Effect of Covalent Crosslinking of Soybean Protein Isolate with Genistein on Protein Characterization and Structure

In order to explore the effects covalent crosslinking between soybean protein isolate (SPI) and genistein on protein characterization and structure, covalent complexes between SPI and genistein, prepared by adding different concentrations (0, 1.2, 1.5 and 2.0 mg/mL) of genistein to SPI solution, were characterized by measuring particle size, zeta potential, turbidity and surface hydrophobicity, and the structural changes of SPI were analyzed by using an ultraviolet spectrophotometer, a fluorescence spectrophotometer and a Fourier transform infrared spectrometer. The results showed that the median particle size of SPI declined from 135.6 to a minimum value of 98.0 μm, the absolute value of zeta potential increased from 15.0 mV to a maximum value of 21.4 mV, the surface hydrophobicity decreased from 216.0 to a minimum value of 115.5, and the total sulfhydryl group content decreased from 31.5 μmol/g to a minimum value of 20.4 μmol/g after covalent complexation with genistein. Compared with the control group, the turbidity of the covalent complexes increased, and the turbidity of the SPI-Ge-1.2 complex was lower than that of the SPI-Ge-1.5 and SPI-Ge-2.0 complexes. Spectral analysis showed that genistein had a quenching effect on SPI. After covalent cross-linking, the hydrophobicity of the microenvironments of tryptophan and tyrosine residues in SPI decreased, the contents of α-helix and β-turn increased, and the contents of β-sheet and random coil decreased. The addition of 1.2 mg/mL genistein had a better effect on the characterization and structure of SPI. The results indicated that the covalent cross-linking of genistein with SPI can affect the characterization and structure of the protein.