Tautomerism and isomerism of guanine-cytosine DNA base pair: Ab initio and density functional theory approaches

The relative stabilities of guanine-cytosine (G-C) DNA base pairs are theoretically investigated with a focus on the keto-enol tautomerism as well as on the cis-trans enol isomerism by using both ab initio method and the density functional theory. The G-C pairs of the keto tautomers turn out to be in general more stable than those of the enol tautomers, 9H-guanine pair, 9KK (see the notation below), being the most stable one. The stability of the G-C pairs appears to be affected by various factors including the keto-enol tautomerization, cis-trans enol isomerization, and the steric hindrance between two tautomeric pairs. Although the B3LYP calculations tend to underestimate the binding energies, in addition, it is shown that the binding energy of G-C pairs interacting through hydrogen bonds can be reasonably calculated with the DFT method, contrary to the base pairs with stacked configurations.