RADIATION-INDUCED FREE-RADICAL FORMATION IN THYMINE DERIVATIVES - EPR/ENDOR OF ANHYDROUS THYMINE SINGLE-CRYSTALS X-IRRADIATED AT 10-K

Anhydrous single crystals of thymine have been X-irradiated at 10 K and studied using K-band EPR, ENDOR, and FSE spectroscopy. In addition to several radical pairs, five monoradicals were formed and stabilized at this temperature. It is demonstrated that the dominating radical pair species analyzed previously (Bergene, R.; Melo, T. B. Int. J. Radiat. Biol. 1973, 23, 255) consists of two radicals, each formed by net hydrogen abstraction from the methyl group. The five monoradicals were identified to be the O4-protonated thymine anion, the thymine-7-yl radical formed by net hydrogen abstraction from the methyl group, the 6-yl radical formed by net hydrogen addition to C5, the 5-yl radical formed by net hydrogen addition to C6, and the I-yl radical formed by net hydrogen abstraction from NI. Upon thermal annealing, the protonated anion decays before 60 K apparently with no successor, and the 6-yl radical decays slowly by storage at room temperature, most probably by a transformation into the 5-yl radical. Possible mechanisms for the formation of these radicals are discussed. These involve processes initiated by primary ionization followed by charge neutralization by protonation/deprotonation or by geminate recombination leading to the formation of hydrogen atoms by homolytic bond scission from molecules in (super)excited states.