Influence of a sugar moiety (rhamnosylglucoside) at 3-O position on the reactivity of quercetin with whey proteins.

The reaction of whey proteins (WP) with quercetin and rutin (quercetin-3-O-rhamnosylglucoside) is influenced by the glycosidic bound sugar moiety. The protein derivatives formed showed a blocking of tryptophan (max. 49%), free amino (max. 32%) and thiol groups (max. 24%). The amount of quercetin and rutin bound covalently (up to 94 and 31nmol mg(-1), respectively) was estimated by their characteristic absorbance between 300 and 340 nm. At least one molecule of the phenolic reactant was covalently bound to a beta-lactoglobulin molecule (beta-Lg). High molecular protein fractions were detected by SDS-PAGE (cross-linking with quercetin). All results confirm that quercetin is more reactive than rutin. The pH-dependent solubility of the derivatives decreased, although their hydrophilic character increased. The structural changes (circular dichroism (CD)) showed that especially rutin causes perturbation of the secondary (decrease of alpha-helix elements accompanied by an increase in random coil) and tertiary structure. The in vitro proteolytic digestibility, especially of the rutin derivatives was elevated, due to an increase in denaturation of the derivatives.