Structuration mechanism of beta-lactoglobulin - acacia gum assemblies in presence of quercetin
Résumé
The interactions of b-lactoglobulin (BLG) with total Acacia gum (TAG) in presence of quercetin have been investigated in aqueous solutions at pH 4.2 and 25 _C. Isothermal titration calorimetry (ITC) has been used to determine the type and magnitude of the energies involved in the complexation process. Dynamic light scattering (DLS), electrophoretic mobility (mE), turbidity measurements (s), optical microscopy and Fourier transform infrared spectroscopy in total attenuated reflection mode (ATR-FTIR) were used as complementary methods to better understand the sum of complicated phenomena at the origin of thermodynamic behaviour. The first structuring stage was characterized by an exothermic signal and was mainly controlled by favourable enthalpy changes due to electrostatic interactions between biopolymers. The second structuring stage was largely endothermic and more entropy driven, probably due to the release of small counterions from the electrical double layer and hydrophobic contribution to the binding process, implying the release of water molecules. The population distribution of the different species in solution and their size were determined through DLS. Dispersion turbidity of particles markedly increased and reached a maximum at 0.013 TAGQ:BLG molar ratio corresponding to the appearance of coacervates. Above TAGQ:BLG molar ratio of 0.015, dispersions turbidity decreased, which might be due to an excess of negative charges onto particles as revealed by electrophoretic mobility measurements. FTIR experiments indicated that BLGeTAG interactions, in presence or in absence of quercetin, induced a change in the a-helical structure of BLG. The results also showed significant loss in b-sheets indicating a change in the environment of BLG hydrophobic amino acids and the formation of proteineflavonoid complexes stabilized by hydrophobic associations. The results presented in this study should provide information about thermodynamic mechanisms of TAGeBLG binding processes in presence of an antioxidant, quercetin and will facilitate the application of the formed supramolecular assemblies as functional ingredients in food and non food systems.