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

摘要

The polar lipid fraction E (PLFE) is a major tetraethernlipid component in the thermoacidophilic archaeon Sulfolobusnacidocaldarius. Using differential scanning and pressure perturbationncalorimetry as well as ultrasound velocity and density measurements,nwe have determined the compressibilities and volume fluctuations ofnPLFE liposomes derived from different cell growth temperatures (Tgn= 68, 76, and 81 °C). The compressibility and volume fluctuationnvalues of PLFE liposomes, which are substantially less than thosendetected from diester lipid membranes (e.g., DPPC), exhibit smallnbut significant differences with Tg. Among the three Tgs employed,n76 °C leads to the least compressible and most tightly packed PLFEnmembranes. This temperature is within the range for optimal cellngrowth (75−80 °C). It is known that a decrease in Tg decreases thennumber of cyclopentane rings in archael tetraether lipids. Thus, our data enable us to present the new view that membranenpacking in PLFE liposomes varies with the number of cyclopentane rings in a nonlinear manner, reaching maximal tightnessnwhen the tetraether lipids are derived from cells grown at optimal Tgs. In addition, we have studied the effects of pressure on totalnlayer thickness, d, and neutron scattering length density, ρn, of a silicon−D2O interface that is covered with a PLFE membranenusing neutron reflectometry (NR). At 55 °C, d and ρn are found to be rather insensitive to pressure up to 1800 bar, suggestingnminor changes of the thickness of the membrane’s hydrophobic core and headgroup orientation upon compression only.