Energetics and mechanism of complexation between β-lactoglobulin and oligochitosan

Abstract

β-lactoglobulin–oligochitosan (MW 9500 Da) system was studied under conditions of protein-polysaccharide incompatibility, the dipole-charge, and charge-charge protein-polysaccharide interactions at pH 3.0, 5.5, and 6.0, respectively, using ITC, DSC, and DLS. At pH 5.5 and 6.0, the ITC data revealed a complex formation between β-lactoglobulin and oligochitosan. The binding curves of the protein to the polysaccharide were obtained. The binding parameters, namely, the number of sites ( n ) and the binding constant ( K b ), were estimated in terms of the Langmuir equation. These are n = 2.8 ± 0.1 and K b = 10 6.4±0.1 M −1 at рН 5.5, while n = 1.5 ± 0.1 and K b = 10 5.6±0.1 M −1 at рН 6.0. At pH 3.0, when both the protein and polysaccharide are positively charged, the denaturation parameters of β-lactoglobulin in the β-lactoglobulin–oligochitosan system and in the absence of the polysaccharide coincide independently of the system composition. Under conditions of the complexation, the denaturation parameters of β-lactoglobulin (the denaturation temperature, enthalpy, entropy, and width) displayed complicated dependences on the oligochitosan content. Their analysis supported by measurements of the diffusion coefficients implied that the conformational stability of β-lactoglobulin in the β-lactoglobulin–oligochitosan complexes was defined by the complex composition and supramolecular structure of oligochitosan. • ⦁BLG conserves its stability under conditions of BLG–OCS thermodynamic incompatibility. • ⦁BLG–OCS complexes are formed at pH ≥ 5.2 by dipole-charge or electrostatic interactions. • ⦁BLG–OCS affinity is enthalpy-driven and unusually high ( K b ∼10 6 M −1 ). • ⦁BLG–OCS complexation stabilizes dimeric or tetrameric forms of bound BLG. ⦁ •Stability of bound BLG decreases at low and increases at high OCS concentrations.