Antioxidant activity of daidzein, a natural antioxidant, and its spectroscopic properties in organic solvents and phosphatidylcholine liposomes.

Daidzein, one of major isoflavones found in soybeans, has a wide spectrum of physiological and pharmacological functions. The observed biological effects involve its interactions with lipid bilayers, usually detected by indirect methods. In this study we use the native fluorescence of daidzein to report changes observed during its interactions with organic solvents and in a phosphatidylcholine membrane. We have investigated interactions of daidzein with lipid bilayers of egg phosphatidylcholine (PC) by absorption and fluorescence methods. The data obtained indicate emission arises from the conjugate anion in excited singlet state. The fluorescence is found to increase with the basicity of the solution and the polarity of the solvent. An increase in fluorescence anisotropy in the presence of membranes suggests partial incorporation of daidzein molecules into the bilayer. Two fluorescence lifetime components, 1.5 ns and 3.5 ns, reflects the partition of daidzein between aqueous and membrane environments, respectively. On the basis of the obtained spectroscopic data we conclude that up to 15% of daidzein is located in hydrophilic region of the membrane whereas the rest is distributed in aqueous bulk and aqueous/membrane interface. For studying the antioxidant activity of daidzein against lipid peroxidation initiated by AAPH the molecule of C11-BODIPY581/591 has been used as a fluorescent oxidation indicator. The results show that the presence of daidzein anions in the membrane interface increases the inhibitory effect on lipid peroxidation compared to the neutral form of daidzein.