| Among three major types, green, oolong, and black tea, black tea is the most popular of tea consumption as indicated by almost 80% in the industry of tea production with 3 billion kilograms. Theaflavins and thearubigin are two main polyphenols that give black tea its characteristic color and taste. Theaflavins are classified as the mixtures of four compounds including theaflavin (TF-1), theaflavin-3-gallate (TF-2a), theaflavin-3'-gallate (TF-2b), and theaflavin-3,3'-digallate (TF-3). There are numerous epidemiological studies both in vitro and vivo have indicated that black tea polyphenols including, theaflavins, and thearubigin, have ability to be antioxidative, anti-inflammatory, anti-mutagenic, and anticancer properties, consequently reducing the risk of non-communicable degenerative diseases such as cancer, and coronary heart disease. Tea polyphenols have been reported to interact with proteins such as salivary proline-rich protein, bovine serum albumin, and milk protein. Some studies reported that the interactions between polyphenols in tea and proteins might lead to the loss of bioavailability of polyphenols and their bioactive capacity.;The structure difference between thearubigin and theaflavins in terms of amount of gallic acid and hydroxyl functional groups could provide the different interaction with proteins. This study aims to monitor the binding processes of thearubigin and theaflavins including theaflavin (TF-1) and theaflavin-3,3'-digallate (TF-3), with bovine serum albumin (BSA) surface using quartz crystal microbalance with dissipation monitoring QCM-D at different environment such as concentration, pH, ionic strength and temperature.;BSA protein was immobilized on the surface of self-assemble monolayer of quartz crystal electrode. The mass and thickness of black tea polyphenols adlayer on BSA surfaces had been determined by QCM-D using Voigt model. Our results showed that the adsorption isotherm of thearubigin on BSA surface can be better described by the Langmuir model than the Freundlich model, suggesting that the thearubigin adsorption on BSA surface was dominated by electrostatic interaction as evidenced by the stronger thearubigin adsorption at pH below the isoelectric point (pI) of BSA. On the other hand, TF-3 had stronger adsorption at the isoelectric point (pI) of BSA and the adsorption isotherm of both TF-3 and TF-1 on BSA surface can be better described by Freundlich model, suggesting that the TF-3 adsorption on BSA surface is dominated by hydrophobic interactions. The much higher adsorption capacity on BSA surface of TF-3 than TF-1 indicates the importance of galloyl group in polyphenol/protein interactions. The addition of salt influenced the thearubigin and theaflavins binding to BSA surfaces. The shape and shifts in the positions of both amides I and II bands in the FTIR spectra of the BSA surface indicated the presence of the hydrogen bonding. |
