[Investigation of tautomeric composition of cytosine by multiphoton excitation spectra. I. Resonance hyper-Raman spectra].

For the first time the quantum-mechanical calculations of intensity distribution in the resonance hyper-Raman spectra of the six tautomeric forms of cytosine were performed. It has been confirmed that in the aqueous solution of cytosine (pH 3.0) there exist several molecular structures: cytosine in the canonical amino-oxo tautomeric form, cation, as well as cys-imino-oxo and cys-amino-hydroxy tautomers; in the gaseous phase at 235 degrees C cys-imino-oxotautomer dominates with the presence of small amount of cys-, trans-, amino-hydroxy tautomers and the caconical form of cytosine.

[Investigation of tautomeric composition of cytosine by multiphoton excitation spectra. II. Two-photon absorption spectra].

For the first time the quantum-mechanical calculations of intensity distribution in the two-photon absorption spectra of the six tautomeric forms of cytosine were performed. It has been confirmed that in the aqueous solution of cytosine (pH 3.0) there exist several tauromeric forms: canonical amino-oxo tautomeric form, cation, as well as cys-imino-oxo and cys-amino-hydroxy tautomers.

[Theoretical study of hydrophobicity and hydrophilicity of uracil and its dimers].

The influence of hydrophilic and hydrophobic properties of the uracil elementary nucleic acids bases on its solubility and structure in aqueous solution was studied. Complexes of uracil with water molecules (from 1 to 14) were then calculated. The geometrical parameters of the hydrogen bridge of uracil and the changes in the frequency of valence vibrations of the bonds participating directly in hydrogen bond formation were calculated. It is shown that for the hydrogen bonds O(w)...HN(1) and O(w)...HN3 the hydrogen atom can tear, it may lead to tautomeric transformation of uracil. The results obtained having calculated the structure of uracil dimers, formed with the hydrogen bonds, in an isolated state and water solution, energy, dipole moments and the hydrogen bridge parameters made it possible to explain low solubility of uracil in water at room temperature. It is shown that water molecules with increase in their number are located mainly at one side of the plane of a pyrimidine uracil ring, that leads to the formation of stacking. Of two possible variants of stacking formation, the most profitable grouping is when a dipole moment of the formed dimer is equal to zero (anti-parallel stacking).

[A theoretical investigation of tautomerism of adenine, purine, guanine, and cytosine].

The parameters of tautomeric equilibrium and stability of lactam-lactim, amino-imino, and N9H <--> N7H tautomeric forms of adenine, purine, guanine, and cytosine for isolated states have been calculated. It was found that the Ade-N9H tautomeric form is most stable; in this case the intramolecular proton transfer and tautomeric transformation Ade-N9H --> Ade-N7H can occur. The parameters of tautomeric equilibrium and stability for seven tautomeric forms of guanine were determined. For cytosine, the simultaneous existence of the amino-hydroxy, imino-oxo and valence amino-hydroxy of tautomeric forms was predicted.