THE SPECIFIC SOLVATION EFFECTS ON THE STRUCTURES AND PROPERTIES OF ADENINE-URACIL COMPLEXES - A THEORETICAL AB-INITIO STUDY

Ab initio quantum chemical studies at the HF level with the 6-31G bas is set were performed for three different Watson-Crick hydrogen bonded adenine-uracil complexes in the gas phase and in a water solution app roximated by the first solvation shell. Full geometry optimizations wi thout any constraints on the planarity of these complexes were carried out. The solvent effect was modeled by explicit inclusion of seven wa ter molecules which creates the first coordination sphere around the a denine-uracil base pair. Single point calculations were also performed at the correlated MP2/6-31//HF/6-31G* level. The interaction and sol vation energies were corrected for the basis set superposition error. It was shown that base pair corresponding to the standard Watson-Crick pair (denoted as AU1) is the lowest energy structure on the potential energy surface both in the gas phase and in a water solution. Only a slight deviation from planarity is observed for these complexes in bot h phases. Furthermore, the relative stability order of the considered WC AU hydrogen bonded complexes remains unchanged upon interaction wit h the water cluster although the zwitterionic form (denoted as AU3) is stabilized more compared to a rare tautomer (denoted as AU2). Some si milarities and differences between the title species and the isocytosi ne-cytosine complexes in both phases are also discussed.