A theoretical investigation of excited-state properties of the adenine-uracil base pair

Molecular geometries of adenine-uracil (AU) base pair were optimized in the ground and selected low-lying singlet pipi* and npi* excited states. The ground-state geometry optimized at the Hartree-Fock level of theory without symmetry restrictions was found to be planar; the predicted planarity was validated by harmonic vibrational frequency calculations. Excited states were generated employing the configuration interaction technique involving singly excited configurations (CIS method) using a ground-state-optimized geometry, and this was followed by excited-state geometry optimizations under planar symmetry. The 6-31++G(d,p) basis set was used in all calculations. The computed electronic transitions of adenine and uracil after linear scaling were found to be in good agreement with the corresponding experimental data. Electronic excitations were found to be localized at either of the monomeric units. It is predicted that among the states studied here the AU base pair has one charge transfer type singlet excited state lying slightly higher in energy. This state is characterized by the excitation of an electron from the occupied orbitals of the adenine moiety to the virtual orbitals of the uracil moiety. In the S-4(pipi*) singlet excited state where the excitation is localized at the uracil moiety of the AU base pair, a large increase in the C'5-C'6 bond length of uracil is revealed. Such a large increase can account for the photophysical reactivity of pyrimidines in view of photodimerization. The base pair geometry is predicted to be largely destabilized under npi* excitations.