"In situ" observation of guanine radicals induced by ultrasoft X-ray irradiation around the K-edge regions of nitrogen and oxygen.

PURPOSE: In order to understand the molecular mechanism of nucleobase damage caused by ultrasoft X-ray irradiation, guanine radicals have been studied using an X-band EPR (electron paramagnetic resonance) spectrometer installed in a synchrotron soft X-ray beamline.

MATERIALS AND METHODS: Guanine pellets were irradiated under vacuum with ultrasoft X-rays obtained from a soft X-ray beamline (BL23SU) in SPring-8. The energy regions around the nitrogen (0.4 keV) and oxygen (0.5 keV) K-edges were chosen for the irradiation. The ultrasoft X-ray irradiation and EPR measurements were carried out simultaneously at low temperature, 20 K and 77 K.

RESULTS: The EPR spectrum observed during irradiation was clearly distinguishable from that of the stable radical, which still exists after exposure to ultrasoft X-rays at 77 K. The spectrum of the short-lived radicals consisted of two components, which exhibited different EPR microwave power saturation. The EPR signal intensities increased linearly with increasing dose rate (photon flux density). These signals immediately disappeared when the beam was turned off, even when irradiated at lower temperature (20 K). At the energy of the oxygen K-resonance excitation (539 eV) the signal intensity was clearly increased to more than five times that obtained on the lower energy side (526 eV). On the other hand, the enhancement was insignificant above and below the nitrogen K-edge (401 eV). The singlet EPR signal of the stable radical was similar to that reported previously in the literature for y-irradiated guanine.

CONCLUSION: The short-lived radical species observed were mainly induced as a result of the final state of the resonant Auger process on oxygen atoms existing solely in the carbonyl group in guanine. Auger events at the other atoms in guanine (namely, carbon and nitrogen) do not induce this radical process to any great extent, even though the abundance of these atoms (i.e. the sum of their photoabsorption cross sections) is dominant in the guanine molecule.