Effects of ionic strength and (−)-epigallocatechin gallate on physicochemical characteristics of soybean 11S and 7S proteins

Polyphenols can interact with proteins to improve their physicochemical and functional properties. This study was aimed to determine the interactions of EGCG with 11S and 7S at different NaCl concentrations. Results of turbidity indicated that the binding affinity of EGCG with 11S and 7S were strongest at 0.3 M NaCl concentration. Zeta potential results confirmed that 11S-EGCG and 7S-EGCG complexes had stronger stability in salt solutions, but the stability decreased with the increase of NaCl concentration to 0.6 M. Fourier transform infrared spectra showed that NaCl could affect hydrogen bonds between soybean proteins and EGCG. Based on Raman spectroscopy, the microenvironment of the tryptophan and tyrosine residues and intermolecular hydrogen of 11S, 7S, 11S-EGCG and 7S-EGCG were influenced by NaCl concentration. The NaCl induced a decrease in α-helix and an increase in β-sheet of 11S protein, while the secondary structure of 7S was not sensitive to NaCl concentration. The bindings of EGCG to 11S and 7S resulted in the secondary structure rearrangement of two proteins. The NaCl addition led the transformation of the α-helix to the β-sheet in 11S-EGCG and 7S-EGCG complexes. The 11S-EGCG and 7S-EGCG complexes had more compact microstructure with a smooth surface than 11S and 7S proteins, while the microstructure of the complexes became undesirable with the increase of NaCl concentration. These results will lay the foundation for the development 11S-EGCG and 7 S-EGCG complexes as a new food material in the food industry.