The interaction between flavonoids and proteins has attracted great interest among researchers. However, few reports have focused on the structure-affinity relationship of flavonoids binding to proteins. This work mainly concerns the effect of glycosylation of flavonoids on the affinities for BSA. Four flavonoid aglycones (baicalein, quercetin, daidzein, and genistein) and their monoglycosides (baicalin, quercitrin, daidzin, and puerarin, genistin) and another poly glycoside (rutin) are studied for their affinities for BSA. The glycosylation of flavonoids significantly affects the quenching/binding process. In general, the glycosylation of flavonoids lowers the affinity for BSA by 1 to 3 orders of magnitude depending on the conjugation site and the class of sugar moiety. The glucopyranosylation (daidzin and genistin) of flavonoids lowered the affinity for BSA by 5-10 times. Rhamnosylation (quercitrin) of flavonoids, however, lowered the affinity for BSA by 5600 times. This result partly supports that flavonoid aglycones are more easily absorbed than flavonoid glycosides. Higher binding affinities for BSA are associated with higher antioxidant activities for flavonoids. Glycosylation also decreases the hydrophobicity of flavonoids, and hydrophobic interaction may play an important role in binding flavonoids to proteins.

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The effects of temperature and common ions on binding (-)-epicatechin gallate (ECG) to bovine serum albumin (BSA) are investigated. The binding constants (Ka) between ECG and BSA are 1.20 Ч 106 (17°C), 1.38 Ч 106 (27°C), and 5.69 x 106 L mol-1 (37°C), and the number of binding sites (n) were 1.14, 1.15, and 1.26, respectively. These results showed that the increasing temperature improves the stability of the ECG-BSA system, which results in a higher binding constant and the number of binding sites of the ECG-BSA system. The presence of Co2+ and Zn2+ ions decreased the binding constants (Ka) and the number of binding sites (n) of ECG-BSA complex. However, the presence of Cu2+ and Ni2+ increased the affinity of ECG for BSA largely. The positive ΔH and positive ΔS indicated that hydrophobic forces might play a major role in the binding between ECG and BSA

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