Solvent effects on stability and 15N NMR shielding of 5-methylcytosine tautomers: A theoretical approach

The structure and energies of the tautomers of 5-methylcytosine in gas phase were predicted using Density Functional Theory (DFT) method. Solvent-induced effects on stability and 15N NMR shielding on the most stable tautomers of 5-methylcytosine were calculated using DFT combined with the polarizable continuum model (PCM) and using the gauge-invariant atomic orbitals (GIAO). In a wide range of solvent dielectrics, the 1-H-oxo-amino form (T6) is predicted as the most stable tautomer and the total electronic energy values of the more stable tautomers in the liquid phase decrease with an increase in the dielectric constant. Direct and indirect solvent effects on 15N NMR shielding of the pyrimidine ring of three dominant tautomers are also calculated. It has been shown that in trivalent nitrogens, the observed solvent-induced shielding variation is more strongly related to the intensity of the solvent reaction field rather than on the change of molecular geometry induced by the solvent.