Complexation of L-Histidine with Pyridinecarboxylic Acid Isomers in an Aqueous Buffer Solution at 298.15 K: A Calorimetric Study

The interaction of the heterocyclic amino acid L-histidine (His) with the structural isomers of pyridinecarboxylic acids: picolinic (PA), nicotinic (NA), and isonicotinic (INA) acids in a phosphate buffer at pH 7.4 and T = 298.15 K was studied by calorimetry. The thermodynamic parameters were determined: binding constants, enthalpies of complexation, Gibbs energies, and entropies. It was established that for His and pyridine monocarboxylic acids, hydrogen bonding and electrostatic interactions are the main forces determining complexation in the buffer solution, as indicated by large negative enthalpy values and positive entropy values. The stability of the resulting complexes depends on the structural isomerism of pyridinecarboxylic acid and increases in the series: PA < NA < INA. It is shown that the enthalpy component of the Gibbs free energy of complexation makes the main contribution to stabilization of the formed complexes.