. This analysis provides the best measure to date for the probability of hole transfer (approximately 67%) into the base stack. It can thus be predicted that the distribution of holes formed in fully hydrated DNA at 4 K will be 78% on the bases and 22% on the sugar-phosphate. Adding the radicals due to electron attachment (confined to the pyrimidine bases), the distribution of all trapped radicals will be 89% on the bases and 11% on the sugar-phosphate backbone. This prediction is supported by partitioning results obtained from the high dose-response curves fitted to the two-component model. These results not only add to our understanding of how the holes redistribute after ionization but are also central to predicting the yield and location of strand breaks in DNA exposed to the direct effects of ionizing radiation.