Yields of${}^{\bullet}{\rm OH}$ in Gamma-Irradiated DNA as a Function of DNA Hydration: Hole Transfer in Competition with${}^{\bullet}{\rm OH}$ Formation

In this work, we report the yields of hydroxyl radicals, as G values and "destruction constants," in the DNA hydration shell as a function of the level of hydration. Electron spin resonance spectroscopy of γ-irradiated DNA at low temperatures is employed for detection of the hydroxyl radical. Due to the glassy nature of the DNA hydration layer at low temperature, the hydroxyl radical gives a broad ESR resonance which is easily distinguished from the hydroxyl radical in a polycrystalline ice phase; thus${}^{\bullet}{\rm OH}$ in both glassy and ice regions is quantified. Three regimes of radiological behavior for waters of hydration in DNA are found. For the first approximately 9 waters/nucleotide (which are glassy), no significant amounts of${}^{\bullet}{\rm OH}$ are found, suggesting hole transfer to DNA. The second regime of hydration waters comprises up to about 12 additional glassy waters/nucleotide (Γ = 21). In this regime, substantial amounts of glassy${}^{\bullet}{\rm OH}$ are found, suggesting that only a few holes which escape recombination in spurs charge-transfer to the DNA. In these two glassy regimes no trapped electrons are found, which is in accord with previous work that has reported that all electrons which do not undergo recombination in spurs transfer to DNA. The third regime of hydration water is comprised of bulk (or bulk-like) polycrystalline ice which forms when levels of hydration over 21 waters/nucleotide are reached. These waters form a separate phase from the DNA/glassy-water system, and neither hole nor substantial electron transfer to the DNA occurs;${}^{\bullet}{\rm OH}$ in this ice phase is observed with G values that vary slightly with the amount of water in the ice phase, but which are close to the values found for pure ice.