Thermodynamics of aromatic amino acid interactions with heterocyclic ligands

Abstract

Interactions between aromatic amino acids (l-tryptophan, l-phenylalanine) and heterocyclic ligands (uracil, nicotinic acid) in aqueous buffer solutions (pH7.35) were studied by calorimetry and UV–vis spectroscopy. Stability constants (lgK) and thermodynamic parameters of 1:2 complex formation (ΔcG, ΔcH, ΔcS) have been calculated and presented in tables. The lgK values obtained by the two methods are in agreement with each other within the limits of experimental error. It was shown that the complex formation of uracil with l-Phe and l-Trp is accompanied by the negative enthalpy and small positive entropy changes. The complex formation between nicotinic acid and amino acid (l-Phe, l-Trp) is characterized by positive values of enthalpy and entropy changes. Uracil was found to be a more suitable complexating agent for l-Trp and l-Phe since the amino acids form more stable complexes with it.