Radiolysis of Aqueous Solutions of Nucleosides Halogenated at the Sugar Moiety

SummaryThe pulse radiolysis of aqueous solutions of nucleosides halogenated at the sugar moiety (2′-bromo-2′-deoxyuridine 4, 3′-deoxy-3′-iodothymidine 5, 5′-deoxy-5′-iodouridine 6) has been studied. G(Hal) were determined by conductometry varying the experimental conditions (pH, saturation with Ar, N2O or air, addition of t-butanol). The results indicate that solvated electrons both add to the nucleobases and eliminate halogen ions from the halogenated sugar moiety. In the case of 4 (and possibly of 5) the radical anion of the base transfers (k ≈ 105s−1) an electron to the sugar-bound halogen atom thus cleaving the C-Hal bond. In competition with this reaction there is a protonation of the radical anion of the base by protons and by water. For the latter reaction a rate constant of k = 5 × 103 M−1s−1 was estimated.Compound 4 has also been investigated by product analysis after 60-Co-γ-irradiation. In aerated solutions erythrose is formed with a G-value of 0·12. Its precursor radical is the 2′-radical generated from 4 by dissociative electron capture which reacts with O2 to the corresponding peroxyl radical. Erythrose is formed after a sequence of reactions, one of which involves the scission of the C-1′–C-2′ bond. Under this condition G(HBr) as measured by pulse radiolysis is 0·8. Thus erythrose is formed in 15 per cent yield with respect to its precursor radical. This result is of importance in assessing the precursor radical of a similar product observed in irradiated DNA.