EGTVEDT, I., SAGSTUEN, E., BERGENE, R., AND HENRIKSEN, T. Radiation Damage to Dihydrouracil. Radiat. Res. 75, 252-268 (1978). Single crystals of dihydrouracil, grown from D20 solutions, were irradiated with 4.0-MeV electrons at 77 K. Electron spin resonance techniques were used to study radiation-induced radicals in the temperature range from 77 to 295 K. Altogether four different radicals were identified. Two radicals, the anion protonated at 0(4) and the cation deprotonated at N(1) were observed at 77 K only. Resonance lines from the dihydrouracil-5-yl (U 5-yl) and dihydrouracil-6-yl (U 6-yl) radicals were also observed at this temperature. The detailed structure of the latter two radicals were studied separately. At room temperature the U 6-yl was the main radical species which completely dominated the resonance spectra. Furthermore, uv exposure at 77 K of crystals previously annealed at 295 K resulted in a conversion of the U 6-yl radical into the U 5-yl species. By repeated annealing at room temperature the process may again be reversed. Thus, the technique used gave ample opportunities for a detailed study of the radical conformation. INDO MO-calculations have been performed both for identification purposes and for attaining information about the radical geometry.