Ab initio MP2 and DFT calculations of geometry and solution tautomerism of purine and some purine derivatives

We present the geometries of purine, adenine, 2-aminopurine, 6-methoxypurine, 6-methylaminopurine and 9-methyladenine as predicted by ab initio calculations. The geometries were obtained at the second order perturbation level of theory, using a medium-sized basis set (MP2/6-31G(d). Stability with respect to basis set changes was investigated with larger basis sets (cc-pVDZ and 6-311 ++ G(2d,p)) for adenine and purine. We used density functional theory (DFT) with the Becke three-parameter (B3) functional for exchange and the Lee-Yang-Parr (LYP) functional for the correlation part to predict equilibrium geometries of selected molecules of the purine series. The basis set dependence was also investigated at the DFT level of theory. The 1-H-9-H-imino tautomer of 2-aminopurine was investigated at the MP2/6-31G(d) level, and it was found to be about 127 kJ/mol less stable in vacuum than the 9-H-2-aminopurine. The 7-H <-> 9-H tautomeric equilibrium of purine and 2-aminopurine was investigated in vacuum and in water solution. The Onsager and polarized continuum models were used in self-consistent reaction field calculations to describe the solvent-solute interaction. In vacuum, the 9-H tautomeric form is favored by 16.5 and 18.8 kJ/mol for purine and 2-aminopurine, respectively. In water solution, the 7-H tautomer is stabilized relative to the 9-H form. The energy difference between the two forms is reduced to 3.0 kJ/mol for purine and to 11.2 kJ/mol for 2-aminopurine.