Multispectroscopic and synergistic antioxidant study on the combined binding of caffeic acid and (-)-epicatechin gallate to lysozyme

The binding of caffeic acid (CA) and/or (-)-epicatechin gallate (ECG) to lysozyme was investigated by multispectroscopic methods and molecular docking. The effects of the single and combined binding on the structure, activity and stability of lysozyme and the synergistic antioxidant activity of CA and ECG were also studied. Fluorescence quenching spectra, time-resolved fluorescence spectra, and UV-vis absorption difference spectra all ascertained the static quenching mechanism of lysozyme by CA/ECG. Thermodynamic parameters indicated that CA and ECG competitively bound to lysozyme, and CA had a stronger binding affinity, which was consistent with the results of molecular docking. Hydrogen bonding, van der Waals' force and electrostatic interaction were the main driving forces for the binding process. Synchronous fluorescence spectra displayed that the interaction of CA/ECG exposed the tryptophan residues of lysozyme to a more hydrophilic environment. Circular dichroism spectroscopy, Fourier transform infrared spectroscopy and dynamic light scattering indicated that the binding of CA and/or ECG to lysozyme resulted in the change of the secondary structure and increased the particle size of lysozyme. The binding of CA and/or ECG to lysozyme inhibited the enzyme activity and enhanced the thermal stability of lysozyme. The combined application of CA and ECG showed antioxidant synergy which was influenced by the encapsulation of lysozyme and cellular uptake. In summary, this work provides theoretical guidance for lysozyme as a carrier for the combined application of CA and ECG.