QUANTUM-CHEMICAL ANALYSIS OF GEOMETRIC AND ENERGETIC CHARACTERISTICS OF HETERO ASSOCIATES m 9 Ade·m 1 Ura IN MAIN TAUTOMERIC FORM

For the first time, on the quantum-mechanical level of theory MP2/6-311++G(2df,pd)// B3LYP/6-311++G(d,p), a full set of hydrogen (H)-bonded hetero associates m 9 Ade·m 1 Ura in the main tautomeric forms consisting of 18 different structures was obtained. Their structural and energetical parameters were characterized and basic physical and chemical characteristics of intermolecular H-bonds that stabilize the investigated complexes were described. It was shown that global minimum of Gibbs free energy corresponds to Hoogsteen pair, after it with a small margin follow Watson–Crick pair, inverse Hoogsteen pair and inverse Watson–Crick pair. It was found that in stabilization of identified hetero associates four types of H-bonds – NH...O, NH...N, CH...O and CH...N are involved. In this case non-canonical CH...O and CH...N bonds show a linear dependence of energy on electron density in the corresponding critical point. Obtained data may be useful for experimental interpretation of the features of non-canonical base pairs association by common methods of spectroscopy, including NMR and vibrational spectroscopy. They are also important for understanding the problem of mutational variability that is caused by formation of non-complementary pairs of nucleotide bases whose role is not clear yet.


INTRODUCTION
Nucleic acids are macromolecular components of cells that play an important role in the conservation and transmission of genetic information. Four nucleotide bases -the purine and pyrimidine derivatives are structural units of these biopolymers. In particular, a unique sequence of nucleic bases determines the DNA functionalities. The presence of two complementary pairs of nucleotide bases, namely Gua·Cyt and Ade·Thy, provides a unique ability of genetic material to replicate with high accuracy. At the same time DNA is able to form functionally important non-canonical conformations that are characterized by presence of non-complementary pairs of nucleotide bases. In addition, problem of non-canonical nucleotide bases pairing is closely related to the emergence of spontaneous point mutations. Unfortunately, the experimental data on structural characteristics of incorrect pairs of nucleotide bases stabilized by the intermolecular H-bonds, remain rather limited. It inspirates researchers to use calculations, including quantum-chemical methods, to receive this biologically important information.

MATERIALS AND METHODS
The H-bonded hetero associates of adenine (Ade) and uracil (Ura), methylated on the 9 th and 1 st position of nitrogen atom respectively, which are in the main tautomeric form were studied. As a subject of study, the geometric and energy characteristics of the above mentioned complexes m 9 Ade·m 1 Ura and basic physicochemical parameters of H-bonds involved in their stabilization were taken.
Quantum chemical calculations of the geometric and electronic structure of the objects were carried out on the level of B3LYP/6-311++G(d,p) theory in the vacuum approximation. All optimized structures were tested for stability in the absence of imaginary frequencies in their vibrational spectra. The study was conducted using a software package "GAUSSIAN03" [5]. All possible starting structures of complexes were generated using the algorithm described in [6]. This approach allows to get a full set of complexes because it is based on up-to-date information about H-bonds and it is not limited by considerations of geometric character.
The distribution of electron density in pairs of bases was investigated by QTAIM method [1] on the basis of wave functions obtained on the levels of MP2/6-311++G(2df,pd)// B3LYP/6-311++G(d,p) theory. The identification of intermolecular H-bonds [10] was carried out in the presence of critical point of type (3,-1) between two valence-unbound atoms and positive values of Laplacian of the electron density ∆ρ at the same point. The electron density topology was analyzed by the software package AIMAll (Version 10.05.04) [8] using standard options. The energy of classic H-bonds was calculated by Iogansen method [7] and a non-canonical CH...O/N H-bonds -by Espinosa-Molins-Lecomte method [4]. In this paper generally accepted numbering of atoms is used [11].

RESULTS AND DISCUSSIONS
For the first time a complete set of H-bonded hetero associates of m 9 Ade·m 1 Ura which includes 18 structures was obtained. The observed complexes lie within a range of relative Gibbs energy 0-9.27 kcal/mol at normal conditions ( Fig. 1, Table 1). It was found that the global minimum of Gibbs free energy corresponds to flat symmetrical pair 1 which is in Hoogsteen configuration and stabilized by three intermolecular H-bonds N6H...O4, N3H...N7, C8H...O2 (Fig. 1, Table 1). Next in the energy scale with small energy gap the plane symmetrical pairs 2 (Watson-Crick configuration), 3 (reversed Hoogsteen conformation) and 4 (reverse Watson-Crick conformation), each of which is stabilized by three intermolecular H-bonds ( Fig. 1) are located. These data are consistent with the crystallographic data available in literature [9]. The total population of 1-4 pairs under normal conditions is 41.7 % (1) + 29.2 % (2) + 24.5 % (3) + 4.4 % (4) = 99.8 %: namely they determine the complexation equilibrium in a free state under normal conditions.  All fixed hetero associates of m 9 Ade·m 1 Ura are polar complexes, whose dipole moment is within a range from 2.07 to 7.46 D. Among them, seven structures -flat symmetrical, another 11 pairs have a non planar structure. Also, the basic physical and chemical characteristics of intermolecular H-bonds that stabilize hetero associates (Table 2)  The dependence of the energy of non-canonical H-bonds CH...N and CH...O on the electron density in the corresponding critical point shows a linear dependence (Fig. 2): E CH…O = 187.4·ρ and E CH…N = 135.1·ρ. On the one hand, it means that the parameter ρ is a convenient descriptor of non-canonical H-binding [2,3]. On the other hand, these dependencies allow to assess the values of electron density which correspond to H-bonds with the lowest possible energy ∼ 0.2 kcal/mol [10]: ρ CH…O = 0.0011 a.u., ρ CH…N = 0.0015 a.u.  Table 2.