l -Lysine and Pyridine Monocarboxylic Acids in Water and Buffer Solvent: Intermolecular Interactions from Volumetric Properties at Various Temperatures

In this work, the interactions of a base amino acid, l -lysine (Lys), with three pyridine monocarboxylic acids (PyCOOH) were studied in liquid media. The density measurements were carried out for binary solutions of (PyCOOH + water) and mixed solutions of (PyCOOH + l -lysine + water or buffer) at temperatures from (288.15 to 313.15) K. The sodium phosphates (NaH 2 PO 4 /Na 2 HPO 4 ) solution at pH 7.4 was used as a buffer. The apparent molar volume of the solute ( V φ ) and limiting apparent molar volume at infinite dilution were obtained. Standard molar volumes of PyCOOH transfer from the water to aqueous Lys solution and from buffer to Lys buffer solution were determined. Nonlinearity in the concentration dependences of V φ was found to be caused by the process of complex formation of PyCOOH with l -lysine in both aqueous and buffer solutions. The apparent molar expansibilities of PyCOOH isomers at infinite dilution and the Hepler parameter were calculated. Positive results obtained imply that PyCOOH isomers promote structure formation in (Lys + water) and (Lys + buffer) solutions. The pyridine monocarboxylic acids exhibit structure-breaking behavior in the water and buffer in the absence of an amino acid. The main types of intermolecular interactions between hydrated solutes were considered using a co-spheres overlap model.