A theoretical study of excited state properties of adenine-thymine and guanine-cytosine base pairs

Geometries of the Watson-Crick adenine-thymine (AT) and guanine-cytosine (GC) base pairs were optimized in the ground and some selected low-lying, singlet pi-pi* and n-pi* excited states. Ground-state geometries were optimized at the Hartree-Fock level of theory without symmetry restriction. Excited states were generated by employing a configuration interaction technique involving singly excited configurations (CIS method) using the ground-state optimized geometry, and this was followed by excited-state geometry optimization under planar symmetry. The standard 6-31 ++G(d,p) basis set was employed in all calculations. Ground-state geometries are found to be planar; the predicted planarity was validated by the harmonic vibrational frequency calculations. Electronic excitations are found to be localized at either of the monomeric units. The existence of higher energy charge-transfer type excited states is also revealed. These states are characterized by them excitation of electrons from occupied orbitals of one moiety to virtual orbitals of the complementary moiety of the base pair. Electronic excitations and subsequent geometrical relaxations of base pairs in which excitations are localized at the pyrimidine moieties (thymine or cytosine) reveal a large increase in the C5C6 bond length of the pyrimidine bases. Furthermore, n-pi* excitations are found to destabilize hydrogen bonding structures.