Electron paramagnetic resonance study of unpaired electron species in thin films of pyrimidine bases induced by nitrogen and oxygen K-shell photoabsorption

In order to clarify the mechanism of DNA-base modification induced by K-shell photoabsorption of nitrogen and oxygen atoms, we measured electron paramagnetic resonance (EPR) spectra of two DNA pyrimidine bases, thymine, and cytosine. The g -factor of 2.000 of the unpaired electron species arising only during irradiation is determined. The EPR intensities for cytosine are two times larger than those simply estimated based on the photoabsorption cross section, whereas those for thymine show similar energy dependence to photoabsorption spectra, suggesting that cytosine favors to form unpaired electron species, rather than thymine, presumably due to the excitation of the enhanced electron capturing.