Effect of pressure on the Prodan fluorescence in bilayer membranes of phospholipids with varying acyl chain lengths

The fluorescence spectra of 6-propionyl-2-(dimethylamino)naphthalene (Prodan) were observed as a function of pressure for the bilayer membrane systems of dilauroylphosphatidylcholine (DLPC), dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), and distearoylphosphatidylcholine (DSPC). The wavelength of the emission maximum, λmax, was found to be 480, 430, and 500 nm for the liquid crystalline (Lα), ripple gel (P′β), and pressure-induced interdigitated gel (LβI) phase, respectively. Since the λmax reflects the solvent property around the probe molecules, we could speculate on the location of the Prodan molecules in the bilayer membranes; in the Lα phase of the lipid bilayer, the Prodan molecules distribute around the phosphate of the lipids (i.e. the polar region). The Lα/P′β phase transition caused the Prodan molecules to move into the less polar region near the glycerol backbone. The fluorescence intensity of the Prodan in the P′β phase was dependent on the chain length of the lipids and on pressure; the shorter the chain length of the lipid, the stronger the fluorescence intensity of the Prodan. Moreover, for the DLPC bilayer membrane system, the fluorescence intensity at 430 nm increased with increasing pressure, indicating that the partition of Prodan into the DLPC bilayer membrane is promoted by applying pressure. In the case of the DPPC and DSPC bilayers, as the pressure increased further, the pressure-induced interdigitation caused the Prodan molecules to squeeze out of the glycerol backbone region and to move the hydrophilic region near the bilayer surface. The ratio of fluorescence intensity at 480 nm to that at 430 nm, F480/F430, showed a sharp change at the phase-transition pressure. In the case of the DPPC and DSPC bilayers, the values of F480/F430 showed an abrupt increase above a certain pressure higher than the Lα/P′β transition pressure, which corresponds to the interdigitation from the P′β to the LβI phase. The plot of F480/F430 versus pressure is available for recognition of the bilayer phase transitions, especially the bilayer interdigitation.