Excited state proton transfer fluorescence of 3-hydroxyflavone in model membranes

3-Hydroxyflavone (3HF), the basic structural moiety of an important group of naturally occurring, biologically active flavonoids, has attracted extensive attention for its intramolecular excited-state proton-transfer (ESPT) and dual fluorescence characteristics. We report here, for the first time, its fluorescence (emission, excitation, polarization anisotropy and lifetime) behaviour in model membranes consisting of small, unilamellar liposomes of synthetic phosphatidylcholine (dimyristoyl phosphatidylcholine (DMPC) and dipalmitoyl phosphatidylcholine (DPPC)). The emission spectrum is conspicuously dominated by the ESPT tautomer fluorescence band. This result, along with other relevant data, indicates that the molecules are embedded in the lipid bilayer matrix facing environments where external H-bonding perturbation effects are minimized. The anisotropy (r) versus temperature (T) profiles dramatically reveal the thermotropic gel-liquid crystalline phase transition properties of the phospholipids, signifying a novel application of the intrinsic (tautomer) fluorescence of 3HF. (C) 1997 Elsevier Science B.V.