Single crystals of the complex 1-methylthymine·9-methyladenine were X-irradiated at 10 and at 65 K and studied in the temperature range 10 to 290 K using K-band EPR, ENDOR and field-swept ENDOR (FSE) techniques. The EPR and ENDOR spectra are dominated by two major and four minor resonances. The two major resonances are: MTMA1, the well-known radical formed by net hydrogen abstraction from the C5 methyl group of the thymine moiety, and MTMA2, the radical formed by net hydrogen abstraction from the N1 methyl group of the thymine moiety. The latter product has not been observed previously in any 1-methylthymine derivative. The four minor resonances are: MTMA3, the anion of 1-methylthymine, possibly protonated at the O4 position; MTMA4, the well-known species formed by net hydrogen addition to C6 of the thymine moiety; MTMA5, the species formed by net hydrogen addition to C2 of the adenine moiety; and MTMA6, the species formed by net hydrogen addition to C8 of the adenine moiety. Radical MTMA3, the O4-protonated thymine anion, was clearly detected at 10 K, but upon thermal annealing at 40 K the lines began to disappear. In crystals irradiated at 65 K MTMA3 was only weakly present. Radical MTMA2 decayed at about 250 K with no detectable successor, and radical MTMA5 disappeared at about 180 K. It was not possible to learn from the data if MTMA5 transformed into MTMA6. The radical distribution in the 1-methylthymine·9-methyladenine crystal system is different from that in crystals of the individual components. Reasons for this behavior are discussed in light of the hydrogen bonding schemes and molecular stacking interactions in each of the crystals. An important feature is the concept of excited-state transfer from the adenine to the thymine moiety, followed by dehydrogenation at the thymine N1-methyl group, the mechanism resulting in radical MTMA2.

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