Temperature, pressure and salt effects on bilayers of an acidic phospholipid, dipalmitoylphosphatidylglycerol

The effects of thermal pretreatments and a monovalent added salt (NaCl) on the thermotropic and barotropic bilayer phase behavior of an acidic phospholipid, dipalmitoylphosphatidylglycerol (DPPG), were investigated by differential scanning calorimetry, fluorometry and light transmittance measurements. At 1.0 mol kg^-1 NaCl concentration, lipid samples with static annealing treatments (i.e., cold storage) showed a transition from a metastable hydrated crystalline phase to the lamellar gel phase in addition to the pre- and the main transition. These transition temperatures increased with pressure and the interdigitated gel phase was induced under high pressure, which is similar to the bilayer phase behavior of dipalmitoylphosphatidylcholine. By contrast, lipid samples with dynamic annealing treatments (i.e., repeated freeze-thaw cycles) exhibited only a transition from the stable hydrated crystalline phase to the liquid crystalline phase accompanied by the morphological change in the bilayer aggregate. The transition temperature also increased with pressure and the phase behavior resembled that of dipalmitoylphosphatidylethanolamine. Regarding the salt effect, we found that the temperature-pressure phase diagrams of the non-annealed DPPG bilayer changed systematically with the salt concentration, demonstrating that the pressure-induced interdigitation is suppressed by the added salt. Thermodynamic quantities of the phase transitions increased with increasing salt concentration, suggesting that the DPPG bilayer was stabilized by the weakening of the electrostatic repulsion among the polar head groups due to the shielding effect. The concentration dependence of the minimum interdigitation pressure indicates the possibility that the interdigitation should occur even under atmospheric pressure in the absence of the salt.