Effect of Ultrasonic Pretreatment on the Structure and Function of Soybean Protein Isolate-Catechin Non-covalent/Covalent Complexes

This study was conducted to explore the effect of ultrasonic pretreatment on the structure and function of non-covalent/covalent complexes of soybean protein isolate (SPI) and catechin. SPI was pretreated with ultrasound before non-covalent/covalent binding to catechin at different pH conditions (3.0, 7.0, 9.0 and 12.0), Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) was used to verify the formation of complexes and the binding degree between protein and polyphenols. Meanwhile, fluorescence spectroscopy, Fourier transform infrared (FTIR) spectroscopy and molecular docking were used to study the interaction between SPI and catechin. The changes in functional properties ofthe complexes were investigated in terms of foamability, foam stability, solubility, turbidity, and antioxidant capacity. The SDS-PAGE profile confirmed the formation of non-covalent/covalent complexes, and that ultrasonic pretreatment could affect the degree of binding. It was found that the highest amount of catechin was bound to ultrasonicated SPI at pH 12. Fluorescence and FTIR spectra indicated that the secondary structure of SPI in the non-covalent/covalent complexes was changed. Compared to the untreated SPI sample, the proportions of α-helix and β-sheet decreased, the proportions of β-turn and random coil increased, and the numbers of exposed tryptophan and tyrosine residues increased. The most significant effect of ultrasonic pretreatment was observed at pH 12; the proportion of β-turn increased to 40.20%, the proportion of random coil increased to 28.61%, and SPI’s structure became unfolded and loosened. After ultrasonic pretreatment, compared to the untreated SPI sample, the solubility, turbidity, foamability, foam stability and antioxidant capacity of SPI and complexes increased. In particular, the scavenging capacity against 1,1-diphenyl-2-picrylhydrazine (DPPH) radical and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical cation of complexes formed at pH 12 were up to 5.5 and 4.8 times of the untreated SPI sample, respectively. In addition, the molecular docking results revealed that the main non-covalent binding interactions between SPI and catechin were hydrogen bonds and hydrophobic interactions. To sum up, ultrasonic-treated SPI had the strongest binding strength to catechin at pH 12, and the resulting covalent complex had the best stability as well as significantly improved foamability and antioxidant activity.