Pressure Perturbation and Differential Scanning Calorimetric Studies of Bipolar Tetraether Liposomes Derived from the Thermoacidophilic Archaeon Sulfolobus acidocaldarius

Differential scanning calorimetry (DSC) and pressure perturbation calorimetry (PPC) were used to characterize thermal phase transitions, membrane packing, and volumetric properties in multilamellar vesicles (MLVs) composed of the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius grown at different temperatures. For PLFE MLVs derived from cells grown at 78°C, the first DSC heating scan exhibits an endothermic transition at 46.7°C, a small hump near 60°C, and a broad exothermic transition at 78.5°C, whereas the PPC scan reveals two transitions at ∼45°C and 60°C. The endothermic peak at 46.7°C is attributed to a lamellar-to-lamellar phase transition and has an unusually low ΔH (3.5 kJ/mol) and ΔV/V (0.1%) value, as compared to those for the main phase transitions of saturated diacyl monopolar diester lipids. This result may arise from the restricted trans-gauche conformational changes in the dibiphytanyl chain due to the presence of cyclopentane rings and branched methyl groups and due to the spanning of the lipid molecules over the whole membrane. The exothermic peak at 78.5°C probably corresponds to a lamellar-to-cubic phase transition and exhibits a large and negative ΔH value (−23.2 kJ/mol), which is uncommon for normal lamellar-to-cubic phospholipid phase transformations. This exothermic transition disappears in the subsequent heating scans and thus may involve a metastable phase, which is irreversible at the scan rate used. Further, there is no distinct peak in the plot of the thermal expansion coefficient α versus temperature near 78.5°C, indicating that this lamellar-to-cubic phase transition is not accompanied by any significant volume change. For PLFE MLVs derived from cells grown at 65°C, similar DSC and PPC profiles and thermal history responses were obtained. However, the lower growth temperature yields a higher ΔV/V (∼0.25%) and ΔH (14 kJ/mol) value for the lamellar-to-lamellar phase transition measured at the same pH (2.1). A lower growth temperature also generates a less negative temperature dependence of α. The changes in ΔV/V, ΔH, and the temperature dependence of α can be attributed to the decrease in the number of cyclopentane rings in PLFE at the lower growth temperature. The relatively low ΔV/V and small ΔH involved in the phase transitions help to explain why PLFE liposomes are remarkably thermally stable and also echo the proposal that PLFE liposomes are generally rigid and tightly packed. These results help us to understand why, despite the occurrence of thermal-induced phase transitions, PLFE liposomes exhibit a remarkably low temperature sensitivity of proton permeation and dye leakage.