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

Abstract

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 °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 QII D and QII P with lattice constants of 105 and 82 Å, respectively. At atmospheric pressure, the vibrational frequency of the CH2 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 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 ∼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 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.

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

“…The broad exothermic transition at 78.5°C in Scan 1 (Fig. 1) corresponds to the phase transition from lamellar to probably inverted bicontinuous cubic phases (Q D II and Q P II ) previously detected at ;74-75°C by small-angle x-ray scattering (22). The broadness of this transition may be attributed to the coexistence of Q D II and Q P II , and the chemical heterogeneity of PLFE.…”

Archaebacterial bipolar tetraether lipids: Physico-chemical and membrane properties

“…PLFE liposomes exhibit two thermal-induced lamellar-to-lamellar phase transitions at ∼47-50 • C and ∼60 • C (Bagatolli et al, 2000;Chong et al, 2003Chong et al, , 2005Gliozzi et al, 2002) and a lamellar-tocubic phase transition at ∼74-78 • C (Chong et al, , 2005, all of which involve small or no volume changes (Chong et al, 2005). A variety of pressure-induced gel-like phases have been detected by high-pressure Fourier transform infrared spectroscopy (Chong et al, 2005).…”

“…BTL assemblies hold great promises for technological applications. The ability to form cubic phases (e.g., Chong et al, 2003) makes BTL lipids appealing for crystallization of membrane-bound proteins (Landau and Rosenbusch, 1996), especially those found in hyperthermophiles. BTL have been used to modify the surface properties of nanoporous aluminum oxide membranes to change their filtration characteristics suitable for sterilization (Muller et al, 2006).…”

Homeoviscous Adaptation of Membranes in Archaea

“…0-100 ı C (Dannenmuller et al 2000;Stewart et al 1990). Small phase transitions have been observed in liposomes of S. acidocaldarius between 42 ı C and 69 ı C (Chong et al 2005;Chong et al 2003). These phase transitions are associated to very small volume changes, which suggests that the polar headgroup region of the lipid may still be rigid and tightly packed through hydrogen-bond network at elevated temperatures.…”