REGULARITIES OF CHANGES IN THERMODYNAMIC PARAMETERS INDUCED BY THE COMPLEXES FORMATION OF URACIL WITH SOME AROMATIC AMINO ACIDS IN A BUFFER SOLUTION AT PH 7.4

Abstract

The increasing interest in the physicochemical studies of nucleic acid bases and their derivatives is due mainly to their biological and pharmaceutical importance. Despite increasing data on the interaction of DNA with proteins, the molecular mechanism of recognition of particles in the resulting DNA-protein complexes is not well understood yet. The investigation of the nature and the forces that stabilize nucleic acid structures and their complexes with other molecules is not only of fundamental significance but may also find practical applications. In this paper, we discussed the interaction of model compounds of nucleic acids and proteins using the example of complexes of uracil with aromatic amino acids: L-histidine, L-tryptophan, and L-phenylalanine in aqueous buffer solutions with physiological value of pH to be 7.4. The available literature data on the thermochemical, volume, and heat capacity properties of these systems were used. It should be noted that a change in the acidity of the medium induces a shift in the equilibrium of ionic forms, tautomers, and this leads to a variation in the reactivity of the reagents. The obtained characteristics of the complex formation process differ markedly from the results obtained for an aqueous solution without the use of buffer. A generalization of data on the change in thermodynamic parameters (equilibrium constants, Gibbs energy, enthalpy, entropy, apparent molar volume, and apparent molar heat capacity) under the formation of uracil complexes with above aromatic amino acids depending on their physicochemical properties is given. The dependence of the thermodynamic parameters of the formation of complexes on the acid-base properties of amino acids as a characteristic of the electron density on the carboxylate group has been established. Changes in the standard apparent molar heat capacity and in the standard apparent molar volume of uracil under the addition of amino acids correlate with the hydrophobicity of the added amino acids.