Chemical Nature of Chain Breaks Produced in DNA by X-Irradiation in Vitro

X-irradiation of aqueous solutions of calf thymus DNA leads to the production of chain breaks, the liberation of inorganic phosphate, the formation of phosphomonoester groups, and the production of a malonic aldehyde-like product due to damage to deoxyribose residues. For each single-chain break produced in DNA by x-irradiation in 0.15 M NaCl, 0.60 molecules of malonic aldehyde-like product were formed; 0.33 molecules of inorganic phosphate were liberated and 1.35 molecules of phosphomonoester groups were produced. Of the phosphomonoester groups liberated, 29% were 5′-phosphoryl groups. The ratio of newly formed 5′-phosphoryl termini to 5′-hydroxyl termini was 32:1. The presence of 0.001 M l-histidine reduced the number of single-chain breaks by a factor of 0.39, the yield of inorganic phosphate by 0.24, the formation of phosphomonoester groups by 0.42, and deoxyribose damage by 0.41.