Biopolymers and cell. 1995; 11 (3-4): 24 - 28

ABOUT PHYSICAL-CHEMICAL MECHANISM OF HYDROGEN-TRITIUM EXCHANGE OF PURINE NUCLEOTIDE BASES C8H8 GROUPS WITH WATER

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

The fact that the mechanism of hydrogen-tritium (H->-T) exchange of C8H8 group of purine nucleotide bases and of C2H2 group of imidazole (Im) with water reduces to the mechanism of rnolecular-zwitterion tautomerism with H8 (H2 in the case of Im) carboproton participation has been established by means of semiempirical quantum mechanical AMI method with optimization (gradient norm <0,01) of all structural parameters. This conclusion was made on the ground that in every purine nucleotide base and Im the main tautomer-zwitterion in the row of tautomer-zwitterions formed by H8 and H2 carboproton migration on the proton acceptor atoms is the tautojner-zwitterion formed by H8 (H2 in case of the Im) carboproton transition on the neighbo¬ring nitrogen atom of imidazole ring, which is connected with C8 (C2 in the case of Im) by double chemical bond. Actually this ilid form is the transitional state of H-*T exchange of C8H8 group of purine nucleotide bases (adenine, guanine, xanthine, hy-poxanthine, purine) and C2H2 group of Im with water. The relay protonation (of purine N7 atom and of imidazole N3 atom) — deprotonalion (of purine H8 atom and of imida¬zole H2 atom) is the molecular-kinetic mechanism of formation of this transitional state in water under the corresponding pH. For example, in the case of adenine it was showed, that H->-T exchange process is sensitive to conformation. It was particulary underlined, that the velosity of H-VT exchange of adenine and guanine C8H8 groups is determined not by nucleic acid conformation, but by indignation of this conformation under adenine and (or) guanine transition from the molecular form to the ilid one. The attempt to elucidate the physical-chemical mechanism of heat induced reaction of 8-oxypurines formation from corresponding purines and 2-oxyimidazole from Im in .water was undertaken from the position of rnolecular-zwitterion tautomerism of purine nucleotide bases and Im with H8 (H2 in the case of Im) participation: this reaction is a interaction of positive charged C8 atom (C2 in Im) of the ilid form with negative charged hydroxyl oxygen atom and this reaction is accompanied by separation of excess hydrogen atom from O8 atom (O2 in Im).