Interaction of psoralens with DNA-bases (I). An ab initio quantum chemical, density functional theory and second-order Møller-Plesset perturbational study

High level quantum mechanical methods were used to study the intercalation complexes of psoralen and 8-methoxy psoralen with DNA. The energetics, structures and properties of isolated psoralen and 8-methoxy psoralen have been investigated. Their interactions with DNA bases and base pair were studied aiming to understand more about the nature of the intercalation binding forces between these intercalators and DNA. The geometry of adducts of psoralens with adenine and thymine as well as adenine-thymine base pair have been optimized in two main orientations, planar and stacked, by means of HF, DFT and MP2 levels of computation employing different basis sets. The different computational methods have been compared and the effect of the basis set has been discussed. The potential energy surface of the psoralen adducts with DNA-bases and base pair was explored and the global minimum was located for the planar adducts. The interaction energy ΔE was corrected for the basis set superposition error with the full counterpoise (CP) method, resulting in a more reliable estimation of the interaction energy. More reliable estimates of the interaction energy have been calculated using coupled-cluster methods (CCSD(T)). Effect of vertical distance and rotational angle between the stacked molecules on the interaction energy were investigated by the above three methods. Only, the MP2 method was shown to be valid for studying intercalation complexes. We found that, of the binding forces, charge transfer and dispersion play an important role with the latter predominating in our investigated systems. Ab initio methods which account for the electron correlation effects are the minimum level for studying the non-covalent interactions.