Low energy electron interactions with uracil: The energetics predicted by theory

Low energy electrons (LEEs) induce strand breaks and base damage in DNA and RNA via fragmentation of molecular bonding. This includes the formation of hydrogen atoms from N-H and C-H bond dissociations in the bases thymine, cytosine and uracil, respectively. To better understand the dissociation of uracil induced by LEEs, we theoretically characterized the potential energy surfaces (PESs) along the N-H and C-H bonds of the uracil anion, as well as the energetics involved. The PESs show that an activation barrier of less than 1 eV exists for the N-1-H dissociation with rather flat PES beyond N-H = similar to1.5 Angstrom. The PESs for C-5-H and C-6-H show larger barriers, which increase monotonically with bond stretching. All the N-H and C-H bond dissociations are endothermic; the adiabatic PESs suggest the energy threshold for formation of hydrogen from N-H and C-H bonds are in the order: 0.78 (N-1-H) < 1.3 (N-3-H) < 2.2 (C-6-H) < 2.7 eV (C-5-H). The H-deleted uracil radicals (U-yl radical family) are found to have exceptionally high adiabatic electron affinities, namely, 3.46 (N-1), 3.8 (N-3), 2.35 (C-5) and 2.67 eV (C-6). During the H bond breaking process of an uracil transient anion, these electron affinities compensate the extra energy needed to break the N-H or C-H bonds. This process may therefore explain the large hydrogen yield found experimentally from uracil upon attachment of LEEs. Potential applications of this process for the synthesis of uracil analogues using LEE irradiation are suggested.