The geometries of hydrogen-bond dimers between DNA bases (adenine, thymine, guanine and cytosine) and guanine, cis-O⁶-alkylguanine and trans-O⁶-alkylguanine have been optimized at the B3LYP/6-311＋G** level. The complete basis-set extrapolation method was employed at the MP2/ cc-pVXZ(X＝D,T)//B3LYP/6-311＋G** levels in order to obtain more accurate interaction energies. Counterpoise correction (CP) scheme was also used to take into account of basis set superposition error (BSSE). The all-electron wave functions of these dimers were calculated at the B3LYP/6-311＋G** level and the weak interactions were analyzed by atoms in molecules (AIM) method. The calculation results show that the alkylation of O⁶-guanine can lead to changes of hydrogen-bond interactions between DNA base-pairs, which results in propeller torsions and various displacements between two bases. Furthermore, the obvious decreases in interaction energies and electron densities of base-pair can also be attributed to the alkylation of guanine. Evidently, the alkylation of O⁶-guanine destabilizes the hydrogen-bond interactions of DNA base-pairs, and the destabilization effect decreases in the order of GC＞GG＞GA≈GT. Our calculation results are basically in agreement with the experiments.

Key words: guanine, alkylation, DNA bases, hydrogen-bond, atom in molecule (AIM)