Abstract
In order 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 270-760eV, which includes the carbon, nitrogen and oxygen K-edges. The experiments were performed at the beamline BL23SU in the SPring-8. Irradiated plasmid DNA was analyzed by agarose gel electrophoresis and the yields of strand breaks were determined by measuring the band intensity of the separated closed circular, open circular and linear forms of the plasmid DNA. The yields of base lesions were determined by the post-irradiation-treatment of the DNA with enzymatic probes (Fpg and Endo III) which excise base lesion. The yields of both pyrimidine and purine base lesions, observed as Nth-sensitive and Fpg-sensitive sites, respectively, are strikingly high (5-6x10-11 lesions/Gy/Da) at the oxygen K-edge (560eV) but extremely low (2-6x10-12 lesions/Gy/Da) at an energy just below the nitrogen K-edge (380eV) as compared with the yields observed at other photon energies. The yields at 560eV are enhanced 10-fold and 27-fold for Nth-sensitive and Fpg-sensitive sites, respectively, compared with those at 380eV. 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 380eV. These results could potentially lead to new methods for selective induction of specific types of DNA damage through tuning the energy of soft X-rays.
