AN INVESTIGATION OF TAUTOMERISM IN ADENINE AND GUANINE

The geometries of an extensive set of adenine and guanine tautomers have been optimized by using the semiempirical AM1 method and ab initio Hartree-Fock method with the 3-21G basis set. Tautomeric energies were evaluated at the AM1 and the Hartree-Fock STO-3G and 3-21G levels. For selected (low-energy) tautomers of both adenine and guanine, correlation effects were evaluated at the MP2 (Møller-Plesset perturbation theory to second order) level with the 6-31G and 6-31G*(5D) basis sets. For the same selected tautomers of guanine, geometry optimizations were performed at the RHF/6-31G*(5D) level. The results suggest that the most stable form of these purines at the AMI, STO-3G, and 3-21G levels is 1A and 2A, i.e., the one usually assumed. At the RMP2/6-31G*(5D)//RHF/6-31G*(5D) level (with the zero-point energy correction estimated at the RHF/3-21G//RHF/3-21G level) there is an enol tautomer of guanine which is less than 2 kcal/mol higher in energy than the most stable (keto) tautomer. All other tautomers are unlikely to be experimentally observed in the gas phase. In these systems, the effects due to polarization functions and correlation do not seem to be additive. © 1990 American Chemical Society.