Adiabatic Electron Affinities of the Polyhydrated Adenine−Thymine Base Pair:  A Density Functional Study

Adiabatic electron affinities (AEAs) of the adenine−thymine (AT) base pair surrounded by 5 and 13 water molecules have been studied by density functional theory (DFT). Geometries of neutral AT·nH₂O and anionic (AT·nH₂O)^- complexes (n = 5 and 13) were fully optimized, and vibrational frequency analysis was performed at the B3LYP/6-31+G** level of theory. The optimized structures of the neutral (AT·nH₂O) and (AT·nH₂O)^- complexes were found to be somewhat nonplanar. Some of the water molecules are displaced away from the AT ring plane and linked with one another by hydrogen bonds. The optimized structures of the complexes are found to be in a satisfactory agreement with the observed experimental and molecular dynamics simulation results. In the optimized anionic complexes, the thymine (T) moiety was found to be puckered, whereas the adenine (A) moiety remained almost planar. Natural population analysis (NPA) performed using the B3LYP/6-31+G** method shows that the thymine moiety in the anionic (AT·nH₂O)^- complexes (n = 5 and 13) has most of the excess electronic charge, i.e., ∼ −0.87 and ∼ −0.81 (in the unit of magnitude of the electronic charge), respectively. The zero-point energy corrected adiabatic electron affinities of the hydrated AT base pair were found to be positive both for n = 5 and 13 and have the values of 0.97 and 0.92 eV, respectively, which are almost three times the AEA of the AT base pair. The results show that the presence of water molecules appreciably enhances the EA of the base pair.