Effect of structure isomerism of pyridine monocarboxylic acids on thermodynamic properties of l-lysine complexation in aqueous buffer solution

Abstract

The thermodynamic characteristics (lgKc, ΔcG, ΔcH, ΔcS) of complexation of l-lysine, a polar basic amino acid, with isomeric pyridine monocarboxylic acids (picolinic acid, nicotinic acid, isonicotinic acid) in buffer solution (pH 7.4) have been obtained at 298.15 K by calorimetry method. The results indicate the formation of molecular complexes between Lys and pyridine carboxylic acids (PyCOOH) with 1:1 stoichiometry in the buffer solution. The complexation of amino acid is influenced by position of –COOH group in the pyridine ring relative to the nitrogen atom. The binding affinity of Lys with PyCOOH isomers follows the order as PA < NA < INA. The measured density values were used to calculate the apparent molar and the partial molar volumes at infinite dilution for PyCOOH isomers in Lys buffer solutions. The results revealed formation of Lys complexes with above pyridine monocarboxylic acids. The positive transfer volumes for the PyCOOH isomers decrease in the series NA ≥ INA > PA. The effects of solute concentration and structure isomerism of PyCOOH on the volume properties were discussed in terms of predominant types of molecular interactions (hydrophilic, hydrophobic, ionic and zwitterionic) on the basis of co-spheres overlap model.