Black lipid membranes of tetraether lipids from Thermoplasma acidophilum

Abstract

Black lipid membranes were formed of tetraether lipids from Thermoplasma acidophilum and compared to the bilayer forming lipids diphytanoylphosphatidylcholine and diphythanylglucosylglycerol. Bilayer-forming lipids varied in thickness of black lipid membranes due to the organic solvent used. Measurements of the specific membrane capacitance ( C m = 0.744 μF/cm 2) showed that the membrane-spanning tetraether lipids from Thermoplasma acidophilum form a monolayer of a constant thickness of 2.5–3.0 nm no matter from which solvent. This finding corresponds to the results of Gliozzi et al. [1] for the lipids of another archaebacterium, Sulfolobus solfataricus. Black lipid membranes were formed at room temperature with a torus from bilayer-forming lipids, however, the torus could also be formed by the tetraetherlipid itself at room temperature and at defined concentration. In these stable black lipid membranes, conductance was measured in the presence of valinomycin, nonactin, and gramicidin. At 10 −7 M concentration, valinomycin mediated higher conductance in membranes from tetraether lipids (200–1200 μS/cm 2) than from bilayer-forming lipids (125–480 μS/cm 2). Nonactin, at 10 −6 M concentration, mediated a 6-fold higher conductance in a tetraether lipid membrane than in a bilayer, whereas conductance, in the presence of 5 · 10 −11 M gramicidin could reach higher values in bilayers than in tetraether lipid monolayers of comparable thickness. Monensin did not increase the conductance of black lipid membranes from tetraether lipids under all conditions applied in our experiments. Poly( l-lysine) destroyed black lipid membranes. Lipopolysaccharides from Thermoplasma acidophilum were not able to form stable black lipid membranes by themselves. The lipopolysaccharide complexes from Thermoplasma acidophilum and from Escherichia coli decreased the valinomycin-mediated conductance of monolayer and bilayer membranes. This influence was stronger than that of the polysaccharide dextran.