Enhancement of Radiation-induced Base Release from Nucleosides in Alkaline Solution: Essential Role of the O^{· −} Radical

Abstract

The effect of pH on base release in the γ-radiolysis of N₂O-saturated solutions of a number of nucleosides (including uridine, 3-methyluridine, 2′,3′-O-isopropylidene-uridine, and adenosine) has been investigated. For all these nucleosides, independent of the base or sugar moiety, base release is very low at pH below 10 (G ≈ (0·3–0·7) × 10⁻⁷ mol J⁻¹), but increases drastically to G ≈ (3–4) × 10⁻⁷ mol J⁻¹ at pH ≥ 13. This phenomenon had already been previously reported and attributed to an OH⁻-induced transfer of a base radical into a sugar radical. However, it is now shown that at pH 12, where base release starts to increase, a lowering of the dose-rate does not affect the yield of free base. The increase in base release is accompanied by an overall reduction of chromophore loss of similar magnitude (with 2′,3′-O-isopropylidene-uridine and 3-methyluridine), as well as by an increase in the yield of oxidizing radicals by a factor of 2 (with uridine). The measured rate constant of the reaction of ^·OH/O^{· −} with the nucleosides is also pH-dependent, as ^·OH reacts faster than O^{· −} with the nucleosides by a factor of 6–7. It is concluded that the increase in base release at high pH is caused by the increasing participation of O^{· −}, which, unlike ^·OH, attacks the nucleosides preferentially at their sugar moieties, and is not due to an OH⁻-induced radical transfer from the base to the sugar moiety.