Interaction of milk α- and β-caseins with tea polyphenols

The interaction of alpha- and beta-caseins with tea polyphenols (+)-catechin (C), (-)-epicatechin (EC), (-)-epigallocatechin (EGC) and (-)-epigallocatechin gallate (EGCG) was examined at a molecular level, using FUR, UV-visible, CD and fluorescence spectroscopic methods as well as molecular modelling. The polyphenol binding mode, the binding constant and the effects of polyphenol complexation on casein stability and conformation were determined. Structural analysis showed that polyphenols bind casein via both hydrophilic and hydrophobic interactions with overall binding constants of KC-alpha-cas = 1.8 (+/- 0.8) x 10(3) M-1, KEC-alpha-cas = 1.8 (+/- 0.6) x 10(3) M-1, KEGC-alpha-cas = 2.4 (+/- 1.1) x 10(3) M-1 and KEGCG-alpha-cas = 7.4 (+/- 0.4) x 10(3) M-1, KC-beta-cas = 2.9 (+/- 0.3) x 10(3) M-1, KEC-beta-cas = 2.5 (+/- 0.6) x 10(3) M-1, KEGC-beta-cas = 3.5 (+/- 0.7) x 10(3) M-1 and KEGCG-beta-cas = 1.59 (+/- 0.2) x 10(4) M-1. The number of polyphenol bound per protein molecule (n) was 1.1 (C), 0.9 (EC), 1.1 (EGC), 1.5 (EGCG) for alpha-casien and 1.0 (C), 1.0 (EC), 1.1 (EGC) and 1.5 (EGCG) for beta-casein. Structural modelling showed the participation of several amino acid residues in polyphenol-protein complexation with extended H-bonding network. Casein conformation was altered by polyphenol with a major reduction of alpha-helix and beta-sheet and increase of random coil and turn structure suggesting further protein unfolding. 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) 2010 Elsevier Ltd. All rights reserved.