Improving gas-water interface properties and bioactivities of ?-lactalbumin induced by three structurally different saponins

Characterization of binding interactions, gas-water interface properties and bioactivities in alpha-lactalbumin (alpha La) with tea saponin (Ts), gynostemma saponin (Gyp) and tribulus saponin (Tr) were elucidated by using molecular docking, multi-spectroscopic techniques, polarization biomicroscopic and cryo-electron microscopic analysis. Specifically, the static quenching ability to intrinsic fluorescence of alpha La decreased according to the order of Ts, Gyp and Tr. Among them, alpha La interacted with Ts and Gyp through hydrogen bond and van der Waals force, while it combined with Tr via hydrophobic interaction force. Furthermore, Ts, Gyp and Tr made foaming ability of alpha La increased by 164.54%, 136.73% and 63.95%, respectively. Under cryo-electron microscope, the descending order of foam integrity and liquid film thickness was alpha La-Gyp, alpha La-Tr, alpha La-Ts. Moreover, the DPPH scavenging ability, ferrous reducing power, and alpha-glucosidase inhibition of alpha La-saponins were higher than those of alpha La, and alpha La-Ts showed the maximum. Meanwhile, molecular docking indicated that there were different binding sites and numbers between alpha La and three saponins. And alpha La-Ts, alpha La-Gyp and alpha La-Tr had 12, 3 and 6 hydrogen bonds, respectively. Moreover, Ts did not significantly affect secondary structure of alpha La, whereas Gyp and Tr decreased its alpha-helix and increased its random coil. The obtained results provided basic data supports for interaction mechanism between natural saponins and proteins, being beneficial to develop efficient and natural protein-saponin foaming agents for food industry.