Discussion on the Mechanism of the “After-Effect” in the Irradiation of Aqueous Solutions of Deoxyribonucleic Acid with X Rays

It has been shown that irradiation of nucleic acids with X rays in aqueous systems leads to some well-defined chemical changes (Scholes and Weiss, 1952, 1953a). At relatively low doses these chemical effects include the formation of ammonia and of inorganic phosphate; by using higher doses, in addition, the formation of certain free bases and of titratable acid groups could be demonstrated. On the basis of these and other experimental findings, a tentative mechanism for the overall attack on the nucleic acid by the radicals produced by the radiation was proposed (loc. cit.). The loss of viscosity observed on irradiation of deoxyribonucleic acid (DNA) solutions has been attributed to a loss of “hydrogen bonding”, particularly through elimination of polar groups by attacks on the constituent bases and possibly, to some extent, by elimination of phosphate end groups