Theoretical Studies of the Hydrogen-bond Interactions between O6-Alkylguanine and DNA Bases

The geometries of hydrogen-bond dimers between DNA bases (adenine, thymine, guanine and cytosine) and guanine, cis-O-6-alkylguanine and trans-O-6-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 O-6-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 O-6-guanine destabilizes the hydrogen-bond interactions of DNA base-pairs, and the destabilization effect decreases in the order of GC>GG>GA approximate to GT. Our calculation results are basically in agreement with the experiments.