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 produ ced in low yield. Due to the glassy nature of this hydration layer at low temperature, the hydroxyl radical gives a broad ESR resonance whic h is not easily detected. Low-field ESR spectra of hydroxyl radicals i n 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 mu mol/J) are found to be greater than those in DNA's fi rst hydration layer (G = 0.035 +/- 0.02 mu mol/J). A large kappa value for destruction of the (OH)-O-. in DNA's hydration layer limits the y ield of (OH)-O-. at high doses. The yield of H2O2 (which likely result s from hydroxyl radical recombinations that occur both during irradiat ion and upon annealing) is found to 0.0035 mu mol/J in the dose range 65 kGy to 195 kGy. The amount of H2O2 formed corresponds to most of th at expected from recombination of the (OH)-O-. trapped at 77 K at the equivalent dose. The low yield of trapped (OH)-O-. radicals in the fir st hydration layer has implications for possible hole transfer to DNA.