The interaction mechanism of β-casein with oligomeric proanthocyanidins and its effect on proanthocyanidin bioaccessibility

The interaction mechanism between β-casein and five types of oligomeric proanthocyanidins, including procyanidins B1, B2, B3, A2, and C1 was investigated using spectroscopy and molecular docking. The fluorescence spectroscopy results showed that B1, B2, B3, and A2 can quench the intrinsic fluorescence of β-casein by static quenching and C1 by both dynamic and static quenching. All five proanthocyanidins can form a binding site with β-casein, and the corresponding binding ability was as follows: procyanidins B1 ( K A = 1317 ± 159 L ⋅ mol − 1 ) > B2 ( K A = 759 ± 179 L ⋅ mol − 1 ) > B3 ( K A = 761 ± 72 L ⋅ mol − 1 ) > C1 ( K A =138 ± 48 L ⋅ mol − 1 ) > A2 ( K A =99 ± 6.1 L ⋅ mol − 1 ). According to thermodynamic analysis and molecular docking, the binding forces of proanthocyanidins with β-casein were mainly induced by hydrophobic interactions, van der Waals forces, and hydrogen bindings. Circular dichroism and Fourier-transform infrared spectroscopy supported the changes in the secondary structure of β-casein induced by proanthocyanidins. The interaction between β-casein and proanthocyanidins did not significantly reduce the digestion of β-casein but increased the recovery rate of procyanidin B1 by 8.9% during in vitro gastrointestinal digestion. This study laid the foundation for the application of β-casein-proanthocyanidins complex in the food industry. • Proanthocyanidins statically quench the intrinsic fluorescence of β-casein. • Proanthocyanidins binding altered the secondary structure of β-casein. • Hydrophobic interactions, van der Waals forces, and hydrogen bindings are the main forces for complex formation. • Complex formation did not significantly reduce the digestion of β-casein but increased the recovery rate of procyanidin B1.