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 (-)-epigalloca techin-3-gallate (EGCg) and b-lactoglobulin (beta-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 beta-Lg by more than EGCg alone, while Mn2+, Al3+ and Zn2+ decrease the quenching efficiency of EGCg to beta-Lg by less than EGCg alone. [beta-Lg-Mn], [beta-Lg-Al], [beta-Lg-Zn] and [beta-Lg-Sn] obviously decreased the apparent binding affinities (K-a) of EGCg to beta-Lg by 95.53%, 81.94%, 44.66% and 38.58%, respectively, whereas [beta-Lg-Cu] significantly increased the K-a of EGCg for beta-Lg by 115.44%. The results of CD spectroscopy experiment showed the a-helix content of bLg decreased with the addition of Cu or Al, indicating that the beta-Lg-Cu and beta-Lg-Al interaction resulted in unfolding of the secondary structure of beta-Lg. (C) 2016 Elsevier Ltd. All rights reserved.