Physical properties of archaeal tetraether lipid membranes as revealed by differential scanning and pressure perturbation calorimetry, molecular acoustics, and neutron reflectometry: effects of pressure and cell growth temperature.

The polar lipid fraction E (PLFE) is a major tetraether lipid component in the thermoacidophilic archaeon Sulfolobus acidocaldarius. Using differential scanning and pressure perturbation calorimetry as well as ultrasound velocity and density measurements, we have determined the compressibilities and volume fluctuations of PLFE liposomes derived from different cell growth temperatures (T(g) = 68, 76, and 81 °C). The compressibility and volume fluctuation values of PLFE liposomes, which are substantially less than those detected from diester lipid membranes (e.g., DPPC), exhibit small but significant differences with T(g). Among the three T(g)s employed, 76 °C leads to the least compressible and most tightly packed PLFE membranes. This temperature is within the range for optimal cell growth (75-80 °C). It is known that a decrease in T(g) decreases the number of cyclopentane rings in archael tetraether lipids. Thus, our data enable us to present the new view that membrane packing in PLFE liposomes varies with the number of cyclopentane rings in a nonlinear manner, reaching maximal tightness when the tetraether lipids are derived from cells grown at optimal T(g)s. In addition, we have studied the effects of pressure on total layer thickness, d, and neutron scattering length density, ρ(n), of a silicon-D(2)O interface that is covered with a PLFE membrane using neutron reflectometry (NR). At 55 °C, d and ρ(n) are found to be rather insensitive to pressure up to 1800 bar, suggesting minor changes of the thickness of the membrane's hydrophobic core and headgroup orientation upon compression only.

Lipid composition of thermophilic Geobacillus sp. strain GWE1, isolated from sterilization oven.

GWE1 strain is an example of anthropogenic thermophilic bacterium, recently isolated from dark crusty material from sterilization ovens by Correa-Llantén et al. (Kor. J. Microb. Biotechnol. 2013. 41(3):278-283). Thermostability is likely to arise from the adaptation of macromolecules such as proteins, lipids and nucleic acids. Complex lipid arrangement and/or type in the cell membrane are known to affect thermostability of microorganisms and efforts were made to understand the chemical nature of the polar lipids of membrane. In this work, we extracted total lipids from GWE1 cell membrane, separated them by TLC into various fractions and characterize the lipid structures of certain fractions with analytical tools such as (1)H, (13)C, (31)P and 2D NMR spectroscopy, ATR-FTIR spectroscopy and MS(n) spectrometry. We were able to identify glycerophosphoethanolamine, glycerophosphate, glycerophosphocholine, glycerophosphoglycerol and cardiolipin lipid classes and an unknown glycerophospholipid class with novel MS/MS spectra pattern. We have also noticed the presence of saturated iso-branched fatty acids with NMR spectra in individual lipid classes.