Ab initio quantum chemical studies at the HF/6-31G* and MP2/6-31G*//HF/6-31G* levels of theory were performed for different three adenine-uracil (AU), four isocytosine-cytosine (iCC), three reverse Watson-Crick RWC iCC hydrogen-bonded complexes in the gas phase and in the water solution. Full geometry optimizations without any constraints on the planarity of these complexes were carried out. The water solution were modeled by explicit inclusion of water molecules, up to seven waters, which creates the first coordination sphere around these base pairs. The interaction and solvation energies were corrected for the basis set superposition error. It was shown that the inclusion of six instead of one, two, or four water molecules has crucial effect on the geometry of the iCC1 base pair. Complexes involving full first hydration shell water molecules become strongly nonplanar in the case of four different WC iCC structures as compared to the case of four or fewer water molecules. The relative stability order changes also when one considers full first hydration shell water molecules, the zwitterionic form of WC iCC and WC AU become the second most stable species after the main WC base pairs, respectively.

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