Observation of complex formation between l-histidine and heterocyclic compounds in water and aqueous buffer solution using calorimetric and spectroscopic methods

Abstract

Interactions between the aromatic amino acid (l-histidine) and heterocyclic ligands (uracil, nicotinic acid) in pure water (pH 5.4) and in aqueous buffer solutions (pH 7.4) were studied by calorimetry and UV–vis spectroscopy. Stability constants (lgKc) and thermodynamic parameters of complex formation (ΔcG, ΔcH, ΔcS) have been calculated and presented in a table. The buffer solution was found to be a more suitable medium for complex formation between l-histidine (His) and uracil (Ura) and nicotinic acid (NA), since the amino acid forms more stable complexes with them in the buffer than in pure water: lgKpH7.4 > lgKpH5.4. It was shown that the complexes of NA with the His are mainly enthalpically stabilized, while those of Ura with His are stabilized by both enthalpic and entropic contributions to the free Gibbs energy for both buffer solution and pure water. The data obtained reveal the presence of complexes between His and ligands (Ura, NA) with 1:1 and 1:2 stoichiometry, respectively, for pure water and buffer solution.