The interpretation of proton magnetic resonance linewidths for lecithin dispersions. Effect of particle size and chain packing

NMR structures of membrane proteins in phospholipid bilayers

Membrane proteins have always presented technical challenges for structural studies because of their requirement for a lipid environment. Multiple approaches exist including X-ray crystallography and electron microscopy that can give significant insights into their structure and function. However, nuclear magnetic resonance (NMR) is unique in that it offers the possibility of determining the structures of unmodified membrane proteins in their native environment of phospholipid bilayers under physiological conditions. Furthermore, NMR enables the characterization of the structure and dynamics of backbone and side chain sites of the proteins alone and in complexes with both small molecules and other biopolymers. The learning curve has been steep for the field as most initial studies were performed under non-native environments using modified proteins until ultimately progress in both techniques and instrumentation led to the possibility of examining unmodified membrane proteins in phospholipid bilayers under physiological conditions. This review aims to provide an overview of the development and application of NMR to membrane proteins. It highlights some of the most significant structural milestones that have been reached by NMR spectroscopy of membrane proteins, especially those accomplished with the proteins in phospholipid bilayer environments where they function.

The influence of headgroup structure and fatty acyl chain saturation of phospholipids on monolayer behavior: a comparative rheological study

This paper compares six phospholipidic monolayers at the water/chloroform interface by performing dilational rheological measurements with a drop tensiometer apparatus. The chosen lipids differ both in their headgroup structure and fatty acyl chain saturation or symmetry. The study concentrated on monolayers formed with DPPC, DPPE, DOPC, DOPE, POPC and POPE. Using a generalized Maxwell rheological model, transposed at the interface, the intimate intermolecular interactions between amphiphilic molecules are studied on and off the monolayer plane. The equilibrium and nonequilibrium phenomena are analyzed and, respectively, correlated with monolayer cohesion and with monolayer/sub-surface interactions. The purpose of this work is to gain further insights into the influences (as slight as they are) of the weak changes in phospholipid structure and on the behavior of the monolayers. The results, widely described, provide further details on nuances existing between very similar molecules, and likewise, on the synergies created between the different effects.

Characterization of the liquid-ordered state by proton MAS NMR

We investigated if magic angle spinning (MAS) 1H NMR can be used as a tool for detection of liquid-ordered domains (rafts) in membranes. In experiments with the lipids SOPC, DOPC, DPPC, and cholesterol we demonstrated that 1H MAS NMR spectra of liquid-ordered domains (lo) are distinctly different from liquid-disordered (ld) and solid-ordered (so) membrane regions. At a MAS frequency of 10 kHz the methylene proton resonance of hydrocarbon chains in the ld phase has a linewidth of 50 Hz. The corresponding linewidth is 1 kHz for the lo phase and several kHz for the so phase. According to results of 1H NMR dipolar echo spectroscopy, the broadening of MAS resonances in the lo phase results from an increase in effective strength of intramolecular proton dipolar interactions between adjacent methylene groups, most likely because of a lower probability of gauche/trans isomerization in lo. In spectra recorded as a function of temperature, the onset of lo domain (raft) formation is seen as a sudden onset of line broadening. Formation of small domains yielded homogenously broadened resonance lines, whereas large lo domains (diameter >0.3 microm) in an ld environment resulted in superposition of the narrow resonances of the ld phase and the much broader resonances of lo. 1H MAS NMR may be applied to detection of rafts in cell membranes.

Domains in binary SOPC/POPE lipid mixtures studied by pulsed field gradient 1H MAS NMR

We studied domain formation in mixtures of the monounsaturated lipids SOPC and POPE as a function of temperature and composition by NMR. Magic angle spinning at kHz frequencies restored resolution of (1)H NMR lipid resonances in the fluid phase, whereas the linewidth of gel-phase lipids remained rather broad and spinning frequency dependent. In regions of fluid- and gel-phase coexistence, spectra are a superposition of resonances from fluid and gel domains, as indicated by the existence of isosbestic points. Quantitative determination of the amount of lipid in the coexisting phases is straightforward and permitted construction of a binary phase diagram. Lateral rates of lipid diffusion were determined by (1)H MAS NMR with pulsed field gradients. At the onset of the phase transition near 25 degrees C apparent diffusion rates became diffusion time dependent, indicating that lipid movement is obstructed by the formation of gel-phase domains. A percolation threshold at which diffusion of fluid-phase lipid becomes confined to micrometer-size domains was observed when approximately 40% of total lipid had entered the gel phase. The results indicate that common phosphatidylethanolamines may trigger domain formation in membranes within a physiologically relevant temperature range. This novel NMR approach may aid the study of lipid rafts.

Difference in Chiral Recognition of Gel and Liquid-Crystalline Phases of Phosphatidylcholine Vesicles as Determined by Circular Dichroism Spectroscopy

The circular dichroism (CD) spectra of three types of (L)phosphatidylcholine (PC) vesicles in aqueous solution showed differences in the sign and intensity of the Cotton effect compared with those of monomers in ethanol, indicating the existence of chiral environments in these vesicles. From the temperature dependence of CD intensities, the main phase transition temperatures between gel (Gel) and liquid-crystalline (LC) phases of the vesicles were estimated to be 40, 23, and 55 degrees C for dipalmitoyl PC, dimyristoyl PC and distearoyl PC, respectively. Furthermore, both low-fluidity Gel and high-fluidity LC phases recognized the chirality of incorporated 2-hydroxymethyl[5]thiaheterohelicene (5HM) with a helical structure, which undergoes a rapid racemization owing to a weak repulsion between the terminal hydrogen atoms. The ability for chiral recognition was evaluated using thermodynamic parameters for the equilibrium between P and M enantiomers of 5HM in the vesicles; the Gel phase manifested a higher recognition ability than the LC phase.

Characterization of the main phase transition in 1,2-dipalmitoyl-phosphatidylcholine luvs by 1H NMR

The main phase transition (Tm) of 100 nm large unilamellar vesicles (LUVs) of 1,2-dipalmitoylphosphatidylcholine (DPPC) was investigated using 1H NMR (proton magnetic resonance) in deuterium oxide, and both DSC (differential scanning calorimetry) and IR (infrared) spectroscopy in water and deuterium oxide. The ability of 1H NMR to determine Tm was demonstrated and the values obtained were in general agreement with those observed with DSC and IR. However, the temperature range of the transition observed by NMR was significantly broader than that observed with either DSC or IR. The effect of deuterium oxide on Tm was studied by comparing results obtained in water and deuterium oxide with DSC and IR. The results showed no significant difference in Tm or temperature range of transition determined in these solvents.

Novel NMR tools to study structure and dynamics of biomembranes

Nuclear magnetic resonance (NMR) studies on biomembranes have benefited greatly from introduction of magic angle spinning (MAS) NMR techniques. Improvements in MAS probe technology, combined with the higher magnetic field strength of modern instruments, enables almost liquid-like resolution of lipid resonances. The cross-relaxation rates measured by nuclear Overhauser enhancement spectroscopy (NOESY) provide new insights into conformation and dynamics of lipids with atomic-scale resolution. The data reflect the tremendous motional disorder in the lipid matrix. Transfer of magnetization by spin diffusion along the proton network of lipids is of secondary relevance, even at a long NOESY mixing time of 300 ms. MAS experiments with re-coupling of anisotropic interactions, like the 13C-(1)H dipolar couplings, benefit from the excellent resolution of 13C shifts that enables assignment of the couplings to specific carbon atoms. The traditional 2H NMR experiments on deuterated lipids have higher sensitivity when conducted on oriented samples at higher magnetic field strength. A very large number of NMR parameters from lipid bilayers is now accessible, providing information about conformation and dynamics for every lipid segment. The NMR methods have the sensitivity and resolution to study lipid-protein interaction, lateral lipid organization, and the location of solvents and drugs in the lipid matrix.

Thermodynamic stability and osmotic sensitivity of small unilamellar phosphatidylcholine vesicles

Evidence is presented to show that small unilamellar phosphatidylcholine vesicles with a diameter of approx. 20 nm are osmotically sensitive. Such vesicles respond to osmotic pressure by swelling or shrinking depending on the direction of the applied salt gradient. This is true for small unilamellar vesicles of egg phosphatidylcholine and dimyristoylphosphatidylcholine below and above their crystal-to-liquid crystal transition temperature. At the transition temperature the vesicles are osmotically insensitive due to the increased bilayer permeability resulting in rapid dissipation of salt gradients. Positive salt gradients produce shrinking and collapse of spherical phospholipid vesicles to disks. Shrinking of vesicles is associated with H2O and solute efflux, but only limited solute influx. Clustering of lipid molecules in the bilayers of the resulting disks can be detected by EPR spin labeling. Negative salt gradients produce swelling of vesicles which is associated with H2O and solute influx. Our experiments are consistent with an osmotically perturbed bilayer. In the presence of osmotic gradients the influx and efflux of H2O is coupled with the movement of ions and small molecules which in the absence of salt gradients or osmotic stress cannot pass the phospholipid bilayer. However, during osmotically induced shrinking and swelling of SUV the integrity of the phospholipid bilayer is maintained to the extent that vesicles do not break, and therefore equilibration between external medium and vesicle cavity does not take place.

Two-dimensional 1H/13C heteronuclear chemical shift correlation spectroscopy of lipid bilayers

Using solid-state magic angle spinning nuclear magnetic resonance (NMR) techniques, we have obtained two-dimensional (2D), 1H/13C chemical shift-correlated spectra of liquid crystalline 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) bilayers in 30 wt% PO4/D2O buffer. Linewidths in both the 13C and the 1H dimensions were less than 0.3 ppm wide. The 2D spectrum consists of chemical shift correlations between all resolvable, directly bonded 13C-1H pairs and exhibits considerably greater spectral dispersion than either ID 1H or 13C MAS spectra. This approach promises to be an important tool in structural studies of biological membranes.

Density-based separation of liposomes by glycerol gradient centrifugation

Sonicated liposomes of soybean phospholipids (asolectin) distribute nearly throughout a 19-22% (v/v) glycerol gradient when centrifuged to near equilibrium. Upon recentrifugation on an identical gradient, liposomes selected from several positions in such a gradient migrate as narrow bands to positions close to their original positions, indicating that the liposome distribution in the first gradient is the result of a density-based fractionation. Molecular sieve chromatography, turbidity, and trapped volume measurements indicate that the liposome densities are qualitatively related to their size, with the larger liposomes more dense than the smaller ones. Size estimates obtained by electron microscopy of negatively stained preparations indicate that the fractionation is effective for liposomes with diameters ranging from 200 to 600 A, with maximum efficiency in the range 200-300 A where the majority of the liposomes is found. Interestingly, high concentrations of liposomes improve the efficiency of the fractionation procedure. The size dependence of liposome density is shown not to be due to differential glycerol permeability or lipid composition, and is therefore most likely due to variations in the specific volumes of the individual phospholipid molecules owing to the curvature of the liposomes. Finally, freezing of the glycerol gradient fractions in liquid N2 and storage at -70 degrees C does not modify the size of the isolated liposomes. It is suggested that glycerol density gradient fractionation of liposomes could be a useful general method for obtaining liposomes of reasonably uniform size in large quantities and high concentrations.

1-Palmitoyl-2-pyrenedecanoyl glycerophospholipids as membrane probes: evidence for regular distribution in liquid-crystalline phosphatidylcholine bilayers

We have synthesized 1-palmitoyl-2-pyrenedecanoyl-sn-glycero derivatives of 3-phosphatidylcholine, 3-phosphatidylethanolamine, 3-phosphatidylserine, 3-phosphatidylglycerol, 3-phosphatidylinositol, and 3-phosphatidic acid and investigated their behavior in monolayers and in neat and mixed bilayers. Fluorescence spectroscopy of neat pyrene phospholipid dispersions revealed a well-defined thermotropic transition at 13.5-19 degrees C depending on the polar head group. An endotherm coinciding with this transition was observed with differential scanning calorimetry, indicating it to be due to the melting of the lipid acyl chains. For pyrenephosphatidylethanolamine, the endotherm was observed at a much higher temperature (70 degrees C). Compression isotherms obtained at an argon/water interface revealed that the pyrene moiety somewhat increases the mean molecular area of a phospholipid molecule but does not prevent the expression of head-group-dependent packing behavior. Partition of the pyrene lipids between coexisting fluid and solid phases was investigated with fluorometry and calorimetry. Both techniques indicate that these lipids prefer the fluid phase and that this preference is independent of the head group. The rates and apparent activation energies of lateral diffusion in fluid bilayers were found to be similar for most pyrene lipids, suggesting that the lateral movement of phospholipids is not critically dependent on interactions at the head-group level. Lateral distribution of the pyrene lipids in gel and fluid phosphatidylcholine bilayers was studied with the excimer technique and calorimetry. In gel-state dipalmitoylphosphatidylcholine bilayers, the pyrene lipids form clusters. These clusters, however, do not consist of pure pyrene lipid but of aggregates (compounds) of the labeled and unlabeled lipid.(ABSTRACT TRUNCATED AT 250 WORDS)

The phospholipid packing arrangement in small bilayer vesicles as revealed by proton magnetic resonance studies at 500 MHz

Proton magnetic resonance spectra of saturated phospholipids in small unilamellar vesicles has been recorded at 500 MHz on a Bruker WM500 spectrometer. The additional spectral dispersion reveals new structure in the acyl chain resonances. At temperatures near the thermal phase transition, the chain methylene and methyl peaks are split, both showing a broad and a relatively sharp component. Magnetization transfer experiments together with studies in the presence of manganese ions inside or outside the vesicles indicate that the sharp component is to be assigned to the protons from the acyl chains in the inner half of the bilayer and the broad component to chains in the outer monolayer. These experiments demonstrate unambiguously that the extreme surface curvature intrinsic to small unilamellar vesicles induces a profound asymmetry in the packing arrangement of the hydrocarbon chains in the two leaflets of the bilayer and causes the two monolayers to exist in markedly different motional states.

Effect of surface curvature on stability, thermodynamic behavior, and osmotic activity of dipalmitoylphosphatidylcholine single lamellar vesicles

The size and surface curvature dependence of the properties and stability of single lamellar vesicles have been investigated by using a variety of physicochemical techniques. Dipalmitoylphosphatidylcholine single lamellar vesicles of sizes ranging between 200 and 900 A in diameter have been prepared by the French press method and characterized with respect to their size distribution, stability, and thermotropic behavior by negative stain electron microscopy, molecular sieve chromatography, nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. Vesicles with a diameter smaller than 400 A are unstable below their transition temperature and fuse spontaneously to form larger single lamellar vesicles. Correlation analysis of experimentally obtained size distributions and calorimetric phase transitions profiles allowed estimation of the size dependence of the transition temperature. The phase transition temperature depends on the vesicle size in a sigmoidal fashion. Throughout the entire 200-700 A diamter range, the phase transition parameters are sensitive to size; however, the size dependence is especially pronounced around 400 A in diameter. The anomalous size dependence of the transition temperature for vesicles smaller than 400 A in diameter has been attributed to a decrease in the effective bilayer curvature due to packing rearrangements of the lipid molecules. Changes in the fractional degree of self-quenching of trapped 6-carboxyfluorescein induced by osmotic stress indicate that large single lamellar vesicles are not spherical under isoosmotic conditions. These vesicles are relatively flexible and can sustain almost a 2-fold increase in their internal aqueous volume without any leakage of the internal content.

Ion and sugar permeabilities of lecithin bilayers: comparison of curved and planar bilayers

Na+ and sugar permeabilities of egg lecithin bilayers were measured using curved bilayers and planar bilayers as represented by single-bilayer vesicles and black lipid films, respectively. The Na+ permeability coefficient measured with single-bilayer vesicles at 25 degrees C is (2.1 +/- 0.6) x 10(-13) cm sec-1. Because of technical difficulties it has been impossible to measure ionic permeabilities of values lower than about 10(-10) cm sec-1 in planar (black) lipid bilayers using tracer methods. The D-glucose and D-fructose permeabilities were measured with both curved and planar bilayers. The permeability coefficients measured with vesicles at 25 degrees C are (0.3 +/- 0.2) x 10(-10) cm sec-1 for glucose and (4 +/- 1) x 10(-10) cm sec-1 for D-fructose; these are in reasonable agreement with the corresponding values obtained for planar (black) lipid bilayers which are (1.1 +/- 0.3) x 10(-10) cm sec-1 for D-fructose, respectively.

The effect of surface curvature on the head-group structure and phase transition properties of phospholipid bilayer vesicles

Proton nuclear magnetic resonance spectra at 360 MHz of small sonicated distearoyl phosphatidylcholine vesicles show easily distinguishable resonances due to choline N-methyl head-group protons located in the inner and outer bilayer halves. A study of the chemical shift of these resonances as a function of temperature reveals that the splitting between them increases below the phase transition. This occurs as a result of an upfield shift of the inner layer resonance at the phase transition. Consideration of the possible causes of this effect results in the conclusion that, at the phase transition, there is a change in the organization of the inner layer head-groups which does not occur for the outer layer head-groups.

Fusion kinetics of phosphatidylcholine-phosphatidic acid mixed lipids vesicles: a proton nuclear magnetic resonance study

The kinetics of Ca2+-induced fusion of phosphatidylcholine-phosphatidic acid vesicles has been studied using the dependence of proton nuclear magnetic resonance linewidths on vesicle size. The linewidth of the lipid acyl chain methylene resonance has been shown to be sensitive to changes in vesicle size but insensitive to vesicle aggregation. For vesicle systems with the same lipid composition, the linewidth increases in a linear fashion with vesicle radius over the range 125--300 A. This dependence has been used to determine quantitatively fusion rates and the dependence of such rates of Ca2+ as well as an vesicle concentration. For vesicle concentrations in the range of 3 . 10(-6)--10(-5) M and Ca2+ concentration at a level approaching 1 : 1 with respect to phosphatidic acid, the initial fusion rates have been found to be fast, with half-times of 1--10 min. An order of reaction of 2.7 with respect to vesicle concentration has been observed. Mechanisms of vesicle fusion are discussed in view of these observations.

Effect of bilayer membrane curvature on activity of phosphatidylcholine exchange protein

Effect of bilayer membrane curvature of substrate phosphatidylcholine and inhibitor phosphatidylserine on the activity of phosphatidylcholine exchange protein has been studied by measuring transfer of spin-labeled phosphatidylcholine between vesicles, vesicles and liposomes, and between liposomes. The transfer rate between vesicles was more than 100 times larger than that between vesicles and liposomes. The transfer rate between liposomes was still smaller than that between vesicles and liposomes and nearly the same as that in the absence of exchange protein. The markedly enhanced exchange with vesicles was ascribed to the asymmetric packing of phospholipid molecules in the outer layer of the highly curved bilayer membrane. The inhibitory effect of phosphatidylserine was also greatly dependent on the membrane curvature. The vesicles with diameter of 17 nm showed more than 20 times larger inhibitory activity than those with diameter of 22 nm. The inhibitory effect of liposomes was very small. The size dependence was ascribed to stronger binding of the exchange protein to membranes with higher curvatures. The protein-mediated transfer from vesicles to spiculated erythrocyte ghosts was about four times faster than that to cup-shaped ghosts. This was ascribed to enhanced transfer to the highly curved spiculated membrane sites rather than greater mobility of phosphatidylcholine in the spiculated ghost membrane.

Physicochemical characterization of 1,2-diphytanoyl-sn-glycero-3-phosphocholine in model membrane systems

We report here on a series of studies aimed at characterization of the structural and dynamical properties of the synthetic lipid diphytanoyl phosphatidylcholine, in multilamellar dispersions and vesicle suspensions. The lipid exhibits no detectable gel to liquid crystalline phase transition over a large temperature range (-120 degrees C to +120 degrees C). Examination of proton nuclear magnetic resonance (NMR) free induction decays obtained from multilayer dispersions of diphytanoyl phosphatidylcholine provided an estimate of the methylene proton order parameter. The estimated magnitude of 0.21 is comparable to those determined for other phospholipids. Sonication of aqueous dispersions of diphytanoyl phosphatidylcholine led to formation of bilayer vesicles as determined by the measurement of the outer/inner choline methyl proton resonances, vesicle sizes in electron micrographs, and comparison of proton NMR linewidths between multilayer and sonicated dispersions. Ultracentrifugation studies of diphytanoyl phosphatidylcholine vesicles in H2O and 2H2O media yielded a value of 1.013 +/- 0.026 ml/g for the partial specific volume of this lipid. We have measured spin lattice relaxation rates for the methyl and methylenemethyne protons of the hydrocarbon chains of diphytanoyl phosphatidylcholine in bilayer vesicles over a range of temperatures and at two NMR frequencies (100 and 220 MHz). The observed relaxation rates for the methylene protons in this system were approximately twice those previously reported for dipalmitoyl phosphatidylcholine at comparable temperatures and resonance frequencies, whereas the relaxation rates measured for the methyl protons were greater than those of the straight chain lipid by an order of magnitude. Measurement of the spin lattice relaxation rates of the hydrocarbon protons of the diphytanoyl phosphatidylcholine in a 10 mol% mixture of the branched-chain lipid in a deuterated host lipid, diperdeuteropalmitoyl phosphatidylcholine, showed a discontinuity in the temperature dependence of the proton NMR longitudinal relaxation rates of the branched-chain lipid in the region of the gel to liquid crystalline phase transition temperature of the deuterated dipalmitoyl phosphatidylcholine host lipid. This result may be taken as evidence of lateral phase separation of a liquid cyrstalline phase enriched in diphytanoyl phosphatidylcholine from a gel phase enriched in diperdeuteropalmitoyl phosphatidylcholine at temperatures below the phase transition temperature of deuterated host lipid. This conclusion is supported by the observation of an abrupt change in the hydrocarbon methylene linewidth (at 100 MHz) of 10 mol% diphytanoyl phosphatidylcholine in diperdeuteropalmitoyl phosphatidylcholine over the temperature range where lateral phase separation is taking place according to differential thermograms.

Kinetic analysis of the interaction of the phosphatidylcholine exchange protein with unilamellar vesicels and multilamellar liposomes

The mode of action of the phosphatidylcholine exchange protein from bovine liver has been studied by using unilamellar vesicles and multilamellar liposomes both of which membranes contain phosphatidylcholine and phosphatidic acid. The protein-mediated exchange of phosphatidylcholine between vesicles and liposomes fit the kinetic model presented in a previous study [V.D. Besselaar et al. (1975) Biochemistry, 1j, 1852]. Kinetic analysis of the rates of exchange indicate that the apparent dissociation constant of the exchange protein-vesicle complex decreases with an increasing phosphatidic acid content of the vesicles. Both vesicles and liposomes of 10 mol% phosphatidic acid show the same dissociation constant; on the other hand, both the formation and the disruption of the protein-membrane complex was 50--100-times higher for the vesicles than for the liposomes. This implies that the exchange protein can discriminate between vesicles and liposomes. Equilibrium gel chromatography of a column of Bio Gel A-5m confirmed that the exchange protein binds more strongly to vesicles of an increased phosphatidic acid content. The protein-mediated exchange of phosphatidylcholine in the vesicle-liposome system demonstrates a pH optimum at 4.0 to 5.5. The kinetic analysis at pH 5.0 as compared to pH 7.4 indicates that the enhanced exchange at pH 5.0 can solely be accounted for by altered interaction of the exchange protein with the liposomes.

Nuclear magnetic resonance studies of lipid-protein interactions. A model of the dynamics and energetics of phosphatidylcholine bilayers that contain cytochrome c oxidase

Reconstituted membrane systems of synthetic phosphatidylcholines and the integral membrane enzyme cytochrome c oxidase were prepared in order to conduct nuclear magnetic resonance studies of lipid-protein interactions. These lipids, labeled with a geminate difluoro group on the 1-position hydrocarbon chain, were combined with the enzyme to give active lipid-protein particles with a well-defined ratio of lipid to protein. The fluorine magnetic resonance spectra of a series of preparations with different lipid/protein ratios suggest that the hydrocarbon chain mobility of the lipid is substantially reduced with increasing amounts of protein. The fluorine spectra of a single lipid-protein preparation show a dramatic increase in the number of the more mobile lipid chains with increasing temperature. The results suggest that the enzyme orders the lipid bilayer well beyond those lipids in direct contact with the protein surface, and that the amount of the lipid restricted by the enzyme is dependent upon temperature. The exchange of lipid between the restricted and the more mobile lipid environments most probably does not occur over the time scale measurable by the magnetic resonance techniques, about 10(-3) s.

A nuclear magnetic resonance study of sphingomyelin in bilayer systems

The physical properties of small single-walled vesicles composed of the zwitterionic phospholipid sphingomyelin have been studied using 1H and 31P nuclear magnetic resonance spectroscopy. The temperature variation of proton line widths and spin-lattice relaxation times and the chemical shift behavior for sphingomyelin vesicles are compared with results previously determined for phosphatidylcholine vesicles. Differences between the two systems are interpreted as indications of the presence of both inter- and intramolecular hydrogen bonding in sphingomyelin bilayers.

More on the motional state of lipid bilayer membranes: interpretation of order parameters obtained from nuclear magnetic resonance experiments

Proton and deuterium order parameters measured for the liquid crystalline phase of unsonicated lipid bilayer membranes are interpreted in terms of two motions: (i) chain reorientation and (ii) chain isomerization via kink diffusion. The observed order parameters are found to be compatible with angular deflections of the chain of about 50 degrees with respect to the bilayer normal, coupled with a probability of trans orientation of a methylene segment in the upper part of the chain of about 0.8-0.9. The motional model can be shown to account for the dynamic properties of the membrane system as measured by nuclear magnetic relaxation measurements, assuming that the chain isomerization occurs at a rate of approximately 10(10) s-1 and chain reorientation at a rate of approximately 10(7) s-1. Analysis of proton and deuterium line-width data in terms of this model shows that sonication has the effect of increasing the rate and amplitude of chain reorientation without substantially changing the isomerization motion along the acyl chain. These conclusions are briefly compared with similar observations recently reported in Raman spectroscopic studies.

Protein-catalyzed exchange of phosphatidylcholine between sonicated liposomes and multilamellar vesicles

Phospholipid exchange protein from beef heart or beef liver does not catalyze the transfer of phosphatidylcholine from multilamellar vesicles of phosphatidylcholine. Certain combinations of phospholipids, however, do yield multilamellar vesicles that will exchange phosphatidylcholine with liposomes in the presence of exchange protein. Multilamellar vesicles of phosphatidylcholine:phosphatidylethanolamine:cardiolipin (70:25:5, mol%) can be used in place of mitochondria or erythrocyte ghosts as an improved acceptor particle in the study of liposome structure with phospholipid exchange proteins. These multilamellar vesicles act as a well-defined reservoir of unlabeled phosphatidylcholine with 7% exchangable phospholipid. When the distribution of phosphatidylcholine in liposomes is studied by the exchange protein technique, results can be influence by the choice of phospholipid acceptor particle. With mitochondria as acceptor particle, the percentage of phosphatidylcholine in the outer monolayer of a liposome appears to be 60%, whereas a value of 70% is obtained when multilamellar vesicles are the acceptor. The discrepancy can be explained by a heterogeneity in liposomes prepared by sonication. A size-dependent fusion or adsorption process occurs between liposomes and mitochondria; the very small liposomal vesicles, obtained by gel filtration, combine nearly quantitatively with the natural membrane. This phenomenon is not seen with multilamellar vesicles. Thus by using multilamellar vesicles one obtains a less biased estimate of phospholipid distribution between inner and outer layers of liposomes.

Effect of lysolecithin on the structure and permeability of lecithin bilayer vesicles

In order to elucidate the role of lysolecithin in membranes, we have examined the effect of lysolecithin on the structure and permeability of lecithin bilayer membranes. Small L-alpha-dimyristoyllecithin (DML) vesicles with myristoyllysolecithin (MLL) incorporated as well as small L-alpha-dipalmitoyllecithin (DPL) vesicles with palmitoyllysolecithin (PLL) were studied by nuclear magnetic resonance (NMR) methods at temperatures both above and below the alpha-gel in equilibrium liquid crystalline phase transition temperature (Tc) and as a function of the concentration of the incorporated lysolecithin. Europium (III) ion was used as a probe to measure the permeability of the vesicular bilayer membrane. At temperatures below Tc, these vesicles were found to be extremely permeable to europium (III) ions. The ion translocation was found to be too fast to be measured by the NMR method under these conditions. However, above the phase transition temperature the ionic permeability decreases to a rate which could be conveniently monitored, and the permeability was shown to increase with temperature and lysolecithin concentration. Analysis of the lysolecithin concentration dependence suggests the formation of ion channels within the lipid bilayer involving four lysolecithin molecules. The data below Tc suggest a phase separation below the phase transition temperature of the host lipid, leading to the formation of patches of lysolecithin molecules within the lecithin matrix. These lysolecithin clusters are presumably long-lived under these conditions and are sufficiently structurally perturbed or disordered to serve as channels for rapid ion permeation.

Biological and model membranes studied by nuclear magnetic resonance of spin one half nuclei

There is an inherent anisotropy in biological and model membrane systems and this anisotropy has a profound influence on the nuclear magnetic resonance spectra of such systems. For nuclei with a quadrupole moment the quadrupole coupling usually dominates the NMR spectrum, while for nuclei with spin quantum numberI= ½ dipolar couplings provides the most important effects. The quadrupole coupling only affects isolated spins and usually gives rise to simple spectra. The dipole interactions on the other hand couple several spins. In a lipid bilayer with a multitude of spin-½ nuclei these couplings can give rise to complicated manybody effects. Thus the interpretation of the NMR spectra of membrane systems poses problems but at the same time there is a lot of information to be gained.

Effect of bilayer curvature on vibrational Raman spectroscopic behavior of phospholipid-water assemblies

In order to clarify the effect of bilayer curvature upon phospholipid conformation, vibrational Raman spectra were recorded for dipalmitoyl and dimyristroyl phosphatidylcholine in the gel state for both multilayer and single-wall vesicle assemblies. An intensity comparison, based upon a nonperturbing internal standard, between the two classes of bilayrer systems reflected a decrease in peak height intensity for the observed hydrocarbon chain transitions in the single shell vesicle form. No intensity change between bilayer form was detected, however, for the two observed head group modes. Trends in the peak height intensity rations for the 1100 cm-1 carbon-carbon stretching vibrations indicated an increase in hydrocarbon chain transgauche isomerization for the vesicle in comparison to the multilayer arrangements. The sensitivity of the methylene carbon-hydrogen stretching modes to interchain interactions was demonstrated by comparisons of the intensity patterns in the 2900 cm-1 region to the intensity characteristics of the carbon-carbon stretching region for polycrystalline, multilayer and vesicle materials. Examination of various carbon-carbon stretching mode intensity ratios for cholesterol doped dipalmitoyl phosphatidylcholine bilayers indicated that while 25 mol% cholesterol increased the transgauche acyl chain isomerization in multilayers, no comparable effect was observed for the vesicle forms. In contrast, the methylene twisting/methylene deformation intensity ratios for the cholesterol containing systems suggested that some further type of interchain perturbation occurs in the vesicle aggregations.