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The physicochemical processes underlying the radiation sensitization of DNA by the K-shell photoabsorption of nitrogen and oxygen atoms are unclear. Accordingly, we evaluated electron paramagnetic resonance spectra of thin films of pyrimidine bases. By altering the substituents of the pyrimidine ring, we found a substituent effect on the yield of unpaired electrons induced by photoabsorption. The yield of the unpaired electrons of the DNA-base containing amino groups increased by the donation of electrons, and the yield of halogenated DNA-bases decreased by the withdrawal of electrons. These different physicochemical processes might affect DNA bases, leading to differences in DNA modification.

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.

Lifetimes of ortho-positronium in mesoporous silica films were measured before and after surface trimethylsilylation of –OH groups. Variations of positronium lifetimes in the mesopores upon the surface modification indicate that the interaction between positronium and the pore surface is weakened in the pores, whose surface is covered with –CH3 groups, in comparison with those covered with –OH groups. This is consistent with the authors’ previous observation that positronium slowing down is less efficient in the pores covered with –CH3 groups. The present work demonstrates that in the porosimetric application of positron annihilation lifetime spectroscopy, the interaction between positronium and the pore surface has to be properly taken into consideration.