Abstract: In this paper, non-covalent binding (pH 7.4, 2 h) and covalent cross-linking (pH 9, 24 h) were used to analyze the variations of protein conformations in soybean protein isolate (SPI)-anthocyanins complexes. The changes in protein structure in the composite systems were studied by turbidity measurement, binding capacity, gel electrophoresis analysis, fluorescence spectroscopy and infrared spectroscopy. The results of gel electrophoresis showed the formation of macromolecular derivatives in complexes 4, 5 and 6 (SPI/anthocyanins ratio = 20:1, 10:1, 5:1, m/m) by covalent cross-linking. The turbidity values of the covalently cross-linked complexes were lower than that of the non-covalently bound ones (1, 2 and 3), and the affinity of the anthocyanins in the covalently cross-linked complexes were stronger. The fluorescence intensity of the proteins was decreased with the increase of anthocyanins in complexes and tryptophan residues were exposed to a more hydrophilic environment. This finding suggested that the fluorescence quenching effect of the covalently cross-linked complexes were obvious. Covalent cross-linking was greater than non-covalent binding; the infrared absorption intensities of the proteins in complexes 1 and 4 were significantly decreased, indicating that the secondary structure of the proteins is changed. In addition, the contents of β-turn and irregular structure in sample 4 were higher, indicating that the covalent cross-linking mechanism of anthocyanins in the complex reveals a stronger ability to unfold the proteins.

Key words: soybean protein isolate, anthocyanins, non-covalent interactions, covalent interactions, conformational change