Abstract: In this paper, we studied the effects of the covalent interaction of soy protein isolate (SPI) and different concentrations of anthocyanin on protein structure under enzymatic and alkaline conditions. The covalent bonding rate and free sulfhydryl content were used to compare the degree of bonding between anthocyanin and SPI in different covalent cross-linking modes. Fourier transform infrared (FT-IR) spectrocopy, UV-Vis absorption spectroscopy and fluorescence spectroscopy were used to analyze the structure and conformation of the anthocyanin-SPI conjugates. The results showed that the bonding rate of anthocyanin-SPI conjugates increased and its content of free sulfhydryl decreased with the increase of anthocyanin concentration. The secondary structure of SPI was changed and the protein was unfolded after anthocyanin was linked covalently to it. And the microenvironment around the aromatic amino acid residues in the protein was also changed, thereby resulting in its conformational change. Furthermore, the anthocyanin-SPI conjugates had a higher bonding rate, a lower content of free sulfhydryl and more obvious structural and conformation changes in under enzymatic conditions than alkaline conditions. This indicated that anthocyanin has a stronger covalent binding ability to SPI under enzymatic conditions than alkaline conditions.

Key words: soybean protein isolate (SPI), anthocyanin, enzymatic method, alkaline method, structure