Molecular complexes of polar basic amino acids (l-lysine, l-histidine) with nicotinic acid in water and buffer solution: A thermodynamic aspects

Abstract

Interactions of the polar basic amino acids, l-lysine (Lys) and l-histidine (His), with heterocyclic ligand, nicotinic acid (NA), in pure water and in phosphate buffer solution (pH 7.4) were studied by calorimetry and densimetry. Under the experimental conditions chosen, thermodynamic parameters of complex formation (lgKc, ΔcG, ΔcH, ΔcS) as well as the hydration enthalpies of different ionized forms of amino acids have been calculated at 298.15 K. The stoichiometry of complexes between Lys/His and NA was determined. The affinity of Lys to binding with NA decreases with increasing pH, the association process being less favorable. The more efficient stabilization of complexes between His and NA was exhibited in buffer media than in water. The apparent molar volumes, partial molar volumes and partial molar volume of transfer of NA from pure solvent (water/buffer) to mixed amino acid solution at infinite dilution have been evaluated. Their values obtained are lower for aqueous solutions than buffer systems. The dependence of thermodynamic characteristics obtained on molecular structure, ionization and hydration state of the solutes was analyzed. The dominant types of driving forces in the complex formation process of polar basic amino acids (Lys, His) with nicotinic acid were found for water and buffer solution.