Characterization of binding interactions of anthraquinones and bovine β-lactoglobulin

Anthraquinones, a class of naturally occurring polyphenolic compounds, exhibit a wide range of bioactivities. However, most free anthraquinones are lipophilic bioactive compounds. Bovine beta-lactoglobulin (beta LG), a major whey protein, has a high affinity for small hydrophobic compounds. In this study, the interactions between anthraquinones (rhein, emodin, and chrysophanol) and beta LG were investigated by using fluorescence, circular dichroism (CD), Fourier-transform infrared spectroscopy (FTIR), and docking studies. These anthraquinones bound to the site near Trp19-Arg124 on beta LG with a binding constant (Ka) between 10(3) and 10(5) L mol(-1) to form complexes, which changed the secondary structure of beta LG, inducing an alpha-helix to beta-sheet structure transition. The order of binding increased with an increasing polarity in the order of rhein > emodin > chrysophanol. In addition, the degree of radical scavenging capacity masking increased with an increasing binding affinity. Complexation with beta LG significantly increases the hydrosolubility of anthraquinones.