Abstract
K-shell photoabsorption of DNA constituent atoms is predicted to induce the critical DNA damage because the constituent atom can be selectively ionized by the K-shell photoabsorption, and ejected secondary low-energy electron tracks cause local multiple damage sites to DNA molecules. To investigate the K-shell photoabsorption effect (photoelectric effect and subsequent secondary electron effect) of DNA damage, dry plasmid DNA (pUC18) films were irradiated with synchrotron monochromatic ultrasoft X-rays. Four photon energies, 270, 380, 435, and 560eV, a value below the carbon K-edge, below and above the nitrogen K-edge, and above the oxygen K-edge, respectively, were chosen for the irradiation experiments. 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 Nth) which excise base lesion. The obtained yields of strand breaks (1∼2×10-10ssb/Gy/Da) were almost the same as that obtained by the irradiation with soft X-rays (2keV), which were investigated by previous reports. The role of ionization effect of carbon, nitrogen and oxygen will be discussed in terms of their efficiency to induce strand breaks, base lesions, and clustered damage sites.
