Biopolymers and cell. 1995; 11 (3-4): 29 - 34

THE INFLUENCE OF PROTONATION AND DEPROTONATION , ON THE ACIDIC-BASIC PROPERTIES OF PURINE, PYRIMIDINE AND IMIDAZOLE .

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

The energies of protonation and deprotonation of neutral, protonated (cation) and de- protonated (anion) forms of purine (Pur), pyrimidine (Pyr) and imidazole (Im) and charges on atoms for all these forms have been calculated in vacuum by means of se- miempirical quantum mechanical AMI method with optimization (gradient norm <0,01) of all structural parameters. It was established that charge state deviation of molecule through protonation or deprotonation changes its acidic-basic properties considerably. So, Pur, Pyr and Im protonation reduces basic properties to zero and considerably in¬ creasing acidic ones; any proton separation depresses acidic properties to zero practi¬ cally, increasing basic ones. The iminogroup with accepted proton is the most acid site and the atom, from which the proton has been removed is the most protonofilic. On the ground of data obtained the important conclusion was made: mutual amplification of two intermolecular hydrogen bonds occures only in the case when the base is donor and acceptor of protons simultaneously. There is mutual decrease of H-bonds in the rest cases. Based on a results of intermolecular H-bonds mutual influence study, the quan¬ tum chemical foundations of Im and Pyr selfassociation through hydrogen bonding are under discussion. • • - .