Ground‐ and excited‐state proton transfer and antioxidant activity of 3‐hydroxyflavone in egg yolk phosphatidylcholine liposomes: absorption and fluorescence spectroscopic studies

Excited‐state intramolecular proton transfer (ESIPT) and dual luminescence behaviour of 3‐hydroxyflavone (3‐HF) have been utilized to monitor its binding to liposomal membranes prepared from egg yolk phosphatydilcholine (EYPC). Additionally, absorption spectrophotometric assay has been performed to evaluate the antioxidant activity of 3‐HF against lipid peroxidation in this membrane system. When 3‐HF molecules are partitioned into EYPC liposomes, a weak long‐wavelength absorption band with λ ～410 nm appears in addition to the principal absorption at ～λ = 345 nm. Selective excitation of the 410 nm band produces the characteristic emission (λ～460 nm) of the ground‐state anionic species, whereas excitation at the higher energy absorption band leads to dual emission with predominatly ESIPT tautomer fluorescence (λ = 528 nm). Both ESIPT tautomer and the anionic species exhibit fairly high fluorescence anisotropy (r) values (r = 0.122 and 0.180, respectively). Biexponential fluorescence decay kinetics are observed for the ESIPT tautomer as well as the ground‐state anionic forms, indicating heterogeneity in the microenvironments of the corresponding emitting species. Furthermore, we demonstrate that lipid peroxidation of EYPC liposomes is significantly inhibited upon 3‐HF binding, suggesting that 3‐HF can be potentially useful as an inhibitor of peroxidative damage of cell membranes. Copyright © 2008 John Wiley & Sons, Ltd.