Chemical action of X-rays on nucleic acids and related substances in aqueous systems. 2. The mechanism of the action of X-rays on nucleic acids in aqueous systems

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Abstract
Aqueous solns. of nucleic acids were irradiated with X-rays, using doses approaching those used in biological expts. and the nature of the chemical changes was investigated. Quantitative detns. of the ammonia and inorganic phosphate produced by the radiation and comparison of the yields when the irradiations were performed in air, in O2, in vacuo and in H allowed the deduction of certain features of the mechanism of attack and of the nature of the reacting species involved. In general, the liberation of ammonia results predominantly from an attack on the constituent amino bases by the oxidative radicals OH and HO2. In the dephosphory-lation process , both these oxidative radicals and presumably H atoms play an important part. Irradiations of adenine, yeast adenylic acid and yeast cytidylic acid confirmed these views. The effect of pH on the yields from these various substances showed that, as in the case of amino acids, the ionic nature of the substrate molecules is an important factor affecting their re-activity. The decrease in viscosity observed on irradiation of DNA solns. may be attributed, in part, to a loss of H bonding as a result of elimination of some of the polar groups (e.g. amino or hydroxyl groups). It appears that hydrolytic processes may contribute largely to the "after effect" (the decrease in viscosity of nucleate solns. which occurs after cessation of irradiation) and these may involve the further slow hydrolysis of labile phosphate esters and also of compounds formed by attack on the pyrimidine ethylenic bonds. Some aspects of the mode of degradation of the nucleic acids by X-rays under various exptl. conditions were discussed.