Polymorphism of POPE/cholesterol system: a 2H nuclear magnetic resonance and infrared spectroscopic investigation

Optical and molecular features of negatively curved surfaces created by POPE lipids: A crucial role of the initial conditions

Membrane fusion is closely related to plasma membrane domains rich in cone-shaped phosphatidylethanolamine (PE) lipids that can reverse membrane curvature under certain conditions. The phase transition of PE-based lipid membranes from the lamellar fluid phase (Lα) to the inverse hexagonal phase (HII) is commonly taken as a general model in reconstructing the membrane fusion pathway, and whose structural features have been mostly described so far using structural and microscopic techniques. The aim of this paper is to decipher the optical and molecular features of Lβ → Lα and especially of Lα → HII transition of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) lipids at pH = 7.0 when they are initially prepared in the form of both multi- and unilamellar liposomes (MLVs and LUVs). The distinction between optical properties of MLS- and LUVs-derived HII phase, provided from turbidity-sensitive temperature-dependent UV-Vis spectra, was attributed to different formation mechanisms of HII phase. Most importantly, from FTIR spectroscopic data of POPE lipids in Lβ (15 °C), Lα (50 °C) and HII (85 °C) phases we identified the changes in molecular features of POPE lipids during phase transitions. Among the latter, by far the most significant is different hydration pattern of POPE lipids in MLVs- and LUVs-derived HII phase which extends from the polar-apolar interface all the way to the terminal amino group of the POPE lipid, along with the changes in the conformation of glycerol backbone as evidenced by the signature of α-methylene groups. Molecular dynamics simulations confirmed higher water penetration in HII phase and provided insight into hydrogen bonding patterns.

Computational Insights into the Role of Cholesterol in Inverted Hexagonal Phase Stabilization and Endosomal Drug Release

Cholesterol is a major component of many lipid-based drug delivery systems, including cationic lipid nanoparticles. Despite its critical role in the drug release stage, the underlying molecular mechanism by which cholesterol assists in endosomal escape remains unclear. An efficient drug release from the endosome requires endosomal disruption. This disruption is believed to involve a lamellar-to-inverted hexagonal (L_α-H_II) phase transition upon fusion of the lipid nanoparticle with the endosomal membrane. We used molecular dynamics simulations to study the structural properties of H_II systems composed of an anionic lipid distearoyl phosphatidylserine (DSPS), an ionizable cationic lipid (KC2H), and cholesterol for several hydration levels and molar ratios. This system corresponds to the lipid mixtures in the hypothesized H_II structure formed upon fusion and is of interest for the rational design of ionizable cationic lipids, including KC2, for an optimal drug release. Simulations suggest a geometry- and symmetry-driven lipid sorting and cholesterol-DSPS co-location around the water cores. Cholesterol preferentially co-locates with negatively charged saturated DSPS lipids at interstitial angles. The observed cholesterol-DSPS co-location results in an overall increase in the DSPS acyl chains' order parameters, which we propose to assist in stabilizing the H_II phase by stretching the DSPS acyl chains for filling the voids formed by three adjacent lipid tubules. Furthermore, a systematic increase in the cholesterol concentration increased the lattice plane spacing and the water core radius but decreased the undulations along the lipid tubule axis. We propose that cholesterol and the degree of saturation/polyunsaturation of the lipid acyl chains, and not the lipid charge, are the main contributors in facilitating the L_α-H_II phase transition and stabilizing/destabilizing the formed H_II phase, whereas the positive charge of the ionizable cationic lipid promotes the LNP-endosomal membrane adhesion and assists in initiating the fusion process at the local contact area. We also propose that the effect of cholesterol on the H_II structure and curvature is the main underlying reason for the well-documented H_II stabilization and destabilization at low and high molar concentrations of cholesterol, respectively.

Membrane elasticity modulated by cholesterol in model of porcine eye lens-lipid membrane

Experimental evidence shows that the eye lens loses its elasticity dramatically with age. It has also been reported that the cholesterol (Chol) content in the eye lens fiber cell plasma membrane increases significantly with age. High Chol content leads to the formation of cholesterol bilayer domains (CBDs) in the lens membrane. The role of high Chol associated with lens elasticity is unclear. The purpose of this research is to investigate the membrane elasticity of the model of porcine lens-lipid (MPLL) membrane with increasing Chol content to elucidate the role of high Chol in lens membrane elasticity. In this study, we used atomic force microscopy (AFM) to study the mechanical properties (breakthrough force and area compressibility modulus (K_A)) of the MPLL membrane with increasing Chol content where K_A is the measure of membrane elasticity. We varied Chol concentration in Chol/MPLL membrane from 0 to ∼71 mol%. Supported Chol/MPLL membranes were prepared by fusion of small unilamellar vesicles (SUVs) on top of a flat mica surface. SUVs of the Chol/MPLL lipid mixture were prepared with the rapid solvent exchange method followed by probe-tip sonication. For the Chol/MPLL mixing ratio of 0, AFM image showed the formation of two distinct phases of the membrane, i.e., liquid-disordered phase (l_d) and solid-ordered phase (s_o) membrane. However, with Chol/MPLL mixing ratio of 0.5 and above, only liquid-ordered phase (l_o) membrane was formed. Also, two distinct breakthrough forces corresponding to l_d and s_o were observed for Chol/MPLL mixing ratio of 0, whereas only one breakthrough force was observed for membranes with Chol/MPLL mixing ratio of 0.5 and above. No significant difference in the membrane surface roughness was measured with increasing Chol content for these membranes; however, breakthrough force and K_A for l_o membrane increased when Chol/MPLL mixing ratio was increased from 0.5 to 1. Interestingly above the Chol/MPLL mixing ratio of 1, both breakthrough force and K_A decreased, indicating the formation of CBDs. Furthermore, these results showed that membrane elasticity increases at high Chol content, suggesting that high Chol content in lens membrane might be responsible for maintaining lens membrane elasticity.

Mechanistic Insight into How PEGylation Reduces the Efficacy of pH-Sensitive Liposomes from Molecular Dynamics Simulations

Liposome-based drug delivery systems composed of DOPE stabilized with cholesteryl hemisuccinate (CHMS) have been proposed as a drug delivery mechanism with pH-triggered release as the anionic form (CHSa) is protonated (CHS) at reduced pH; PEGylation is known to decrease this pH sensitivity. In this manuscript, we set out to use molecular dynamics (MD) simulations with a model with all-atom resolution to provide insight into why incorporation of poly(ethyleneglycol) (PEG) into DOPE–CHMS liposomes reduces their pH sensitivity; we also address two additional questions: (1) How CHSa stabilizes DOPE bilayers into a lamellar conformation at a physiological pH of 7.4? and (2) how the change from CHSa to CHS at acidic pH triggers the destabilization of DOPE bilayers? We found that (A) CHSa stabilizes the DOPE lipid membrane by increasing the hydrophilicity of the bilayer surface, (B) when CHSa changes to CHS by pH reduction, DOPE bilayers are destabilized due to a reduction in bilayer hydrophilicity and a reduction in the area per lipid, and (C) PEG stabilizes DOPE bilayers into the lamellar phase, thus reducing the pH sensitivity of the liposomes by increasing the area per lipid through penetration into the bilayer, which is our main focus.

Formation of cholesterol Bilayer Domains Precedes Formation of Cholesterol Crystals in Membranes Made of the Major Phospholipids of Human Eye Lens Fiber Cell Plasma Membranes

Purpose/Aim: The goal of this study is to reveal how age-related changes in phospholipid (PL) composition in the fiber cell plasma membranes of the human eye lens affect the cholesterol (Chol) content at which Chol bilayer domains (CBDs) and Chol crystals start to form.Materials and Methods: Saturation-recovery electron paramagnetic resonance with spin-labeled cholesterol analogs and differential scanning calorimetry were used to determine the Chol contents at which CBDs and cholesterol crystals, respectively, start to form in in membranes made of the major PL constituents of the plasma membrane of the human eye lens fiber cells. To preserve compositional homogeneity throughout the membrane suspension, the lipid multilamellar dispersions investigated in this work were prepared using a rapid solvent exchange method. The cholesterol content changed from 0 to 75 mol%.Results: The saturation recovery electron paramagnetic resonance results show that CBDs start to form at 33, 50, 46, and 48 mol% Chol in the phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, and sphingomyelin bilayers, respectively. The differential scanning calorimetry results show that Chol crystals start to form at 50, 66, 70, and 66 mol% Chol in the phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, and sphingomyelin bilayers, respectively.Conclusions: These results, as well those of our previous studies, indicate that the formation of CBDs precedes the formation of Chol crystals in all of the studied systems, and the appearance of each depends on the type of PL forming the bilayer. These findings contribute to a better understanding of the molecular mechanisms involved in the regulation of Chol-dependent processes in eye lens fiber cell membranes.

Nonlamellar-Phase-Promoting Colipids Enhance Segregation of Palmitoyl Ceramide in Fluid Bilayers

Ceramide is an important intermediate in sphingolipid homeostasis. We examined how colipids, with negative intrinsic curvature and which may induce curvature stress in the bilayers, affected the segregation of palmitoyl ceramide (PCer). Such colipids include 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and tetra-linoleoyl cardiolipin (CL). In 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayers, PCer formed ordered, gel-like domains at concentrations above 10 mol% at 23°C, as evidenced by the change in the average lifetime of the trans-parinaric acid emission. When POPE or DOPE were included in the DOPC bilayer (at 20:80 or 40:60 POPE or DOPE to DOPC, by mol), the lateral segregation of PCer was facilitated in a concentration-dependent manner, and less PCer was required for the formation of the ordered ceramide-rich domains. Inclusion of CL in the DOPE bilayer (at 10:90 or 20:80 CL to PC, by mol) also caused a similar facilitation of the lateral segregation of PCer. The PCer-rich domains formed in the presence of POPE, DOPE, or CL in DOPC bilayers were slightly more thermostable (by 2-10°C) when compared to PCer-rich domains in DOPC-only bilayers. Nonlamellar phases were not present in bilayers in which the effects of POPE or DOPE on PCer segregation were the largest, as verified by ³¹P NMR. When palmitoyl sphingomyelin was added to the different bilayer compositions at 5 mol%, relative to the phospholipids, PCer segregated into gel domains at lower concentrations (2-3 mol% PCer), and the effect of POPE on PCer segregation was eliminated. We suggest that the effects of POPE, DOPE, and CL on PCer segregation was in part influenced by their effects on membrane curvature stress and in part because of unfavorable interactions with PCer due to their unsaturated acyl chains. These lipids are abundant in mitochondrial membranes and are likely to affect functional properties of saturated ceramides in them.

Ceramide-C16 Is a Versatile Modulator of Phosphatidylethanolamine Polymorphism

Ceramide-C16 (CerC16) is a sphingolipid associated with several diseases like diabetes, obesity, Parkinson disease, and certain types of cancers. As a consequence, research efforts are devoted to identify the impact of CerC16 on the behavior of membranes, and to understand how it is involved in these diseases. In this work, we investigated the impacts of CerC16 (up to 20 mol %) on the lipid polymorphism of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), using differential scanning calorimetry, and sequential ²H and ³¹P solid-state nuclear magnetic resonance spectroscopy. A partial phase diagram is proposed. The results indicate that the presence of CerC16 leads to an upshift of the temperature of the gel-to-liquid crystalline (L_β - L_α) phase transition, leading to a large L_β/L_α phase coexistence region where gel-phase domains contain ∼35 mol % CerC16. It also leads to a downshift of the temperature of the lamellar-to-inverted hexagonal (L - H_II) phase transition of POPE. The opposite influence on the two-phase transitions of POPE brings a three-phase coexistence line when the two transitions overlap. The resulting H_II phase can be ceramide enriched, coexisting with a L_α phase, or ceramide depleted, coexisting with a L_β phase, depending on the CerC16 proportions. The uncommon capability of CerC16 to modulate the membrane fluidity, its curvature propensity, and the membrane interface properties highlights its potential as a versatile messenger in cell membrane events.

Lipid Interactions and Organization in Complex Bilayer Membranes

Bilayer lipids influence the lateral structure of the membranes, but the relationship between lipid properties and the lateral structure formed is not always understood. Model membrane studies on bilayers containing cholesterol and various phospholipids (PLs) suggest that high and low temperature melting PLs may segregate, especially in the presence of cholesterol. The effect of different PL headgroups on lateral structure of bilayers is also not clear. Here, we have examined the formation of lateral heterogeneity in increasingly complex (up to five-component) multilamellar bilayers. We have used time-resolved fluorescence spectroscopy with domain-selective fluorescent probes (PL-conjugated trans-parinaric acid), and (2)H NMR spectroscopy with site or perdeuterated PLs. We have measured changes in bilayer order using such domain-selective probes both as a function of temperature and composition. Our results from time-resolved fluorescence and (2)H NMR showed that in ternary bilayers, acyl chain order and thermostability in sphingomyelin-rich domains were not affected to any greater extent by the headgroup structure of the monounsaturated PLs (phosphatidylcholine, phosphatidylethanolamine, or phosphatidylserine) in the bilayer. In the complex five-component bilayers, we could not detect major differences between the different monounsaturated PLs regarding cholesterol-induced ordering. However, cholesterol clearly influenced deuterated N-palmitoyl sphingomyelin differently than the other deuterated PLs, suggesting that cholesterol favored N-palmitoyl sphingomyelin over the other PLs. Taken together, both the fluorescence spectroscopy and (2)H NMR data suggest that the complex five-component membranes displayed lateral heterogeneity, at least in the lower temperature regimen examined.

Photoinduced Bilayer-to-Nonbilayer Phase Transition of POPE by Photoisomerization of Added Stilbene Molecules

The photocontrol of a bilayer-to-nonbilayer phase transition (the liquid-crystalline Lα phase to the inverted hexagonal HII phase) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) by the photoisomerization of incorporated stilbene molecules was examined by utilizing differential scanning calorimetry, small-angle X-ray diffraction, ultraviolet (UV)/visible absorption, and attenuated total reflectance Fourier transform infrared spectroscopies. cis-Stilbene lowered the transition temperature, Th, to a greater extent than did trans-stilbene, and the difference was at most ca. 10 °C. At temperatures higher than the Th of POPE/cis-stilbene but lower than that of POPE/trans-stilbene, the photoisomerization from the trans to the cis form of the stilbene molecules by irradiation with UV light caused a Lα-HII phase transition. The UV irradiation partially induced the HII phase at a constant temperature because of the incomplete photoisomerization of stilbene (ca. 60%). The reduction in Th by the incorporation of stilbenes was caused mainly by the reduction in the spontaneous radius of curvature of the lipid monolayer, R0. The greater bulkiness of cis-stilbene as compared to the trans form resulted in a more effective reduction in R0 and stabilization of the HII phase.

Probing the Role of Ceramide Headgroup Polarity in Short-Chain Model Skin Barrier Lipid Mixtures by ²H Solid-State NMR Spectroscopy

The thermoptropic phase behaviors of two stratum corneum model lipid mixtures composed of equimolar contributions of either Cer[NS18] or Cer[NP18] with stearic acid and cholesterol were compared. Each component of the mixture was specifically deuterated such that the temperature-dependent (2)H NMR spectra allowed disentanglement of the complicated phase polymorphism of these lipid mixtures. While Cer[NS] is based on the sphingosine backbone, Cer[NP] features a phytosphingosine, which introduces an additional hydroxyl group into the headgroup of the ceramide and abolishes the double bond. From the NMR spectra, the individual contributions of all lipids to the respective phases could be determined. The comparison of the two lipid mixtures reveals that Cer[NP] containing mixtures have a tendency to form more fluid phases. It is concluded that the additional hydroxyl group of the phytosphingosine-containing ceramide Cer[NP18] in mixture with chain-matched stearic acid and cholesterol creates a packing defect that destabilizes the orthorhombic phase state of canonical SC mixtures. This steric clash favors the gel phase and promotes formation of fluid phases of Cer[NP] containing lipid mixtures at lower temperature compared to those containing Cer[NS18].

How cholesterol could be drawn to the cytoplasmic leaf of the plasma membrane by phosphatidylethanolamine

In the mammalian plasma membrane, cholesterol can translocate rapidly between the exoplasmic and cytoplasmic leaves, so that its distribution between them should be given by the equality of its chemical potential in the leaves. Due to its favorable interaction with sphingomyelin, which is almost entirely in the outer leaf, one expects the great majority of cholesterol to be there also. Experimental results do not support this, implying that there is some mechanism attracting cholesterol to the inner leaf. We hypothesize that it is drawn there to reduce the bending free energy of the membrane caused by the presence of PE (phosphatidylethanolamine). It does this in two ways: first by simply diluting the amount of PE in the inner leaf, and second by ordering the tails of the PE to reduce its spontaneous curvature. Incorporating this mechanism into a model free energy for the bilayer, we find that between 50 and 60% of the total cholesterol should be in the inner leaf of human erythrocytes.

Long-Range Lipid-Water Interaction as Observed by ATR-FTIR Spectroscopy

It is commonly assumed that the structure of water at a lipid-water interface is influenced mostly in the first hydration layer. However, recent results from different experimental methods show that perturbation extends through several hydration layers. Due to its low light penetration depth, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy is specifically suited to study interlamellar water structure in multibilayers. Results obtained by this technique confirm the long-range water structure disturbance. Consequently, in confined membrane environments nearly all water molecules can be perturbed. It is important to note that the behavior of confined water molecules differs significantly in samples prepared in excess water and in partially hydrated samples. We show in what manner the interlamellar water perturbation is influenced by the hydration level and how it is sequentially modified with a step-by-step dehydration of samples either by water evaporation or by osmotic pressure. Our results also indicate that besides different levels of hydration the lipid-water interaction is modulated by different lipid headgroups and different lipid phases as well. Therefore, modification of interlamellar water properties may clarify the role of water-mediated effects in biological processes.

Lamellar self-assemblies of single-chain amphiphiles and sterols and their derived liposomes: distinct compositions and distinct properties

Typically, single-chain amphiphiles and sterols do not form fluid lamellar phases once hydrated individually. Most of the single-chain amphiphiles form actually micelles in aqueous environments, while sterols display a very limited solubility in water. However, under certain conditions, mixtures of single-chain amphiphiles and sterols lead to the formation of stable fluid bilayers. Over the past decade, several of these systems leading to fluid lamellar self-assemblies have been identified and this article reviews the current knowledge relative to these non-phospholipid bilayers made of single-chain amphiphiles and sterols. It presents an integrated view about the molecular features that are required for their stability, the properties they share, and the origin of these characteristics. It was also shown that these lamellar systems could lead to the formation of unilamellar vesicles, similar to phospholipid based liposomes. These vesicles display distinct properties that make them potentially appealing for technological applications; they display a limited permeability, they are stable, they are formed with molecules that are relatively chemically inert (and relatively cheap), and they can be readily functionalized. The features of these distinct liposomes and their technological applications are reviewed. Finally, the putative biological implications of these non-phospholipid fluid bilayers are also discussed.

Formation of pH-sensitive cationic liposomes from a binary mixture of monoalkylated primary amine and cholesterol

It has been shown that mixtures of monoalkylated amphiphiles and sterols can form liquid-ordered (lo) lamellar phases. These bilayers can be extruded using conventional methods to obtain large unilamellar vesicles (LUVs) that have very low permeability and a specific response to a given stimulus. For example, pH variations can trigger the release from LUVs formed with palmitic acid and sterols. In the present work, the possibility to form non phospholipid liposomes with mixtures of stearylamine (SA) and cholesterol (Chol) was investigated. The phase behavior of these mixtures was characterized by differential scanning calorimetry, infrared, and (2)H NMR spectroscopy. It is found that this particular mixture can form a lo lamellar phase that is pH-sensitive as the system undergoes a transition from a lo phase to a solid state when pH is increased from 5.5 to 12. LUVs have been successfully extruded from equimolar SA/Chol mixtures. Release experiments as a function of time revealed the relatively low permeability of these systems. The fact that the stability of these liposomes is pH dependent implies that these LUVs display an interesting potential as new cationic carriers for pH-triggered release. This is the first report of non phospholipid liposomes with high sterol content combining an overall positive charge and pH-sensitivity.

DHA-fluorescent probe is sensitive to membrane order and reveals molecular adaptation of DHA in ordered lipid microdomains

Docosahexaenoic acid (DHA) disrupts the size and order of plasma membrane lipid microdomains in vitro and in vivo. However, it is unknown how the highly disordered structure of DHA mechanistically adapts to increase the order of tightly packed lipid microdomains. Therefore, we studied a novel DHA-Bodipy fluorescent probe to address this issue. We first determined if the DHA-Bodipy probe localized to the plasma membrane of primary B and immortal EL4 cells. Image analysis revealed that DHA-Bodipy localized into the plasma membrane of primary B cells more efficiently than EL4 cells. We then determined if the probe detected changes in plasma membrane order. Quantitative analysis of time-lapse movies established that DHA-Bodipy was sensitive to membrane molecular order. This allowed us to investigate how DHA-Bodipy physically adapted to ordered lipid microdomains. To accomplish this, we employed steady-state and time-resolved fluorescence anisotropy measurements in lipid vesicles of varying composition. Similar to cell culture studies, the probe was highly sensitive to membrane order in lipid vesicles. Moreover, these experiments revealed, relative to controls, that upon incorporation into highly ordered microdomains, DHA-Bodipy underwent an increase in its fluorescence lifetime and molecular order. In addition, the probe displayed a significant reduction in its rotational diffusion compared to controls. Altogether, DHA-Bodipy was highly sensitive to membrane order and revealed for the first time that DHA, despite its flexibility, could become ordered with less rotational motion inside ordered lipid microdomains. Mechanistically, this explains how DHA acyl chains can increase order upon formation of lipid microdomains in vivo.

How SNARE molecules mediate membrane fusion: recent insights from molecular simulations

SNARE molecules are the core constituents of the protein machinery that facilitate fusion of synaptic vesicles with the presynaptic plasma membrane, resulting in the release of neurotransmitter. On a molecular level, SNARE complexes seem to play a quite versatile and involved role during all stages of fusion. In addition to merely triggering fusion by forcing the opposing membranes into close proximity, SNARE complexes are now seen to also overcome subsequent fusion barriers and to actively guide the fusion reaction up to the expansion of the fusion pore. Here, we review recent advances in the understanding of SNARE-mediated membrane fusion by molecular simulations.

Deuterium NMR study of the effect of ergosterol on POPE membranes

We study the effect of ergosterol on the physical properties of 1-[(2)H(31)]palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) multibilayers using deuterium nuclear magnetic resonance. NMR spectra were taken as a function of temperature and ergosterol concentration up to 70 mol %. The spectral first moments show that there is a dramatic difference in the ability of ergosterol to disorder the gel phase and to order the liquid-crystalline phase of POPE membranes, an unusual behavior among lipid/sterol systems studied up to now. Further investigation of the liquid-crystalline phase shows that ergosterol (erg) increases the chain order of POPE-d31, but that this effect saturates at 10 mol % ergosterol. This is in marked contrast to the effect of cholesterol (chol) on POPE membranes: the chain order of POPE increases with cholesterol to at least 45 mol %. Moreover, we found that at higher ergosterol concentrations (>40 mol %) ergosterol decreases the POPE-d31 chain order, which, to our knowledge, has not been directly observed in other lipid/sterol systems. The temperature-composition phase diagram is presented. Finally, at all ergosterol concentrations, the chain order of liquid-crystalline-phase POPE is much smaller than that of comparable POPE/chol membranes. This implies that there is no liquid-ordered phase behavior for POPE/erg membranes.

Formation of fluid lamellar phase and large unilamellar vesicles with octadecyl methyl sulfoxide/cholesterol mixtures

Systems composed of a monoalkylated amphiphile and a sterol have been shown to form stable liquid-ordered (lo) lamellar phases; these include negatively charged mixtures of unprotonated palmitic acid/cholesterol (Chol) or cholesterol sulfate (Schol) and mixtures of positively charged cetylpyridinium chloride/Schol. Large unilamellar vesicles (LUVs) could be formed by these systems, using conventional extrusion methods. The passive permeability of these LUVs was drastically limited, a phenomenon associated with the high sterol content. In the present paper, we showed that octadecyl methyl sulfoxide (OMSO), a neutral monoalkylated amphiphile, can form, in the presence of cholesterol, LUVs that are stable at room temperature. Differential scanning calorimetry, infrared spectroscopy, and nuclear magnetic resonance spectroscopy of deuterium were used to characterize the phase behavior of OMSO/Chol mixtures. A temperature-composition diagram summarizing the behavior of the OMSO/Chol system is proposed; it includes a eutectic with an OMSO/Chol molar ratio of 5/5. It is found that the fluid phase observed at temperature higher than 43 degrees C is metastable at room temperature, and this situation allows extruding these mixtures to form stable LUVs at room temperature. This distinct behavior is associated with the strong H-bond capability of the sulfoxide group. The properties associated with this neutral formulation expand the potential of these non-phospholipid liposomes for applications in several areas such as drug delivery.

Influence of the nature of the sterol on the behavior of palmitic acid/sterol mixtures and their derived liposomes

The phase behavior of mixtures formed with palmitic acid (PA) and one of the following sterols (dihydrocholesterol, ergosterol, 7-dehydrocholesterol, stigmasterol and stigmastanol), in a PA/sterol molar ratio of 3/7, has been characterized by IR and (2)H NMR spectroscopy at different pH. Our study shows that it is possible to form liquid-ordered (lo) lamellar phases with these binary non-phospholipid mixtures. The characterization of alkyl chain dynamics of PA in these systems revealed the large ordering effect of the sterols. It was possible to extrude these systems, using standard extrusion techniques, to form large unilamellar vesicles (LUVs), except in the case of ergosterol-containing mixture. The resulting LUVs displayed a very limited passive permeability consistent with the high sterol concentration. In addition, the stability of these PA/sterol self-assembled bilayers was also found to be pH-sensitive, therefore, potentially useful as nanovectors. By examining different sterols, we could establish some correlations between the structure of these bilayers and their permeability properties. The structure of the side chain at C17 of the sterol appears to play a prime role in the mixing properties with fatty acid.

Oleic- and docosahexaenoic acid-containing phosphatidylethanolamines differentially phase separate from sphingomyelin

A central tenet of the lipid raft model is the existence of non-raft domains. In support of this view, we have established in model membranes that a phosphatidylethanolamine (PE)-containing docosahexaenoic acid (DHA) forms organizationally distinct non-raft domains in the presence of sphingomyelin (SM) and cholesterol (Chol). We have shown that formation of DHA-rich domains is driven by unfavorable molecular interactions between the rigid Chol molecule and the highly flexible DHA acyl chain. However, the molecular interactions between SM and the DHA-containing PE, which could also contribute to the formation of DHA-rich non-raft domains, have not been sufficiently investigated. To address this issue, we use differential scanning calorimetry (DSC) to study the phase behavior of mixtures of SM with either 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphoethanolamine (16:0-22:6PE) or 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (16:0-18:1PE), an oleic acid (OA)-containing control, over a wide range of concentrations. Deconvolution of binary DSC scans shows that both 16:0-22:6PE and 16:0-18:1PE phase separate from SM. Analysis of transition temperatures and partial phase diagrams, constructed from the DSC scans for the first time, shows that 16:0-22:6PE displays greater non-ideal mixing with SM compared to 16:0-18:1PE. Our findings support a model in which DHA- and OA-containing PEs differentially phase separate from SM over a wide range of molar ratios to initiate the formation of non-raft domains, which is greatly enhanced by DHA, but not OA, in the presence of cholesterol.

Cationic detergent/sterol mixtures can form fluid lamellar phases and stable unilamellar vesicles

In recent studies, it has been shown that mixtures of palmitic acid (PA), and cholesterol or cholesterol sulfate (Schol), in a PA/sterol molar ratio of 30/70 lead to the formation of liquid-ordered (lo) lamellar phases. The extrusion of these systems gave large unilamellar vesicles (LUVs) that displayed a very limited passive permeability, a property associated with their high sterol content. In this study, we showed that the formation of lo-phase bilayers was also possible when mixing a cationic detergent (cetylpyridinium chloride, CPC) and sterol in a 30/70 molar ratio. The existence of this phase was established using IR and 2H NMR spectroscopy. Moreover, 2H NMR allowed us to study the orientation and dynamics of CPC and cholesterol in these self-assemblies. The extrusion of the CPC/Schol bilayers leads to the formation of LUVs, and their passive permeability was found to be very limited, making them interesting candidates as nanovectors.

A raft-associated species of phosphatidylethanolamine interacts with cholesterol comparably to sphingomyelin. A Langmuir-Blodgett monolayer study

Background

Specific interactions between sphingomyelin (SM) and cholesterol (Ch) are commonly believed to play a key role in the formation of rafts in the biological membranes. A weakness of this model is the implication that these microdomains are confined to the outer bilayer leaflet. The cytoplasmic leaflet, which contains the bulk of phosphatidylethanolamine (PE), phosphatidylserine (PS) and phosphatidylinositol (PI), is thought also to harbour half of the membrane cholesterol. Moreover, SLPE (1-stearoyl-2-linoleoyl-sn-glycero-3-phosphatidyl-ethanolamine) has recently been shown to be enriched in isolated detergent-resistant membranes (DRM), and this enrichment was independent of the method of isolation of DRM.

Methodology/Principal Findings

Here we present quantitative evidence coming from Langmuir-Blodgett monolayer experiments that SLPE forms complex with Ch similar to that between SM and Ch. The energies of these interactions as calculated form the monolayer studies are highly negative. FRAP analysis showed that NBD-Ch recovery was similar in liposomes composed of DOPC/Ch SM or SLPE but not DPPE, providing further evidence that SLPE may form an l_o phase in the presence of high Ch concentration. Experiments on the solubility of DOPC liposomes containing DPPE/Ch (1∶1), SM/Ch (1∶1) or SLPE/Ch (1∶1) showed the presence of Triton X-100 insoluble floating fraction (TIFF) in the case of SM/Ch or SLPE/Ch but not in DPPE/Ch containing liposomes. Quantitative determination of particular lipid species in the TIFF fraction confirms the conclusion that SLPE (or similar PE species) could be an important constituent of the inner leaflet raft.

Conclusion

Such interactions suggest a possible existence of inner-leaflet nanoscale assemblies composed of cholesterol complexes with SLPE or similar unsaturated PE species.

Docosahexaenoic acid enhances segregation of lipids between : 2H-NMR study

Solid-state (2)H-NMR of [(2)H(31)]-N-palmitoylsphingomyelin ([(2)H(31)]16:0SM, PSM*), supplemented by differential scanning calorimetry, was used for the first time, to our knowledge, to investigate the molecular organization of the sphingolipid in 1:1:1 mol mixtures with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (16:0-18:1PE, POPE) or 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphoethanolamine (16:0-22:6PE, PDPE) and cholesterol. When compared with (2)H-NMR data for analogous mixtures of [(2)H(31)]16:0-18:1PE (POPE*) or [(2)H(31)]16:0-22:6PE (PDPE*) with egg SM and cholesterol, molecular interactions of oleic acid (OA) versus docosahexaenoic acid (DHA) are distinguished, and details of membrane architecture emerge. SM-rich, characterized by higher-order, and PE-rich, characterized by lower-order, domains <20 nm in size are formed in the absence and presence of cholesterol in both OA- and DHA-containing membranes. Although acyl chain order within both domains increases on the addition of sterol to the two systems, the resultant differential in order between SM- and PE-rich domains is almost a factor of 3 greater with DHA than with OA. Our interpretation is that the aversion that cholesterol has for DHA--but not for OA--excludes the sterol from DHA-containing, PE-rich (nonraft) domains and excludes DHA from SM-rich/cholesterol-rich (raft) domains. We attribute, in part, the diverse health benefits associated with dietary consumption of DHA to an alteration in membrane domains.

Analysis of Quil A–phospholipid mixtures using drift spectroscopy

The aim of this study was to investigate molecular interactions between Quil A and phosphatidylcholine in the solid state using diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). Analysis of the interactions was characterized on the different regions of phosphatidylcholine: hydrophobic chain, interfacial and headgroup regions. The spectra of the hydrocarbon region of phosphatidylcholine alone compared to that for the binary mixture of Quil A and phosphatidylcholine were similar. These findings suggest that Quil A did not cause conformational disorder of the fatty acyl chains of the phospholipid. In contrast, a shift in the wavenumber of the choline group and a broad band in this moiety indicate a modification of the phospholipid in the headgroup region due to interaction between Quil A and phosphatidylcholine. These results suggest possibly ionic interactions between the negatively charged glucuronic acid moiety of the Quil A molecule with the positively charged choline group. The findings could also be the result of conformational changes in the choline group because of the intercalation of sugar moieties in Quil A between the choline and phosphate groups due to hydrogen bonding. Shift of wavenumbers to lower values on the carbonyl group was observed suggesting hydrogen bonding between Quil A and phosphatidylcholine. The difference in degrees of wavenumber shift (choline>phosphate>carbonyl group) and observed broad bands indicated that Quil A preferentially interacted with phosphatidylcholine on the hydrophilic headgroup. Cholesterol influenced such interactions at relatively high concentration (60%, w/w).

Phase Behavior of Palmitic Acid/Cholesterol/Cholesterol Sulfate Mixtures and Properties of the Derived Liposomes

The phase behavior of mixtures formed by palmitic acid (PA), cholesterol (Chol), and sodium cholesteryl sulfate (Schol) has been characterized by differential scanning calorimetry and infrared and 2H NMR spectroscopy. It is reported that it is possible to form, with PA/sterol mixtures, fluid lamellar phases where the sterol content is very high (a sterol mole fraction of 0.7). As a consequence of the rigidifying ability of the sterols, the PA acyl chains are very ordered. The stability of these self-assembled bilayers is found to be pH-dependent. This property can be controlled by the Chol/Schol molar ratio, and it is proposed that this parameter modulates the balance between the intermolecular interactions between the constituting species. A phase-composition diagram summarizing the behavior of these mixtures as a function of pH, at room temperature, is presented. It is also shown that it is possible to produce large unilamellar vesicles (LUVs) from these mixtures, using standard extrusion techniques. The resulting LUVs display a very limited passive release of the entrapped material. In addition, these LUVs constitute a versatile vector for pH-triggered release.

Cholesterol supports headgroup superlattice domain formation in fluid phospholipid/cholesterol bilayers

Fluorescence and Fourier transform infrared (FTIR) spectroscopic techniques were used to explore the effect of added cholesterol on the composition-dependent formation of putative phospholipid headgroup superlattices in fluid 1-palmitoyl-2-oleoyl-phosphatidylethanolamine/1-palmitoyl-2-oleoyl-phosphatidylcholine/cholesterol (POPE/POPC/CHOL) bilayers. Steady-state fluorescence anisotropy measurements of diphenylhexatriene (DPH) chain-labeled phosphatidylcholine (DPH-PC) revealed significant dips at several POPE-to-phospholipid mole fractions (X(PE)'s) when the cholesterol-to-lipid mole fraction (X(CHOL)) was fixed at 0.00, 0.35, 0.40, and 0.50. Most of the observed dips occur at or close to critical X(PE)'s predicted by the Headgroup Superlattice (SL) model, suggesting that phospholipid headgroups of different structures tend to adopt regular distributions even in the presence of cholesterol. Time-resolved fluorescence anisotropy measurements revealed that DPH-PC senses a disordered and highly mobile microenvironment in the POPE/POPC/CHOL bilayers at those critical X(PE)'s, indicating that this probe may partition to defect regions in the bilayers. The presence of coexisting packing defect regions and regularly distributed SL domains is a key feature predicted by the Headgroup SL model. Importantly, probe-free FTIR measurements of acyl chain C-H, interfacial carbonyl, and headgroup phosphate stretching peak frequencies revealed the presence of abrupt changes at X(PE)'s close to those observed in the fluorescence data. When X(PE) was varied from 0.60 to 0.72 and X(CHOL) from 0.34 to 0.46, a clear dip at the lipid composition coordinates (X(PE), X(CHOL)) approximately (0.68, 0.40) was observed in the three-dimensional surface plots of DPH-PC anisotropy as well as the carbonyl and phosphate stretching frequencies. The critical X(CHOL) at 0.40 agrees with the Cholesterol SL model, which assumes that cholesterol and phospholipid form SL domains at the lipid acyl chain level. In conclusion, this study provides evidence that cholesterol supports formation of phospholipid headgroup SLs in fluid state ternary lipid bilayers. The feasibility of the parallel existence of SLs at the lipid headgroup and acyl chain levels supports the relevance of the lipid SL model for the membranes of eukaryotic cells that typically contain significant amounts of cholesterol. We speculate that lipid SL formation may play a central role in the regulation of membrane lipid compositions, maintenance of organelle boundaries, and other crucial phenomena in those cells.

Interaction of the C-terminal region of the Ggamma protein with model membranes

Heterotrimeric G-proteins interact with membranes. They accumulate around membrane receptors and propagate messages to effectors localized in different cellular compartments. G-protein-lipid interactions regulate G-protein cellular localization and activity. Although we recently found that the Gbetagamma dimer drives the interaction of G-proteins with nonlamellar-prone membranes, little is known about the molecular basis of this interaction. Here, we investigated the interaction of the C-terminus of the Ggamma(2) protein (P(gamma)-FN) with model membranes and those of its peptide (P(gamma)) and farnesyl (FN) moieties alone. X-ray diffraction and differential scanning calorimetry demonstrated that P(gamma)-FN, segregated into P(gamma)-FN-poor and -rich domains in phosphatidylethanolamine (PE) and phosphatidylserine (PS) membranes. In PE membranes, FN increased the nonlamellar phase propensity. Fourier transform infrared spectroscopy experiments showed that P(gamma) and P(gamma)-FN interact with the polar and interfacial regions of PE and PS bilayers. The binding of P(gamma)-FN to model membranes is due to the FN group and positively charged amino acids near this lipid. On the other hand, membrane lipids partially altered P(gamma)-FN structure, in turn increasing the fluidity of PS membranes. These data highlight the relevance of the interaction of the C-terminal region of the Ggamma protein with the cell membrane and its effect on membrane structure.

Molecular dynamics investigation of the structural properties of phosphatidylethanolamine lipid bilayers

We report a 14 ns microcanonical (NVE) molecular dynamics simulation of a fully hydrated bilayer of 1-stearoyl-2-oleoyl-phosphatidyethanolamine. This study describes the structure of the bilayer in terms of NMR order parameters and radial distribution functions, and compares them to experimental results and simulations of other lipids. A focus of this work is the characterization of the lipid-water interface, particularly the hydrogen bonding network of the phosphatidylethanolamine (PE) headgroups. We find that hydrogen bonding between the primary amine and phosphate groups has a pronounced effect on the structure of PE relative to phosphatidylcholine, and is evident in, for example, the P-N radial distribution functions.

Molecular Organization of Cholesterol in Unsaturated Phosphatidylethanolamines: X-ray Diffraction and Solid State2H NMR Reveal Differences with Phosphatidylcholines

The major mammalian plasma membrane lipids are phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), and cholesterol. Whereas PC-cholesterol interactions are well studied, far less is known about those between PE and cholesterol. Here, we investigated the molecular organization of cholesterol in PEs that vary in their degree of acyl chain unsaturation. For heteroacid sn-1 saturated (palmitoyl), sn-2 unsaturated (various acyl chain) PEs, cholesterol solubility determined by X-ray diffraction was essentially identical with 1 (oleoyl, 51 +/- 3 mol %) and 2 (linoleoyl, 49 +/- 2 mol %) double bonds before decreasing progressively with 4 (arachidonyl, 41 +/- 3 mol %) and 6 (docosahexaenoyl, 31 +/- 3 mol %) double bonds. With 6 double bonds in each chain, cholesterol solubility was further reduced to 8.5 +/- 1 mol %. However, (2)H NMR experiments established that the orientation of cholesterol in the same heteroacid PE membranes was unaffected by the degree of acyl chain unsaturation. A tilt angle of 15 +/- 1 degrees was measured when equimolar [3alpha-(2)H(1)]cholesterol was added, regardless of the number of double bonds in the sn-2 chain. The finding that solubility of cholesterol in sn-1 saturated PEs depends on the amount of polyunsaturation in the sn-2 chain of PE differs from the equivalent PCs that universally incorporate approximately 50 mol % sterol. Unlike PCs, a differential in affinity for cholesterol and tendency to drive lateral segregation is inferred between polyunsaturated PEs. This distinction may have biological implications reflected by the health benefits of dietary polyunsaturated fatty acids that are often taken up into PE > PC.

Cholesterol reduces membrane electroporation and electric deformation of small bilayer vesicles

Electric fields, similar in the order of magnitude of the natural membrane fields of cellular lipid/protein membranes, and chemical relaxation spectrometry can be used as tools to quantify the rigidifying effect of cholesterol in membranes. Small unilamellar vesicles of radius a=50+/-3 nm, prepared form phosphatidylcholine, phosphatidylserine and phosphatidyl-glycerol in the molar ratio 1:1:1 and containing the optical lipid probe molecule 2-(3-diphenyl-hexatrienyl) propanoyl)-1-palmitoyl-sn-glycerol-3-phosphocholine (beta-DPH pPC), serve as examples for curved lipid membranes. The data of electrooptical turbidity and absorbance relaxations at the wavelength lambda=365 nm are analysed in terms of membrane bending rigidity kappa and membrane stretching modulus K. Both kappa and K increase with increasing mole fraction x of cholesterol up to x=0.5. The cholesterol induced denser packing of the lipids reduces the extent of both membrane electroporation (ME) and electroelongation of the vesicles. Further on, cholesterol in the lipid phase and sucrose in the aqueous suspension reduce the extent of membrane undulation and electro-stretching.

Structure of phospholipid-cholesterol membranes: an x-ray diffraction study

We have studied the phase behavior of mixtures of cholesterol with dipalmitoyl phosphatidylcholine (DPPC), dimyristoyl phosphatidylcholine (DMPC), and dilauroyl phosphatidylethanolamine (DLPE), using x-ray diffraction techniques. Phosphatidylcholine (PC)-cholesterol mixtures are found to exhibit a modulated phase for cholesterol concentrations around 15 mol % at temperatures below the chain melting transition. Lowering the relative humidity from 98% to 75% increases the temperature range over which it exists. An electron density map of this phase in DPPC-cholesterol mixtures, calculated from the x-ray diffraction data, shows bilayers with a periodic height modulation, as in the ripple phase observed in many PCs in between the main- and pretransitions. However, these two phases differ in many aspects, such as the dependence of the modulation wavelength on the cholesterol content and thermodynamic stability at reduced humidities. This modulated phase is found to be absent in DLPE-cholesterol mixtures. At higher cholesterol contents the gel phase does not occur in any of these three systems, and the fluid lamellar phase is observed down to the lowest temperature studied (5 degrees C).

Oleic and docosahexaenoic acid differentially phase separate from lipid raft molecules: a comparative NMR, DSC, AFM, and detergent extraction study

We have previously suggested that the omega-3 polyunsaturated fatty acid, docosahexaenoic acid (DHA) may in part function by enhancing membrane lipid phase separation into lipid rafts. Here we further tested for differences in the molecular interactions of an oleic (OA) versus DHA-containing phospholipid with sphingomyelin (SM) and cholesterol (CHOL) utilizing (2)H NMR spectroscopy, differential scanning calorimetry, atomic force microscopy, and detergent extractions in model bilayer membranes. (2)H NMR and DSC (differential scanning calorimetry) established the phase behavior of the OA-containing 1-[(2)H(31)]palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (16:0-18:1PE-d(31))/SM (1:1) and the DHA-containing 1-[(2)H(31)]palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphoethanolamine (16:0-22:6PE-d(31))/SM (1:1) in the absence and presence of equimolar CHOL. CHOL was observed to affect the OA-containing phosphatidylethanolamine (PE) more than the DHA-containing PE, as exemplified by >2 x greater increase in order measured for the perdeuterated palmitic chain in 16:0-18:1PE-d(31)/SM (1:1) compared to 16:0-22:6PE-d(31)/SM (1:1) bilayers in the liquid crystalline phase. Atomic force microscopy (AFM) experiments showed less lateral phase separation between 16:0-18:1PE-rich and SM/CHOL-rich raft domains in 16:0-18:1PE/SM/CHOL (1:1:1) bilayers than was observed when 16:0-22:6PE replaced 16:0-18:1PE. Differences in the molecular interaction of 16:0-18:1PE and 16:0-22:6PE with SM/CHOL were also found using biochemical detergent extractions. In the presence of equimolar SM/CHOL, 16:0-18:1PE showed decreased solubilization in comparison to 16:0-22:6PE, indicating greater phase separation with the DHA-PE. Detergent experiments were also conducted with cardiomyocytes fed radiolabeled OA or DHA. Although both OA and DHA were found to be largely detergent solubilized, the amount of OA that was found to be associated with raft-rich detergent-resistant membranes exceeded DHA by almost a factor of 2. We conclude that the OA-PE phase separates from rafts far less than DHA-PE, which may have implications for cellular signaling.

Characterization of a quasicrystalline phase in codispersions of phosphatidylethanolamine and glucocerebroside

Synchrotron x-ray diffraction, differential scanning calorimetry, and electron spin resonance spectroscopy have been employed to characterize a quasicrystalline phase formed in aqueous dispersions of binary mixtures of glucocerebroside and palmitoyloleoylphosphatidylethanolamine. Small- and wide-angle x-ray scattering intensity patterns were recorded during temperature scans between 20 degrees and 90 degrees C from mixtures of composition 2, 5, 10, 20, 30, and 40 mol glucocerebroside per 100 mol phospholipid. The quasicrystalline phase was characterized by a broad lamellar d-spacing of 6.06 nm at 40 degrees C and a broad wide-angle x-ray scattering band centered at approximately 0.438 nm, close to the gel phase centered at approximately 0.425 nm and distinct from a broad peak centered at 0.457 nm observed for a liquid-crystal phase at 80 degrees C. The quasicrystalline phase coexisted with gel and fluid phase of the pure phospholipid. An analysis of the small-angle x-ray scattering intensity profiles indicated a stoichiometry of one glucosphingolipid per two phospholipid molecules in the complex. Structural transitions monitored in cooling scans by synchrotron x-ray diffraction indicated that a cubic phase transforms initially into a lamellar gel. Thermal studies showed that the gel phase subsequently relaxes into the quasicrystalline phase in an exothermic transition. Electron spin resonance spectroscopy using spin labels located at positions 7, 12, and 16 carbons of phospholipid hydrocarbon chains indicated that order and motional constraints at the 7 and 12 positions were indistinguishable between gel and quasicrystalline phases but there was a significant decrease in order and increase in rate of motion at the 16 position on transformation to the quasicrystalline phase. The results are interpreted as an arrangement of polar groups of the complex in a crystalline array and a quasicrystalline packing of the hydrocarbon chains predicated by packing problems in the bilayer core requiring disordering of the highly asymmetric chains. The possible involvement of quasicrystalline phases in formation of membrane rafts is considered.

Dynamic molecular structure of DPPC-DLPC-cholesterol ternary lipid system by spin-label electron spin resonance

The hydrated ternary lamellar lipid mixture of dipalmitoyl-PC/dilauroyl-PC/cholesterol (DPPC/DLPC/Chol) has been studied by electron spin resonance (ESR) to reveal the dynamic structure on a molecular level of the different phases that exist and coexist over virtually the full range of composition. The spectra for more than 100 different compositions at room temperature were analyzed by nonlinear least-squares fitting to provide the rotational diffusion rates and order parameters of the end-chain labeled phospholipid 16-PC. The ESR spectra exhibit substantial variation as a function of composition, even though the respective phases generally differ rather modestly from each other. The Lalpha and Lbeta phases are clearly distinguished, with the former exhibiting substantially lower ordering and greater motional rates, whereas the well-defined Lo phase exhibits the greatest ordering and relatively fast motional rates. Typically, smaller variations occur within a given phase. The ESR spectral analysis also yields phase boundaries and coexistence regions which are found to be consistent with previous results from fluorescence methods, although new features are found. Phase coexistence regions were in some cases confirmed by observing the existence of isosbestic points in the absorption mode ESR spectra from the phases. The dynamic structural properties of the DPPC-rich Lbeta and DLPC-rich Lalpha phases, within their two-phase coexistence region do not change with composition along a tie-line, but the ratio of the two phases follows the lever rule in accordance with thermodynamic principles. The analysis shows that 16-PC spin-label partitions nearly equally between the Lalpha and Lbeta phases, making it a useful probe for studying such coexisting phases. Extensive study of two-phase coexistence regions requires the determination of tie-lines, which were approximated in this study. However, a method is suggested to accurately determine the tie-lines by ESR.

Hydrophobic match between cholesterol and saturated fatty acid is required for the formation of lamellar liquid ordered phases

Palmitic acid and cholesterol have been shown to form, under certain conditions, bilayers in the liquid ordered (lo) phase. In the present work, the contribution of the hydrophobic match between cholesterol (chol), and the acyl chain of saturated fatty acids (FA) has been examined. The behavior of FA/chol mixtures where the FA acyl chain length was varied between 12 and 24 carbon atoms was investigated by infrared and 2H NMR spectroscopy, as well as by differential scanning calorimetry. It was found that only fatty acids with acyl chains of 14-18 carbon atoms lead to the formation of lo phase bilayers. The length of these chains corresponds, in fact, to the length of the long axis of the cholesterol molecule. Therefore, the hydrophobic match between the apolar parts of the molecular constituents appears to be a requisite for the formation of lamellar lo phases.

Interaction of Cholesterol with a Docosahexaenoic Acid-Containing Phosphatidylethanolamine: Trigger for Microdomain/Raft Formation?†

Docosahexaenoic acid (DHA, 22:6) containing phospholipids have been postulated to be involved in promoting lateral segregation within membranes into cholesterol- (CHOL-) rich and CHOL-poor lipid microdomains. Here we investigated the specific molecular interactions of phospholipid bilayers composed of 1-[(2)H(31)]palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphoethanolamine (16:0-22:6PE-d(31)) or 1-[(2)H(31)]palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (16:0-18:1PE-d(31)) with equimolar CHOL using solid-state (2)H NMR spectroscopy and low- and wide-angle X-ray diffraction (XRD). Moment analysis of (2)H NMR spectra obtained as a function of temperature reveals that the main chain melting transition and the lamellar-to-inverted hexagonal (H(II)) phase transition of 16:0-22:6PE-d(31) remain in the presence of equimolar CHOL, whereas addition of equimolar CHOL essentially obliterates the gel-to-liquid crystalline transition of 16:0-18:1PE-d(31). (2)H NMR order parameter measurements show that the addition of equimolar CHOL in the lamellar liquid crystalline phase causes a smaller increase in order for the perdeuterated sn-1 chain by 22% for 16:0-22:6PE-d(31) as opposed to 33% for 16:0-18:1PE-d(31). XRD experiments determined markedly lower solubility of 32 +/- 3 mol % for CHOL in 16:0-22:6PE bilayers in contrast to the value of approximately 51 mol % for 16:0-18:1PE. Our findings provide further evidence that cholesterol has a low affinity for DHA-containing phospholipids and that this reduced affinity may serve as a mechanism for triggering the formation of lipid microdomains such as rafts.