PRODAN AS A MEMBRANE-SURFACE FLUORESCENCE PROBE - PARTITIONING BETWEEN WATER AND PHOSPHOLIPID PHASES

Fluorescence spectra features of 6-propionyl-2-dimethylaminonaphthalen e (Prodan) in phospholipid vesicles of different phase states are inve stigated. Like the spectra of 6-lauroyl-2-dimethylaminonaphthalene (La urdan), the steady-state excitation and emission spectra of Prodan are sensitive to the polarity of the environment, showing a relevant shif t due to the dipolar relaxation phenomenon. Because of the different l engths of their acyl residues, the partitioning of the two probes betw een water and the membrane bilayer differs profoundly. To account for the contribution of Prodan fluorescence arising from water, we introdu ce a three-wavelength generalized polarization method that makes it po ssible to separate the spectral properties of Prodan in the lipid phas e and in water, and to determine the probe partitioning between phosph olipid and water and between the gel and the liquid-crystalline phases of phospholipids. In contrast to Laurdan, Prodan preferentially parti tions in the liquid-crystalline phase with respect to the gel and is s ensitive to the polar head pretransition, and its partition coefficien t between the membrane and water depends on the phase state, i.e., on the packing of the bilayer. Prodan is sensitive to polarity variations occurring closer to the bilayer surface than those detected by Laurda n.