MODEL STUDY OF THE RECOGNITION CENTER INVOLVING PYRIMIDINE BASES AND AMIDE GROUP OF AMINO ACIDS. I.K. Galetich and V.S. Shelkovsky, Institute for Low Temperature Physics and Engineering of the Academy of Sciences of Ukraine

In the frames of the problem of molecular recognition this work is aimed at the investigation of hydrogen bond formation of the amide group with nucleic acid bases in the crystal structure of a specific repressor-operator complex of bacteriofage 434. The interactions between the monomers of nucleic acids and proteins, which lead to hydrogen bonds formation, were studied on the systems (consisting of nitrogen bases 1,3-dimetUra and 1-metThy and acrylamide) which are good models of interactions, involving specific aminoacid side chains of Gln and Asn and free atomic groups of nucleic base Thy in single or double stranded DNA. Thermodynamic parameters of interactions were obtained using a method of temperature dependent field ionization mass spectrometry and theoretically by the atom-atom potential functions calculations. Under the mass spectrometric conditions the associates of molecules were formed in the gas phase; variation of the temperature allowed to obtain relative association constants and to calculate the enthalpies of associates formation (DH, kcal/mole) using Vant-Hoff plots. A good agreement between experimental and theoretical data for acrylamide complexes with 1,3-dimetUra (9.7 vs 1O.1) and 1-metThy (6.8 vs 6.6) was observed. Proposed schemes of the energetically favourable hydrogen bonding are as follows: a) for the complex of acrylamide with 1,3- dimetUra: N1-H...O2 ; b) for the complex of acryl- amide with 1-metThy: O1...H-N2 or N1-H...O2. The data obtained enabled us to determine hydrogen bond for formation and mutual positions of molecules in the crystals and can be used for quantitative es-timates of the probability of recognition.