A density functional approach of prototropic tautomerism of guanine Original Research Article

Seven tautomeric species of guanine are studied through density functional calculations with various exchange-correlation functionals XαPL, XαLYP, and B3LYP, as well as the Hartree–Fock ab initio, using the 6-31G(d), 6-31+G(d,p) and D95V(d) basis sets. Corrections for the zero-point vibrational energy are included. Based on the self-consistent reaction field calculations with the Onsager or polarized continuum model approximation, it is found that guanine N(7)H[G5] is more stable than N(9)H[G1] tautomer in vacuum with 55 kcal mol−1 high energy barrier, which is inverse in water solution. We also investigate the thermodynamic effect on the distribution of the prototropic tautomers using transition state theory.