Hydrogen-Atom Abstraction from the Adenine−Uracil Base Pair^†

The hydrogen-abstracted radicals from the adenine−uracil (AU) base pair have been studied at the B3LYP/DZP++ level of theory. The A(N9)−U and A−U(N1) radicals, which correspond to hydrogen-atom abstraction at the adenine N9 and uracil N1 atoms, respectively, were predicted to be the two lowest-lying among the nine (AU − H) radicals studied in this study. The removal of the amino hydrogen of the adenine moiety that forms a hydrogen bond with the uracil O4 atom in the AU pair resulted in radical A(N6a)−U, which has the smallest base-pair dissociation energy, 5.9 kcal mol^-1. This radical is more likely to dissociate into the two isolated bases than to recover the hydrogen bond with the O4 atom through N6−H bond rotation along the C6−N6 bond. In general, the radicals generated by C−H bond breaking were higher in energy than those arising from N−H bond cleavage, because the unpaired electrons in the carbon-centered radicals were mainly localized on the carbon atom from which the hydrogen atom was removed. However, the highest-lying radical was found to arise from removal of the N3 hydrogen of uracil. The most remarkable structural feature of this radical is a very short C−H···O distance of 2.094 Å, consistent with a substantial hydrogen bond. Although this radical lost the N1···H−N3 hydrogen bond between the two bases, its dissociation energy was predicted to be 12.9 kcal mol^-1, similar to that of the intact AU base pair. This is due to the transfer of electron density from the adenine N1 atom to the uracil N3 atom.