Pressure effects on the physical properties of lipid bilayers detected by trans-parinaric acid fluorescence decay.

The effects of hydrostatic pressure on the physical properties of large unilamellar vesicles of single lipids dipalmitoyl phosphatidylcholine (DPPC) and dimyristoyl phosphatidylcholine (DMPC) and lipid mixtures of DMPC/DPPC have been studied from time-resolved fluorescence of trans-parinaric acid. Additional experiments were carried out using diphenylhexatriene to compare the results extracted from both probes. Fluorescence decays were analyzed by the maximum entropy method. Pressure does not influence the fluorescence lifetime distribution of trans-parinaric acid in isotropic solvents. However, in pressurized lipid bilayers an abrupt change was observed in the lifetime distribution which was associated with the isothermal pressure-induced phase transition. The pressure to temperature equivalence values, dT/dP, determined from the midpoint of the phase transitions, were 24 and 14.5 degrees C kbar-1 for DMPC and POPC, respectively. Relatively moderate pressures of about 500 bar shifted the DMPC/DPPC phase diagram 11.5 degrees C to higher temperatures. The effects of pressure on the structural properties of these lipid vesicles were investigated from the anisotropy decays of both probes. Order parameters for all systems increased with pressure. In the gel phase of POPC the order parameter was smaller than that obtained in the same phase of saturated phospholipids, suggesting that an efficient packing of the POPC hydrocarbon chains is hindered.