Quantum-chemical ab initio study on the adenine-difluorotoluene complex--a mimic for the adenine-thymine base pair

Recent experiments have shown that difluorotoluene (F), a nonpolar isostere for thymine (T), codes efficiently and specifically for adenine (A) in DNA replication. F has almost the same shape as thymine but it is unable to form conventional hydrogen bonds with adenine. Therefore, it has been claimed that not hydrogen bonding but shape complementary may be important for the selection of the correct bases by DNA-replicating enzymes. In order to gain deeper insight into structure, charge distribution and energetics of the A-F and A-T base pairs we have performed quantum-chemical ab initio and density functional calculations at the HF, MP2 and B3LYP levels. The interaction energy of the A-F complex amounts to -3.8 kcal/mol (MP2) and is thus substantially smaller than typical ab initio interaction energies for Watson-Crick or non-canonical base pairs. The A-T and A-F complexes are planar and their overall geometries are similar (root-mean-square deviation: 0.4 A). The calculated donor acceptor atom distances in A-T are in good agreement with the experimental mean values obtained from an analysis of 21 high resolution DNA structures. However, A-F shows a base pair opening as compared to A-T. Even though the interaction energy in the A-F base pair is small, the distances for the N6-H...F and N1...H-C3 contacts are still below the sum of the van-der-Waals radii, which means that the interaction is not governed by van-der-Waals forces alone. If the experimental findings can be confirmed, then our results indicate that DNA polymerase is able to retain high fidelity with base pairs of much smaller interaction energies than found for the conventional Watson-Crick and non-canonical base pairs.