*3.1.2. Analysis of the electron and geometric structures of hydrogen bonds in complementary NA pairs by non-empirical ab initio calculations in the 6-31G\* basis set*

Calculations were performed again by the аb initio method with the 6-31G\* basis set with complete optimization of the geometric structure (Fig. 5).

*Ab initio* calculations in the 6-31G\* basis also demonstrated the formation of proper hydrogen bonds in complementary pairs. The energy estimates of the bonds were about twice higher than with the PM3 method, the lengths of the bonds were about 0.2 Å greater, but their bond orders remained the same. Electron density analysis according to Mulliken

(Mulliken R.S.,1955) showed that the charge of the hydrogen atoms is positive, about +0.5е, while the interacting O and N atoms have a negative charge ranging from –0.68 to –0.83е. As above, these results suggest quite stable hydrogen bonds.

Then, we performed *ab initio* calculations for noncomplementary pairs of adenine with other nucleotides (Table 5).

Е = -11.78 kcal/mol Е = -25.39 kcal/mol

**Figure 5.** Hydrogen bonding in the complementary pairs АТ and CG. Quantum chemical computation by the *аb initio* method in the 6-31G\* basis. The charges computed according to Mulliken are shown for the atoms involved in hydrogen bonding.


Note: Energy (kcal/mol) was estimated by *аb initio* calculations in the 6-31G\* basis.

**Table 5.** Energy of hydrogen bonds in pairs of A with T, A, C, and G

As Table 5 demonstrates, selection of complementary T for A is not most advantageous in terms of energy. The interactions AC and AG are more advantageous. The selectivity of nucleotide matching is due to the specific geometric structure of the NA double helix (Fig. 4) (Singer M., Berg P. 1998). Thus, *ab initio* calculations in the 6-31G\* basis confirmed that complementarity in the AT and CG pairs is determined mostly by the orientation of hydrogen bonds formed between the interacting nucleotides and the distance between deoxyribose residues in the NA double helix, rather than by the energy of hydrogen bonding.
