Energetics and mechanism of complexation between β-lactoglobulin and oligochitosan

beta-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 beta-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 pH 5.5, while n = 1.5 +/- 0.1 and K-b = 10(5.6 +/- 0.1) M-1 at pH 6.0. At pH 3.0, when both the protein and polysaccharide are positively charged, the denaturation parameters of beta-lactoglobulin in the beta-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 beta-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 beta-lactoglobulin in the beta-lactoglobulin-oligochitosan complexes was defined by the complex composition and supramolecular structure of oligochitosan.

Automated summary

The complex formation and protein-polysaccharide interactions in the beta-lactoglobulin-oligochitosan system were studied under different pH conditions using ITC, DSC, and DLS. The results showed the formation of a complex between beta-lactoglobulin and oligochitosan at pH 5.5 and 6.0. At pH 3.0, where both the protein and polysaccharide are positively charged, the denaturation parameters of beta-lactoglobulin remained the same regardless of the presence of the polysaccharide.