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 % D2O showed two orders of lamellar Bragg reflections over the temperature range 5 -75 degreesC. The lamellar repeat distances d indicate three lamellar regions: 5-50 degreesC (d, similar to49-50 Angstrom), 50-60 degreesC (similar to50-54 Angstrom), and 60-74 degreesC (similar to54-56 Angstrom). The 4-Angstrom increase in d-spacing from 50 to 60 degreesC 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 degreesC, 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 Angstrom, respectively. At atmospheric pressure, the vibrational frequency of the CH2 symmetric stretching mode showed transitions at similar to46-48, 61, and 74 degreesC, in agreement with the SAXS data. The overall 2 cm(-1) increase in wavenumber between 61 and 74 degreesC indicates a transition from a rigid, gellike lipid conformation to a chain conformation with considerable disorder, as expected from cubic phases which appear above similar to74 degreesC. The pressure dependence (up to 15 kbar) of the symmetric CH2 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 similar to2 cm(-1) increase in wavenumber, which appears at 8.0 kbar at 60 degreesC, 8.4 kbar at 43 degreesC, and 10.8 kbar at 20 degreesC, giving an unusual negative dT/dp 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.