A density functional approach of prototropic tautomerism of guanine

Seven tautomeric species of guanine are studied through density functional calculations with various exchange-correlation functionals X alpha PL, X alpha LYP, 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. (C) 2001 Elsevier Science B.V. All rights reserved.