INTERACTIONS BETWEEN PRESSURE AND ETHANOL ON THE FORMATION OF INTERDIGITATED DPPC LIPOSOMES - A STUDY WITH PRODAN FLUORESCENCE

Steady-state fluorescence of 6-propionyl-2-(dimethylamino)naphthalene (Prodan) has been employed to study the interacting effects between ethanol and pressure on the formation of the fully interdigitated dipalmitoylphosphatidylcholine (DPPC). At 1 atm and 20-degrees-C, a dramatic change in the emission spectrum of Prodan fluorescence is observed at about 1.1-1.3 M ethanol. The emission maximum shifts to longer wavelengths, and the intensity ratio of Prodan fluorescence at 435 nm to that at 510 nm, F435/F510, decreases abruptly with increasing ethanol content. The spectral changes are correlated to the ethanol-induced phase transition of DPPC from the noninterdigitated gel state to the fully interdigitated gel state [Rowe, E.S. (1983) Biochemistry 22, 3299-3305; Simon, S.A., & McIntosh, T.J. (1984) Biochim. Biophys. Acta 773, 169-172]. The spectral changes are attributed to the probe relocation from a less polar environment to a more polar environment due to lipid interdigitation. This relocation is either due to the bulky terminal methyl group of the lipids or due to the partition of Prodan into the bulk solution or both. The present study demonstrates that Prodan is a useful probe in monitoring the formation of the ethanol-induced fully interdigitated DPPC gel phase. Pressure is found to produce spectral changes similar to those induced by ethanol when the ethanol content amounts to 0.8-1.1 M. At lower (e.g., < 0.4 M) and higher ethanol (e.g., > 2.4 M) concentrations, pressure is unable to induce such spectral changes. The critical ethanol concentrations for the formation of the fully interdigitated DPPC gel phase (Cr) have been determined. Cr decreases with increasing pressure in a nonlinear manner, and the Cr's at 50-degrees-C are less than those at 20-degrees-C. These results suggest that high pressure and high temperature assist ethanol in forming the fully interdigitated gel phase in DPPC. The results imply that ethanol "toxicity" as a result of lipid interdigitation can be enhanced under pressure.