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 N1−H dissociation with rather flat PES beyond N−H = ∼1.5 Å. The PESs for C5−H and
C6−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 (N1−H) < 1.3 (N3−H) < 2.2 (C6−H) < 2.7 eV (C5−H).
The H-deleted uracil radicals (U-yl radical family) are found to have exceptionally high adiabatic electron
affinities, namely, 3.46 (N1), 3.8 (N3), 2.35 (C5), and 2.67 eV (C6). 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.