734. Chemical action of ionising radiations in solutions. Part XVI. Formation of labile phosphate esters from purine and pyrimidine ribonucleotides by irradiation with X-rays in aqueous solution

Abstract

Irradiation of purine and pyrimidine ribonucleotides in aqueous solution with X-rays (200 kv) results in the formation of intermediate labile phosphate esters which lead to a post-irradiation release of inorganic phosphate. This release of phosphate, which occurs after irradiation in the presence or absence of oxygen, follows a first-order rate law and is strongly base-catalysed. It is suggested that the radiation-induced lability of the nucleotides is due to the formation of activating carbonyl groups in the sugar components, PREVIOUS studies of the action of X-rays (200 kv) on purine and pyrimidine ribonucleotides in aqueous solutions have shown that a number of chemical reactions take place. Both the base and the pentose constituents are attacked, ammonia and inorganic phosphate being formed. Some free base is also liberated but no nucleoside or free pentose is produced. Such chemical changes induced by ionising radiations in dilute aqueous systems are due, in the first instance, to the production of free radicals from the water according to the net process :The radicals thus formed may then react with the solute. Because of the possible rBle of labile phosphate esters in the radiation-induced degradation of nucleic acids in aqueous systems,3 we have studied the effects of X-rays on a number of phosphate esters. The ribonucleotides are of obvious interest from this point of view and in the following we report our observations with adenylic and cytidylic acid, which are typical of purine and pyrimidine ribonucleotides respectively.

Dissociation of a Model DNA Compound dApdA by Monochromatic Soft X-rays in Solids and Comments on the High Selectivity for 3′ Breakage in the Phosphoester Bond

“…Raleigh et al (1973) reported that the release of inorganic phosphates by y-radiation from AMP at neutral pH was 2-4 times higher in the 3' isomer than in the 5' isomer . A similar preference was noted earlier in the Xirradiation experiments by Daniels et al (1956) . With dApdA preferential cleavage of the 3' end of the internucleotide bond, producing more 5'-dAMP than 3'-dAMP, has been reported (Raleigh et al .…”

Chemical Nature of Radiation-Induced Single-Strand Breaks in the DNA of Dormant Barley Seeds in Vivo

“…3), it can be estimated that only about 40% of the total phosphomonoester groups formed are 5' -phosphomono esters. One cannot rule out whether all the other phosphomonoesters are 3'-phos phomonoesters, since Daniels et al (24) showed that radical attack on the C atom of the sugar moiety results in the formation of a labile phosphate group with the eventual release of inorganic phosphate. Therefore, additional studies, perhaps employing a specific 3' -phosphomonoesterase will be necessary to identify the nature of the 3'-termini.…”