Structure and hydrogen bonding in polyhydrated complexes of guanine

13, 16, and 17 water molecules were calculated using B3LYP/6-311G(d,p) level of theory. Interaction with water results in some deformation of geometrical parameters of guanine, which can be described as contribution of zwitter-ionic resonant form into the structure of DNA base. Saturation of water binding sites within guanine creates possibilities for the formation of the N center dot center dot center dot H-O hydrogen bond where the nitrogen atom of amino group acts as proton acceptor. The NBO analysis of guanine-water interactions reveals that hydrogen bonds involving the N(3) and N(7) atoms of guanine represent a case of mixed N center dot center dot center dot H-O/pi center dot center dot center dot H-O hydrogen bonds where contribution of pi-system into total energy of interaction varies from 3% to 41 %. This contribution significantly depends on orientation of the hydrogen atom of water molecule with respect to plane of purine bicycle and influence of neighboring water molecules.