An interacting spin label study of lateral expansion in dipalmitoyllecithin-cholesterol bilayers

Fluorescence sensors for imaging membrane lipid domains and cholesterol

Lipid membrane domains are supramolecular lateral heterogeneities of biological membranes. Of nanoscopic dimensions, they constitute specialized hubs used by the cell as transient signaling platforms for a great variety of biologically important mechanisms. Their property to form and dissolve in the bulk lipid bilayer endow them with the ability to engage in highly dynamic processes, and temporarily recruit subpopulations of membrane proteins in reduced nanometric compartments that can coalesce to form larger mesoscale assemblies. Cholesterol is an essential component of these lipid domains; its unique molecular structure is suitable for interacting intricately with crevices and cavities of transmembrane protein surfaces through its rough β face while "talking" to fatty acid acyl chains of glycerophospholipids and sphingolipids via its smooth α face. Progress in the field of membrane domains has been closely associated with innovative improvements in fluorescence microscopy and new fluorescence sensors. These advances enabled the exploration of the biophysical properties of lipids and their supramolecular platforms. Here I review the rationale behind the use of biosensors over the last few decades and their contributions towards elucidation of the in-plane and transbilayer topography of cholesterol-enriched lipid domains and their molecular constituents. The challenges introduced by super-resolution optical microscopy are discussed, as well as possible scenarios for future developments in the field, including virtual ("no staining") staining.

Formation of multilayered vesicles from water/organic-solvent (w/o) emulsions: theory and practice

Multilayered liposome (MLV) formation from water/organic-solvent (W/O) emulsions was studied. A fundamental liposome population parameter, the E(ratio), was defined and used to estimate the bilayer number and water spacing in MLV liposomes. MLVs prepared from W/O emulsions have optimum drug entrapment at an emulsion-lipid/emulsion-water ratio of ∼0.33. Drug entrapment is typically 50 to 65% under these optimal conditions.

Monovalent ions and water dipoles in contact with dipolar zwitterionic lipid headgroups-theory and MD simulations

The lipid bilayer is a basic building block of biological membranes and can be pictured as a barrier separating two compartments filled with electrolyte solution. Artificial planar lipid bilayers are therefore commonly used as model systems to study the physical and electrical properties of the cell membranes in contact with electrolyte solution. Among them the glycerol-based polar phospholipids which have dipolar, but electrically neutral head groups, are most frequently used in formation of artificial lipid bilayers. In this work the electrical properties of the lipid layer composed of zwitterionic lipids with non-zero dipole moments are studied theoretically. In the model, the zwitterionic lipid bilayer is assumed to be in contact with aqueous solution of monovalent salt ions. The orientational ordering of water, resulting in spatial variation of permittivity, is explicitly taken into account. It is shown that due to saturation effect in orientational ordering of water dipoles the relative permittivity in the zwitterionic headgroup region is decreased, while the corresponding electric potential becomes strongly negative. Some of the predictions of the presented mean-field theoretical consideration are critically evaluated using the results of molecular dynamics (MD) simulation.

Are lipid phase transitions responsible for chilling damage in human platelets?

In previous studies we have proposed that the well-known chilling-induced activation of human blood platelets can be ascribed at least in part to a thermotropic phase transition in membrane lipids. The evidence that this is the case is reviewed and amplified in this review, followed by an examination of the available physical data concerning phase transitions in lipid mixtures that mimic the mixture found in platelet membranes. Assuming complete mixing at all temperatures and equal contributions of the members of the mixture to the phase transition, the lipid mixture found in platelets should give values for Tm ranging from about 1 degrees C to about 16 degrees C, depending on the isomers present in the mixture. (The former value is not in agreement with the observed Tm, but the latter is in excellent agreement.) However, examination of the phase diagram for a binary pair of lipids found in platelet membranes shows that ideal mixing almost certainly does not occur; instead of a linear phase diagram, a convex one was obtained. This shape for the phase diagram, which would displace Tm to an unexpectedly elevated temperature, is in agreement with previously published phase diagrams for mixtures of this type. The prediction, based on thermodynamic properties of lipids found in the platelets, is that Tm will be displaced upward in more complex mixtures of the composition found in platelets, a prediction that requires experimental testing.

Probing DMPG vesicle surface with a cationic aqueous soluble spin label

A small, highly aqueous soluble, deuterated, cationic spin label, 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-d17-1-oxyl iodide (dCAT1), was used to directly monitor the negatively charged DMPG vesicle surface in order to test a recent suggestion (Riske et al., Chem. Phys. Lipids, 89 (1997) 31-44) that alterations in the surface potential accompanied apparent phase transitions observed by light scattering. The temperature dependence of the label partition between the lipid surface and the aqueous medium indicated an increase in the surface potential at the gel to liquid-crystal transition, supporting the previous suggestion. Results at the phase transition occurring at a higher temperature were less definitive. Although some change in the dCAT1 ESR spectra was observed, the interpretation of the phenomena is still rather unclear. DMPG surface potentials were estimated from the dCAT1 partition ratios (surface label moles/total label moles), using a simple two-sites model, where the electrostatic potential is zero everywhere but at the vesicle surface, and the interaction between the spin label and the membrane surface is chiefly electrostatic. The Gouy-Chapman-Stern model predicts surface potentials similar to those observed, although the measured decrease in the surface potential with ionic strength is somewhat steeper than that predicted by the model.

Charge-induced tilt in ordered-phase phosphatidylglycerol bilayers evidence from X-ray diffraction

X-ray diffraction studies have been performed, as a function of water content, on dipalmitoyl phosphatidylglycerol bilayers, both in the charged state at pH 8.0 and in the protonated state at pH 1.5, using buffers of 1.5 M salt concentration. Measurements were made at 20 degrees C, and the high-angle reflections indicated that the bilayers were in the ordered phase at both pH values. Lamellar diffractions were observed under all conditions studied. THe lamellar repeat reached a limiting value of 62.4 A (6.24 nm) at a water/lipid ratio of 0.24 at pH 8.0, and a limiting value of 67.3 A (6.73 nm) at a water/lipid ratio of 0.22 at pH 1.5. The area per lipid molecule in the plane of the bilayer, deduced from the bilayer thickness and the lipid partial specific volume, is 48 A2 (0.48 nm2) at pH 8.0 and 37 A2 (0.37 nm2) at pH 1.5. The area per molecule in the plane perpendicular to the chain axes, deduced from the X-ray short spacings, is 40.5 A2 (0.405 nm2) at pH 8.0 and 39.2 A2 (0.392 nm2) at pH 1.5. Thus the lipid molecules are tilted by approx. 30 degrees relative to the bilayer normal at pH 8.0, but are essentially untilted at pH 1.5.

The use of n-(9-anthroyloxy) fatty acids to determine fluidity and polarity gradients in phospholipid bilayers

A set of n-(9-anthroyloxy) fatty acid probes (n = 2, 6, 9, 12) have been used to examine gradients in fluorescence polarization, lifetime (tau F), relative quantum yield (phi rel) and positions of emission maxima (lambda max) through bilayers composed of synthetic phospholipids. The fluorophores of these probes report the environment at a graded series of depths from the surface to the centre of the bilayer structure. 1. Polarizations decrease as the fluorophore is moved deeper into the bilayer indicating greater rotational motion of the fluorophore in the hydrocarbon core of the bilayer. 2. The different responses of the probe diphenylhexatriene and the anthroyloxy fatty acids to the action of cholesterol on lipid bilayers are discussed in terms of the orientation of these probes in the bilayer and the types of anisotropic rotational motions which result in depolarization of fluorescence. 3. Stearic acid derivatives which have the fluorophore in the 6-, 9- and 12-positions along the acyl chain have a similar response to solvent polarity as measured by values of lambda max and phi rel in a variety of organic solvents. 4. The position of the emission maximum has little dependence on solvent viscosity, but viscosity does change the degree of vibrational structure seen in the emission spectrum. The vibrational structure itself may be used as an indication of the 'mciroviscosity' gradient in the transverse plane of the bilayer. 5. Values of lambda max, tau F and phi rel indicate that a gradient of polarity exists from the surface to the centre of the bilayer. For dipalmitoyl phosphatidylcholine in the crystalline phase, cholesterol acts to make this polarity gradient shallower.

Electrostatic interactions at charged lipid membranes. Hydrogen bonds in lipid membrane surfaces

Hydrogen-bonded structures within lipid membrane surfaces are not disrupted by water and are of thermodynamic and therefore potential structural importance in biological systems.

On two-dimensional passive random walk in lipid bilayers and fluid pathways in biomembranes

The lateral mobility of pyrene, pyrene decanoic acid, and 1-palmitoyl-2-pyrene decanoyl-phosphatidyl choline (pyrene lecithin) in lipid bilayers is determined by the excimer formation technique. This method is applied to vesicles of lecithins differing in chain length and in the degree of saturation of the hydrocarbon chains. These values are compared with results in cephalins of different chain length and in dipalmitoyl phosphatidic acid at variable pH. The influence of cholesterol is investigated. The results are analyzed in terms of the Montroll model of two-dimensional random walk. The jump frequency of the probe molecule within the lipid lattice is obtained. The advantage of this measure of transport in lipid layers is that it does not involve lipid lattice parameters. The main results of the present work are: (i) The lateral mobility of a given solute molecule in lamellae of saturated lecithins is independent of hydrocarbon chain length and rather a universal function of temperature. (ii) In unsaturated dioleyl lecithin the amphiphatic molecules have lateral mobilities of the same size as in saturated lipids. The jump frequency of pyrene, however, is by a factor of two larger in the unsaturated lecithin. (iii) The jump frequencies in phosphatidyl ethanolamines are about equal to those in lecithins. (iv) In phosphatidic acid layers the hopping frequencies depend on the charges of the head groups of both the lipids and the probes. (v) Cholesterol strongly reduces the jump frequency in fluid layers. (vi) The lateral mobility in biological membranes is comparable to that in artificial lipid bilayers. The experimental results are discussed in terms of the free volume model of diffusion in fluids. Good agreement with the predictions made from this model is found. A striking result is the observation of a tilt in dioleyl-lecithin bilayer membranes from the hopping frequencies of pyrene and pyrene lecithin. A tilt angle of phi = 17 degrees is estimated.

Morphology of egg phosphatidylcholine-cholesterole single-bilayer vesicles, studied by freeze-etch electron microscopy

Homogeneous, small, single-bilayer vesicles were prepared from egg phosphatidylcholine with various concentrations of cholesterol by ultrasonic dispersion in 0.1 M KCl, 0.01 M Tris, pH 8.0, buffer, followed by gel chromatography. The shape and size distributions of the fractionated vesicles were investigated for preparations with cholesterol compositions from 0 to 50 moles/100 moles, using freeze-etch electron microscopy. The size distribution was estimated from the shadow width of vesicles which were exposed by etching and the vesicle shape was checked by comparing the images obtained by tilting the replicas. The widths of the vesicle diameter distributions were relatively broad, corresponding to standard deviations in the range 60--90 A, but showing no systematic variation with cholesterol composition. In all cases it was found that 70% of the vesicle diameters lay within 150 A of the modal value. The apparent vesicle diameters remained constant for cholesterol compositions up to 20 moles/100 moles (modal diameter = 330 +/- 20 A, mean diameter = 350 +/- 3 A), but there was a sharp net increase in diameter at 30 moles cholesterol/100 moles reaching a model diameter of 430 +/- 20 A (mean diameter = 430 +/- 3 A) at 50 moles cholesterol/100 moles. Using the tilted microscope stage it was found that all vesicles were spherical at all cholesterol compositions studied, including those above 30 moles cholesterol/100 moles. The molecular mechanism by which cholesterol controls the vesicle size is discussed in terms of the asymmetric distribution of cholesterol across the vesicle bilayer.

Molecular motion and order in oriented lipid multibilayer membranes evaluated by simulations of spin label ESR spectra. Effects of temperature, cholesterol and magnetic field

A simulation method to interpret electron spin resonance (ESR) of spin labelled amphiphilic molecules in oriented phosphatidylcholine multibilayers in terms of a restricted motional model is presented. Order and motion of the cholestane spin label (3-spiro-doxyl-5alpha-cholestane) incorporated into egg yolk phosphatidylcholine, dipalmitoylphosphatidylcholine and dimyristoylphosphatidylcholine, pure and in mixture with cholesterol, were studied at various temperatures. With egg yolk phosphatidylcholine identical sets of motional parameters were obtained from simulations of ESR spectra obtained at three microwave frequencies (X-, K- and Q-band). With dipalmitoylphosphatidylcholine and dimyristoylphosphatidylcholine analyses of the spectra show that phase transitions occur in samples containing up to 30 mol % cholesterol. The activation energy for the motion of the spin label is about three times larger above than below the phase transition, indicating a more collective motion in the lipid crystalline state than in the gel state. In the liquid crystalline state the activation energy is larger in the pure phosphatidylcholines than with cholesterol added. Additions of cholesterol to egg phosphatidylcholine induces a higher molecular order but does not appreciably affect correlation times. This is in contrast to dipalmitoylphosphatidylcholine where both order and correlation times are affected by the presence of cholesterol. The activation energies follow the same order as the transition temperatures: dipalmitoylphosphatidylcholine greater than dimyristoylphosphatidylcholine greater than egg yokd phosphatidylcholine, suggesting a similar order of the cooperativity of the motion of the lipid molecules. Magnetic field-induced effects on egg phosphatidylcholine multibilayers were found at Q-band measurements above 40 degrees C. The cholestane spin label mimics order and motion of cholesterol molecule incorporated into the lipid bilayers. This reflects order and motion of the portions of the lipid molecules on the same depth of the bilayer as the rigid steroid portions of the intercalated molecules.

Cooperativity of the phase transition in single- and multibilayer lipid vesicles

The effect of membrane morphology on the cooperativity of the ordered-fluid, lipid phase transition has been investigated by comparing the transition widths in extended, multibilayer dispersons of dimyristoyl phosphatidyl-choline, and also of dipalmitoyl phosphatidylcholine, with those in the small, single-bilayer vesicles obtained by sonication. The electron spin resonance spectra of three different spin-labelled probes, 2,2,6,6-tetramethylpiperdine-N-oxyl, phosphatidylcholine and stearic acid, and also 90 degrees light scattering and optical turbidity measurements were used as indicators of the phase transition. In all cases the transition was broader in the single-bilayer vesicles than in the multibilayer dispersions, corresponding to a decreased cooperativity on going to the small vesicles. Comparison of the light scattering properties of centrifuged and uncentrifuged, sonicated vesicles suggests that these are particularly sensitive to the presence of intermediate-size particles, and thus the spin label measurements are likely to give a more reliable measure of the degree of cooperativity of the small, single-bilayer vesicles. Application of the Zimm and Bragg theory ((1959) J. Chem. Phys. 31, 526-535) of cooperative transitions to the two-dimensional bilayer system shows that the size of the cooperative unit, 1/square root sigma, is a measure of the mean number of molecules per perimeter molecule, in a given region of ordered or fluid lipid at the centre of the transition. From this result it is found that it is the vesicle size which limits the cooperativity of the transition in the small, single-bilayer vesicles. The implications for the effect of membrane structure and morphology on the cooperativity of phase transitions in biological membranes, and for the possibility of achieving lateral communication in the plane of the membrane, are discussed.

Vibrational Raman spectra of lipid systems containing amphotericin B

Resonance-enhanced and normal vibrational Raman spectra were observed for both multilamellar and single-wall vesicle assemblies of dimyristoyl phosphatidylcholine containing amphotericin B, a channel-forming polyene antibiotic, and cholesterol. The decrease in the frequency of the polyene antibiotic C = C stretching mode at 1556 cm-1 and the increase in intensity of the C-C-H in-plane deformation mode at 1002 cm-1 indicate that amphotericin B is ordered in a lipid-cholesterol medium similarly to the solid, but is surrounded by a slightly more polar environment. The intensity of the C = C stretching mode I1556 decreases 4-fold during the broadened gel to liquid crystalline phase transition (16--32 degrees C) of dimyristoyl lecithin-cholesterol (4 : 1) multilayers. Other resonance-enhanced vibrations of amphotericin B exhibit similar behavior. For amphotericin B in pure dimyristoyl lecithin multilayer or vesicle systems, however, the vibrational intensity associated with the C = C stretching mode remains constant during the melting of lipid hydrocarbon chains. In addition, a third effect occurs in liquid crystalline egg lecithin-cholesterol (4 : 1, mol ratio) multilayers in which I 1556 first increases by 25% between 3 and 25 degrees C, in parallel with the loss of active channels, and then remains constant as the temperature increases from 25 to 42 degrees C. This latter intensity pattern is masked in the dimyristoyl lecithin-cholesterol system by the overwhelming effect upon the C = C mode from changes in the lipid chain packing characteristics which occur during the phase transition. The broadened phase transition in 4 : 1 dimyristoyl lecithin-cholesterol multilayers (16--32 degrees C), as followed by the ratio of intensities at 2880 and 2850 cm-1 (asymmetric and symmetric methylene C-H stretching modes, respectively) is slightly narrowed by the addition of amphotericin B, and effect from which a binding stoichiometry at 24 degrees of 1 : 1 amphotericin B: cholesterol is estimated. This stoichiometry was confirmed by differential calorimetric scans, which also show the presence of a peak proportional to cholesterol content. Raman I2880/2850 peak height ratios in pure dimyristoyl lecithin bilayers were increased over the 14--38 degrees C range by amphotericin B, a spectral effect which suggests an ordering of the lipid matrix perhaps as a consequence of the polyene binding to the bilayer surface. For bilayers containing cholesterol, the ratios of intensities of the 2935 cm-1 feature, composed mainly of acyl chain terminal methyl and underlying methylene C-H stretching modes, to the 2850 cm-1 feature are significantly increased by amphotericin B. This effect indicates that the antibiotic penetrates the bilayer in the lipid-sterol system.

Electrostatic interactions at charged lipid membranes. I. Effects of pH and univalent cations on membrane structure

Electrostatic interactions at charged lipid membranes make a significant contribution to the free energy of the system, and can be varied within a wide range by alteration either of the membrane's surface charge density or of the concentration of electrolytes in the surrounding medium. Changes in the charged membrane's structure, such as the ordered in equilibrium fluid phase transition, can thus be induced at constant temperature by variations in pH and salt concentration. An adequate quantitative description of these phenomena is obtained from the Gouy--Chapman theory. The good agreement between theory and experiment confirms that the expression derived for the electrostatic free energy especially in respect of its positive sign is correct. The classical expression derived for the electrostatic free energy, especially in respect of its positive sign, is correct. The classical expression for the "free energy of the double layer" derived by Verwey and Overbeek, which has a negative sign, is not applicable to lipid membranes with ionizable polar groups.