Mechanistic Aspects of Radiation-Induced Free Radical Formation in Frozen Aqueous Solutions of DNA Constituents: Consequences for DNA?

Frozen aqueous solutions of 1 M thymidine-5′-monophosphate were X-irradiated 77 K. The free radicals formed were analyzed by electron spin resonance spectroscopy between 77 K and about 260 K and were shown to result nearly exclusively from electron reaction at 77 K forming the thymine base anion, which converts into the well known 5-thymyl radical upon annealing. Primary oxidation of the substrate was not detectable. A minority species denoted <tex-math>${\rm TOH}^{\bullet}$</tex-math>, which appeared at about 200 K, was suggested to result from <tex-math>${\rm OH}^{\bullet}$</tex-math> addition to carbon C₆ of the base, perhaps via intermediate oxidation involving H₂ O₂ or from direct reaction of <tex-math>${\rm OH}^{\bullet}$</tex-math> with the base. Another minority species at 77 K up to about 150 K, which was strongly enhanced by H₂ O₂, was shown to be the allyl radical formed by reaction of the <tex-math>${\rm OH}^{\bullet}$</tex-math> with the methyl group. Support for this was given from experiments using <tex-math>${\rm BeF}_{2}$</tex-math> glasses. The possible spectral features for the cation of dTMP were extracted from aqueous pastes of the Ca²⁺ salt at 77 K. The mechanistic aspects derived from the results are in conflict with previous assumptions and are discussed for DNA model compounds and DNA.