ESR Detection at 77 K of the Hydroxyl Radical in the Hydration Layer of Gamma-Irradiated DNA

Abstract

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.