Induction of DNA strand breaks, base lesions and clustered damage sites in hydrated plasmid DNA films by ultrasoft X rays around the phosphorus K edge

Yokoya, Akinari; Cunniffe, S. M. T.*; Watanabe, Ritsuko; Kobayashi, Katsumi*; O'Neill, P.*

To characterize the DNA damage induced by K-shell ionization of phosphorus atom in DNA backbone on the level of hydration, the yields of DNA strand breaks and base lesions arising from the interaction of ultrasoft X-rays (USX) with energies around the phosphorus K-edge were determined using dry and fully hydrated pUC18 plasmid DNA samples. Base lesions and bistranded clustered DNA damage sites were revealed by post-irradiation treatment with the base excision repair proteins, Nth and Fpg. The complex types of damage, prompt and enzymatically induced DSB are preferentially induced by phosphorus K-resonance at 2153 eV rather than simple SSB and isolated base lesions particularly in hydrated conditions. It is concluded that not only the phosphorus K-resonance and resulting emission of low energy LMM-Auger electrons (120 eV) but also the level of hydration plays an important role in the induction of complex damage in plasmid DNA.