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Free radicals induced in DNA by oxygen and nitrogen K-shell ionization/excitation observed by an EPR apparatus installed in a soft X-ray undulator beamline

Yokoya, Akinari; Fujii, Kentaro; Ukai, Masatoshi*

We have developed an EPR apparatus installed in a soft X-ray beamline, BL23SU in SPring-8, to study DNA radical processes induced by K-shell photoionization/excitation at specific elements constituting DNA. "${it In situ}$" EPR measurement for thin layers of DNA related molecule reveals that (1) short-lived radicals are observed in evaporated thin film of guanine and adenine bases only when irradiated with soft X-rays, (2) the EPR spectra are different between oxygen and nitrogen K-edge region, (3) the photon energy dependence of the radical yields are similar to those of XANES. Interestingly, (4) physically adsorbed water on sample surface of the evaporated base film seem to quench the base radical generation, and (5) photon energy dependence of total radical yield of a calf thymus DNA film shows a large peak at oxygen K-edge region. These results indicate that hydrated waters to DNA might strongly modify the short-lived radical process.

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