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 al pha 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 vibr ational energy are included. Based on the self-consistent reaction field ca lculations 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 v acuum with 55 kcal mol(-1) high energy barrier, which is inverse in water s olution. We also investigate the thermodynamic effect on the distribution o f the prototropic tautomers using transition state theory. (C) 2001 Elsevie r Science B.V. All rights reserved.