Effects of (−)-Epigallocatechin-3-gallate on the Functional and Structural Properties of Soybean Protein Isolate

In the present study, soy protein isolate (SPI) was noncovalently modified by (−)-epigallocatechin-3-gallate (EGCG), and its foaming, emulsifying, and antioxidant properties were all significantly increased. Fluorescence analysis revealed that the fluorescence quenching of SPI by EGCG was static quenching. EGCG mainly changed the folding state of SPI around Trp and Tyr residues, and the binding site was closer to Trp. UV–vis spectra further proved that more hydrophobic residues of SPI were exposed to a hydrophilic microenvironment. Circular dichroism spectra indicated that the contents of ordered structures were transforming into random coils with the reduce of α-helix, β-sheet, and β-turns by 3.8%, 2.0%, and 1.2%, respectively. Meanwhile, the binding stoichiometry of two molecules of EGCG per one molecule of SPI was obtained from isothermal titration calorimetry, and the interaction was a spontaneous endothermic process with a noncovalent complex preferentially formed. According to thermodynamic parameters and molecular docking model, hydrophobic force and hydrogen bonds were considered to be the main interaction forces between SPI and EGCG. Overall, after modification through the high affinity to EGCG, the structure of SPI became looser and exposed more active groups, thus resulting in an improvement of its foaming, emulsifying, and antioxidant properties.