Quenching of nucleotide-derived radicals by bisbenzimidazole derivative Hoechst-33258 in aqueous solution

The pulse radiolysis technique has been employed to investigate the reactio n of DNA-minor-groove ligand bisbenzimidazole Hoechst 33258 with pyrimidine and purine nucleotide-derived radicals. Formation of an N-centred Hoechst- 33258 radical is observed. Bimolecular rate constants and the yields of Hoe chst-33258 radical have been evaluated. While the rate constant for the rea ction of pyrimidine-derived radicals with Hoechst-33258 remained the same ( 1-2) x 10(9) dm(3) mol(-1) s(-1), th, yields of the Hoechst-33258 radical v aried from 25% (5'-cytidine monophosphate) to 75% (5'-guanosine monophospha te) under anoxic conditions. The rate constant values for the reaction of p urine-derived radicals with Hoechst-33258, under oxic and anoxic conditions , remained the same whereas with pyrimidine-derived radicals, the rate cons tant value under oxic conditions was about two orders of magnitude lower th an under anoxic conditions. The difference in the yields of Hoechst-33258 r adical with various nucleotide-derived radicals suggest the formation of di fferent types of radicals and that the reaction mainly occurs by electron t ransfer from Hoechst-33258 to the nucleotide radicals.