Production mechanism of DNA bases radical induced by inner-shell excitation

To reveal the precursor of oxidative nucleobase damages, such as 8-oxo-G in a DNA molecule, induced by ionizing radiation, DNA base radicals were examined around oxygen and nitrogen K-edge region using an electron paramagnetic resonance (EPR) spectrometer installed in a synchrotron soft X-ray beamline (BL23SU) in SPring-8. In situ measurements of EPR spectrum of guanine base revealed that short-lived transient radical species are specifically induced by photoexcitation of a 1s electron to σ* antibonding orbital at the carbonyl oxygen atom. They promptly disappear by beam-off''. On the other hand, a long-lived radical whose EPR spectrum is consistent with previous reports for guanine cation radical was accumulated during the irradiation. The stable radical shows a smaller yield by the oxygen σ* resonance than that by irradiation at off-resonance energy. These results indicate that these two radical processes would be induced competitively after decay from the Auger final state. (author)