Specific Solvation Effects on Structures and Properties of Isocytosine−Cytosine Complexes:  A Theoretical ab Initio Study

Ab initio quantum chemical studies at the Hartree−Fock (HF) level with the 6-31G* basis set were performed for four different hydrogen-bonded isocytosine−cytosine (iCC) complexes in the gas phase and in a water solution. Full geometry optimizations without any constraints on the planarity of these complexes were carried out. The water solution was modeled by explicit inclusion of different numbers of water molecules, up to six, which creates the first coordination sphere around the iCC base pair. Single point calculations were also performed at the correlated MP2/6-31G*//HF/6-31G* level. The interaction and solvation energies were corrected for the basis set superposition error by using the full Boys−Bernardi counterpoise correction scheme. It was shown that the base pair corresponding to the standard Watson−Crick pair (denoted as iCC1) is the global minimum on the potential energy surface both in the gas phase and in a water solution. Inclusion of six instead of four water molecules has crucial effects both on the geometries and relative stabilities of iCC complexes. Complexes involving six water molecules become strongly nonplanar, whereas in the case of four or fewer water molecules, only a slight deviation from planarity is observed. Moreover, the relative stability order changes when one considers six water molecules, and the zwitterionic form (denoted as iCC4) becomes the second most stable species after the Watson−Crick iCC1 base pair. Since the structure of isocytosine mimics the six-membered parts of guanine, the results of this study could provide important insights into the structures and properties of analogous guanine−cytosine complexes in a water solution.