ESR studies on the lipid bilayers of separated outer and cytoplasmic membranes of a moderately halophilic bacterium.

Abstract

Electron spin resonance (ESR) studies were performed on the isolated outer and cytoplasmic membranes from cells of a moderately halophilic bacterium Pseudomonas halosaccharolytica ATCC 29423, which were grown in 2 M NaCl medium at different temperatures. The cytoplasmic membrane contained more acidic phospholipids (50% or more; including most of the diphosphatidylglycerol) than the outer membrane (40%) in 25 and 30 °C grown cells, while in both membranes of 35 °C grown cells the relative content of acidic phospholipids was almost the same. With elevated growth temperature C19-cyclopropanoic fatty acids in both membranes increased.The physical properties of lipid bilayers of both membranes were studied by a spin-label method using N-oxyl spin-labelled stearic acids. Isolated membranes suspended in a medium with or without 2 M NaCl showed no appreciable differences in ESR spectra on changing the sodium chloride concentration in the medium. The lipid bilayers of both membranes gave almost uniform rotational correlation times for the first 12 carbon atoms of the hydrocarbon chains starting from the polar surface of the bilayers. The difference between rotational correlation times of either membrane and those of whole cells and liposomes of extractable lipids of this bacterium was that the rotational correlation times of both membranes gave similar absolute values regardless of change in the growth temperature and varied similarly with the temperature of ESR measurements. The surface lipid layers up to C12 had a phase transition temperature near the growth temperature of the bacterium. The rotational activation energies were 5–10 kcal/mol (1 cal = 4.1868 J) for the liquid phase and 10–25 kcal/mol for the solid phase. In general, the order parameters of both membranes decreased gradually with increasing temperature of ESR measurements, except for rapid decreases and discontinuous drops near the growth temperature with the spin-probe labelled at C12 of stearic acid (12NS). Such discontinuous decreases in order parameter of 12NS were not observed for the whole cell membrane nor for liposomes of total extractable lipids of this bacterium.