Radiation products of thymine, 1-methylthymine, and uracil investigated by density functional theory

The geometries, relative energies, and hyperfine coupling constants of dehydrogenated, hydrogenated, and hydroxylated radiation products, as well as the anion and the cation, of thymine (T), 1-methylthymine (1-MeT), and uracil (U) are calculated through the use of density functional theory. The results for T and 1-MeT are subsequently compared to accurate ENDOR experiments performed by Sagstuen and co-workers (Sagstuen, E.; Hole, E. O.; Nelson, W. H., Close, D. M. J. Phys. Chem. 1989, 93, 5974 and references therein). The theoretical coupling constants support the experimental assignment of the observed radicals. It is suggested that the hydrogen added to form the O4-hydrogenated product is located at an angle of 50 degrees out of the molecular plane in T. Unfavorable interactions of this hydrogen with the N3 and the methyl hydrogens are shown to lead to this out-of-plane position for O4-H. For the similar radical in 1-MeT, hydrogen bonding in the crystal leads to an in-plane hydrogen position. Notable geometry alterations are found for the radicals formed upon hydrogen addition to the C5 position in T and 1-MeT. Hydroxyl radical addition to either the C5 or the C6 positions in all three bases also leads to considerable distortions.