Ab initio quantum chemical studies at the HF level with the 6-31G* basis set were performed for three different Watson-Crick hydrogen bonded adenine-uracil complexes in the gas phase and in a water solution approximated by the first solvation shell. Full geometry optimizations without any constraints on the planarity of these complexes were carried out. The solvent effect was modeled by explicit inclusion of seven water molecules which creates the first coordination sphere around the adenine-uracil base pair. Single point calculations were also performed at the correlated MP2/6-31*//HF/6-31G* level. The interaction and solvation 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 both phases. Furthermore, the relative stability order of the considered WC AU hydrogen bonded complexes remains unchanged upon interaction with the water cluster although the zwitterionic form (denoted as AU3) is stabilized more compared to a rare tautomer (denoted as AU2). Some similarities and differences between the title species and the isocytosine-cytosine complexes in both phases are also discussed.