Studies of the orientational order and bilayer thickness in biological and model membranes

The hydrocarbon order of a membrane bilayer in the hydrocarbon region is suggested to play a fundamental role in maintaining functional integrity in the biological membrane system, Acholeplasma laidlawii strain B. The hydrocarbon order profile, which can be considered to be linearly related to hydrophobic thickness, has been measured by211 NMR methods for a range of Acholeplasma laidlawii membranes containing exogenously incorporated perdeuteriated palmitic acid and a second fatty acid of increasing unsaturation. The microorganism was grown under conditions where de novo fatty acid biosynthesis was suppressed. At 37°C, the growth temperature, there exists a range of hydrocarbon order compatible with good growth characteristics of the microorganism and outside of which the organism grows poorly or not at all. When grown in the presence of cholesterol which is known to increase the orientational order in the hydro-carbon region, it appears that a small fraction of the cholesterol is solubilized by the membrane. A significant fraction of membrane cholesterol, however, is excluded from the lipid bilayer of the microorganism or is in a membrane domain separate from the rest of the lipid. 211 NMR measurements show that this pool of cholesterol is solid –like or crystalline but is available for membrane incorporation using several solubilization methods. It is suggested that the range of hydrocarbon order compatible with good A.laidlawii growth characteristics is maintained even in the presence of cholesterol. Using model membrane systems, T2 relaxation anisotropy measurements have been made for multilamellar vesicles and macroscopically aligned multibilayers by 211 NMRquadrupolar echo techniques. The dominant relaxation mechanism in the multilamellar system is found to be fundamentally different from that in the macroscopically aligned multi bilayers as suggested by the T2 anisotropy found for each. The multi lamellarvesicles show an orientation dependence consistent with such mechanisms as collective lipid motions or surface undulations. The macroscopically aligned multi-layers, however, appear to damp out several of these motional modes as suggested by the anisotropy and also an increase in the magnitudes of the relaxation times obtained. A phenomenological theory developed on the basis of similar experimental results suggests that fluctuations in bilayer thickness could be the mechanism responsible for T2relaxation.Medicine, Faculty ofBiochemistry and Molecular Biology, Department ofGraduat