Effects of Hydration on the Induction of Strand Breaks, Base Lesions, and Clustered Damage in DNA Films by α-Radiation

The yields of DNA single (ssb)- and double-strand breaks (dsb) as well as base lesions, which are converted into detectable ssb by base excision repair enzymes, induced at 278 K by densely ionizing α-radiation have been determined as a function of the level of hydration (Γ, number of water molecules per nucleotide) of films of supercoiled plasmid DNA (pUC18). The yields of prompt ssb induced by α-radiation are independent of Γ, from vacuum-dried up to 35 water molecules per nucleotide, indicating that diffusible hydroxyl radicals or H<INF>2</INF>O^•+, if induced in the hydrated layer by α-radiation, do not significantly contribute to the induction of ssb. In contrast, the yield of prompt dsb does increase with increasing hydration level. At a Γ of 35, the yield of dsb is about twice that for γ-irradiation. Treatment of α-particle-irradiated DNA with the enzymatic probes, endonuclease III (Nth), and formamidopyrimidine-DNA glycosylase (Fpg), does not lead to significant levels of additional ssb and dsb. It is proposed that (i) base lesions induced by direct energy deposition in the DNA−water complex by high LET radiation, such as α-particles, are generally present in clustered DNA damage, e.g., two or more lesions produced within a few tens of base pairs, and (ii) the complexity of the clustered damage, lesion density, is greater for densely (compared with sparsely) ionizing radiation. As a consequence, the majority of DNA base lesions induced by high LET radiation, in contrast to those by low LET radiation, and which are substrates for Fpg and Nth become refractory to excision repair due to their formation within more complex, clustered DNA damage.