Nature of Bonding in Nine Planar Hydrogen-Bonded Adenine···Thymine Base Pairs

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Abstract
In this work, we investigate the mode of binding of all nine hydrogen-bonded structures of the adenine···thymine base pair. The planar H-bonded structures were optimized at the MP2/cc-pVTZ level, and the respective interaction energies, corrected for the basis set superposition error, were determined with the aug-cc-pVDZ basis set. The energy components were obtained from the DFT-SAPT procedure using the aug-cc-pVDZ basis set. The charge-transfer character of the single structures was estimated using NBO characteristics. It was established that dipole−dipole interaction itself cannot explain the preferred structure of the pair. Of the various energy components, first-order electrostatic energy plays the most important role. Second-order energy (the sum of induction and dispersion energies) amounts to about 56% of the electrostatic energy. The δ(HF) term covering among others the charge-transfer energy is rather large. The importance of δ(HF) is reflected by the NBO characteristics and especially by the NBO charge-transfer energy. The sum of the second-order energy and the δ(HF) term is only slightly smaller than the electrostatic energy (75−77%), which reflects the importance of the nonelectrostatic terms even in the case of strong H-bonded complexes. The WC structure, which exists in DNA, represents the seventh local minimum, while the three most stable structures utilize the N9−H proton donor group of the five-membered ring.

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