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.

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