Complexation of L-histidine with pyridine carboxylic acid isomers in aqueous buffer solution at 298.15 K: a calorimetric study

The peculiarities of interaction of heterocyclic amino acid of L-histidine (His) with structural isomers of pyridine carboxylic acid: picolinic (PA), nicotinic (NA), and isonicotinic (INA) acids in the phosphate buffer, pH 7.4 at T = 298.15 K, are studied by the method of solution calorimetry. Thermodynamic parameters, viz. binding constants, enthalpies of complexation, Gibbs energies and entropies, are determined. For His and pyridine monocarboxylic acids, the formation of hydrogen bonds and electrostatic interactions is found to be the main force determining the formation of complexes between them in the buffer solution, as evidenced by large negative enthalpy values and positive entropy values. The stability of the obtained complexes depends on the structural isomerism of pyridine carboxylic acid and increases in the series: PA < NA < INA. The main contribution to the stabilization of the formed complexes is shown to be made by the enthalpic component of the Gibbs free energy of complexation.