Insight into the complex formation of β-alanyl-L-histidine with nicotinic acid in water and buffer aqueous solution

Abstract

Interactions between the peptide, L-carnosine (Car), and heterocyclic ligand, nicotinic acid (NA), in pure water and in aqueous buffer solution (pH 7.4) were studied by calorimetry, UV–vis spectroscopy and densimetry. Under the experimental conditions chosen, stability constants (lgKc) and thermodynamic parameters of complex formation (ΔcG, ΔcH, ΔcS) were determined for the reaction of Car with NA at 298.15 K. The complexation of L-carnosine with nicotinic acid is influenced by the pH of medium and ionic state of the reagents investigated. The results indicate the formation of a more stable complex between Car and NA with 1:2 stoichiometry in water than in buffer. Different negative enthalpic contributions to the free Gibbs energy accompanied by close positive values of entropic terms were obtained for Car/NA complexes in water and buffer. Using the measured density values of NA solutions in Car – water and Car – buffer systems, the apparent molar volumes, partial molar volumes at infinite dilution and their derivatives with respect to temperature have been obtained. The effects of solute concentration, pH and temperature (from 293.15 to 318.15 K) on the volume properties derived were discussed in terms of predominant types of molecular interactions (hydrophilic, hydrophobic, ionic and zwitterionic) on the basis of co-spheres overlap model.