Alkali-labile Sites and Post-irradiation Effects in Single-stranded DNA Induced by H Radicals

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Abstract
Single-stranded .vphi.X174 DNA in aqueous solutions was irradiated in the absence of O2, under conditions in which only H radicals react with the DNA. H radical reactions result in breaks, which contribute approximately 10% inactivation. Two types of alkali-labile sites are formed. One is lethal and gives rise to single-strand breaks by alkali and is most probably identical with post-irradiation heat damage and contributes about 33% to the inactivation mentioned above. The other consists of non-lethal damage, partly dihydropyrimidine derivatives, and is converted to lethal damage by alkali. This follows from experiments in which the DNA was treated with osmium tetroxide, which oxidizes thymine to 5,6-dihydroxydihydrothymine. Treatment with alkali of this DNA gives the same temperature dependence as found for the non-lethal alkali-labile sites in irradiated DNA. A similar temperature dependence is found on dihydrothymine and irradiated pyrimidines with alkali.