Hydrolysable tannin induced non-covalent binding with walnut (Juglans regia L.) globulin: insight from aggregation...

The current study focused on the transition mechanism underling hydrolyzable tannins (HT) induced the insoluble aggregation of walnut (Juglans regia L.) globulin isolate (WGI). First, pedunculagin and castalagin were employed to generate non-covalent complexes with prepared WGI. As the HT concentration increased in the range of 5-40 μmol/L, the formation of large complex aggregates was confirmed by the results of tannin bound equivalents, UV turbidity and dynamic light scattering (DLS). The significant changes of tertiary conformation in the complex were evidenced by UV and fluorescence spectroscopy. Fourier transform infrared spectroscopy (FTIR) further indicated that the tannins acted as a "bridge" to cross-link the protein through a static quenching mechanism. Meanwhile, the intermolecular cross-linking led to a significant increase in random coil and β-turn contents, while different degrees of decrease in α-helix and β-sheet contents, inducing an regular and stable conformation. Combined with molecular docking and molecular dynamics (MD) simulations, it was illustrated that the HT non-covalently bound to walnut globulin primarily through hydrophobic interaction and electrostatic interaction, ultimately causing precipitation of insoluble aggregate. In addition, castalagin were more favorable to the complexation with walnut globulin due to more benzene rings and phenol hydroxyl groups in its structure.