Gamma-Ray Induced Damage in Thymine in Mononucleotide Mixtures, and in Single- and Double-Stranded DNA

In this work some of the factors which are expected to influence the radiation chemical reactivity of the nucleic acid bases in situ in the living cell were investigated. Two γ-ray-induced reactions of thymine, the production of³ H₂ O from$[{}^{3}{\rm H}]\text{thymine-methyl}$ and of products of the 6-(hydroxy or hydroperoxy)-dihydrothymine type ($t^{\dagger}$), were compared in mononucleotide mixtures, single-stranded φX174 DNA, and double-stranded E. coli DNA. Effect of polymerization and strandedness: The initial rates of³ H₂ O and$t^{\dagger}$ formation were similar for the mononucleotide mixture and single-stranded DNA irradiated under nonprotective conditions. For native E. coli DNA the initial rates for³ H₂ O formation were lower by a factor of 9 and for the formation of$t^{\dagger}$ by a factor of 12 relative to φX174 DNA. Effect of protective media: The protective media in all cases suppressed the reactions of thymine more efficiently in the polymer relative to the mononucleotide mixtures, e.g., the formation of$t^{\dagger}$ in tryptone-glucose-yeast extract medium was 67 times more efficient in the mononucleotide mixture than in E. coli DNA.