PNNL

Abstract

We characterized valence-type and dipole-bound anionic states of uracil using various electronic structure methods. We found that the most stable anion is related to neither the canonical 2,4-dioxo nor a rare imino-hydroxy tautomer. Instead, it is related to an imino-oxo tautomer, in which the N1H proton is transferred to the C5 atom. This valence anion, , is characterized by an electron vertical detachment energy (VDE) of 1267 meV and it is adiabatically stable with respect to the canonical neutral by 3.93 kcal/mol. It is also more stable by 2.32 and 5.10 kcal/mol than the dipole-bound ( ) and valence anion ( ), respectively, of the canonical tautomer. The VDE values for and are 73 and 506 meV, respectively. Another, anionic, low-lying imino-oxo tautomer with a VDE of 2499 meV has a proton transferred from N3H to C5 ( ). It is less stable than the neutral canonical tautomer by 1.38 kcal/mol. The mechanism of formation of anionic tautomers with the carbon C5 protonated may involve intermolecular proton transfer or dissociative electron attachment to the canonical neutral tautomer followed by a barrier-free attachment of a hydrogen atom to C5. The six-member ring structure of anionic tautomers with carbon atoms protonated might be unstable upon an excess electron detachment. Indeed, the neutral systems resulting from electron detachment from and evolve along barrier-free decomposition pathways to a linear or a bicyclo structure, respectively, which might be viewed as lesions to RNA. Within the PCM hydration model, the low-lying valence anions become adiabatically bound with respect to the canonical neutral; remains the most stable, being followed by , and .