Biopolymers and cell. 1995; 11 (6): 41 - 44

PROTOTROPIC MOLECULAR-ZWITTERION TAUTOMERISM OF IMIDAZOLE AND PYRIMIDINE

D. M. Govorun, 1. V. Kondratyuk, N. V. Zheltovsky

The prototropic molecular-zwitterion tautomerism of imidazole dm) and of pyrimidine (Pyr) was investigated in vacuum by means of semiempirical quantum mechanical AMI method with full optimization of all parameters. It was established that molecular-zwitterion family of Im occupies an energetical range about 46 kcal/mol and consists of three planar structural isomers — one molecular one (the main tautomer) and two zwitterionic ones. Such family of Pyr occupies an energetical range 116 kcal/moie and consists of 10 structural isomers — one molecular one (the main tautomer) and 9 zwitterionic ones. All they, except the pair of zwitterionic-enantiomers, are planar. In both molecules — Im and Pyr — the ilid forms (result of proton transition from C2 atom to the neighbouring atom N3) are the most prevalent zwitterionic-tautomers. The relative energy of ilid form of Pyr is twice large as the Im one. From the data obtained the conclusion has been made that carboproton at the