Interaction of the antioxidant flavonoid quercetin with planar lipid bilayers

Our capacitance and conductance measurements on reconstituted planar lipid bilayers (BLM) suggest an insertion of the flavonoid quercetin (QCT) in the membranes, which is concentration- and pH-dependent. Interaction of the fl avonoid with the membrane has no impact on either structure or integrity of the lipid bilayer. The QCT molecules penetrate the lipid bilayer by interc alating between the flexible acyl chains of the phospholipids, the deepest insertion occuring in acidic medium, when QCT is neutral and completely lip osoluble. Results indicated that aggregation of QCT within the hydrophobic core is accompanied by an increase of the transmembrane conductance followi ng an alteration of the hydrophobic barrier for small electrolytes. By cont rast, within alkaline media where QCT is deprotonated, the reaction site of the flavonoid is restricted to the hydrophilic domain of the membrane. Thi s significantly changes the double layer capacitance as the negatively char ged QCT molecules become sandwiched between polar headgroups at the bilayer surface. At highest alkaline pH, the transmembrane conductance was not aff ected, since QCT did not perturb the molecular packing of the hydrocarbonic acyl chains of the phospholipids. Results also demonstrated that changes i n physical properties of the lipid bilayers following interstitial QCT embe dding within either the hydrophobic domain or the polar headgroup domain ma y be related to both its lipophilic nature and interactions with the electr ic dipole moments of the polar headgroups of phospholipids. Data also demon strated that translocation of QCT in the polar part of the lipid bilayer, a t physiological pH and salt conditions. may be correlated with its optimize d radical scavenging activity. This paper discusses the significance of the free radical scavenging capacity and antioxidant efficiency of QCT. (C) 20 00 Elsevier Science B.V. All rights reserved.