Monitoring the binding processes of (–)-epigallocatechin gallate and theaflavin-3,3’-digallate to alpha-casein surface using quartz crystal microbalance with dissipation

Quartz crystal microbalance with dissipation monitoring (QCM-D) has been employed to study the interactions between (–)-epigallocatechin gallate (EGCG) - the powerful green tea antioxidant polyphenol - and theaflavin-3,3’-digallate (TF-3) - the black tea polyphenols, which gives black tea its unique color and taste with α-casein, surface. The adsorbed mass and thickness of EGCG and TF-3 adlayer on α-casein surface at various concentration, pH, and sodium chloride concentrations have been determined by QCM-D using Voigt model. The results show that the Freundlich model can be used to describe the adsorption isotherm of both EGCG and TF-3 on α-casein surface, suggesting that the adsorption on α-casein surface are dominated by hydrophobic nonspecific interactions, which are supported by stronger adsorption at the isoelectric point (pI) of α-casein for both EGCG and TF-3. The addition of salt slightly reduces the EGCG binding to protein surfaces. The shifts in the positions of both amides I and II bands in the FTIR spectra of the α-casein surface with EGCG adsorption indicate the presence of the hydrogen bonding.