Hydration effects on free radical yields in DNA X-irradiated at 4 K

Variably hydrated sodium DNA was X-irradiated and observed at 4 K using Q-band EPR spectroscopy. The free radical yield, G, was measured as a function of the mass ratio of water:DNA, gamma. Both humidified and dehumidified samples showed large scatter when G was plotted against gamma in the range gamma < 1. For gamma > 1, the scatter in G(gamma) is greatly reduced. Also, as gamma is increased above 1.4, G(gamma) decreases toward the G of pure ice. There is no evidence of the ice radicals, OH. and H., until the water content exceeds 40 waters per nucleotide. The scatter in the G(gamma) data is suggested to be a consequence of DNA packing and/or conformation. The absence of ice radicals in DNA samples with twice the water content of fully hydrated DNA is suggested to be a consequence of (1) damage transfer from water of solvation to DNA and (2) differential recombination in which hydration water is less efficient at trapping free radicals.