Biopolymers and cell. 1996; 12 (1): 49 - 52

The quantum mechanical calculations evidence molecular-zwitterionic features of prototropic tautomerism of canonical nucleotide bases. 2. Purines

D. M. Govorun, 1. V. Kondratyuk

The prototropic zwitterionic tautomerism of free adenine (Ade) and guanine ( Gua) was investigated by the semiempirical quantum-mechanical AMI method at the full optimization of structural parameters. This property ofpurine nucleotide bases was shown to be of molecular-zwitterionic features. Unlike pyrimidine bases, energetic intervals ofpurine molecular and zwitterionic tautomers are overlapped. Therewith zwit¬terionic tautomer with the most low energy is formed from molecular tautomeric ground state by the migra¬tion of the C8H carboproton to neighbour endocyclic N7 nitrogen atom, its relative energy being equal to 17,06 and 14,50 kcal/mol respectively for Ade and Gua.