Modulation of liposomal membrane fluidity by flavonoids and isoflavonoids

The polyphenolic structures of flavonoids and isoflavonoids confer them wit h the ability to scavenge free radicals and to chelate transition metals, a basis for their potent antioxidant abilities, Another possible contributor y mechanism toward their antioxidant activities is their ability to stabili ze membranes by decreasing membrane fluidity, In this study, the effects of representative flavonoids, isoflavonoids, and their metabolites on membran e fluidity and their preferential localization in the membrane were investi gated using large unilamellar vesicles (LUVs) as the membrane models, These results were compared with those of cholesterol and alpha-tocopherol. Chan ges in fluorescence anisotropy values for a series of n-(9-anthroyloxy) fat ty acid probes (n = 6, 12, 16) upon addition of the test compounds were use d to monitor alterations in membrane fluidity at graded depths in lipid bil ayer. The results of the study suggest that the flavonoids and isoflavonoid s, similar to cholesterol and alpha-tocopherol, partition into the hydropho bic core of the membrane and cause a dramatic decrease in lipid fluidity in this region of the membrane. Localization of flavonoids and isoflavonoids into the membrane interiors and their resulting restrictions on fluidity of membrane components could sterically hinder diffusion of free radicals and thereby decrease the kinetics of free radical reactions. (C) 2000 Academic Press.