Thymine and some of its derivatives have been exposed to ionizing radiation, thermal hydrogen and deuterium atoms. The radicals induced have been studied by ESR spectroscopy. Special attention has been given to the formation and relative significance of the 5-thymyl radical. The 5-thymyl radical is formed either via a precursor radical or by a direct radical reaction. On exposure to hydrogen atoms an addition reaction takes place in the thymine-containing substances, whereas in dihydrothymine a corresponding abstraction reaction operates. These radical reactions can partly be hindered or intercepted by sulfur-containing substances. They are temperature-dependent with activation energies in the range 1-3 kcal/mole. In thymine and thymidylic acid the 5-thymyl radical hardly accounts for more than up to approximately 25% of the resonances induced by ionizing radiation, whereas in thymidine and dihydrothymine the radical attains greater importance. The yield of radicals induced in thymine by ionizing radiation depends largely on the physical form of the sample. In freeze-dried or amorphous samples the yield was 3-4 times larger than that obtained for polycrystalline samples, suggesting that lattice imperfections and disorder are involved in radical trapping.

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