Yields of OH in gamma-irradiated DNA as a function of DNA hydration: hole transfer in competition with 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 gamma-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 .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 .OH are found, suggesting hole transfer to DNA. The second regime of hydration waters comprises up to about 12 additional glassy waters/nucleotide (gamma = 21). In this regime, substantial amounts of glassy .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; .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.

ESR detection at 77 K of the hydroxyl radical in the hydration layer of gamma-irradiated DNA.

Previous ESR reports of gamma-irradiated DNA at low temperatures have suggested that hydroxyl radicals are not formed in the first hydration layer of DNA. In this report we show that hydroxyl radicals are produced in low yield. Due to the glassy nature of this hydration layer at low temperature, the hydroxyl radical gives a broad ESR resonance which is not easily detected. Low-field ESR spectra of hydroxyl radicals in an irradiated 6 M CsF aqueous glass are shown to be nearly identical to those found in DNA; however, the yields in the aqueous glass (G = 0.087 to 0.13 mumol/J) are found to be greater than those in DNA's first hydration layer (G = 0.035 +/- 0.02 mumol/J). A large kappa value for destruction of the OH in DNA's hydration layer limits the yield of OH at high doses. The yield of H2O2 (which likely results from hydroxyl radical recombinations that occur both during irradiation and upon annealing) is found to 0.0035 mumol/J in the dose range 65 kGy to 195 kGy. The amount of H2O2 formed corresponds to most of that expected from recombination of the OH trapped at 77 K at the equivalent dose. The low yield of trapped OH radicals in the first hydration layer has implications for possible hole transfer to DNA.