β-Carotene as a membrane antioxidant probed by cholesterol-anchored daidzein.

β-Carotene is found to be more effective as an antioxidant in phosphatidylcholine (PC) liposomes when protecting against hydrophilic radicals compared to lipophilic radicals, as measured by the rate of formation of conjugated dienes. Daidzein alone is without effect, but decreases the antioxidative effect of β-carotene for hydrophilic initiation and increases the effect for lipophilic initiation. The newly synthesized 7-cholesterylglycol daidzein has the opposite effect for β-carotene as antioxidant, with a strong enhancement for hydrophilic initiation and a slight decrease for lipophilic initiation. Redistributing β-carotene to membrane surfaces by cholesterol-anchoring of daidzein enhances protection against aqueous radicals significantly at the expense of protection against lipid-derived radicals. Anchoring of daidzein to cholesterol is concluded to be useful as a mechanistic tool for controlling antioxidant distribution in membranes sensitive to radical damage, as supported by quantum mechanical calculation within the density function theory and further supported by fluorescence probes and fluorescence polarization probes.