The uncoupling efficiency and affinity of flavonoids for vesicles

The relative hydrophobicity and interaction of flavonoids with artificial m embranes using vesicles was studied. At the same degree of hydroxylation, f lavones were slightly more hydrophobic than flavanones. Flavonoids possess a hydrophobic character and are weak acids. For this reason, their uncoupli ng efficiency of the membrane potential was studied using cytochrome c oxid ase vesicles. With emphasis on naringenin, it was shown that flavonoids aff ect both the transmembrane potential difference (V) and the transmembrane p H difference (V). Flavones were slightly more effective in uncoupling the m embrane potential than flavanones; the 7OH group seems to play an important role. Hydroxylation of the exocyclic phenyl group decreased the uncoupling efficiency for all flavonoids studied. The flavonol quercitin exhibited ha rdly any uncoupling activity. Glycosylation abolished all uncoupling activi ty. The affinity of flavonoids for vesicle membranes was also studied using the fluorescence quenching of the membrane probe diphenylhexatriene. Flavo nols exhibited a substantially higher affinity for liposomes than flavanone s. This difference in affinity is assumed to be caused by the far more plan ar configuration of the flavonols in comparison with the tilted configurati on of flavanones, Due to this planar configuration, it seems reasonable to assume that flavonols could more easily intercalate into the organised stru ctures of the phospholipids within the vesicle membranes than flavanones. I t is concluded that, in vivo, hardly any uncoupling activity of flavonoids can be anticipated. However, the quercitin plasma concentration in vivo can be such that, based on the affinity study, part of this flavonol could be associated with biological membranes to function there as, for example, an antioxidant. (C) 2000 Elsevier Science Inc.