Isoguanine: From Base Pair to Tetrad

Abstract

Five different isoguanine (isoG) base pairs and five isoG tetrads have been located as the local minima on the potential energy surfaces at the B3LYP/6-311G(d,p) level of theory. An analysis of the isoG base pair enables us to evaluate the hydrogen bond energy in isoG−iosG interactions which is useful in the study of the energy properties of the isoG tetrad. All isoG tetrads in cyclic form are more stable than guanine tetrads. The difference in the stabilization energy between the planar and nonplanar conformers of the isoG tetrad is negligible. With the bond energy assignment, we are able to investigate the cooperative effect of hydrogen bonding in the cyclic tetrads. The cooperative effects of hydrogen bonding are crucial for the highly stabilized isoG tetrads. The balance between proton donation and the accepting of protons in the isoG tetrad governs the cooperative effects. Bifurcated H-bonding is important in the formation of isoG tetrads. They provide alternatives for isoG to develop various stable tetrad conformers which have the potential to create diversity in biological systems. The electrostatic potential (ESP) maps reveal that the nonplanarity of the isoG base pairs originates from the electrostatic repulsion between the monomers in the pair. ESP maps for isoG tetrads also indicate that placing a cation, especially alkali metal ions, in the central area of the isoG tetrads will inevitably lead to the bowl-like structure.