Structural analysis of hydrated egg lecithin and cholesterol bilayers. I. X-ray diffraction

The importance of the phospholipid bilayer and the length of the cholesterol molecule in membrane structure

The properties of mixtures of phosphatidylcholine and analogues of cholesterol bearing side chains of varying lengths were examined by a variety of methods. The incorporation of the analogues into sonicated liposomes and their effect on the rate of osmotic shrinking of multilamellar liposomes were determined. The ordering of a steroid spin label was studied in an oriented multibilayer system and the effect of the analogues on the phase transition of dipalmitoyl phosphatidylcholine monitored using the spin label TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl). Mixtures of analogues and phospholipid were also studied in monolayers. In all the bilayer systems studied cholesterol caused the greatest 'rigidifying' effect, the analogues with shorter or longer side chains being less effective. However, in the monolayer experiments the length of the sterol molecule was found to be much less critical. It is suggested that cholesterol is anchored in position in a phospholipid bilayer by virtue of the molecule being the precise length required to maximise interactions between neighbouring molecules without disturbing the bilayer structure.

Threshold effects on the lectin-mediated aggregation of synthetic glycolipid-containing liposomes

Cholesterol analogs containing sugar residues linked by spacer groups to the cholesterol O can be incorporated into egg yolk lecithin small unilamellar liposomes. The synthetic glycolipid analogs distribute evenly on both sides of the bilayer. These liposomes are aggregated by the appropriate lectin. For example, when the sugar residue is a beta-galactoside the liposomes are aggregated by ricin and when it is an alpha-mannoside they are aggregated by Con A. The lectin-mediated aggregation of these liposomes is reversed by the addition of the appropriate sugar. The rates but not the extents of aggregation of these liposomes are highly sensitive to the amount of glycolipid incorporated. Below approximately 5% glycolipid incorporation the rate of the lectin-mediated aggregation of these liposomes is exceedingly slow, whereas above this level rapid aggregation proceeds. At all concentrations studied the synthetic glycolipids are incorporated in a unimodal fashion so that the observed threshold effects cannot be based on possible differences in the manner in which the glycolipids are incorporated at different concentrations. This conclusion is based on 1) studies with galactose oxidase that show that the percentage of galactose oxidation in a liposome prepared from a galactosyl-containing glycolipid is independent of glycolipid concentration, and 2) studies on the aggregation of liposomes containing mixed glycolipids in which the glycolipids are shown to behave independently. The importance of a critical density of membrane-bound receptors in order for aggregation to occur is discussed.

Effect of cholesterol on the molecular motion in the hydrocarbon region of lecithin bilayers studied by nanosecond fluorescence techniques

Effects of cholesterol on the dynamic structure of the hydrocarbon region of dipalmitoyllecithin vesicles were examined. Decays of the emission anisotropy and the fluorescence intensity of 1,6-diphenyl-1,3,5-hexatriene embedded in lecithin-cholesterol vesicles were measured over a temperature range of 10--60 degrees C. The emission anisotropy decreased rapidly with time and then leveled off. The rotational motion of the probe was analyzed by a model of wobbling diffusion confined in a cone. Cholesterol (10--50 mol%) decreased the cone angle in the liquid-crystalline phase and increased it in the gel phase. In the presence of 33 mol% cholesterol, the wobbling diffusion constant increased in the gel phase and changed little in the liquid-crystalline phase. The viscosity in the cone decreased in the gel phase and remained almost unchanged in the liquid-crystalline phase in the presence of 33 mol% cholesterol. The total fluorescence intensity followed a singel exponential decay independently of the cholesterol content 0--50 mol%.

The effect of cholesterol on the structure of phosphatidylcholine bilayers

The effect of cholesterol on the structure of phosphatidylcholine bilayer was investigated by X-ray diffraction methods. Electron density profiles at 5 A resolution along with chain tilt and chain packing parameters were obtained and compared for phosphatidylcholine/cholesterol bilayers and for pure phosphatidylcholine bilayers in both the gel and liquid crystalline states. The cholesterol in the bilayers was localized by noting the position of discrete elevations in the electron density profiles. Cholesterol can either increase or decrease the width of the bilayer depending on the physical state and chain length of the lipid before the introduction of cholesterol. For saturated phosphatidylcholines containing 12--16 carbons per chain, cholesterol increases the width of the bilayer as it removes the chain tilt from gel state lipids or increases the trans conformations of the chains for liquid crystalline lipids. However, cholesterol reduces the width of 18 carbon chain bilayers below the phase transition temperature as the long phospholipid chains must deform or kink to accomodate the significantly shorter cholesterol molecule. Although cholesterol has a marked effect on hydrocarbon chain organization, it was found that, within the resolution limits of the data, the phosphatidylcholine head group conformation is unchanged by the addition of cholesterol to the bilayer. The head group is oriented parallel to the plane of the bilayer for phosphatidylcholine in the gel and liquid crystalline states and this orientation is not changed by the addition of cholesterol.

Structure determination of lipid bilayers

A method of determining the phases of X-ray reflections from oriented model membrane systems at low resolution is described. The method involves deconvolution and requires that d less than or equal to 2v where v is the width of the head group region within the bilayer and d is the thickness of the bilayer. The method can be used with a single set of X-ray data and applies to lipid bilayers which have a relatively constant density in the hydrocarbon region. Phases for the first five or six orders of phosphatidylethanolamine and lecithin are derived. A refined analysis based upon deconvolution but using information inherent in the Fourier profile is also described.

Where do general anaesthetics act?

General anaesthetics were found to have no effect on lipid bilayer structures when studied using X-ray and neutron diffraction. Combined gaseous and aqueous phase solubility data suggested that the primary site of action of general anaesthetics has both polar and nonpolar characteristics, and probably involves protein.

Head-group conformation in phospholipids: a phosphorus-31 nuclear magnetic resonance study of oriented monodomain dipalmitoylphosphatidylcholine bilayers

Angular-dependent 31P NMR spectra of oriented biaxial monodomain DPPC.H2O multilayers are employed to study head-group conformation in this phospholipid. The results indicate that the O-P-O plane of the phosphate, where the O's are the nonesterified oxygens of the phosphodiester, is tilted at 47 +/- 5 degrees with respect to the bilayer normal. This PO4 orientation could result in the choline moiety being extended parallel to the bilayer plane, and it will explain the breadth of the axially symmetric 31P powder spectrum observed for DPPC in excess water. This work is the first direct observation of this conformation for lecithins and it illustrates the utility of high-resolution solid-state NMR in structural studies of disordered systems.

Optimum interaction of sterol side chains with phosphatidylcholine

The specificity of the interaction between the cholesterol side chain and egg phosphatidylcholine was precisely defined by examining the effect of three new analogues of cholesterol with modified side chains on the ordering of two steroid spin labels in liposomes. The complete side chain of cholesterol was shown to be required for maximum ordering. Sterols with side chains shorter or longer than cholesterol caused significantly less ordering.

Influence of cholesterol on the polar region of phosphatidylcholine and phosphatidylethanolamine bilayers

The structural changes in the polar head group region of unsonicated bilayer membranes of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine produced by addition of cholesterol have been determined using deuterium and phosphorus-31 NMR. Incorportion of up to 50 mol percent cholesterol produces little change in the phosphorus-31 chemical shielding anisotropies, compared with the values in pure bilayers above the phase transition temperatures, while some of the deuterium quadrupole splittings are reduced by almost a factor of two. Adjustment of the head group torsion angles by only a few degrees accounts for the observed spectral changes. Addition of cholesterol therefore has opposite effects on the hydrocarbon and polar regions of membranes: although cholesterol makes the hydrocarbon region gel-like, with an increased probability of trans conformations, the conformation of the polar head groups is very similar to that found in the liquid crystalline phase of pure phospholipid bilayers.

13C NMR studies on [4-13C] cholesterol incorporated in sonicated phosphatidylcholine vesicles

1. 90.5 MHz 13C NMR linewidth measurements were performed on mixed sonicated [4-13C] cholesterol/phosphatidylcholine vesicles of different fatty acid composition. 2. From the Dy3+ -induced shift of the C4 resonance of cholesterol it suggested that this part of the molecule is localized in the ester bond region of the bilayer. 3. The local motion of the cholesterol ring system is restricted and independent of fatty acid composition. 4. At cholesterol concentrations below 30 mol percent the ring system becomes more immobilised when the fatty acids of the phosphatidylcholine molecules enter the gel state.

Orientation and flexibility of the choline head group in phosphatidylcholine bilayers

The average orientation and flexibility of the phosphorylcholine group are deduced from deuterium and phosphorus-31 nuclear magnetic resonance measurements of unsonicated phosphatidylcholine bilayers in the liquid crystalline state. The experimental data are consistent with a model in which the polar head group exhibits a restricted flexibility characterized by rapid transitions between two enantiomeric conformations. A completely flexible or a completely rigid head group structure can be excluded. The phosphorylcholine residue is found to be bent at the position of the phosphate group, due to a gauche-gauche conformation of the phosphodiester linkage. The choline dipole is aligned parallel to the plane of the membrane, which is in agreement with X-ray and neutron diffraction studies. The average orientation of the phosphorylcholine group is therfore the same as that of the phosphorylethanolamine head group.

Configurations of fatty acyl chains in egg phosphatidylcholine-cholesterol mixed bilayers

Based on the structural properties of cholesterol and egg phosphatidylcholine, and on the assumption that the van der Waals' type attactive interaction between the steroid nucleus and the fatty acyl chains provides a stabilizing force for the cholesterol-egg phosphatidylcholine complex, some specific orientation and configurations of the fatty acyl chains around the steroid nucleus in the interacting system are proposed in terms of an optimal packing. The proposed model suggests thathe saturated chains are largely facing the flattened (alpha) surface of the steroid nucleus of cholesterol, while the unsaturated chains can interact with both the alpha and beta surfaces of the steroid nucleus. It is also suggested that the angular methyl groups on the beta surface of the steroid nucleus lock the unsaturated fatty acyl chain in a relatively immobile configuration. Experimental evidence which provides support for the proposed stereochemical model is presented.

The molecular organization of asymmetric lipid bilayers and lipid-peptide complexes

Oriented fatty acid bilayers with asymmetric distributions of lipid head group types, hydrocarbon chain lengths, and associated polypeptides have been analyzed by a combined use of high resolution electron microscopy and X-ray diffraction techniques. The exclusion of fixatives, stains, and embedding materials has made it possible to relate unequivocally microscopic images to molecular composition. The ultrastructure of asymmetric bilayers has been determined by a novel analysis in which one half of the bilayer serves as a structural reference for the entire bilayer. Absolute electron density profiles at 7 A resolution have been computed for bilayers formed from long and short chain length lipids either segregated to opposite sides or mixed together in both sides of the bilayer. The data indicate that the two lipids self organize in a specific paired configuration. Detailed analysis of bilayers associated with poly-L-lysine shows that although this hydrophilic peptide resides near the lipid head group region, its presence alters the arrangement of the bilayer hydrocarbon chains.

A structural model for the cholesterol-phosphatidylcholine complexes in bilayer membranes

Based on the structural properties of phospholipid and cholesterol molecules, and making use of the known structural and motional effects of cholesterol and its analogs on phospholipid bilayers, a model for the cholesterol-phosphatidylcholine complex is proposed. In this model, the 3beta-hydroxyl group of cholesterol is assumed to engage in hydrogen bonding with the carbonyl oxygen of the fatty acyl groups in phospholipids. Some specific configurations of the saturated and unsaturated fatty acyl chains of the phospholipid are suggested to participate in van der Waals attractive interactions with the apha and beta surface of the steroid nucleus.

A deuterium nuclear magnetic resonance study of the condensing effect of cholesterol on egg phosphatidylcholine bilayer membranes. I. Perdeuterated fatty acid probes

Deuterium nuclear magnetic resonance quadrupole splittings Dq and the related acyl chain segmental order parameters Smol have been determined for perdeuteriostearic acid intercalated as a molecular probe in the lamellar liquid crystalline phase of egg yolk phosphatidylcholine-cholesterol-water mixtures. The 2H NMR data show that cholesterol induces a high degree of order in the acyl chains of the phospholipid while maintaining the general profile of high order near the head group and relatively low order in the middle of the bilayer. This results in a pronounced thickening of the bilayer and concomitant decrease in the average molecular area of the fatty acyl chains. The giometrical changes in the bilayer due to cholesterol are discussed in terms of trans-gauche isomerization in the fatty acyl chains. The picture of the condensing effect revealed by 2H NMR is consistent with that from previous X-ray diffraction experiments. No evidence for a specific complex between cholesterol and phosphatidylcholine is apparent. The condensing effect of cholesterol is attributed to interaction between the fatty acyl chains and the rigid steroid nucleus, and to solvation of the 3beta-hydroxyl group at the aqueous interface.