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 larg e unilamellar vesicles of single lipids dipalmitoyl phosphatidylcholin e (DPPC) and dimyristoyl phosphatidylcholine (DMPC) and lipid mixtures of DMPC/DPPC have been studied from time-resolved fluorescence of tra ns-parinaric acid. Additional experiments were carried out using diphe nylhexatriene to compare the results extracted from both probes. Fluor escence decays were analyzed by the maximum entropy method. Pressure d oes not influence the fluorescence lifetime distribution of trans-pari naric acid in isotropic solvents. However, in pressurized lipid bilaye rs an abrupt change was observed in the lifetime distribution which wa s associated with the isothermal pressure-induced phase transition. Th e pressure to temperature equivalence values, dT/dP, determined from t he midpoint of the phase transitions, were 24 and 14.5-degrees-C kbar- 1 for DMPC and POPC, respectively. Relatively moderate pressures of ab out 500 bar shifted the DMPC/DPPC phase diagram 11.5-degrees-C to high er temperatures. The effects of pressure on the structural properties of these lipid vesicles were investigated from the anisotropy decays o f both probes. Order parameters for all systems increased with pressur e. 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.