Ab initio and DFT studies on tautomerism of 5-methyl cytosine in gaseous phase

Abstract

Ab initio and DFT methods have been used to study the seven tautomeric forms of 5-methylcytosine molecule.<br />The related tautomer in gas phase have been studied at HF/6-31G, HF/6-31G* and B3LYP/6-31G* levels of<br />theory. The structures,enthalpies,entropies,Gibbs free energies,relative tautomerization energies of tautomers<br />and tautomeric equilibrium constants were compared and analyzed along with full geometry optimization. The<br />calculations showed that the Oxo-amino(6), Oxo-imino(7) and Hydroxy-amino(4) tautomers are the most stable<br />in the gas phase. The results are in a good agreement with the available experimental data. The entropy effect<br />on the Gibbs free energy of the 5-methylcytosine bases is very small and it has a little significance on the<br />tautomeric equilibria of pyrimidine bases.Therefore the enthalpic term is dominant in the determination of the<br />equilibrium constants. 13C-NMR studies have been carried out for these tautomers and the results are<br />discussed.We have also evaluated the hybridation coefficient for bonds and hetero atom LPS in the aromatic<br />ring for the stable tautomers. Natural Bond Orbital Theory (NBO) calculation showed that the stable tautomers<br />must be considered aromatic.