The effect of cholesterol on the structure of phosphatidylcholine bilayers

The effect of cholesterol on the structure of phosphatidylcholine bilayers was investigated by X-ray diffraction methods. Electron density profiles at 5 Å 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 bilayer 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.     

Interpretation of small angle X-ray measurements guided by molecular dynamics simulations of lipid bilayers

Reconstruction and interpretation of lipid bilayer structure from X-ray scattering often rely on assumptions regarding the molecular distributions across the bilayer. It is usually assumed that changes in head-head spacings across the bilayer, as measured from electron density profiles, equal the variations in hydrocarbon thicknesses. One can then determine the structure of a bilayer by comparison to the known structure of a lipid with the same headgroup. Here we examine this procedure using simulated electron density profiles for the benchmark lipids DMPC and DPPC. We compare simulation and experiment in both real and Fourier space to address two main aspects: (i) the measurement of head-head spacings from relative electron density profiles, and (ii) the determination of the absolute scale for these profiles. We find supporting evidence for the experimental procedure, thus explaining the robustness and consistency of experimental structural results derived from electron density profiles. However, we also expose potential pitfalls in the Fourier reconstruction that are due to the limited number of scattering peaks. Volumetric analysis of simulated bilayers allows us to propose an improved, yet simple method for scale determination. In this way we are able to remove some of the restrictions imposed by limited scattering data in constructing reliable electron density profiles.     

Area per molecule and distribution of water in fully hydrated dilauroylphosphatidylethanolamine bilayers

The area per lipid molecule for fully hydrated dilauroylphosphatidylethanolamine (DLPE) has been obtained in both the gel and liquid-crystalline states by combining wide-angle X-ray diffraction, electron density profiles, and previously published dilatometry results [Wilkinson, D. A., & Nagle, J. F. (1981) Biochemistry 20, 187-192]. The molecular area increases from 41.0 +/- 0.2 to 49.1 +/- 1.2 A2 upon melting from the gel to liquid-crystalline phase. The thickness of the bilayer, as measured from the electron density profiles, decreases about 4 A upon melting, from 45.2 +/- 0.3 to 41.0 +/- 0.6 A. A somewhat unexpected result is that the fluid layer between fully hydrated bilayers is the same in both gel and liquid-crystalline phases and is only about 5 A thick. From these data, plus the volume of the anhydrous DLPE molecule, it is possible to determine the number of water molecules per lipid and their approximate distribution relative to the lipid molecule. Our analysis shows that there are about 7 and 9 waters per DLPE molecule in the gel and liquid-crystalline phases, respectively. About half of the water is located in the fluid space between adjacent bilayers, and the remaining waters are intercalated into the bilayer, presumably in the head group region. There are significantly fewer water molecules in the fluid spaces between DLPE bilayers than in the fluid spaces in gel- or liquid-crystalline-phase phosphatidylcholine bilayers. This small fluid space in PE bilayers could arise from interbilayer hydrogen bond formation through the water molecules or electrostatic interactions between the amine and phosphate groups on apposing bilayers.     

Hydration force and bilayer deformation: a reevaluation

The hydration repulsive force between lipid bilayers and the deformability of both gel and liquid-crystalline bilayers have been quantitated by an X-ray diffraction analysis of osmotically stressed liposomes. Both sampling theorem reconstructions and electron density distributions were calculated from diffraction data obtained from multilayers with applied osmotic pressures of 0-50 atm. The bilayer thickness and area per lipid molecule remain nearly constant (to within about 4%) in this pressure range, as adjacent bilayers move from their equilibrium separation in excess water to within 2-4 A of each other. This analysis indicates that the bilayers are relatively incompressible. This results differs from previously published X-ray diffraction studies of bilayer compressibility but agrees with direct mechanical measurements of the bilayer compressibility modulus. It is also found that the hydration repulsive force decays exponentially with separation between bilayers with a decay constant of 1.4 A for gel-state dipalmitoylphosphatidylcholine and 1.7 A for liquid-crystalline egg phosphatidylcholine bilayers. This implies that the exponential decay constant is not necessarily equal to the diameter of a water molecule, as has been previously suggested on experimental and theoretical grounds.     

The structure of lipid bilayers and the effects of general anaesthetics: An X-ray and neutron diffraction study

We have looked for the effects of three clinically used inhalational anaesthetics (nitrous oxide, halothane and cyclopropane) on the structure of lecithin/ cholesterol bilayers. The anaesthetics were delivered to the membranes in the gaseous phase, so that effects at clinical concentrations could be determined.High-resolution X-ray diffraction patterns were recorded out to 4 Å and analyzed using swelling experiments. Parallel neutron diffraction experiments were performed and analyzed using H₂O-²H₂O exchange. Methods were developed which enabled us to obtain confidence limits for the X-ray and neutron structure factors.The resultant X-ray and neutron scattering density profiles clearly define the positions of the principal molecular groups in the unperturbed bilayer. In particular, the high-resolution electron density profiles reveal features directly attributable to the cholesterol molecule. A comparison with the neutron scattering density profiles shows that cholesterol is anchored with its hydroxyl group at the water/hydrocarbon interface, aligned with the fatty acid ester groups of the lecithin molecule. We suggest that this positioning of the cholesterol molecule allows it to act as a thickness buffer for plasma membranes.In the presence of very high concentrations of general anaesthetics, the bilayers show increased disorder while maintaining constant membrane thickness. At surgical concentrations, however, there are no significant changes in bilayer structure at 95% confidence levels. We briefly review the literature previously used to support lipid bilayer hypotheses of general anaesthesia. We conclude that the lipid bilayer per se is not the primary site of action of general anaesthetics.     

Amiodarone-liposome interaction: a multinuclear NMR and X-ray diffraction study

Amiodarone, a potent antiarrhythmic drug, is widely used in cardiology. Its electrophysiological effects, as well as many of its side effects, seem to involve lipids. We report here a multinuclear NMR and X-ray diffraction study of amiodarone in egg phosphatidylcholine liposomes and lipid multilayers. In proton NMR experiments, amiodarone alters the signal from the lipid trimethyl ammonium group for pH values ranging from 3.2 to 8.4; cholesterol does not cause this alteration. The addition of SCN- changes both the proton and phosphorus NMR spectra of liposomes containing amiodarone. For both proton and carbon NMR, amiodarone modifies the signal from the lipid methylene groups, but to a far lesser extent than does cholesterol. Incorporation of amiodarone in EPC bilayers also modifies the low-angle X-ray diffraction patterns, decreasing the lamellar repeat period at low water contents, but swelling the fluid spaces between bilayers at high water contents. Electron density profiles and modeling studies using the X-ray data indicate that amiodarone decreases the bilayer thickness and adds electron density at the interfacial region of the bilayer. Our analysis of the NMR and X-ray data indicates that the iodine atoms of amiodarone are located near the hydrocarbon/water interface and that the tertiary amine of amiodarone is in the headgroup region of the bilayer.     

High-resolution electron density profiles reveal influence of fatty acids on bilayer structure.

Small-angle x-ray diffraction studies were performed on gel phase-oriented bilayers of dipalmitoylphosphatidylcholine (DPPC) and DPPC containing 40 mol% of either palmitic acid (PA) or palmitic acid brominated at the 2-position (BPA). Oriented samples were prepared using a method developed by us, which is as simple as powder sample preparations while offering all the advantages of oriented samples made by traditional methods. Phases were determined using swelling experiments with structure factors plotted in reciprocal space, creating a relatively smooth curve as the amount of water between the bilayers was changed. Continuous Fourier transforms were also calculated to further test the consistency of the phase assignments. The diffraction data were used to calculate absolute electron density profiles for different bilayers to a resolution of 5-6 A. Analysis indicates the following: (a) The electron density profiles for the three preparations are virtually identical in the hydrocarbon chain region. (b) There is a decrease in the electron density of the glycerol backbone-headgroup region and d-space in DPPC-PA compared to DPPC. (c) The bromine of fatty-acid brominated at the 2-position is in the vicinity of the glycerol backbone. (d) The bilayer thickness of DPPC containing either brominated or unbrominated fatty acid remains relatively constant with increased levels of hydration, unlike DPPC bilayers.     

Closer look at structure of fully hydrated fluid phase DPPC bilayers

X-ray data are presented for the benchmark dipalmitoylphosphatidylcholine lipid bilayer in the most biologically relevant state in which the bilayers are fully hydrated and in the fluid (liquid-crystalline) phase. Form factors F(q(z)) are obtained from a combination of two sample preparations, oriented stacks of bilayers for q(z) extending to 0.85 A(-1) and unilamellar vesicles for smaller q(z). Modeling obtains the electron density profile and values for the area per molecule, for the locations of the component groups, and for the different types of thicknesses of the bilayer, such as the hydrocarbon thickness and the steric thickness.     

Effects of cyclosporin A on model lipid membranes

Cyclosporin A (CSA) is a widely used immunosuppressant drug for transplant therapy, however its limitation is its toxicity. The effect of CSA on model membranes such as dimyristoyl phosphatidylcholine (DMPC) bilayers was studied using small-angle X-ray diffraction and differential scanning calorimetry (DSC). CSA abolishes the pretransition and affects the transition of DMPC model membranes in a concentration-related manner as is shown by DSC. CSA induces a second peak at the high temperature side of the main transition, which is interpreted as a phase separation between areas rich and poor in CSA concentration. Small angle X-ray diffraction shows that the repeat distance of the DMPC bilayers in the lamellar Lalpha state increases as a function of concentration up to 10 mol% and remains constant thereafter. Furthermore, CSA affects the fatty acyl chains of the bilayer, especially the part of the chain proximal to the head group. In conclusion, CSA, as both small-angle X-ray diffraction and DSC show, affects in a concentration-wise manner the DMPC model membranes and perturbs the bilayer, in particular the acyl chain region.     

The orientation of (-)-delta 9-tetrahydrocannabinol in DPPC bilayers as determined from solid-state 2H-NMR

The orientation of the motional axis of (-)-delta 9-tetrahydrocannabinol in dipalmitoylphosphatidylcholine model membrane was calculated from the 2H quadrupolar splittings (delta nu Q) of individual deuterons strategically located on the cannabinoid tricyclic component. The molecule assumes an orientation in which its long axis is nearly perpendicular to the phospholipid chains and its most ordered axis is almost in the plane of the aromatic ring. This 'awkward' cannabinoid orientation in the membrane presumably occurs in order to allow the phenolic hydroxyl group to direct itself towards the polar bilayer interface.     

Structure of polymerizable lipid bilayers. I--1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine, a tubule-forming phosphatidylcholine

This report presents the first X-ray diffraction data on diacetylenic phospholipids. The tubule-forming polymerizable lipid, 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC8,9PC), was studied by low angle X-ray diffraction from partially dehydrated oriented multibilayers in both polymerized and unpolymerized form. Bilayers of this material were found to be highly ordered, yielding as many as 16 orders of lamellar diffraction, in both the polymerized and unpolymerized states. The unit cell dimension was very small for a lipid of this size. In addition to the features usually observed in the electron density profile structure of phospholipid bilayers, the electron-dense diacetylenic portions of the fatty acyl chain produced electron density maxima at two well-defined levels on each side of the bilayer approximately 15 A and 9 A from the bilayer midplane. A model molecular conformation deduced from the one-dimensional electron density map features all-trans acyl chains tilted at approximately 28 degrees from the bilayer normal that are interdigitated with chains of the opposing monolayer by approximately two carbons at the bilayer center. The linear diacetylene moieties on beta- and gamma-chains appear at different positions along the bilayer normal axis and are roughly parallel to the bilayer surface. This model is discussed in terms of a polymerization mechanism.     

Order-disorder transition in bilayers of diphytanoyl phosphatidylcholine

A comparative study on bilayers of diphytanoyl phosphatidylcholine (DPhPC) and bilayers of dimyristoyl phosphatidylcholine (DMPC) was made by X-ray lamellar diffraction as a function of temperature and the degree of hydration. An order-disorder phase transition of DPhPC reveals an interesting contrast to the standard model of DMPC. Electron density profiles allow us to deduce the conformational changes which occur in the headgroup-glycerol region and in the chain region. The important conclusion is that the lipid headgroup may have different conformational energetics in lipids of different chains. We explain why this is important to protein-membrane interactions.     

Effects of cannabinoids in membrane bilayers containing cholesterol.

The thermotropic and dynamic properties of the biologically active Delta(8)-tetrahydrocannabinol (Delta(8)-THC) and its inactive congener O-methyl-Delta(8)-tetrahydrocannabinol (Me-Delta(8)-THC) in DPPC/cholesterol (CHOL) bilayers have been studied using a combination of DSC and solid-state NMR spectroscopy. The obtained results showed differential effects of the two cannabinoids under study. These are summarized as follows: (a) the presence of the active compound fluidizes more significantly the DPPC/CHOL bilayers than the inactive analog as it is revealed by DSC and NMR spectroscopy results; (b) cholesterol seems to play a significant role in the way cannabinoids act in membrane bilayers; (c) the observed additional peaks in (13)C/MAS-NMR spectra which were cannabinoid specific offer an evidence of their different dynamic properties in membranes. In particular, the aromatic part of the inactive cannabinoid appears more mobile than that of the active one. This finding is in agreement with previously obtained X-ray data which locate the inactive cannabinoid in the hydrophobic core of the bilayer while the active one in the polar region; and (d) the observed downfield shift of C-1 carbon in the preparation containing the active cannabinoid is a strong evidence that Delta(8)-THC resides nearby the polar region where also cholesterol is well known to locate itself. Such downfield shift is absent when Me-Delta(8)-THC is resided in the membrane bilayer. These differential effects of the two cannabinoids propose that the phospholipid/cholesterol core of the membrane may play an important role in the mode of cannabinoid action by regulating their thermotropic and dynamic properties.     

Cadmium-induced changes in the membrane of human erythrocytes and molecular models

The structural effects of cadmium on cell membranes were studied through the interaction of Cd(2+) ions with human erythrocytes and their isolated unsealed membranes (IUM). Studies were carried out by scanning electron microscopy and fluorescence spectroscopy, respectively. Cd(2+) induced shape changes in erythrocytes, which took the form of echinocytes. According to the bilayer couple hypothesis, this result meant that Cd(2+) ions located in the outer monolayer of the erythrocyte membrane. Fluorescence spectroscopy measurements in IUM indicated a disordering effect at both the polar headgroup and the acyl chain packing arrangements of the membrane phospholipid bilayer. Cd(2+) ions also interacted with molecular models of the erythrocyte membrane consisting in bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE), representing classes of phospholipids located in the outer and inner monolayers the erythrocyte membrane, respectively. X-ray diffraction indicated that Cd(2+) ions induced structural perturbation of the polar headgroup and of the hydrophobic acyl regions of DMPC, while the effects of cadmium on DMPE bilayers were much milder. This conclusion is supported by fluorescence spectroscopy measurements on DMPC large unilamellar vesicles (LUV). All these findings point to the important role of phospholipid bilayers in the interaction of cadmium on cell membranes.     

The bilayer nature of deposits occurring in Gaucher's disease

Evidence is presented for the structure of the Gaucher cell deposit existing as a series of bilayers that are each 60 Å thick and are gradually twisted along their length. This evidence was obtained by freeze-etching studies and by X-ray diffraction studies that were used to calculate possible electron density profiles for each bilayer. A model is presented which shows the probable arrangement of the aggregated glucocerebroside molecules.     

Cholesterol modifies the short-range repulsive interactions between phosphatidylcholine membranes

Pressure versus distance relationships have been obtained for egg phosphatidylcholine bilayers containing a range of cholesterol concentrations. Water was removed from between adjacent bilayers by the application of osmotic pressures in the range of 0.4-2600 atm (4 x 10(5)-2.6 x 10(9) dyn/cm2), and the distance between adjacent bilayers was obtained by Fourier analysis of X-ray diffraction data. For applied pressures up to about 50 atm and bilayer surface separations of 15-5 A, the incorporation of up to equimolar cholesterol has little influence on plots of pressure versus bilayer separation. However, for the higher applied pressures, cholesterol reduces the interbilayer separation distance by an amount that depends on the cholesterol concentration in the bilayer. For example, the incorporation of equimolar cholesterol reduces the distance between bilayers by as much as 6 A at an applied pressure of 2600 atm. At this applied pressure, electron density profiles show that the high-density head-group peaks from apposing bilayers have merged. This indicates that equimolar concentrations of cholesterol spread the lipid molecules apart in the plane of the bilayer enough to allow the phosphatidylcholine head groups from apposing bilayers to interpenetrate as the bilayers are squeezed together. All of these X-ray and pressure-distance data indicate that, by reducing the volume fraction of phospholipid head groups, cholesterol markedly reduces the steric repulsion between apposing bilayers but has a much smaller effect on the sum of the longer ranged repulsive hydration and fluctuation pressures. Increasing concentrations of cholesterol monotonically increase the dipole potential of egg phosphatidylcholine monolayers, from 415 mV with no cholesterol to 493 mV with equimolar cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)     

A Molecular Dynamics Study of DMPC Lipid Bilayers Interacting with Dimethylsulfoxide–Water Mixtures

The diffusion process of dimethylsulfoxide (DMSO) through zwitterionic dimyristoylphosphatidylcholine (DMPC) lipid bilayer was studied by means of molecular dynamics (MD) simulations. To account for the cryoprotectant concentration difference between the inside and the outside of the cell, dual DMPC lipid bilayers which separate two aqueous reservoirs with and without DMSO were modeled. The initial configuration of the simulation model had DMSO molecules present in one of the aqueous phases (outside the cell) at two different concentrations of ~3 and ~6?mol%. MD simulations were performed on the systems for 50?ns at 323?K and 1?bar. Although the simulation time considered in the study was insufficient for the DMSO molecules to reach the other aqueous phase and equilibrium, early stages of the diffusion process indicated that DMSO molecules had a tendency to diffuse towards the other aqueous phase. The effects of DMSO on bilayer structural characteristics during the diffusion process were investigated. Simulations were analyzed to correlate the following properties of lipid bilayers in the presence of two different aqueous phases: area per lipid, lipid thickness, mass density profiles, lipid tail order parameter and water dipole orientation. Area per lipid calculated for the leaflet facing the aqueous DMSO?Cwater mixture did not show any significant difference compared to area per lipid for the DMSO-free pure DMPC bilayer. Mass density profiles revealed that DMSO molecules had a strong tendency to diffuse toward the aqueous phase with pure water. The lipid tail order parameter calculated for the sn-1 tail of the leaflet facing the aqueous DMSO?Cwater mixture showed that the ordering of lipid tails decreased compared to the leaflet exposed to pure water. However, the ordering of lipid tails in a system where a single bilayer is hydrated by an aqueous DMSO?Cwater mixture is far lower.     

Multibilayer structure of dimyristoylphosphatidylcholine dihydrate as determined by energy minimization

Complete energy minimization was carried out on the multibilayer crystal structure of 1,2-dimyristoyl-sn-glycero-3-phosphocholine dihydrate (DMPC.2H2O), starting from the X-ray structure determination reported by Pearson and Pascher (1979) Nature 281, 499-501. The asymmetric unit contains two nonidentical DMPC molecules and four water molecules. Minimization removed the acyl chain disorder present in the X-ray structure and caused the carbon planes of the acyl chains to become mutually parallel. Two energy-minimized structures (structures I and II) were found which mainly differed in the hydrogen-bonding arrangement of the waters of hydration. In structure I as in the X-ray structure, one of the water molecules forms a hydrogen-bonded bridge between successive bilayers; but in structure II, all hydrogen bonds are satisfied on the same bilayer. Structure II corresponds to the global energy minimum and is also a suitable structure for single bilayers. The lattice constants and cell volume of the minimized structures are close to the experimental values. The electrostatic force between DMPC bilayers is attractive. The mean hydration energy of the water is -14.2 kcal/mol, which is 2.5 kcal/mol lower than the binding energy of ice.     

The structure of oriented sphingomyelin bilayers

X-ray diffraction from oriented bilayers of sphingomyelin gave up to 14 orders of diffraction of a lamellar repeat of 68.5 Å on the meridian and up to eight reflections, including a strong reflection at 4.2 Å, on the equator. The diffraction spacings did not change when the sphingomyelin bilayers were exposed to different humidities. A direct analysis of the low resolution X-ray data, using deconvolution is presented. A comparison of the Patterson functions of sphingomyelin with those of phosphatidylcholine and phosphatidylethanolamine suggests that the molecular structure of sphingomyelin in oriented bilayers resembles the structure of both phosphatidylcholine and phosphatidylethanolamine. Molecular model calculations for sphingomyelin bilayers have also been performed. Electron density profiles of sphingomyelin bilayers at resolution of about 6 Å and about 2.5 Å are presented. Our results indicate that the phosphorylcholine head group of sphingomyelin is in the plane of the membrane and at right angles to the hydrocarbon chains, the hydrocarbon chains are nearly parallel to each other, and there is only a limited, if any, interdigitation of the hydrocarbon chains of the adjacent sphingomyelin molecules in the bilayer.     

Cholesterol esterase accelerates intestinal cholesterol absorption

Lipid bilayers of dimyristoyl phosphatidylcholine (DMPC) containing opioid peptide dynorphin A(1-17) are found to be spontaneously aligned to the applied magnetic field near at the phase transition temperature between the gel and liquid crystalline states (T(m)=24 degrees C), as examined by 31P NMR spectroscopy. The specific interaction between the peptide and lipid bilayer leading to this property was also examined by optical microscopy, light scattering, and potassium ion-selective electrode, together with a comparative study on dynorphin A(1-13). A substantial change in the light scattering intensity was noted for DMPC containing dynorphin A(1-17) near at T(m) but not for the system containing A(1-13). Besides, reversible change in morphology of bilayer, from small lipid particles to large vesicles, was observed by optical microscope at T(m). These results indicate that lysis and fusion of the lipid bilayers are induced by the presence of dynorphin A(1-17). It turned out that the bilayers are spontaneously aligned to the magnetic field above T(m) in parallel with the bilayer surface, because a single 31P NMR signal appeared at the perpendicular position of the 31P chemical shift tensor. In contrast, no such magnetic ordering was noted for DMPC bilayers containing dynorphin A(1-13). It was proved that DMPC bilayer in the presence of dynorphin A(1-17) forms vesicles above T(m), because leakage of potassium ion from the lipid bilayers was observed by potassium ion-selective electrode after adding Triton X-100. It is concluded that DMPC bilayer consists of elongated vesicles with the long axis parallel to the magnetic field, together with the data of microscopic observation of cylindrical shape of the vesicles. Further, the long axis is found to be at least five times longer than the short axis of the elongated vesicles in view of simulated 31P NMR lineshape.