YIELDS OF (OH)-O-CENTER-DOT IN GAMMA-IRRADIATED DNA AS A FUNCTION OF DNA HYDRATION - HOLE TRANSFER IN COMPETITION WITH (OH)-O-CENTER-DOT FORMATION

In this work, we report the yields of hydroxyl radicals, as G values a nd ''destruction constants,'' in the DNA hydration shell as a function of the level of hydration. Electron spin resonance spectroscopy of ga mma-irradiated DNA at low temperatures is employed for detection of th e hydroxyl radical. Due to the glassy nature of the DNA hydration laye r at low temperature, the hydroxyl radical gives a broad ESR resonance which is easily distinguished from the hydroxyl radical in a polycrys talline ice phase; thus (OH)-O-. in both glassy and ice regions is qua ntified. Three regimes of radiological behavior for waters of hydratio n in DNA are found. For the first approximately 9 waters/nucleotide (w hich are glassy), no significant amounts of (OH)-O-. are found, sugges ting hole transfer to DNA. The second regime of hydration waters compr ises up to about 12 additional glassy waters/nucleotide (Gamma = 21). In this regime, substantial amounts of glassy (OH)-O-. are found, sugg esting that only a few holes which escape recombination in spurs charg e-transfer to the DNA. In these two glassy regimes no trapped electron s are found, which is in accord with previous work that has reported t hat 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 ov er 21 waters/nucleotide are reached. These waters form a separate phas e from the DNA/glassy-water system, and neither hole nor substantial e lectron transfer to the DNA occurs; (OH)-O-. in this ice phase is obse rved 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. (C) 1 996 by Radiation Research Society.