[The mechanism of tautomeric transitions of nucleic acid bases with limited access to water molecules].

Pathways of tautomeric transformations of nucleic acid bases have been considered in the conditions when only few separate water molecules can interact with the bases. Field ionization mass-spectrometric study has demonstrated that substitution of one of the labile hydrogen atoms for deuterium takes place when thymine (or 1-methylthymine) interacts with heavy water in vacuo. This observation was explained by a supposition that a transient water-thymine complex is formed and then tautomeric transition of the base occurs via double proton transfer along hydrogen bonds. Calculations of the interaction energy between base and water molecules allowed us to suggest the structure of such a complex in which water molecule lays in the plane of the base. Formation of such complexes during biosynthesis of nucleic acids can give rise to point errors, probability of their appearance being dependent on the structure and properties of a macromolecular DNA-enzyme-nucleotide complex.