[Show abstract][Hide abstract] ABSTRACT: We created novel non-phospholipid photosensitive liposomes from a mixture of a monoacylated azobenzene amphiphile (AzoC10N+), and cholesterol sulfate (Schol). This system belongs to the family of sterol-enriched non-phospholipid liposomes that were shown to form stable large unilamellar vesicles (LUVs) with enhanced impermeability. Fluid bilayers were successfully prepared from AzoC10N+/Schol (25/75, mol/mol) mixtures and LUVs could be derived at room temperature using standard extrusion methods. The isomerization process of the bilayer-inserted AzoC10N+ was characterized. Leakage from these liposomes could be induced by the photoconversion of the AzoC10N+ from its trans to its cis form. This photo-controlled release is obtained from fluid liposomes, contrasting with phospholipid-based azo-containing liposomes, which are generally required to be in the gel phase in order to be photosensitive. It is proposed that the very high conformational order of the monoalkylated amphiphile and the tight packing of the hydrophobic core of the AzoC10N+/Schol liposomes make them responsive to the presence of the bulky cis azo isomer. Interestingly, the liposome impermeability could be fully restored by the photoisomerization of the cis form back to the trans form, providing a sharp on-and-off control of payload release. In addition, these non-phospholipid liposomes display a very limited passive release. Therefore, it is shown that AzoC10N+/Schol LUVs can be used as nanocontainers, whose content can be released by light in a controlled and switchable manner.
[Show abstract][Hide abstract] ABSTRACT: Liposomes made of single-chain amphiphiles and a large amount of sterols display several advantages including a limited permeability. In the present paper, we examine the possibility to prepare such non-phospholipid liposomes with interfacial polyethylene glycol (PEG) in order to improve their circulation in the blood stream. Cholesterol (Chol) was chosen as the PEG anchor.
Journal of Colloid and Interface Science 08/2014; 428:111–120. · 3.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Despite the fact that palmitic acid (PA) and cholesterol (Chol) do not form fluid bilayers once hydrated individually, giant unilamellar vesicles (GUVs) were formed from a mixture of palmitic acid and cholesterol, 30/70 mol/mol. These free-floating GUVs were stable over weeks, did not aggregate and were shown to be highly stable in alkaline pH compared to conventional phospholipid-based GUVs. Acidic pH-triggered payload release from the GUVs was associated with the protonation state of palmitic acid that dictated the mixing lipid properties, thus affecting the stability of the fluid lamellar phase. The successful formation of PA-Chol GUVs reveals the possibility to create monoalkylated amphiphile-based GUVs with distinct pH stability/sensitivity.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Cetylpyridinium chloride (CPC) is a surfactant that binds strongly to bacteria and bacterial biofilms. In this study, fluorescence-based techniques were used to determine the penetration and adhesion of CPC when it was introduced in liposomes. In spite of a reduced adhesion as compared to pure CPC micelles, CPC-containing liposomes adhered significantly to the biofilms of Streptococcus mutans. In contrast, no binding was observed for liposomes that were composed of phosphatidylcholine-cholesterol. The influence of the charge of the liposome on its adhesion to biofilms was studied using cholesterol (Chol) and cholesterol sulfate (Schol). In spite of similar binding to the biofilms, positively charged CPC/Chol liposomes were located mainly in the core of the biofilm microcolonies, whereas the negatively charged CPC/Schol liposomes were mainly concentrated at their periphery. This effect may be attributed to the different availability of the CPC head group. In summary, this work demonstrates the high potential for tailoring drug nanovectors by modulating sterol selection in order to selectively target and bind biofilms.
[Show abstract][Hide abstract] ABSTRACT: The skin acts mainly as a protective barrier from the external environment, thanks to the stratum corneum which is the outermost layer of the skin. As in vitro tests on skin are essential to elaborate new drugs, the development of skin models closer to reality becomes essential. It is now possible to produce in vitro human skin substitutes through tissue engineering by using the self-assembly method developed by the Laboratoire d'Organogénèse Expérimentale. In the present work, infrared microspectroscopy imaging analyses were performed to get in-depth morpho-spectral characterization of the three characteristic layers of human skin substitutes and normal human skin, namely the stratum corneum, living epidermis, and dermis. An infrared spectral analysis of the skin is a powerful tool to gain information on the order and conformation of the lipid chains and the secondary structure of proteins. On one hand, the symmetric stretching mode of the lipid methylene groups (2,850 cm(-1)) is sensitive to the acyl chain conformational order. The evolution profile of the frequency of this vibrational mode throughout the epidermis suggests that lipids in the stratum corneum are more ordered than those in the living epidermis. On the other hand, the frequencies of the infrared components underneath the envelop of the amide I band provide information about the overall protein conformation. The analysis of this mode establishes that the proteins essentially adopt an α-helix conformation in the epidermis, probably associated with the presence of keratin, while modifications of the protein content are observed in the dermis (extracellular matrix made of collagen). Finally, the lipid organization, as well as the protein composition in the different layers, is similar for human skin substitutes and normal human skin, confirming that the substitutes reproduce essential features of real skin and are appropriate biomimetics.
Analytical and Bioanalytical Chemistry 06/2013; · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The bovine seminal plasma contains phosphocholine (PC)-binding proteins, which associate to sperm membranes upon ejaculation. These binder-of-sperm (BSP) proteins then induce a phospholipid and cholesterol efflux from these membranes. In this work, we determined physical and chemical parameters controlling this efflux by characterizing the lipid extraction induced by BSP1, the most abundant of BSP protein in bull seminal plasma, from model membranes with different composition. The model membranes were formed from binary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (Lyso-PC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) or cholesterol. The modulation of BSP1-induced lipid extraction from membranes by their chemical composition and their physical properties brings us to propose a 3-step extraction mechanism. First, the protein associates with membranes via specific binding to phosphocholine groups. Second, BSP1 penetrates in the membrane, essentially in the external lipid leaflet. Third, BSP1 molecules solubilize a lipid patch coming essentially from the outer lipid leaflet, without any lipid specificity, to ultimately form small lipid/protein auto-assemblies. The stoichiometry of these complexes corresponds to 10-15 lipids per protein. It is also shown that fluid-phase membranes are more prone to BSP1-induced lipid extraction than gel-phase ones. The inhibition of the lipid extraction in this case appears to be related to the inhibition of the protein penetration in the membrane (step 2) and not to the protein association with PC head groups (step 1). These findings contribute to our understanding of the mechanism by which BSP1 modify the lipid composition of sperm membranes, a key event in sperm capacitation.
Biochimica et Biophysica Acta 08/2012; · 4.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Ethanol is used in a variety of topical products. It is known to enhance the permeability of the skin by altering the ability of the stratum corneum (SC) intercellular membranes to form an effective barrier. In addition, ethanol and other alcohols are key components of antiseptic gels currently used for hand wash. Using infrared and deuterium NMR spectroscopy as well as calorimetry, we have investigated the effect of ethanol on a model membrane composed of lipids representing the three classes of SC lipids, an equimolar mixture of N-palmitoylsphingosine (ceramide), palmitic acid and cholesterol. Ethanol is found to influence the membrane in a dose dependent manner, disrupting packing and increasing lipid motion at low concentrations and selectively extracting lipids at moderate concentrations.
Biochimica et Biophysica Acta 02/2012; 1818(5):1410-9. · 4.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that it is possible to form non-phospholipid fluid bilayers in aqueous milieu with a mixture of palmitic acid (PA), cholesterol (Chol), and cholesterol sulfate (Schol) in a molar proportion of 30/28/42. These self-assemblies are shown to be bilayers in the liquid ordered phase. They are stable between pH 5 and 9. Over this pH range, the protonation/deprotonation of PA carboxylic group is observed but this change does not appear to alter the stability of these bilayers, a behavior contrasting with that observed for binary mixtures of PA/Chol, and PA/Schol. The multilamellar dispersions formed spontaneously from the PA/Chol/Schol mixture could be successfully extruded to form Large Unilamellar Vesicles (LUVs). These LUVs show interesting permeability properties, linked with their high sterol content. These non-phospholipid liposomes can sustain a pH gradient (pHinternal 8/pHexternal 6) 100 times longer than LUVs made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and cholesterol, with a molar ratio of 60/40. Moreover, the non-phospholipid LUVs are shown to protect ascorbic acid from an oxidizing environment (1 mM iron(III)). Once entrapped in liposomes, ascorbic acid displays a degradation rate similar to that obtained in the absence of iron(III). These results show the possibility to form novel nanocontainers from a mixture of a monoalkylated amphiphile and sterols, with a good pH stability and showing interesting permeability properties.
[Show abstract][Hide abstract] ABSTRACT: Milk has been used routinely as an extender for sperm preservation. Caseins, the major proteins in milk, are proposed to be the protective constituents of milk during sperm preservation. It is unclear whether the whey proteins in milk are also implicated in the protection of sperm. Our previous studies have shown that the major proteins of bovine seminal plasma (recently named as binder of sperm or BSP, which comprises BSP1, BSP3, and BSP5 proteins) mediate a continuous phospholipid and cholesterol efflux from the sperm plasma membrane that is detrimental for sperm preservation. In this study, we investigated whether the protective effect of milk could be due to an interaction between BSP proteins and milk proteins. The binding of BSP proteins to milk proteins was demonstrated by gel filtration chromatography. Milk was fractionated into three fractions: the first containing whey protein aggregates and kappa-casein, the second containing all milk proteins, and the third containing small peptides, salts, and sugars. BSP1 has a higher affinity for the milk proteins in the milk fractions as compared to BSP3 and BSP5. The binding of BSP proteins to milk proteins was further characterized by isothermal titration calorimetry. We demonstrated that BSP1 binds to caseins and the titration could be simulated with a Scatchard approach, leading to an affinity constant (K(a)) of 350 mM(-1) and a stoichiometric parameter for the association (n) of 4.5 BSP1 per casein. The association between BSP1 and alpha-lactalbumin was characterized by a K(a) of 240 mM(-1) and an n value of 0.8. These results indicate the existence of an interaction between BSP proteins and milk proteins that could be the origin of the protection of sperm during preservation in milk.
Biology of Reproduction 05/2011; 85(3):457-64. · 3.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Egg yolk is used in extender to protect sperm from cold shock and freezing during preservation. It is the low-density lipoprotein (LDL) fraction of egg yolk that protects sperm. Even though essential for sperm capacitation, the major proteins from bull seminal plasma, the Binder of SPerm (BSP) proteins, are detrimental for sperm preservation because they induce a continual phospholipids and cholesterol efflux from sperm membranes. The BSP proteins were proposed to bind to egg yolk LDL, preventing the sperm membrane damage. We characterized the binding between the BSP proteins and the LDL by isothermal titration calorimetry, providing the thermodynamics and quantitative description of this putative association. The association between BSP1 (major BSP proteins) and LDL is characterized by an affinity constant (Ka) of 3.4±0.4μM−1. A protein/LDL ratio of 104±5 was determined indicating that 104 molecules of BSP1 would bind to one LDL particle. This stoichiometry leads to proposing that the association involves 1.6±0.1 phosphatidylcholines (PC) per BSP protein. This finding is satisfactorily consistent with the fact that each BSP1 protein has 2 binding sites for choline group. In conclusion, the formation of a high affinity complex between BSP1 and LDL is proposed to be important for the protection of sperm by egg yolk extender.
[Show abstract][Hide abstract] ABSTRACT: Curdlan, a bacterial polysaccharide, can form different types of thermogels, having the very same chemical composition, but whose structures depend on the incubation temperature. Structural characterization of 10% (w/v) low-set and high-set curdlan gels was carried out by Fourier transformed infrared (FT-IR) imaging and environmental scanning electron microscopy (eSEM) in the hydrated state. Considerable differences were observed between the two gels, the high-set one being overall more homogeneous. The self-diffusion coefficients of a series of analytes of different sizes (water, phosphate, glucose-6-phosphate, polyphosphate, polyethylene glycol, and dextran labelled with rhodamine B) were measured in aqueous solution (D(s)(sln)) and in both types of curdlan gels (D(s)(gel)) using (1)H and (31)P pulsed field gradient nuclear magnetic resonance (PFG NMR) spectroscopy. The mutual-diffusion coefficients (D(m)(gel)) of dextran in the curdlan gels were determined from release experiments based on fluorescence spectroscopy. The dependence of the relative diffusion coefficient (D(s)(gel)D(s)(sln)) on the size of the analyte, expressed by its hydrodynamic radius (R(h)), could be expressed by D(s)(gel)D(s)(sln) ∝ exp(-R(h)(0.46)), valid for both types of gels. The self-diffusion measurements for the largest investigated analytes were not compatible with a single diffusion coefficient and, therefore, were analysed using an approach based on a normal distribution of self-diffusion coefficients. In the hydrogels, broadening of the self-diffusion coefficient distribution increased as a function of the analyte size. This phenomenon was associated with the limited distance travelled by the analytes during the measurements, and it is inferred that the distribution of diffusion coefficients is representative of the distribution of local environments of the individual analyte. It was found that the structural differences observed between both types of curdlan gels are not correlated with the gel transport properties, highlighting the complexity of the relationship between structural details and transport properties in gels.
Journal of Colloid and Interface Science 02/2011; 357(2):419-27. · 3.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Binder-of-sperm (BSP) proteins interact with sperm membranes and are proposed to extract selectively phosphatidylcholine and cholesterol from these. This change in lipid composition is a key step in sperm capacitation. The present work demonstrates that the interactions between the protein BSP1 and model membranes composed with phosphatidylcholine lead to drastic changes in the morphology of the lipidic self-assemblies. Using cryo-electron microscopy and fluorescence microscopy, we show that, in the presence of the protein, the lipid vesicles elongate, and form bead necklace-like structures that evolve toward small vesicles or thread-like structures. In the presence of multilamellar vesicles, where a large reservoir of lipid is available, the presence of BSP proteins lead to the formation of long nanotubes. Long spiral-like threads, associated with lipid/protein complexes, are also observed. The local curvature of lipid membranes induced by the BSP proteins may be involved in lipid domain formation and the extraction of some lipids during the sperm maturation process.
Biochemical and Biophysical Research Communications 08/2010; 399(3):406-11. · 2.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: It has been shown that mixtures of palmitic acid (PA) and cholesterol (Chol) or cholesterol sulfate (Schol) can form fluid bilayers. These bilayers could be extruded using standard extrusion techniques to obtain nonphospholipid large unilamellar vesicles (LUVs). These LUVs displayed a very limited passive permeability, associated with their high sterol content (typically 70 mol %). In addition, they showed a pH-dependent behavior dictated by the electrostatic interfacial interactions, which are drastically modulated by the protonation state of PA. Interestingly, the LUVs prepared with cholesterol were stable at high pH and the release of the content could be triggered by a pH decrease (i.e., the protonation of PA). In contrast, the LUVs including Schol were stable at low pH and a pH increase (leading to the deprotonation of PA) would induce the release. In the present study, we demonstrate that the pH triggering the release in these two systems can be dictated in a predictable manner by selecting a fatty acid with an appropriate pK(a). The pK(a) of the fatty acids was modulated by the presence of an electro-withdrawing group (hydroxyl or fluoro) in the alpha position of the carboxylic function. The fatty acid protonation state is shown to be a critical factor for the modulation of the liposome permeability. The described systems display a remarkable versatility regarding the pH-sensitivity because the nature of the sterol controls the overall pH stability of the LUVs while the fatty acid pK(a) fine-tunes the pH-induced release. Therefore, it is possible to rationally design LUVs with controlled release at a specific pH; this original aspect is beneficial to the use of LUVs for encapsulation, vectorization, and controlled release of active agents.
[Show abstract][Hide abstract] ABSTRACT: 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.
Biochimica et Biophysica Acta 02/2010; 1798(6):1144-52. · 4.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In controlled drug delivery, the drug diffusion coefficient in a given matrix is a key factor for predicting its release rate. In this work, we compare (31)P nuclear magnetic resonance (NMR) profiling for obtaining the mutual-diffusion coefficient (D(m)(gel)) of a drug in hydrogel with results obtained from conventional source/sink experiments. Despite the fact that NMR profiling is a powerful approach for measuring transport properties, it is rarely used for characterizing drug diffusion in gel matrices in pharmaceutical sciences. This work provides an illustration of the applicability of this technique and highlights its advantages for studying drug release systems. The comparison with results obtained from the source/sink experiment clearly establishes the validity of the NMR profiling approach. Alendronate was used as a model drug while curdlan, a gel-forming bacterial polysaccharide, served as a model biomaterial. The determined (D(m)(gel)) value (5.6 ± 0.3 × 10(-10) m(2)/s) agrees with the one obtained from a conventional source/sink experiment (5.4 ± 0.5 × 10(-10) m(2)/s). In addition, the alendronate self-diffusion coefficients in solution (D(s)(sln)) and in the hydrogel (D(s)(gel)) were measured on the same system using pulse-field gradient (PFG) (31)P NMR. These supplementary parameters provided a more detailed characterization of the drug transport properties in the gel matrix.
Pharmaceutical Development and Technology 01/2010; 16(6):651-6. · 1.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dimethyl sulfoxide (DMSO), an efficient transdermal enhancer, is proposed to alter the skin barrier by, at least partially, disturbing the lipid phase of the stratum corneum (SC). We have investigated, using differential scanning calorimetry and vibrational microspectroscopy, the effect of DMSO on the phase behavior of a lipid mixture formed by N-palmitoyl-d-erythro-sphingosine, deuterated palmitic acid, and cholesterol, mimicking the SC lipid phase. Our results reveal that DMSO favors the disordering of the lipid acyl chains. Moreover, the effect of DMSO is strongly concentration dependent and this dependence is reminiscent of that describing the DMSO transdermal enhancement. DMSO-induced fluidification affects primarily the fatty acid in the mixture. Therefore, it is proposed that the molecular mechanism of the transdermal transport enhancement caused by DMSO is associated with its H-bonding properties; its presence alters the interfacial H-bond network involving the fatty acid molecules and consequently the cohesive lipid packing.
Chemistry and Physics of Lipids 09/2009; · 2.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The phase behavior and lipid mixing properties of an equimolar mixture of nonhydroxylated palmitoyl ceramide (Cer16), palmitic acid (PA), and cholesterol have been investigated using 2H NMR and vibrational spectroscopy. This mixture is formed by the three main classes of lipids found in the stratum corneum (SC), the top layer of the epidermis, and provides an optimized hydrophobic matching. Therefore, its behavior highlights the role played by hydrophobic matching on the phase behavior of SC lipids. We found that, below 45 degrees C, the mixture is essentially formed of coexisting crystalline domains with a small fraction of lipids (less than 20%) that forms a gel or fluid phase, likely ensuring cohesion between the solid domains. Upon heating, there is the formation of a liquid ordered phase mainly composed of PA and cholesterol, including a small fraction of Cer16. This finding is particularly highlighted by correlation vibrational microspectroscopy that indicates that domains enriched in cholesterol and PA include more disordered Cer16 than those found in the Cer16-rich domains. Solubilization of Cer16 in the fluid phase occurs progressively upon further heating, and this leads to the formation of a nonlamellar self-assembly where the motions are isotropic on the NMR time scale. It is found that the miscibility of Cer16 with cholesterol and PA is more limited than the one previously observed for ceramide III extracted from bovine brain, which is heterogeneous in chain composition and includes, in addition to Cer16, analogous ceramide with longer alkyl chains that are not hydrophobically matched with cholesterol and PA. Therefore, it is inferred that, in SC, the chain heterogeneity is a stronger criteria for lipid miscibility than chain hydrophobic matching.