Milk β-lactoglobulin complexes with tea polyphenols

The effect of milk on the antioxidant capacity of tea polyphenols is not fully understood. The complexation of tea polyphenols with milk proteins can alter the antioxidant activity of tea compounds and the protein secondary structure. This study was designed to examine the interaction of beta-lactogolobulin (beta-LG) with tea polyphenols (+)-catechin (C), (-)-epicatechin (EC), (-)-epicatechin gallate (ECG) and (-)-epigallocatechin gallate (EGCG) at molecular level, using FTIR, CD and fluorescence spectroscopic methods as well as molecular modelling. The polyphenol binding mode, the binding constant and the effects of polyphenol complexation on beta-LG stability and secondary structure were determined. Structural analysis showed that polyphenols bind beta-LG via both hydrophilic and hydrophobic interactions with overall binding constants of KC-beta-LG = 2.2 (+/- 0.8) x 10(3) M-1, KEC-beta-LG = 3.2 (+/- 1) x 10(3) M-1, KECG-beta-LG = 1.1 (+/- 0.6) x 10(4) M-1 and KEGCG-beta-LG = 1.3 (+/- 0.8) x 10(4) M-1. The number of polyphenols bound per protein molecule (n) was 1.1 (C), 0.9 (EC), 0.9 (ECG) and 1.3 (EGCG). Molecular modelling showed the participation of several amino acid residues in polyphenol-protein complexation with extended H-bonding network. The beta-LG conformation was altered in the presence of polyphenols with an increase in beta-sheet and alpha-helix suggesting protein structural stabilisation. These data can be used to explain the mechanism by which the antioxidant activity of tea compounds is affected by the addition of milk. (C) 2011 Elsevier Ltd. All rights reserved.