Interactions between β-Lactoglobulin and Three Major Polyphenols in Coffee under Different pH Environments

The purpose of this study was to explore the non-covalent interactions between three major polyphenols (chlorogenic acid, caffeic acid and ferulic acid) in coffee and β-lactoglobulin (β-LG) at different pH values and their effects on protein structure by multispectroscopic techniques and molecular docking. The results showed that all three coffee polyphenols statically quenched the fluorescence of β-lactoglobulin under two pH conditions. At pH 7.4 and 298 K, the quenching constants (K_sv) of chlorogenic acid, caffeic acid and ferulic acid for β-lactoglobulin were 6.53 × 10⁴, 3.16 × 10⁴ and 3.09 × 10⁴ L/mol with 1.20, 1.02 and 1.14 binding sites and energy transfer efficiency of 34.55%, 24.56% and 21.35%, respectively. At pH 3.0 and 298 K, the K_sv were 7.18 × 10⁴, 5.24 × 10⁴ and 7.12 × 10⁴ L/mol with 1.28, 1.18 and 1.25 binding sites and energy transfer efficiency of 34.70%, 30.42% and 29.65%, respectively. The results of circular dichroism showed that the α-helix content of β-LG decreased and the β-sheet content increased at pH 7.4, and the β-sheet content increased and the random coil content decreased at pH 3.0. The results of molecular docking showed that the three polyphenols could bind to the hydrophobic pocket of β-lactoglobulin mainly through hydrogen bonds, van der Waals force and hydrophobic interaction. This study preliminarily revealed the interaction mechanism between β-lactoglobulin and chlorogenic acid, caffeic acid and ferulic acid in coffee milk beverage under different pH conditions.