Structure and thermotropic properties of hydrated N-stearoyl sphingomyelin bilayer membranes

Molecular Structure of Sphingomyelin in Fluid Phase Bilayers Determined by the Joint Analysis of Small-Angle Neutron and X-ray Scattering Data

We have determined the fluid bilayer structure of palmitoyl sphingomyelin (PSM) and stearoyl sphingomyelin (SSM) by simultaneously analyzing small-angle neutron and X-ray scattering data. Using a newly developed scattering density profile (SDP) model for sphingomyelin lipids, we report structural parameters including the area per lipid, total bilayer thickness, and hydrocarbon thickness, in addition to lipid volumes determined by densitometry. Unconstrained all-atom simulations of PSM bilayers at 55 °C using the C36 CHARMM force field produced a lipid area of 56 Ų, a value that is 10% lower than the one determined experimentally by SDP analysis (61.9 Ų). Furthermore, scattering form factors calculated from the unconstrained simulations were in poor agreement with experimental form factors, even though segmental order parameter (S_CD) profiles calculated from the simulations were in relatively good agreement with S_CD profiles obtained from NMR experiments. Conversely, constrained area simulations at 61.9 Ų resulted in good agreement between the simulation and experimental scattering form factors, but not with S_CD profiles from NMR. We discuss possible reasons for the discrepancies between these two types of data that are frequently used as validation metrics for molecular dynamics force fields.

Surface Shear Viscosity and Interleaflet Friction from Nonequilibrium Simulations of Lipid Bilayers

Nonequilibrium simulation protocols based on shear deformations are applied to determine the surface viscosity and interleaflet friction of lipid bilayers. At high shear rates, a non-Newtonian shear thinning regime is observed, but lower shear rates yield a Newtonian plateau and results that are consistent with equilibrium measurements based on fluctuation-dissipation theorems. Application to all-atom bilayers modeled with the CHARMM36 parameter set yields values for the surface viscosity that are consistent with microscopic measurements based on membrane protein diffusion but are approximately 10 times lower than more macroscopic experimental measurements. The interleaflet friction is about 10 times lower than experimental measurements. Trends across different lipids, temperatures, and ternary liquid-disordered phase mixtures produce results that are consistent with experimental diffusion constants. Application of the protocol to the liquid-ordered phase fails to yield a Newtonian plateau, suggesting more complex rheology.

Mechanical properties of bilayers containing sperm sphingomyelins and ceramides with very long-chain polyunsaturated fatty acids

Sphingomyelins (SM) and ceramides (Cer) with very long chain polyunsaturated fatty acids (V) are important components of spermatozoa membranes. In this study, the mechanical properties of bilayers of SM and Cer with nonhydroxy (n-V) and 2-hydroxy (h-V) fatty acid (30:5) were studied by molecular dynamics simulation at different temperatures and in the presence and the absence of salt. From our results, it was evidenced how n-V SM and h-V SM bilayers showed similar behavior. When n-V Cer was added to a h-V SM bilayer, the Gaussian curvature modulus and E_curve of binary bilayers decreased. This variation in the mechanical properties of the bilayer can be associated with an incipient step during the fecundation process.

Detection of Sphingomyelin Clusters by Raman Spectroscopy

Sphingomyelin (SM) is a major sphingolipid in mammalian cells that forms specific lipid domains in combination with cholesterol (Chol). Using molecular-dynamics simulation and density functional theory calculation, we identified a characteristic Raman band of SM at ∼1643 cm(-1) as amide I of the SM cluster. Experimental results indicate that this band is sensitive to the hydration of SM and the presence of Chol. We showed that this amide I Raman band can be utilized to examine the membrane distribution of SM. Similarly to SM, ceramide phosphoethanolamine (CerPE) exhibited an amide I Raman band in almost the same region, although CerPE lacks three methyl groups in the phosphocholine moiety of SM. In contrast to SM, the amide I band of CerPE was not affected by Chol, suggesting the importance of the methyl groups of SM in the SM-Chol interaction.

Miscibility of Sphingomyelins and Phosphatidylcholines in Unsaturated Phosphatidylcholine Bilayers

Polyunsaturated phospholipids are common in biological membranes and affect the lateral structure of bilayers. We have examined how saturated sphingomyelin (SM; palmitoyl and stearoyl SM (PSM and SSM, respectively)) and phosphatidylcholine (PC; dipalmitoyl PC and 1-palmitoyl-2-stearoyl PC (DPPC and PSPC, respectively)) segregate laterally to form ordered gel phases in increasingly unsaturated PC bilayers (sn-1: 16:0 and sn-2: 18:1...22:6; or sn-1 and sn-2: 18:1...22:6). The formation of gel phases was determined from the lifetime analysis of trans-parinaric acid. Using calorimetry, we also determined gel phase formation by PSM and DPPC in unsaturated PC mixed bilayers. Comparing PSM with DPPC, we observed that PSM formed a gel phase with less order than DPPC at comparable bilayer concentrations. The same was true when SSM was compared with PSPC. Furthermore, we observed that at equal saturated phospholipid concentration, the gel phases formed were less ordered in unsaturated PCs having 16:0 in sn-1, as compared to PCs having unsaturated acyl chains in both sn-1 and sn-2. The gel phases formed by the saturated phospholipids in unsaturated PC bilayers did not appear to achieve properties similar to pure saturated phospholipid bilayers, suggesting that complete lateral phase separation did not occur. Based on scanning calorimetry analysis, the melting of the gel phases formed by PSM and DPPC in unsaturated PC mixed bilayers (at 45 mol % saturated phospholipid) had low cooperativity and hence most likely were of mixed composition, in good agreement with trans-parinaric acid lifetime data. We conclude that both interfacial properties of the saturated phospholipids and their chain length, as well as the presence of 16:0 in sn-1 of the unsaturated PCs and the total number of cis unsaturations and acyl chain length (18 to 22) of the unsaturated PCs, all affected the formation of gel phases enriched in saturated phospholipids, under the conditions used.

Direct visualization of the action of Triton X-100 on giant vesicles of erythrocyte membrane lipids

The raft hypothesis proposes that microdomains enriched in sphingolipids, cholesterol, and specific proteins are transiently formed to accomplish important cellular tasks. Equivocally, detergent-resistant membranes were initially assumed to be identical to membrane rafts, because of similarities between their compositions. In fact, the impact of detergents in membrane organization is still controversial. Here, we use phase contrast and fluorescence microscopy to observe giant unilamellar vesicles (GUVs) made of erythrocyte membrane lipids (erythro-GUVs) when exposed to the detergent Triton X-100 (TX-100). We clearly show that TX-100 has a restructuring action on biomembranes. Contact with TX-100 readily induces domain formation on the previously homogeneous membrane of erythro-GUVs at physiological and room temperatures. The shape and dynamics of the formed domains point to liquid-ordered/liquid-disordered (Lo/Ld) phase separation, typically found in raft-like ternary lipid mixtures. The Ld domains are then separated from the original vesicle and completely solubilized by TX-100. The insoluble vesicle left, in the Lo phase, represents around 2/3 of the original vesicle surface at room temperature and decreases to almost 1/2 at physiological temperature. This chain of events could be entirely reproduced with biomimetic GUVs of a simple ternary lipid mixture, 2:1:2 POPC/SM/chol (phosphatidylcholine/sphyngomyelin/cholesterol), showing that this behavior will arise because of fundamental physicochemical properties of simple lipid mixtures. This work provides direct visualization of TX-100-induced domain formation followed by selective (Ld phase) solubilization in a model system with a complex biological lipid composition.

Solubilization of binary lipid mixtures by the detergent Triton X-100: the role of cholesterol

The solubilization of lipid bilayers of different composition and phase by the detergent Triton X-100 (Triton X-100) was investigated using optical and fluorescence microscopy of giant unilamellar vesicles (GUVs) and light scattering of large unilamellar vesicles (LUVs). The compositions explored were 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), in the liquid-disordered (Ld) phase, sphingomyelin (SM), in the gel phase, and binary mixtures of these phospholipids with 30 mol % cholesterol (chol), resulting in bilayers in the Ld and liquid-ordered (Lo) phases, respectively. We show that the phospholipid bilayers are completely soluble in TX-100, but optical microscopy reveals that whereas fluid POPC is gradually solubilized by TX-100, gel SM is first shattered in bilayer fragments. Incorporation of TX-100 in the membrane leads to increase in GUV area, which was quantified and expressed as bound detergent-to-lipid molar ratio. The partition of TX-100 in POPC is very high, decreases in POPC/chol, and is negligible in SM/chol. Fluorescence microscopy shows that TX-100 induces Lo/Ld phase separation in previously homogeneous POPC/chol GUVs, and insoluble bilayer fragments/vesicles are detected with optical microscopy and light scattering. Vesicles of SM/chol, in the Lo phase, are virtually insoluble in TX-100. Taken together, our results show that the presence of cholesterol is the origin of membrane resistance to solubilization, which depending on the specific lipid mixture can result in either partially (POPC/chol) or completely (SM/chol) insoluble mixtures.

Phase diagram and tie-line determination for the ternary mixture DOPC/eSM/cholesterol

We propose a novel, to our knowledge, method for the determination of tie lines in a phase diagram of ternary lipid mixtures. The method was applied to a system consisting of dioleoylphosphatidylcholine (DOPC), egg sphingomyelin (eSM), and cholesterol (Chol). The approach is based on electrofusion of single- or two-component homogeneous giant vesicles in the fluid phase and analyses of the domain areas of the fused vesicle. The electrofusion approach enables us to create three-component vesicles with precisely controlled composition, in contrast to conventional methods for giant vesicle formation. The tie lines determined in the two-liquid-phase coexistence region are found to be not parallel, suggesting that the dominant mechanism of lipid phase separation in this region changes with the membrane composition. We provide a phase diagram of the DOPC/eSM/Chol mixture and predict the location of the critical point. Finally, we evaluate the Gibbs free energy of transfer of individual lipid components from one phase to the other.

Phase behavior and domain size in sphingomyelin-containing lipid bilayers

Membrane raft size measurements are crucial to understanding the stability and functionality of rafts in cells. The challenge of accurately measuring raft size is evidenced by the disparate reports of domain sizes, which range from nanometers to microns for the ternary model membrane system sphingomyelin (SM)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/cholesterol (Chol). Using Förster resonance energy transfer (FRET) and differential scanning calorimetry (DSC), we established phase diagrams for porcine brain SM (bSM)/dioleoyl-sn-glycero-3-phosphocholine (DOPC)/Chol and bSM/POPC/Chol at 15 and 25°C. By combining two techniques with different spatial sensitivities, namely FRET and small-angle neutron scattering (SANS), we have significantly narrowed the uncertainty in domain size estimates for bSM/POPC/Chol mixtures. Compositional trends in FRET data revealed coexisting domains at 15 and 25°C for both mixtures, while SANS measurements detected no domain formation for bSM/POPC/Chol. Together these results indicate that liquid domains in bSM/POPC/Chol are between 2 and 7nm in radius at 25°C: that is, domains must be on the order of the 2-6nm Förster distance of the FRET probes, but smaller than the ~7nm minimum cluster size detectable with SANS. However, for palmitoyl SM (PSM)/POPC/Chol at a similar composition, SANS detected coexisting liquid domains. This increase in domain size upon replacing the natural SM component (which consists of a mixture of chain lengths) with synthetic PSM, suggests a role for SM chain length in modulating raft size in vivo.

Lipid sorting by ceramide and the consequences for membrane proteins

We mimicked the effect of sphingomyelinase activity on lipid mixtures of palmitoyl-oleoyl-phosphatidylcholine, sphingomyelin, ceramide, and 10 mol % cholesterol. Using x-ray diffraction experiments in combination with osmotic stress we found, in agreement with previous studies, that ceramide induces a coexistence of L(α) and L(β) domains. A detailed structural analysis of the coexisting domains demonstrated an increase of lipid packing density and membrane thickness in the L(α) domains upon increasing overall ceramide levels. This provides evidence for a ceramide-driven accumulation of cholesterol in the L(α) domains, in support of previous reports. We further determined the bending rigidities of the coexisting domains and found that the accumulation of cholesterol in the L(α) domains stabilizes their bending rigidity, which experiences a dramatic drop in the absence of cholesterol. Deriving experimental estimates for the spontaneous curvature and Gaussian modulus of curvature, we show, using a simple geometric model for ion channels, that in this way changes in the conformational equilibrium of membrane proteins can be kept small.

Another Piece of the Membrane Puzzle: Extending Slipids Further

To be able to model complex biological membranes in a more realistic manner, the force field Slipids (Stockholm lipids) has been extended to include parameters for sphingomyelin (SM), phosphatidylglycerol (PG), phosphatidylserine (PS) lipids, and cholesterol. Since the parametrization scheme was faithful to the scheme used in previous editions of Slipids, all parameters are consistent and fully compatible. The results of careful validation of a number of key structural properties for one and two component lipid bilayers are in excellent agreement with experiments. Potentials of mean force for transferring water across binary mixtures of lipids and cholesterol were also computed in order to compare water permeability rates to experiments. In agreement with experimental and simulation studies, it was found that the permeability and partitioning of water is affected by cholesterol in lipid bilayers made of saturated lipids to the largest extent. With the extensions of Slipids presented here, it is now possible to study complex systems containing many different lipids and proteins in a fully atomistic resolution in the isothermic-isobaric (NPT) ensemble, which is the proper ensemble for membrane simulations.

Effect of hydrophobic mismatch and interdigitation on sterol/sphingomyelin interaction in ternary bilayer membranes

Sphingomyelin (SM) is a major phospholipid in most cell membranes. SMs are composed of a long-chain base (often sphingosine, 18:1(Δ4t)), and N-linked acyl chains (often 16:0, 18:0 or 24:1(Δ15c)). Cholesterol interacts with SM in cell membranes, but the acyl chain preference of this interaction is not fully elucidated. In this study we have examined the effects of hydrophobic mismatch and interdigitation on cholesterol/sphingomyelin interaction in complex bilayer membranes. We measured the capacity of cholestatrienol (CTL) and cholesterol to form sterol-enriched ordered domains with saturated SM species having different chain lengths (14 to 24 carbons) in ternary bilayer membranes. We also determined the equilibrium bilayer partitioning coefficient of CTL with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes containing 20mol% of saturated SM analogs. Ours results show that while CTL and cholesterol formed sterol-enriched domains with both short and long-chain SM species, the sterols preferred interaction with 16:0-SM over any other saturated chain length SM analog. When CTL membrane partitioning was determined with fluid POPC bilayers containing 20mol% of a saturated chain length SM analog, the highest affinity was seen with 16:0-SM (both at 23 and 37°C). These results indicate that hydrophobic mismatch and/or interdigitation attenuate sterol/SM association and thus affect lateral distribution of sterols in the bilayer membrane.

Sphingomyelin analogs with branched N-acyl chains: the position of branching dramatically affects acyl chain order and sterol interactions in bilayer membranes

Sphingolipids have been found to have single methyl branchings both in their long-chain base and in their N-linked acyl chains. In this study we determined how methyl-branching in the N-linked acyl chain of sphingomyelin (SM) affected their membrane properties. SM analogs with a single methyl-branching at carbon 15 (of a 17:0 acyl chain; anteiso) had a lower gel-liquid transition temperature as compared to an iso-branched SM analog. Phytanoyl SM (methyls at carbons 3, 7, 11 and 15) as well as a SM analog with a methyl on carbon 10 in a hexadecanoyl chain failed to show a gel-liquid transition above 10 degrees C. Only the two distally branched SM analogs (iso and anteiso) formed ordered domains with cholesterol in a 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayer. However, domains formed by the branched SM analogs appeared to contain less sterol when compared to palmitoyl SM (PSM) as the saturated phospholipid. Sterol-enriched domains formed by the anteiso SM analog were also less stable against temperature than domains formed by PSM. Both the 10-methyl and phytanoyl SM analogs failed to form sterol-enriched domains in the POPC bilayer. Acyl chain branching weakened SM/sterol interactions markedly when compared to PSM, as also evidenced from the decreased affinity of cholestatrienol to bilayers containing branched SM analogs. Our results show that methyl-branching weakened intermolecular interactions in a position-dependent manner.

Using 31P MAS NMR to monitor a gel phase thermal disorder transition in sphingomyelin/cholesterol bilayers

The impact of low cholesterol concentrations on an egg sphingomyelin bilayer is investigated using 31P magic angle spinning (MAS) NMR spectroscopy. The magnitude of the isotropic 31P MAS NMR line width is used to monitor the main gel to liquid crystalline phase transition, along with a unique gel phase pretransition. In addition, the 31P chemical shift anisotropy (CSA) and spin-spin relaxation times (T2), along with the effects of spinning speed, proton decoupling and magnetic field strength, are reported. The variation of this unique gel phase thermal pretransition with the inclusion of 5 through 21 mol% cholesterol is presented and discussed.

Cholesterol-sphingomyelin interactions: a molecular dynamics simulation study

Stearoylsphingomyelin (SSM) bilayers containing 0, 22, and 50 mol % cholesterol (Chol) and a pentadecanoyl-stearoylphosphatidylcholine (15SPC) bilayer containing 22 mol % Chol were molecular dynamics simulated at two temperatures (37 degrees C and 60 degrees C). 15SPC is the best PC equivalent of SSM. The Chol effect on the SSM bilayer differs significantly from that on the 15SPC bilayer. At the same temperature and Chol content, H-bonding of Chol with SSM is more extensive than with 15SPC. SSM-Chol H-bonding anchors the OH group of Chol in the lower regions of the SSM-Chol bilayer interface. Such a location strengthens the influence of Chol on the SSM chains. In effect, the phase of the SSM-Chol bilayer containing 22 mol % Chol at 37 degrees C is shifted from the gel to the liquid-ordered phase, and the bilayer displays similar properties below and above the main phase-transition temperature for a pure SSM bilayer of approximately 45 degrees C. In contrast, due to a higher location, Chol is not able to change the phase of the 15SPC-Chol bilayer, which at 37 degrees C remains in the gel phase. Chol affects both the core and interface of the SSM bilayer. With increasing Chol content, the order of SSM chains and hydration of SSM headgroups increase, whereas polar interactions between lipids decrease.

Lipid lateral diffusion in bilayers with phosphatidylcholine, sphingomyelin and cholesterol

Pulsed field gradient (pfg)-NMR measurements of the lipid lateral diffusion coefficients in several macroscopically aligned bilayer systems were summarized from previous and new studies. The aim was to carry out a comparison of the translational dynamics for bilayers with various mixtures of l,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), l,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and chicken egg yolk sphingomyelin (eSM), with or without cholesterol. New useful information was obtained on the dynamics in these lipid bilayers that has not been previously appreciated. Thus, we were able to propose that the driving force behind the phase separation into l(d)and l(o)phases evolves from the increasing difficulty to incorpotate DOPC into a highly ordered phase. Our results suggest that DOPC has a preference to be located in a disordered phase, while DPPC and eSM prefer the ordered phase. Quite unexpectedly, CHOL seems to partition into both phases to roughly the same extent, indicating that CHOL has no particular preference for any of the l(d)or l(o) phases, and there are no specific interactions between CHOL and saturated lipids.

Effect of hydrophobic mismatch on phase behavior of lipid membranes

We investigate the competing effects of hydrophobic mismatch and chain stretching on the morphology and evolution of domains in lipid membranes via Monte Carlo techniques. We model the membrane as a binary mixture of particles that differ in their preferred lengths, with the shorter particles mimicking unsaturated nonraft lipids and the longer particles mimicking saturated raft lipids. We find that phase separation can be induced upon increasing either the ratio J/kappa of the hydrophobic surface tension J to the compressibility modulus kappa. J/kappa determines the decay length for thickness changes. When this decay length is larger than the system size the membrane remains mixed. Furthermore, increasing the thickness relaxation time can induce transient phase separation.

Sphingomyelin structure influences the lateral diffusion and raft formation in lipid bilayers

Liquid-disordered/liquid-ordered two-phase coexistence regions in hydrated bilayers have been investigated for sphingomyelins (SMs) of three different origins: egg, brain, and milk with the pulsed-field gradient NMR technique for lateral diffusion measurement. It is found that the three SMs have the same diffusional behavior in bilayers of SM alone, but in the multicomponent systems of dioleoylphosphatidylcholine/SM/cholesterol, the ability to form domains differs for the three SMs. The two-phase area is more extended for egg SM than for brain SM, and no two-phase coexistence is found for milk SM. The differences in behavior are correlated with the homogeneity of the SM hydrocarbon chain compositions, in which egg SM has the most homogeneous and milk SM has the most heterogeneous composition. The results indicate that a crucial element in the domain-forming process is the formation of highly packed bilayers of SM and cholesterol rather than specific interactions between SM and cholesterol.

Influence of chain length and unsaturation on sphingomyelin bilayers

Sphingomyelins (SMs) are among the most common phospholipid components of plasma membranes, usually constituting a mixture of several molecular species with various fatty acyl chain moieties. In this work, we utilize atomistic molecular dynamics simulations to study the differences in structural and dynamical properties of bilayers comprised of the most common natural SM species. Keeping the sphingosine moiety unchanged, we vary the amide bonded acyl chain from 16 to 24 carbons in length and examine the effect of unsaturation by comparing lipids with saturated and monounsaturated chains. As for structural properties, we find a slight decrease in average area per lipid and a clear linear increase in bilayer thickness with increasing acyl chain length both in saturated and unsaturated systems. Increasing the acyl chain length is found to further the interdigitation across the bilayer center. This is related to the dynamics of SM molecules, as the lateral diffusion rates decrease slightly for an increasing acyl chain length. Interdigitation also plays a role in interleaflet friction, which is stronger for unsaturated chains. The effect of the cis double bond is most significant on the local order parameters and rotation rates of the chains, though unsaturation shows global effects on overall lipid packing and dynamics as well. Regarding hydrogen bonding or properties related to the lipid/water interface region, no significant effects were observed due to varying chain length or unsaturation. The significance of the findings presented is discussed.

Simulation of the early stages of nano-domain formation in mixed bilayers of sphingomyelin, cholesterol, and dioleylphosphatidylcholine

It is known from experimental studies that lipid bilayers composed of unsaturated phospholipids, sphingomyelin, and cholesterol contain microdomains rich in sphingomyelin and cholesterol. These domains are similar to "rafts" isolated from cell membranes, although the latter are much smaller in lateral size. Such domain formation can be a result of very specific and subtle lipid-lipid interactions. To identify and study these interactions, we have performed two molecular dynamics simulations, of 200-ns duration, of dioleylphosphatidylcholine (DOPC), sphingomyelin (SM), and cholesterol (Chol) systems, a 1:1:1 mixture of DOPC/SM/Chol, and a 1:1 mixture of DOPC/SM. The simulations show initial stages of the onset of spontaneous phase-separated domains in the systems. On the simulation timescale cholesterol favors a position at the interface between the ordered SM region and the disordered DOPC region in the ternary system and accelerates the process of domain formation. We find that the smooth alpha-face of Chol preferentially packs next to SM molecules. Based on a comparative analysis of interaction energies, we find that Chol molecules do not show a preference for SM or DOPC. We conclude that Chol molecules assist in the process of domain formation and the process is driven by entropic factors rather than differences in interaction energies.

Structure and dynamics of sphingomyelin bilayer: insight gained through systematic comparison to phosphatidylcholine

Sphingomyelin, one of the main lipid components of biological membranes, is actively involved in various cellular processes such as protein trafficking and signal transduction. In particular, specific lateral domains enriched in sphingomyelin and cholesterol have been proposed to play an important functional role in biomembranes, although their precise characteristics have remained unclear. A thorough understanding of the functional role of membranes requires detailed knowledge of their individual lipid components. Here, we employ molecular dynamics simulations to conduct a systematic comparison of a palmitoylsphingomyelin (PSM, 16:0-SM) bilayer with a membrane that comprises dipalmitoylphosphatidylcholine (DPPC) above the main phase transition temperature. We clarify atomic-scale properties that are specific to sphingomyelin due to its sphingosine moiety, and further discuss their implications for SM-rich membranes. We find that PSM bilayers, and in particular the dynamics of PSM systems, are distinctly different from those of a DPPC bilayer. When compared with DPPC, the strong hydrogen bonding properties characteristic to PSM are observed to lead to considerable structural changes in the polar headgroup and interface regions. The strong ordering of PSM acyl chains and specific ordering effects in the vicinity of a PSM-water interface reflect this issue clearly. The sphingosine moiety and related hydrogen bonding further play a crucial role in the dynamics of PSM bilayers, as most dynamic properties, such as lateral and rotational diffusion, are strongly suppressed. This is most evident in the rotational motion characterized by spin-lattice relaxation times and the decay of hydrogen bond autocorrelation functions that are expected to be important in complexation of SM with other lipids in many-component bilayers. A thorough understanding of SM bilayers would greatly benefit from nuclear magnetic resonance experiments for acyl chain ordering and dynamics, allowing full comparison of these simulations to experiments.

Structure of sphingomyelin bilayers: a simulation study

We have carried out a molecular dynamics simulation of a hydrated 18:0 sphingomyelin lipid bilayer. The bilayer contained 1600 sphingomyelin (SM) molecules, and 50,592 water molecules. After construction and initial equilibration, the simulation was run for 3.8 ns at a constant temperature of 50 degrees C and a constant pressure of 1 atm. We present properties of the bilayer calculated from the simulation, and compare with experimental data and with properties of dipalmitoyl phosphatidylcholine (DPPC) bilayers. The SM bilayers are significantly more ordered and compact than DPPC bilayers at the same temperature. SM bilayers also exhibit significant intramolecular hydrogen bonding between phosphate ester oxygen and hydroxyl hydrogen atoms. This results in a decreased hydration in the polar region of the SM bilayer compared with DPPC. Since our simulation system is very large we have calculated the power spectrum of bilayer undulation and peristaltic modes, and we compare these data with similar calculations for DPPC bilayers. We find that the SM bilayer has significantly larger bending modulus and area compressibility compared to DPPC.

Combined Monte Carlo and molecular dynamics simulation of hydrated 18:0 sphingomyelin–cholesterol lipid bilayers

We have carried out atomic level molecular dynamics and Monte Carlo simulations of hydrated 18:0 sphingomyelin (SM)-cholesterol (CHOL) bilayers at temperatures of 20 and 50 degrees C. The simulated systems each contained 266 SM, 122 CHOL, and 11861 water molecules. Each simulation was run for 10 ns under semi-isotropic pressure boundary conditions. The particle-mesh Ewald method was used for long-range electrostatic interactions. Properties of the systems were calculated over the final 3 ns. We compare the properties of 20 and 50 degrees C bilayer systems with each other, with experimental data, and with experimental and simulated properties of pure SM bilayers and dipalmitoyl phospatidyl choline (DPPC)-CHOL bilayers. The simulations reveal an overall similarity of both systems, despite the 30 degrees C temperature difference which brackets the pure SM main phase transition. The area per molecule, lipid chain order parameter profiles, atom distributions, and electron density profiles are all very similar for the two simulated systems. Consistent with simulations from our lab and others, we find strong intramolecular hydrogen bonding in SM molecules between the phosphate ester oxygen and the hydroxyl hydrogen atoms. We also find that cholesterol hydroxyl groups tend to form hydrogen bonds primarily with SM carbonyl, methyl, and amide moieties and to a lesser extent methyl and hydroxyl oxygens.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Conformational studies of sphingolipids by NMR spectroscopy. II. Sphingomyelin

Sphingomyelin (SM) is the most prevalent sphingolipid in the majority of mammalian membranes. Proton and 31P nuclear magnetic resonance spectral data were acquired to establish the nature of intra- and intermolecular H-bonds in the monomeric and aggregated forms of SM and to assess possible differences between this lipid and dihydrosphingomyelin (DHSM), which lacks the double bond between carbons 4 and 5 of the sphingoid base. The spectral trends suggest the formation of an intramolecular H-bond between the OH group of the sphingosine moiety and the phosphate ester oxygen of the head group. The narrower linewidth and the downfield shift of the resonance corresponding to OH proton in SM suggest that this H-bond is stronger in SM than in DHSM. The NH group appears to be involved predominantly in intramolecular H-bonding in the monomer. As the concentration of SM increases and the molecules come in closer proximity, these intramolecular bonds are partially disrupted and the NH group becomes involved in lipid-water interactions. The difference between the SM and DHSM appears to be not in the nature of these interactions but rather in the degree to which these intermolecular interactions prevail. As SM molecules cannot come as close together as DHSM molecules can, both the NH and OH moieties remain, on average, more intramolecularly bonded as compared to DHSM.

Conformational studies of sphingolipids by NMR spectroscopy. I. Dihydrosphingomyelin

The conformational features of dihydrosphingomyelin (DHSM), the major phospholipid of human lens membranes, were investigated by 1H and 31P nuclear magnetic resonance spectroscopy. Several postulates emerge from the observed trends: (a) in partially hydrated samples of DHSM in CDCl3 above 13 mM, at which lipid-lipid interactions prevail, the amide proton is mostly involved in intermolecular H-bonds that link neighboring phospholipids through bridging water molecules. In the absence of water, the NH group is involved in an intramolecular H-bond that restricts the mobility of the phosphate group. (b) In the monomeric form of the lipid molecule, the amide proton of the major conformer is bound intramolecularly with one of the anionic and/or ester oxygens of the phosphate group. A minor conformer may also be present in which the NH proton participates in an intramolecular H-bond linking to the OH group of the sphingoid base. (c) Complete hydration leads to an extension of the head group as water molecules bind to the phosphate and NH groups via H-bonds, thus disrupting the intramolecular H-bonds prevalent at low concentrations.

Separation and purification of sphingomyelin diastereomers by high-performance liquid chromatography

All naturally occurring sphingomyelins have the d-erythro-(2S,3R) configuration of the sphingoid base. We have developed a normal-phase HPLC method for the separation of this natural stereoisomer from the l-threo-sphingomyelin, which is the other stereoisomer commonly present in semisynthetic preparations of acyl-chain defined sphingomyelins. The chromatographic method was developed by modification of a previously reported method for phospholipid separation on a normal-phase diol column. The separation was accomplished by a binary gradient of solvent mixtures (A) hexane:isopropanol:acetic acid (82:17:1.0 by vol) and (B) isopropanol:water:acetic acid (85:14:1.0 by vol) with 0.08 vol% triethylamine added to both solvent mixtures. The program of gradient elution was optimized for maximal separation of sphingomyelin diastereomers. For detection of the lipids, a light-scattering detector was used. This analytical scale HPLC method was also used for purification of the stereoisomers (up to 0.5 mg of N-oleoyl-sphingomyelin in a single injection). The purified stereoisomers were at least 99% pure according to high-performance thin-layer chromatography and analytical HPLC.

Analysis of natural and synthetic sphingomyelins using high-performance thin-layer chromatography

The chromatographic behaviour of molecular species of sphingomyelin on HPTLC was investigated. Sphingomyelin gave a double band pattern on HPTLC plates developed using chloroform/methanol/acetic acid/water (25 : 15 : 4 : 2, v/v) or chloroform/methanol/water (25 : 10 : 1.1, v/v). HPTLC analysis of acyl chain-defined sphingomyelins showed that the Rf values increased linearly with the length of the N-linked acyl chain. A double-banded pattern was therefore seen for natural sphingomyelins with a bimodal fatty acid composition. Racemic sphingomyelins also gave a double band pattern on HPTLC, where the lower band represented the Derythro and the upper band the Lthreo isomer. We also showed that Derythro-N-16:0-dihydrosphingomyelin migrated faster on HPTLC than Derythro-N-16:0-sphingomyelin. The upper and lower band sphingomyelins from two different cell lines (human skin fibroblasts and baby hamster kidney cells) were separately scraped off the HPTLC plates and the fatty acid and long-chain base profiles were studied using GC-MS. The lower bands contained short-chain fatty acids and most of the fatty acids in the upper bands were long. The predominant long-chain base was sphingosine, which was found in both upper and lower bands, but sphinganine was found only in the upper bands. To conclude, there are at least three possible reasons for the sphingomyelin double bands on HPTLC; acyl chain length, long-chain base composition and stereochemistry. These reasons might sometimes overlap and, therefore, HPTLC alone is insufficient for complete analysis of the molecular species of sphingomyelin.

Comparison of the biophysical properties of racemic and d-erythro-N-acyl sphingomyelins

In this study stereochemically pure d-erythro-sphingomyelins (SMs) with either 16:0 or 18:1(cisDelta9) as the N-linked acyl-chain were synthesized. Our purpose was to examine the properties of these sphingomyelins and acyl-chain matched racemic (d-erythro/l-threo) sphingomyelins in model membranes. Liquid-expanded d-erythro-N-16:0-SM in monolayers was observed to pack more densely than the corresponding racemic sphingomyelin. Cholesterol desorption to beta-cyclodextrin was significantly slower from d-erythro-N-16:0-SM monolayers than from racemic N-16:0-SM monolayers. Significantly more condensed domains were seen in cholesterol/d-erythro-N-16:0-SM monolayers than in the corresponding racemic mixed monolayers, when [7-nitrobenz-2-oxa-1, 3-diazol-4-yl]phosphatidylcholine was used as a probe in monolayer fluorescence microscopy. With monolayers of N-18:1-SMs, both the lateral packing densities (sphingomyelin monolayers) and the rates of cholesterol desorption (mixed cholesterol/sphingomyelin monolayers) was found to be similar for d-erythro and racemic sphingomyelins. The phase transition temperature and enthalpy of d-erythro-N-16:0-SM in bilayer membranes were slightly higher compared with the corresponding racemic sphingomyelin (41.1 degrees C and 8.4 +/- 0.4 kJ/mol, and 39.9 degrees C and 7.2 +/- 0.2 kJ/mol, respectively). Finally, d-erythro-sphingomyelins in monolayers (both N-16:0 and N-18:1 species) were not as easily degraded at 37 degrees C by sphingomyelinase (Staphylococcus aureus) as the corresponding racemic sphingomyelins. We conclude that racemic sphingomyelins differ significantly in their biophysical properties from the physiologically relevant d-erythro sphingomyelins.

Interactions of N-stearoyl sphingomyelin with cholesterol and dipalmitoylphosphatidylcholine in bilayer membranes

Differential scanning calorimetry and x-ray diffraction have been utilized to investigate the interaction of N-stearoylsphingomyelin (C18:0-SM) with cholesterol and dipalmitoylphosphatidylcholine (DPPC). Fully hydrated C18:0-SM forms bilayers that undergo a chain-melting (gel -->liquid-crystalline) transition at 45 degrees C, delta H = 6.7 kcal/mol. Addition of cholesterol results in a progressive decrease in the enthalpy of the transition at 45 degrees C and the appearance of a broad transition centered at 46.3 degrees C; this latter transition progressively broadens and is not detectable at cholesterol contents of >40 mol%. X-ray diffraction and electron density profiles indicate that bilayers of C18:0-SM/cholesterol (50 mol%) are essentially identical at 22 degrees C and 58 degrees C in terms of bilayer periodicity (d = 63-64 A), bilayer thickness (d rho-p = 46-47 A), and lateral molecular packing (wide-angle reflection, 1/4.8 A-(1)). These data show that cholesterol inserts into C18:0-SM bilayers, progressively removing the chain-melting transition and altering the bilayer structural characteristics. In contrast, DPPC has relatively minor effects on the structure and thermotropic properties of C18:0-SM. DPPC and C18:0-SM exhibit complete miscibility in both the gel and liquid-crystalline bilayer phases, but the pre-transition exhibited by DPPC is eliminated at >30 mol% C18:0-SM. The bilayer periodicity in both the gel and liquid-crystalline phases decreases significantly at high DPPC contents, probably reflecting differences in hydration and/or chain tilt (gel phase) of C18:0-SM and DPPC.

X-ray diffraction and calorimetric study of N-lignoceryl sphingomyelin membranes

Differential scanning calorimetry and x-ray diffraction have been used to investigate hydrated multibilayers of N-lignoceryl sphingomyelin (C24:0-SM) in the hydration range 0-75 wt % H2O. Anhydrous C24:0-SM exhibits a single endothermic transition at 81.3 degrees C (delta H = 3.6 kcal/mol). At low hydration (12.1 wt % H2O), three different endothermic transitions are observed: low-temperature transition (T1) at 39.4 degrees C (transition enthalpy (delta H1) = 2.8 kcal/mol), intermediate-temperature transition (T2) at 45.5 degrees C, and high-temperature transition (T3) at 51.3 degrees C (combined transition enthalpy (delta H2 + 3) = 5.03 kcal/mol). On increasing hydration, all three transition temperatures of C24:0-SM decrease slightly to reach limiting values of 36.7 degrees C (T1), 44.4 degrees C (T2), and 48.4 degrees C (T3) at approximately 20 wt % H2O. At 22 degrees C (below T1), x-ray diffraction of C24:0-SM at different hydration levels shows two wide-angle reflections, a sharp one at 1/4.2 A-1 and a more diffuse one at 1/4.0 A-1 together with lamellar reflections corresponding to bilayer periodicities increasing from d = 65.4 A to a limiting value of 71.1 A. Electron density profiles show a constant bilayer thickness dp-p approximately 50 A. In contrast, at 40 degrees C (between T1 and T2) a single sharp wide-angle reflection at approximately 1/4.2 A-1 is observed. The lamellar reflections correspond to a larger bilayer periodicity (increasing from d = 69.3-80.2 A) and there is some increase in dp-p (52-56 A) with hydration. These structural parameters,together with calculated lipid thickness and molecular area considerations, suggest that the low temperature endotherm(T1) of hydrated C24:0-SM corresponds to a transition from a tilted, gel state (Gel I) with partially interdigitated chains to an untilted, or less tilted, gel state (Gel 11). At 600C (above T3), the usual liquid-crystalline La bilayer structure (d = 59.5-66.3A; dp p -46 A) is present at all hydrations. Comparison with the behavior of C18:0-SM indicates that the in equivalence of length of the sphingosine (C18) and lignoceryl (C24) chains results in a more complex gel phase polymorphism for the sphingosine (C18) and lignoceryl (C24) chains results in a more complex gel phase polymorphism for C24:0-SM.

Phases and phase transitions of the sphingolipids

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. Herein, we present a review of the LIPIDAT data subset referring to sphingolipids together with an analysis of these data. It includes data collected over a 40-year period and consists of 867 records obtained from 112 articles in 25 different journals. An analysis of these data has allowed us to identify trends in hydrated sphingolipid phase behavior reflecting differences in fatty acyl chain length, saturation and hydroxylation, head group type, and sphingoid base identity. Information on the mesomorphism of biologically-derived and dry sphingolipids is also presented. This review includes 161 references.