MEMBRANE THICKNESS AND MOLECULAR ORDERING IN ACHOLEPLASMA-LAIDLAWII STRAIN-A STUDIED BY H-2 NMR-SPECTROSCOPY

Since Acholeplasma laidlawii can be restricted to incorporating fatty acids from the growth medium into its membrane lipids, it is possible to study the effects of the length of the acyl chains on the propertie s of the membrane of the organism. A. laidlawii strain A-EF22 was grow n with mixtures of one perdeuterated saturated fatty acid and one mono unsaturated fatty acid. The average length (<C-n>) of the acyl chains in the membrane Lipids varied from 14.6 to 19.9, and the degree of uns aturation ranged from 21 to 79 mol %. H-2 nuclear magnetic resonace (N MR) spectra were recorded on whole cells, on intact membranes, and on lipids extracted from these membranes. It was found that the NMR spect ra for all three cases were very similar, yielding deuterium quadrupol ar splittings typical for the lamellar liquid-crystalline phase (L(alp ha)) found in model membrane systems. The use of a perdeuterated acyl chain as a reporter molecule allowed for the calculation of order para meters averaged over the entire system. These measurements yielded a w ide range of average order parameters varying from 0.136 to 0.186 for the membranes and from 0.137 to 0.181 for the extracted lipids. From t he order parameters the average acyl chain length can be calculated, w hich is related to the average membrane thickness. This value ranged f rom 23.2 to 30.6 Angstrom. When either the order or the membrane thick ness of the intact membranes was compared to that of the extracted lip ids, only slight or even undetectable differences were found. This imp lies that the proteins associated with the membranes do not have any l arge effect on the overall packing of the membrane lipids, even though the membrane thickness varied by approximately 8 Angstrom over the se ries studied. A decrease in the ordering of the acyl chains was observ ed when the length of the acyl chains incorporated from the growth med ium was increased in either the membranes or the extracted lipids. Thi s decrease correlated with the decrease in the fraction of monoglucosy ldiacylglycerol (MGlcDAG) found in the membrane. Since both the averag e order and the membrane thickness varied, it is proposed that by chan ging the mole fraction of MGlcDAG the organism regulates either the me mbrane curvature energy or the permeability, both of which are related to lipid packing in the bilayer.