Deoxynucleotides–models for Studying Mechanisms of Strand Breakage in DNA

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Abstract
Reactions which cause release of inorganic phosphate from thymidine 3′5′-diphosphate are equivalent to strand-break-producing reactions in DNA. When the former is irradiated in dilute aqueous solution, reactions of the OH· radical induce the liberation of inorganic phosphate. Three types of phosphate release are distinguished: (1) Immediately after irradiation, G = 0·17 (O2), G = 0·21 (N2); (2) On standing in neutral solution, G = 0·46 (O2), G = 0·12 (N2); (3) On treatment with alkali G = 1·8 (O2), G = 0·7 (N2). The sites of OH· radical attack responsible for these three processes are identified. Chromatographic separations indicate that in all cases both sugar phosphate bonds of the parent molecule are broken and no monophosphate products are formed. The yields of formation of strand breaks produced by irradiation of DNA in vivo and the mechanisms by which they are repaired are discussed in terms of the three processes by which sugar phosphate bond breakage occurs in thymidine 3′5′-diphosphate.