Structure and Conformation of Bipolar Tetraether Lipid Membranes Derived from Thermoacidophilic ArchaeonSulfolobus acidocaldariusas Revealed by Small-Angle X-ray Scattering and High-Pressure FT-IR Spectroscopy

The phase behavior, conformation, and structure of bipolar tetraether liposomes composed of the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius have been studied by small-angle X-ray scattering (SAXS) and high-pressure Fourier transform infrared spectroscopy (FT-IR) at pD 2.15. The SAXS data on PLFE multilamellar vesicles in 85 wt % D 2 O showed two orders of lamellar Bragg reflections over the temperature range 5−75 °C. The lamellar repeat distances d indicate three lamellar regions: 5−50 °C ( d, ∼49−50 Å), 50−60 °C (∼50−54 Å), and 60−74 °C (∼54−56 Å). The 4-Å increase in d -spacing from 50 to 60 °C is probably due to an increase in hydration at the PLFE polar headgroups and/or a stretch of the polar headgroups toward the bulk aqueous phase. At 74−75 °C, a lamellar-to-cubic phase transition occurs. The reciprocal spacings of the cubic phases correspond to the coexistence of the inverse bicontinuous cubic phases Q II D and Q II P with lattice constants of 105 and 82 Å, respectively. At atmospheric pressure, the vibrational frequency of the CH 2 symmetric stretching mode showed transitions at ∼46−48, 61, and 74 °C, in agreement with the SAXS data. The overall 2 cm -1 increase in wavenumber between 61 and 74 °C indicates a transition from a rigid, gellike lipid conformation to a chain conformation with considerable disorder, as expected from cubic phases which appear above ∼74 °C. The pressure dependence (up to 15 kbar) of the symmetric CH 2 stretching vibrational wavenumber at various temperatures has also been examined. A variety of new gellike phases are observed at elevated pressures, showing again a rich polymorphism in PLFE liposomes. The most prominent pressure-induced transition involves a ∼2 cm -1 increase in wavenumber, which appears at 8.0 kbar at 60 °C, 8.4 kbar at 43 °C, and 10.8 kbar at 20 °C, giving an unusual negative d T /d p value. The possible source of this anomaly has been discussed in terms of the temperature attenuation of the hydrogen bond network in the polar headgroup region.