Effect of fatty acyl chain length on some structural and functional parameters of Acholeplasma membranes

Two membrane preparations from Acholeplasma laidlawii were obtained by supplementing the growth media with either arachidic or lauric acid. The cells grown with arachidic acid supplementation yielded membrane lipids greatly enriched with the arachidoyl group and those supplemented with lauric acid yielded membrane lipids enriched with lauroyl, myristoyl, and palmitoyl groups. The cell size (0.1–0.7 μm), the membrane thickness (70 ± 14 Å), and the cell shape (coccoid) showed no difference for these two preparations. The arachidoyl-enriched membrane had a greater buoyant density (1.195 g/ml vs. 1.189 g/ml), a smaller permeability to glycerol, and a greater sensitivity to osmotic shock compared to the membrane enriched with shorter acyl groups. The spin label 2-(10-carboxydecyl)-2-hexyl-4,4-dimethyl-3-oxazolidinyloxyl is less mobile in the arachidoyl-enriched membrane (rotational correlation time at 22 °C, τc = 10 ns) than in the shorter acyl groups enriched membrane (τc = 6 ns). This difference in apparent membrane fluidity can account for the differences observed for the membrane properties.