Molecular structures of complexes of adenine with 12, 13, 14, and 16 water molecules were calculated using the B3LYP/6-31G(d) method. The location of water molecules on one side with respect to the adenine mean plane leads to a significant deformation of the nucleobase geometry. It results in unusual changes to the amino group geometry and a slight increase in the nonplanarity of the purine fragment of the title molecule. The formation of N-H‚‚‚O hydrogen bonds with the participation of an amino group results in a disruption of the relationship between the pyramidality of the nitrogen atom of the amino group and the length of the C-N bond. Because of the specific geometry of the H bonds, a shortening of the C(6)-N(10) bond in the adenine‚12H 2 O complex does not entail a flattening of the nitrogen atom as compared to that of isolated adenine. Our data reveal the possibility of the formation of unusual hydrogen bonds in polyhydrated complexes of adenine.

Molecular structure of complexes of guanine with 12, 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 NÁÁÁH-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 NÁÁÁH-O/pÁÁÁH-O hydrogen bonds where contribution of p-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.

A comprehensive computational study of all stable monohydrates of the oxo-amino N9H (Gua9), oxo-amino N7H (Gua7), and hydroxy-imino N9H (Gua9*) tautomers of guanine has been performed at the MP2/ 6-31G(d) level of theory. Five stable complexes for the Gua9 tautomer and seven for the Gua7 and Gua9* tautomers have been revealed. They can be divided into three groups. The first group includes hydrates where the nucleobase appears to act as a proton donor and acceptor simultaneously. The second group contains hydrates where guanine is either a proton donor or a proton acceptor. Two additional complexes of water with the Gua7 and Gua9* tautomers are stabilized by non-conventional hydrogen bonds: C-HÁ Á ÁO and NÁ Á ÁH-O with participation of the nitrogen atom of the amino group. All hydrogen bonds between the nucleobase and water are rather weak. It was demonstrated that the geometry of the amino and carbonyl groups and the N1-C6 bond are the most sensitive to interactions with water. Method of calculationThe molecular structures of isolated oxo-amino N9H (Gua9), oxo-amino N7H (Gua7), and hydroxy-imino N9H (Gua9*) tautomers of guanine and their monohydrates were optimized at the second order of the closed shell restricted Møller-Plesset y Electronic supplementary information (ESI) available: Geometries of a guanine moiety in isolated molecules and monohydrates of the Gua9, Gua7 and Gua9* tautomers (Tables S1, S2 and S3). See

Molecular structures of mono- and diamino derivatives of N,N‘-dimethyluracil and their complexes with water are investigated by ab initio quantum chemical methods at the MP2/6-31G(d,p) level of theory. The results of the calculations demonstrate that the formation of the N···H−O hydrogen bonds with participation of the nitrogen atom of the amino group in the N,N‘-dimethyl-5,6-diaminouracil complex with water is caused by the intrinsic properties of this substituent. An analysis of the geometries and water−amine interaction energies for different monohydrates of mono- and diamino derivatives of N,N‘-dimethyluracil indicates that the formation of this H bond requires the presence of the neighboring proton donor group. The energy of the N···H−O hydrogen bonds depends on a degree of conjugation between the lone pair of the nitrogen atom and the π system of the rest of the molecule. In the case of amino derivatives of uracil, the weakest conjugation is observed for the substituent at the C(5) atom. Therefore, the N···H−O hydrogen bond is formed by this amino group. A comparison of the amine−water interaction energies and the relative stability of isomeric monohydrates allows for the conclusion that the formation of such nonstandard hydrogen bonds is the most favorable way for interactions of 1,2-diamines with water and other proton donors.