Nucleobase lesions and strand breaks in dry DNA thin film selectively induced by monochromatic soft X-rays.

To verify the possibility of "selective damage induction" in DNA, the yields of base lesions as well as strand breaks have been measured in dry plasmid DNA films irradiated with highly monochromatized soft X-rays in the energy region of 270-760 eV, which includes the carbon, nitrogen, and oxygen K-edges. The yields of both pyrimidine and purine base lesions, observed as Nth-sensitive and Fpg-sensitive sites, respectively, are strikingly high at the oxygen K-edge (560 eV) but extremely low at an energy just below the nitrogen K-edge (380 eV) as compared with the yields observed at other photon energies. The yields at 560 eV are enhanced 9.6-fold and 27-fold for Nth-sensitive and Fpg-sensitive sites, respectively, compared with those at 380 eV. The yield of prompt single strand breaks is also enhanced at the oxygen K-ionization energy, but only 2-fold, as compared with that at 380 eV. Our results strongly suggest that (1) the K-shell ionization of oxygen in both the nucleobases as well as in other parts of DNA and in the hydrating water molecules bound to DNA, but not the K-shell ionization of nitrogen in the nucleobases, most likely contributes to the induction of nucleobase lesions and that (2) migration of electrons and holes is involved differentially in the production of each type of DNA lesion. These results could potentially lead to new methods for "partially selective induction" of specific types of DNA damage through tuning the energy of soft X-rays.