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, Γ. Both humidified and dehumidified samples showed large scatter when G was plotted against Γ in the range Γ < 1. For Γ > 1, the scatter in G(Γ) is greatly reduced. Also, as Γ is increased above 1.4, G(Γ) 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(Γ) 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.