Volumetric and heat capacity studies of heterocyclic compounds in aqueous amino acids buffer solutions at 298.15 K

Highlights: • Interactions of Phe and Trp with nicotinic acid and uracil in buffer were studied. • Apparent and partial molar properties were calculated. • Role of the hydrophobic – hydrophilic balance in molecular interactions was shown. -- Abstract: The heterocyclic ligand–amino acid complexes used as models of drug binding with plasma proteins have been studied. In this paper, we report the new data on densities and heat capacities of uracil and nicotinic acid in aqueous buffer solutions (pH 7.4) with aromatic amino acids (l-phenylalanine, l-tryptophan) at 298.15 K using the density meter DSA 5000 M (Anton Paar) and the microdifferential scanning calorimeter SCAL-1 (Pushchino, Russia). From these experimental data the apparent molar properties (V_φ, C_p,φ) and the partial molar parameters of volume and heat capacity at infinite dilution (V_φ^o, C_p,φ^o) were obtained for all the systems. The partial molar properties of transfer of heterocyclic compounds from buffer to buffer amino acid solutions are determined. The data have been discussed in terms of molecular interactions (hydrophilic, hydrophobic and zwitterionic interactions) on the basis of a cosphere overlap model. These results show that the interactions of l-phenylalanine and l-tryptophan with uracil and nicotinic acid are accompanied by complex formation. Analysis of the resulting data shows that all the complexes formed have ∼1:2 stoichiometry.