The interaction mechanism of cyanidin-3-glucoside (C3G) with β-conglycinin and glycinin was investigated by multiple spectroscopies and molecular docking. The results showed that C3G quenched the intrinsic fluorescence of β-conglycinin/glycinin strongly in both static and dynamic modes and C3G had stronger binding affinity toward glycinin than β-conglycinin. However, the major interaction force involved in C3G binding to glycinin was different from that for β-conglycinin. As judged from the number of binding sites, a stable C3G-soy protein complex with a molar ratio of 1:1 was formed. C3G induced partial unfolding of the secondary structure of soy protein and a conformational switch from α-helix to β-sheet, thus resulting in conformational unfolding of soy protein. The addition of C3G could reduce the hydrophobicity of the microenvironment around tryptophan residues in β-conglycinin, but had no significant effect on the microenvironment around amino acid residues in glycinin. In addition, the majority of the phenolic hydroxyl groups in the C3G molecule were mainly involved in hydrogen bonds and hydrophobic interaction with soy protein. Compared with β-conglycinin, glycinin showed great potential as a biological carrier for the stabilization and delivery of C3G. However, it may be unfavorable for the biological activity of C3G.

The interaction between β-conglycinin (7S)/glycinin (11S) and (–)-epigallocatechin-3-gallate (EGCG) under neutral conditions was characterized by fluorescence spectroscopy, ultraviolet-visible (UV-Vis) absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy and molecular docking. The results showed that EGCG and 7S/11S could interact with each other at pH 7.0, which induced a change in the microenvironment of amino acid residues. EGCG could quench the intrinsic fluorescence of 7S/11S in dynamic and static manners. EGCG had higher affinity to 11S than to 7S. The reaction between EGCG and 7S/11S was a spontaneous binding process, resulting in the formation of a complex at 1:1 molar ratio by hydrogen bonding and van der Waals force. EGCG could reduce the surface hydrophobicity of 7S/11S. With increasing EGCG concentration, 11S showed a greater change in surface hydrophobicity. FTIR and molecular docking studies suggested that in addition to hydrogen bonds, hydrophobic interactions were also involved in the formation of complexes. Binding to EGCG could cause different changes in the secondary structure of 7S/11S subsequently resulting in protein unfolding.