EPR spectrometer installed in a soft X-ray beamline at SPring-8 for biophysical studies

We have developed an Electron paramagnetic resonance (EPR) system combined with a synchrotron beamline (Synchrotron Light Excited Electron Paramagnetic Resonance Spectrometer (SLEEPRS)) to detect radicals in biomolecules caused by irradiating with soft X-rays below 2 keV. SLEEPRS was installed in a soft X-ray undulator beamline equipped with a grazing incidence grating monochromator (BL23SU, SPring-8). The cavity of the X-band microwave was set in a high vacuum chamber connected with the beamline transport channel. The sample temperature was controlled from 10 to 300 K by a closed-cycle cryogenic system during the soft X-ray irradiation and the EPR measurements. Typical EPR signals of a deaminated alanine radical from l-α-alanine were observed by irradiating 1.5 keV soft X-rays. The calculated absorbed dose shows that a dose of the same order or less gives the same EPR signal intensity as that generated by a conventional 100 kVp X-ray source. Thus the combination of an EPR spectrometer and synchrotron soft X-ray beamline may open a way for investigating the radical processes involved in biomolecular damages induced by a selective K-photoabsorption of a specific atom.