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内容記述 |
The aim of this study is to examine the severity of DNA damage induced by phosphorus (P) K-shell ionization and resonant excitation, with particular emphasis on DNA strand breaks and their susceptibility to cellular repair mechanisms. Phosphorus atoms are essential components of the DNA backbone. Dry thin films of plasmid DNA molecules encoding enhanced green fluorescent protein (EGFP) were irradiated with monochromatic soft X-rays at energies near the P-K edge: 2147 eV (below the K-edge), 2153 eV (corresponding to the prominent K-shell resonance peak), and 2160 eV (above the K-edge). The irradiated plasmids were analyzed by agarose gel electrophoresis. The yields of DNA single-strand breaks (SSBs) were nearly identical regardless of P-K photoabsorption. Subsequently, plasmids were exposed to soft X-rays at a dose that reduced the fraction of intact parental plasmids to 1/e, corresponding to an average of one SSB per plasmid. These plasmids were then transfected into non-irradiated human cells according to our previous work, and DNA repair efficiency was accessed by quantifying EGFP expression. Cells transfected with plasmids irradiated with P-K resonance and above K-edge exhibited slightly lower EGFP expression rates compared to those irradiated below K-edge. These findings suggest that phosphorus K-shell resonant excitation and ionization induce DNA damage that is relatively difficult to repair compared to non-P-K photoabsorption. Additional base lesions formed by Auger electrons from phosphorus in close proximity to SSBs may lead to damage clustering, thereby hindering the cellular SSB repair process. |
