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Radiation energy transfer and trapping in single crystals of hemihydrate and hydrochloride of 5- methylcytosine doped with 5-methylcytosine - An EPR study

Two different crystals of 5-methylcytosine with 5- methylthiocytosine as impurity, irradiated with ionising radiation, exhibit quite different types of paramagnetic centres associated with thio impurities. In 5-methylcytosine hemihydrate there are two types of radicals with the unpaired spin located mainly on sulphur. Both are of thiyl structure, presumably derived by a loss of an electron from 5- methylthiocytosine. In doped 5-methylcytosine hydrochloride two distinct paramagnetic species with high spin density on chlorine have been detected. One of them has been spectroscopically characterised and assigned to an electron-deficient species with the unpaired electron shared by a sulphur and a chlorine atom, with some delocalization over the pyrimidine ring. Both crystal lattices of 5-methylcytosine support migration of electrons/holes. In the hemihydrate lattice, the base stacking interaction, although weak in comparison to that in DNA or some other molecular crystals of the nucleic-acid bases, might be responsible for that. In the dihydrochloride lattice, the chlorine-ion strings or layers might provide the means for the hole transport. Both neutral and positively charged 5-methylthiocytosine is a good radiation energy trap (hole trap).

EPR ; 5-Methylthiocytosine ; Radiation damage ; Sulphur-centered radicals ; Transfer of radiation energy

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