PRIMARY REDUCTION AND OXIDATION OF THYMINE DERIVATIVES - ESR/ENDOR OF THYMIDINE AND 1-METHYLTHYMINE X-IRRADIATED AT 10-K

Single crystals of thymidine and 1-methylthymine were X-irradiated at 10 K and studied by using K-band ESR, ENDOR, and FSE spectroscopy. In thymidine, six primary radicals were identified. The major species were the O4-protonated anion (TI), the radical formed by net H addition to the C6 position of the base (TII), the radical formed by net H abstraction from the methyl group (TIII), and the alkoxy radical (TVI). The two minority radicals were one formed by net H addition to the C5 position of the base (TIV) and one in which the unpaired spin was localized on the sugar moiety, thought to be a Cl' H abstraction radical (TV). A small, nonexchangeable coupling in radical TI was shown to be due to a coupling to the methyl group. Evidence is presented that the characteristics of this coupling tensor indicates the protonation state of the thymine anion radical. Upon annealing, the anion decayed at about 40 K with no detectable successor. The alkoxy radical decays in the same temperature range. In 1-methylthymine, three major radical species were identified. These were the O4 protonated anion radical (MTI), the radical formed by net H abstraction from C5 methyl group (MTII), and the radical formed by net H addition to the C5 position of the base (MTIII). The protonation state of the anion was interpreted from the characteristics of a small coupling with the methyl group. The anion decayed at about 40 K with no detectable successor. The thymin-5-yl radical slowly grew in upon prolonged annealing above 250 K. In both crystals the protonated anion is the reduction product. Comparison of the O4 protonated product's magnetic parameters with those of products thought to be thymine anions in other thymine systems in the solid state led to the proposal that all were similarly protonated. Finally, evidence is reviewed for reactions in which thymin-5-yl radicals may arise from the O4 protonated anions or other precursors.