Effect of metal ions on the binding reaction of (-)-epigallocatechin gallate to β-lactoglobulin.

The effects of metal ions (Cu2+, Sn2+, Zn2+, Mn2+, Al3+ and Fe3+) on the interaction between (-)-epigallocatechin-3-gallate (EGCg) and β-lactoglobulin (β-Lg) were investigated via fluorescence and circular dichroism (CD) spectroscopy. The results of the fluorescence titration experiment demonstrated that the addition of Cu2+ increases the quenching efficiency of EGCg to β-Lg by more than EGCg alone, while Mn2+, Al3+ and Zn2+ decrease the quenching efficiency of EGCg to β-Lg by less than EGCg alone. [β-Lg-Mn], [β-Lg-Al], [β-Lg-Zn] and [β-Lg-Sn] obviously decreased the apparent binding affinities (Ka) of EGCg to β-Lg by 95.53%, 81.94%, 44.66% and 38.58%, respectively, whereas [β-Lg-Cu] significantly increased the Ka of EGCg for β-Lg by 115.44%. The results of CD spectroscopy experiment showed the α-helix content of β-Lg decreased with the addition of Cu or Al, indicating that the β-Lg-Cu and β-Lg-Al interaction resulted in unfolding of the secondary structure of β-Lg.