Chemical action of X-rays on nucleic acids and related substances in aqueous systems. 1. Degradation of nucleic acids, purines, pyrimidines, nucleosides and nucleotides by X-rays and by free radicals produced chemically

Abstract

Well-defined chemical changes take place when aqueous solns. of nucleic acids (RNA and DNA) are irradiated with X-rays. Fragmentation of the polynucleotides is accompanied by an increase in number of titratable acid groups and by liberation of inorganic phosphate and of small amts. of free purine bases. Liberation of NH3 and some ring fission of ihe constituent purine and pyrimidine bases also occurs. Glycosidic, ester, and internucleotide linkages are thus broken. Corresponding chemical effects observed on treatment of the nucleates with H2O2-ferrous salt (Fenton''s) reagent have confirmed that the mechanism of the action of X-rays on these dilute aqueous solns. is via free radicals formed by the initial splitting of water molecules. Aqueous solns. of purines and pyrimidines and of some of their corresponding ribo-nucleosides and ribonucleotides were irradiated with X-rays and the nature of the concomitant chemical changes investigated. The presence of an amino group in the base (e. g. in adenine and guanine) enhances production of NH3. In both the purine and pyrimidine systems the ring N atoms may also contribute to the NH3 yield. Purines also yield small amts. of oxalic acid, guanine in addition being degraded with the formation of small amts. of guanidine. Mild alkaline hydrolysis of irradiated uracil solns. results in the formation of urea and oxalic acid. Competition reactions of uracil and of adenine with OH radicals has confirmed that uracil reacts more rapidly with these radicals than adenine. Chemical changes occurring on irradiation of adenosine and of yeast adenylic acid include NH3-liberation, decarboxylation, increase in titratable acid groups and liberation of small amts. of free purine base. From the nucleoside some free ribose is produced. The nucleotide suffers some dephos-phorylation, but does not yield any uncombined pentose or any of the corresponding nucleoside. Irradiation of uridine results in the formation of some free uracil and some free ribose. Somewhat similar effects are induced on treatment of these various substances with Fenton''s reagent.