The post-SCF quantum chemistry characteristics of the guanine-guanine stacking B-DNA.

The stacking interactions of two guanine molecules were analyzed detail at the DF-MP2/aug-cc-pVDZ level of theory for conformations appearing B-DNA. The dependence of intermolecular interaction energies on the pairs of step parameters (shift, slide, rise, tilt, roll and twist) was determined. The values of these parameters were chosen to cover the whole range of variability appearing crystallographic data. The scanning procedure was performed by subsequent changes of two variables with fixed values of the remaining base-pair and base-step BDNA parameters. Additionally, the hybrid variational-perturbational scheme was applied for the decomposition of the interaction energy into physically meaningful contributions at the MP2 level of theory. The significant impact of the mutual orientations of guanine bases was observed not only on the total intermolecular energy but also on its components. The second-order dispersion interaction is the most significant contribution to stabilization energy and is about eight times larger compared to the first-order electrostatic term with relaxation effects, which is also of stabilizing character. The dispersion interactions may vary up to 9.6 kcal mol(-1) between different guanine-guanine conformations. The parameters defining the mutual orientation of stacked guanine molecules have a different impact on the stabilization of the investigated complex. The following base-step parameters have only a minor impact on the stabilization energies: shift-slide, shift-roll, shift-twist, slide-twist and roll-twist. On the other hand, parameters such as rise and tilt significantly influence intermolecular interactions, i.e. strong attraction occurs only for a limited range of their values.