Effects of tannic acid on the structure and proteolytic digestion of bovine lactoferrin

Tannic acid (TA) can strongly interact with proteins to improve the stability of protein-based nanoparticles. In the present study, the complexation of TA with bovine lactoferrin (LF) was investigated at pH 6.0. The size and turbidity of the LF-TA complexes did not change significantly until the LF/TA molar ratio was over a critical value of 1:8. Zeta potential of LF-TA was gradually negative as the TA content increased, indicating that there were electrostatic interactions between LF and TA. The circular dichroism (CD) spectroscopy confirmed that TA could further reduce the α-helix and increase the β-sheet content of LF significantly, resulting in a more compact structure of LF. The fluorescence spectra revealed that the interaction between LF and TA caused intramolecular quenching of LF, and the values of Ksv increased with temperature, indicating that static quenching was involved in the interaction between LF and TA. Thermodynamic parameters indicated that the reaction between LF and TA proceed spontaneously, and hydrophobic interactions were the major interaction force. Molecular docking suggested that the binding site of TA and LF was in the inter-leaf region where the two lobes were connected by a spiral. Besides, the proteolytic digestion experiments testified that the addition of TA could reduce the digestibility of LF. Information derived from this work suggests that the addition of TA in LF systems could improve the digestive stability of bio-complex for oral delivery of functional constituents.