Isothermal titration calorimetric and spectroscopic studies of β-lactoglobulin-water-soluble fraction of Persian gum interaction in aqueous solution

• Soluble complexes were formed at a pHc above protein's pI.
• A decrease in Pr:Ps MR and TC shifted pHφ1 and pHopt toward more acidic regions.
• Divalent ions had more effects in preventing complex formation.
• Binding enthalpy was independent from the studied temperature range.

In this work, the interaction between β-lactoglobulin (β-lg) and water-soluble fraction of Persian gum (WPG) was studied under the effects of extrinsic parameters including pH, protein to polysaccharide mixing ratio (MR 8:1–1:4), total biopolymer concentration (TC 0.1–0.6% (w/w)), ion type (Na+ and Ca2+), ionic strength (0–100 mM) and temperature (25, 40 and 55 °C). Soluble complexes were formed at a pH above protein's isoelectric point (pI). MR and TC had significant effects on the values of critical pHφ1 (formation of insoluble complexes) and pHopt (maximum optical density). A decrease in MR (at a constant TC) and in TC (at a constant MR) shifted the values toward more acidic domains; while, the critical pHc (formation of soluble complexes) remained constant. The effect of divalent ions (Ca2+) in preventing the complex coacervation was more than that of monovalent ones (Na+). Increasing the ionic strength had significant effect in decreasing the interaction. In this study, the effects of temperature on the thermodynamic parameters were obvious. Biopolymers binding enthalpy as obtained by isothermal titration calorimetry (ITC) was independent from the temperature in the studied range. Analysis of the temperature effect showed that electrostatic interaction, hydrogen bonding and hydrophobic interactions were involved in the complexation process.

Absorbance as a function of pH for different β-lg-WPG aqueous mixtures (total concentration 0.3% (w/w)).Figure optionsDownload as PowerPoint slide