Phase behavior of an equimolar mixture of N-palmitoyl-D-erythro-sphingosine, cholesterol, and palmitic acid, a mixture with optimized hydrophobic matching

Effects of imidazolium ionic liquids on skin barrier lipids - Perspectives for drug delivery

Ionic liquids (ILs) have great potential to facilitate transdermal and topical drug delivery. Here, we investigated the mechanism of action of amphiphilic ILs 1-methyl-3-octylimidazolium bromide (C8MIM) and 3-dodecyl-1-methylimidazolium bromide (C12MIM) in skin barrier lipid models in comparison to their complex effects in human skin. C8MIM incorporated in a skin lipid model was a better permeation enhancer than C12MIM for water and model drugs, theophylline and diclofenac. Solid state 2H NMR and X-ray diffraction indicated that both ILs prefer the cholesterol-rich regions in skin lipids without significantly perturbing their lamellar arrangement and that C8MIM induces the formation of an isotropic lipid phase to a greater extent compared to C12MIM. C12MIM applied topically to the lipid model or human skin as a pretreatment was more potent than C8MIM. When co-applied with the drugs to human skin, aqueous C12MIM was more potent than C8MIM in enhancing theophylline permeation, but neither IL affected (even decreased) diclofenac permeation. Thus, the IL's ability to permeabilize skin lipid barrier is strongly modulated by its ability to reach the site of action and its interactions with drug and solvent. Such an interplay is far from trivial and requires detailed investigation to realize the full potential of ILs.

The skin barrier: An extraordinary interface with an exceptional lipid organization

The barrier function of the skin is primarily located in the stratum corneum (SC), the outermost layer of the skin. The SC is composed of dead cells with highly organized lipid lamellae in the intercellular space. As the lipid matrix forms the only continuous pathway, the lipids play an important role in the permeation of compounds through the SC. The main lipid classes are ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs). Analysis of the SC lipid matrix is of crucial importance in understanding the skin barrier function, not only in healthy skin, but also in inflammatory skin diseases with an impaired skin barrier. In this review we provide i) a historical overview of the steps undertaken to obtain information on the lipid composition and organization in SC of healthy skin and inflammatory skin diseases, ii) information on the role CERs, CHOL and FFAs play in the lipid phase behavior of very complex lipid model systems and how this knowledge can be used to understand the deviation in lipid phase behavior in inflammatory skin diseases, iii) knowledge on the role of both, CER subclasses and chain length distribution, on lipid organization and lipid membrane permeability in complex and simple model systems with synthetic CERs, CHOL and FFAs, iv) similarity in lipid phase behavior in SC of different species and complex model systems, and vi) future directions in modulating lipid composition that is expected to improve the skin barrier in inflammatory skin diseases.

The Intriguing Molecular Dynamics of Cer[EOS] in Rigid Skin Barrier Lipid Layers Requires Improvement of the Model

Omega-O-acyl ceramides such as 32-linoleoyloxydotriacontanoyl sphingosine (Cer[EOS]) are essential components of the lipid skin barrier, which protects our body from excessive water loss and the penetration of unwanted substances. These ceramides drive the lipid assembly to epidermal-specific long periodicity phase (LPP), structurally much different than conventional lipid bilayers. Here, we synthesized Cer[EOS] with selectively deuterated segments of the ultralong N-acyl chain or deuterated or ¹³C-labeled linoleic acid and studied their molecular behavior in a skin lipid model. Solid-state ²H NMR data revealed surprising molecular dynamics for the ultralong N-acyl chain of Cer[EOS] with increased isotropic motion towards the isotropic ester-bound linoleate. The sphingosine moiety of Cer[EOS] is also highly mobile at skin temperature, in stark contrast to the other LPP components, N-lignoceroyl sphingosine acyl, lignoceric acid and cholesterol, which are predominantly rigid. The dynamics of the linoleic chain is quantitatively described by distributions of correlation times and using dynamic detector analysis. These NMR results along with neutron diffraction data suggest an LPP structure with alternating fluid (sphingosine chain-rich), rigid (acyl chain-rich), isotropic (linoleate-rich), rigid (acyl-chain rich), and fluid layers (sphingosine chain-rich). Such an arrangement of the skin barrier lipids with rigid layers separated with two different dynamic "fillings" i) agrees well with ultrastructural data, ii) satisfies the need for simultaneous rigidity (to ensure low permeability) and fluidity (to ensure elasticity, accommodate enzymes or antimicrobial peptides), and iii) offers a straightforward way to remodel the lamellar body lipids into the final lipid barrier.

Cholesterol Sulfate Fluidizes the Sterol Fraction of the Stratum Corneum Lipid Phase and Increases its Permeability

Desulfation of cholesterol sulfate (CholS) to cholesterol (Chol) is an important event in epidermal homeostasis and necessary for stratum corneum (SC) barrier function. The CholS/Chol ratio decreases during SC maturation but remains high in pathological conditions, such as X-linked ichthyosis, characterized by dry and scaly skin. The aim of this study was to characterize the influence of the CholS/Chol molar ratio on the structure, dynamics, and permeability of SC lipid model mixtures. We synthesized deuterated CholS and investigated lipid models with specifically deuterated components using ²H solid-state NMR spectroscopy at temperatures from 25°C to 80°C. Although the rigid acyl chains in ceramides and fatty acids remained essentially rigid upon variation of the CholS/Chol ratio, both sterols were increasingly fluidized in lipid models containing higher CholS concentrations. We also show the X-ray repeat distance of the lipid lamellar phase (105 Å) and the orthorhombic chain packing of the ceramide’s acyl chains and long free fatty acids did not change upon the variation of the CholS content. However, the Chol phase separation visible in models with high Chol concentration disappeared at the 50:50 CholS/Chol ratio. This increased fluidity resulted in higher permeabilities to model markers of these SC models. These results reveal that a high CholS/Chol ratio fluidizes the sterol fraction and increases the permeability of the SC lipid phase while maintaining the lamellar lipid arrangement with an asymmetric sterol distribution.

Skin hydration as a tool to control the distribution and molecular effects of intermediate polarity compounds in intact stratum corneum

The barrier function of the skin is mainly assured by its outermost layer, stratum corneum (SC), which consists of dead keratin-filled cells embedded in a lipid matrix. The skin is daily exposed to an environment with changing conditions in terms of hydration and different chemicals. Here we investigate how a molecule that has reasonable solubility in both hydrophobic and hydrophilic environments can be directed to certain regions in SC by changing the skin hydration. We use 1,2,3-trimethoxy propane (TMP) as a model substance and solid-state NMR on natural abundance ¹³C to obtain atomically resolved information on the molecular dynamics of TMP as well as SC lipid and protein components at varying hydration conditions. Upon dehydration, TMP redistributes from the hydrophilic corneocytes to the hydrophobic SC lipid regions. In this way, TMP can act to prevent the fluid-solid lipid transition in drying conditions and be present in the corneocytes in more humid conditions. Hydration can thereby be used as a switch to control the location and action of TMP or similar compounds in complex materials like SC. The general principles described here can also have impact on other applications including lipid-based formulations in food, drug delivery and cosmetics.

The Importance of Free Fatty Chain Length on the Lipid Organization in the Long Periodicity Phase

The skin's barrier ability is an essential function for terrestrial survival, which is controlled by intercellular lipids within the stratum corneum (SC) layer. In this barrier, free fatty acids (FFAs) are an important lipid class. As seen in inflammatory skin diseases, when the lipid chain length is reduced, a reduction in the barrier's performance is observed. In this study, we have investigated the contributing effects of various FFA chain lengths on the lamellar phase, lateral packing. The repeat distance of the lamellar phase increased with FFA chain length (C20-C28), while shorter FFAs (C16 to C18) had the opposite behaviour. While the lateral packing was affected, the orthorhombic to hexagonal to fluid phase transitions were not affected by the FFA chain length. Porcine SC lipid composition mimicking model was then used to investigate the proportional effect of shorter FFA C16, up to 50% content of the total FFA mixture. At this level, no difference in the overall lamellar phases and lateral packing was observed, while a significant increase in the water permeability was detected. Our results demonstrate a FFA C16 threshold that must be exceeded before the structure and barrier function of the long periodicity phase (LPP) is affected. These results are important to understand the lipid behaviour in this unique LPP structure as well as for the understanding, treatment, and development of inflammatory skin conditions.

Very Long Chain Lipids Favor the Formation of a Homogeneous Phase in Stratum Corneum Model Membranes

The stratum corneum (SC), the outermost layer of mammal epidermis, acts as a barrier dictating the rate of absorption of exogenous molecules through the skin, as well as to prevent excessive water loss from the body. The SC consists of protein-rich corneocytes embedded into a complex lipid mixture. The lipid fraction is mainly constituted of an equimolar mixture of ceramides (Cer), free fatty acids (FFA), and cholesterol (Chol), forming a solid phase in the intracellular space; this lipid phase is supposed to play a fundamental role in the SC barrier function. An unusual characteristic of this biological membrane is that its lipids generally bear very long acyl chains, with the 24-carbon long ones being the most abundant. In this work, we used Raman microspectroscopy and infrared spectroscopy to study the influence of the acyl chain length on the lipid mixing properties in SC model membranes. Our results revealed that the combination of ceramides and FFA bearing a very long chain is required for the formation of homogeneous lipid mixtures, while lipids with shorter chains (16-carbon and 20-carbon atom long) lead to domains with micrometer dimensions. It is proposed that the biological machinery necessary for acyl chain elongation occurring at the mammalian skin level is required to inhibit lipid phase separation, a critical feature in the proper barrier functioning.

The Sphingosine and Acyl Chains of Ceramide [NS] Show Very Different Structure and Dynamics That Challenge Our Understanding of the Skin Barrier

The lipid phase of the uppermost human skin layer is thought to comprise highly rigid lipids in an orthorhombic phase state to protect the body against the environment. By synthesizing sphingosine-d28 deuterated N-lignoceroyl-d-erythro-sphingosine (ceramide [NS]), we compare the structure and dynamics of both chains of that lipid in biologically relevant mixtures using X-ray diffraction, 2 H NMR analysis, and infrared spectroscopy. Our results reveal a substantial fraction of sphingosine chains in a fluid and dynamic phase state at physiological temperature. These findings prompt revision of our current understanding of the skin lipid barrier, where an extended ceramide [NS] conformation is preferred and a possible domain structure is proposed. Mobile lipid chains may be crucial for skin elasticity and the translocation of physiologically important molecules.

State of the Art in Stratum Corneum Research. Part II: Hypothetical Stratum Corneum Lipid Matrix Models

This review is the second part of a series which presents the state of the art in stratum corneum (SC) lipid matrix (LM) research in depth. In this part, the various hypothetical models which were developed to describe the structure and function of the SC LM as the skin's barrier will be discussed. New as well as a cumulative assortment of older results which change the view on the different models are considered to conclude how well the different models are holding up today. As a final conclusion, a model, factoring in as much of the known data as possible, is concluded, unifying the varying different models into one which can be developed further, as new results are found in the future. So far, the model is described with a single crystalline or gel-like phase with a certain amount of nanocrystallites of concentrated ceramides (CERs) and free fatty acids and more fluid nanodomains caused by a fluidizing effect of the cholesterol. These domains are dynamically resolved and reformed and do not impair the barrier function. The chain conformation is not completely clear yet; however, an equilibrium of fully extended and hairpin-folded CERs with ratios depending on the properties of each individual CER species is proposed as most likely. An overlapping middle layer as described for the tri-layer model in part I of this series would be present for both conformations. The macroscopic broad-narrow-broad layering, observed in electron micrographs, is explained by an external templating by the lipid envelope, and an internal templating by short and long lipid chains each preferentially show a homophilic association, forming thicker and thinner bilayers, respectively. The degree of influence of the very long ω-hydroxy-CERs is discussed as well.

Quantification of the amount of mobile components in intact stratum corneum with natural-abundance 13C solid-state NMR

The outermost layer of the skin is the stratum corneum (SC), which is mainly comprised of solid proteins and lipids. Minor amounts of mobile proteins and lipids are crucial for the macroscopic properties of the SC, including softness, elasticity and barrier function. Still this minor number of mobile components are not well characterized in terms of structure or amount. Conventional quantitative direct polarization (Q-DP) 13C solid-state NMR gives signal amplitudes proportional to concentrations, but fails to quantify the SC mobile components because of spectral overlap with the overwhelming signals from the solids. Spectral editing with the INEPT scheme suppresses the signals from solids, but also modulates the amplitudes of the mobile components depending on their values of the transverse relaxation times T2, scalar couplings JCH, and number of covalently bound hydrogens nH. This study describes a quantitative INEPT (Q-INEPT) method relying on systematic variation of the INEPT timing variables to estimate T2, JCH, nH, and amplitude for each of the resolved resonances from the mobile components. Q-INEPT is validated with a series of model systems containing molecules with different hydrophobicity and dynamics. For selected systems where Q-DP is applicable, the results of Q-INEPT and Q-DP are similar with respect to the linearity and uncertainty of the obtained molar ratios. Utilizing a reference compound with known concentration, we quantify the concentrations of mobile lipids and proteins within the mainly solid SC. By melting all lipids at high temperature, we obtain the total lipid concentration. These Q-INEPT results are the first steps towards a quantitative understanding of the relations between mobile component concentrations and SC macroscopic properties.

Lamellar Phases Composed of Phospholipid, Cholesterol, and Ceramide, as Studied by 2H NMR

Sphingolipids constitute a significant fraction of cellular plasma membrane lipid content. Among sphingolipids, ceramide levels are usually very low. However, in some cell processes like apoptosis, cell membrane ceramide levels increase markedly because of the activation of enzymes like acid sphingomyelinase. This increase can change the physical state of the membrane by promoting molecular order and inducing solid-ordered (So) phase domains. This effect has been observed in a previous 2H NMR study on membranes consisting of palmitoyl sphingomyelin (PSM) and palmitoyl ceramide (PCer). Cholesterol (Chol), too, is present at high concentrations in mammalian plasma membranes and has a favorable interaction with sphingomyelin (SM), together forming domains in the liquid-ordered phase in model membranes. There are reports that Chol is able to displace ceramide (Cer) in SM bilayers and abolish the So phase domains formed by SM:Cer. This ability of Chol appears to be concentration dependent; in membranes with low Chol and high Cer contents, So phase domains rich in Cer coexist with the continuous fluid phase of the membrane. Here, we studied the effect of increasing PCer concentration in PSM:Chol bilayers, using 2H NMR. Chol:PCer mole ratios were 3:1, 3:2, and 3:3, at a fixed 7:3 phospholipid:cholesterol mol ratio. Both PSM and PCer were monitored in separate samples for changes in their physical state by introducing a perdeuterated palmitoyl chain in either molecule. Moreover, the effect of replacing PSM with DPPC was investigated to test the impact on membrane phase behavior of replacing the sphingosine with a palmitoylated glycerol backbone. We found that PCer can increase acyl chain order in both PSM:Chol and DPPC:Chol bilayers. Especially in bilayers with Chol:PCer 1:1 molar ratios, PCer induces highly stable So phase domains in both PSM and DPPC bilayers near 37°C. However, PCer has a more pronounced ordering effect on PSM compared to DPPC bilayers.

Cholesterol Effects on the Physical Properties of Lipid Membranes Viewed by Solid-state NMR Spectroscopy

In this chapter, we review the physical properties of lipid/cholesterol mixtures involving studies of model membranes using solid-state NMR spectroscopy. The approach allows one to quantify the average membrane structure, fluctuations, and elastic deformation upon cholesterol interaction. Emphasis is placed on understanding the membrane structural deformation and emergent fluctuations at an atomistic level. Lineshape measurements using solid-state NMR spectroscopy give equilibrium structural properties, while relaxation time measurements study the molecular dynamics over a wide timescale range. The equilibrium properties of glycerophospholipids, sphingolipids, and their binary and tertiary mixtures with cholesterol are accessible. Nonideal mixing of cholesterol with other lipids explains the occurrence of liquid-ordered domains. The entropic loss upon addition of cholesterol to sphingolipids is less than for glycerophospholipids, and may drive formation of lipid rafts. The functional dependence of ²H NMR spin-lattice relaxation (R _1Z) rates on segmental order parameters (S _CD) for lipid membranes is indicative of emergent viscoelastic properties. Addition of cholesterol shows stiffening of the bilayer relative to the pure lipids and this effect is diminished for lanosterol. Opposite influences of cholesterol and detergents on collective dynamics and elasticity at an atomistic scale can potentially affect lipid raft formation in cellular membranes.

State of the art in Stratum Corneum research: The biophysical properties of ceramides

This review is summarizing an important part of the state of the art in stratum corneum research. A complete overview on discoveries about the general biophysical and physicochemical properties of the known ceramide species' is provided. The ceramides are one of the three major components of the lipid matrix and mainly govern its properties and structure. They are shown to exhibit very little redundancy, despite the minor differences in their chemical structure. The results are discussed, compared to each other as well as the current base of knowledge. New interesting aspects and concepts are concluded or suggested. A novel interpretation of the 3-dimensional structure of the lipid matrix and its influence on the barrier function will be discussed. The most important conclusion is the presentation of a new and up to date theoretical model of the nanostructure of the short periodicity phase. The model suggests three perpendicular layers: The rigid head group region, the rigid chain region and, a liquid-like overlapping middle layer. The general principle of the skin barrier function is highlighted in regard to this structure and the ceramides biophysical and physicochemical properties. As a result of these considerations, the entropy vs. enthalpy principle is introduced, shedding light on the function as well as the effectiveness of the skin barrier. Additionally, general ideas to effectively overcome this barrier principle for dermal and transdermal delivery of actives or how to use it for specific targeting of the stratum corneum are proposed.

Solid and fluid segments within the same molecule of stratum corneum ceramide lipid

The outer layer of the skin, stratum corneum (SC) is an efficient transport barrier and it tolerates mechanical deformation. At physiological conditions, the majority of SC lipids are solid, while the presence of a small amount of fluid lipids is considered crucial for SC barrier and material properties. Here we use solid-state and diffusion nuclear magnetic resonance to characterize the composition and molecular dynamics of the fluid lipid fraction in SC model lipids, focusing on the role of the essential SC lipid CER EOS, which is a ceramide esterified omega-hydroxy sphingosine linoleate with very long chain. We show that both rigid and mobile structures are present within the same CER EOS molecule, and that the linoleate segments undergo fast isotropic reorientation while exhibiting extraordinarily slow self-diffusion. The characterization of this unusual self-assembly in SC lipids provides deepened insight into the molecular arrangement in the SC extracellular lipid matrix and the role of CER EOS linoleate in the healthy and diseased skin.

Impact of the ceramide subspecies on the nanostructure of stratum corneum lipids using neutron scattering and molecular dynamics simulations. Part I: impact of CER[NS]

For this study mixtures based on the ceramides [NS] (NS = non-hydroxy-sphingosine) and [AP] (AP = α-hydroxy-phytosphingosine) in a 2:1 and 1:2 ratio, together with cholesterol and lignoceric acid, were investigated. These mixtures are modelling the uppermost skin layer, the stratum corneum. Neutron diffraction, utilizing specifically deuterated ceramide molecules, was used to obtain a maximum amount of experimental detail. Highly detailed molecular dynamics simulations were used to generate even more information from the experimental data. It was possible to observe a single lamellar phase for both systems. They had a lamellar repeat distance of 5.43 ± 0.05 nm for the [NS]/[AP] 2:1 and a slightly shorter one of 5.34 ± 0.05 nm for the 1:2 system. The structure and water content was uninfluenced by excess humidity. Both the experimental and simulation data indicated slightly tilted ceramides, with their C24 chains overlapping in the lamellar mid-plane. This arrangement is well comparable to systems investigated before. The structure of both systems, except for the differing repeat distance, looks similar at first. However, on a smaller scale there were various distinct differences, demonstrating only low redundancy between the different ceramide species, despite only minor chemical differences. The mainly ceramide [AP] determined 1:2 system has a slightly smaller repeat distance. This is a result of a tighter arrangement of the lipids chain along the bilayer normal and increased overlapping of the long chains in the lamellar middle. For the CER[NS] some novel features could be shown, despite it being the overall most investigated ceramide. These include the low adaptability to changed lateral interactions, leading to an increased chain opening. This effect could explain its low miscibility with other lipids. The investigated model systems allows it to directly compare results from the literature which have used ceramide [NS] to the most recent studies using the phytosphingosine ceramides such as ceramide [AP].

Enhancement of Galantamine HBr Skin Permeation Using Sonophoresis and Limonene-Containing PEGylated Liposomes

This study aimed to investigate the effect of low-frequency sonophoresis (SN) and limonene-containing PEGylated liposomes (PL) on the transdermal delivery of galantamine HBr (GLT). To evaluate the skin penetration mechanism, confocal laser scanning microscopy (CLSM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) were employed. The application of SN led to more GLT penetration into and through the skin than GLT solution alone. The liposomes also improved GLT permeation, and 2% limonene-containing PL (PL-LI2%) exhibited the highest GLT permeation, followed by PL-LI1%, PL-LI0.1%, and PL. The CLSM images of PL-LI2% resulted in the highest fluorescence intensity of fluorescent hydrophilic molecules in the deep skin layer, and the rhodamine PE-labeled liposome membrane was distributed in the intercellular region of the stratum corneum (SC). PL-LI2% induced significant changes in intercellular lipids in the SC, whereas SN had no effect on intercellular lipids of the SC. DSC thermograms showed that the greatest decrease in the lipid transition temperature occurred in PL-LI2%-treated SC. SN might improve drug permeation through an intracellular pathway, while limonene-containing liposomes play an important role in delivering GLT through an intercellular pathway by increasing the fluidity of intercellular lipids in the SC. Moreover, a small vesicle size and high membrane fluidity might enhance the transportation of intact vesicles through the skin.

Probing the role of ceramide hydroxylation in skin barrier lipid models by 2H solid-state NMR spectroscopy and X-ray powder diffraction

In this work, we studied model stratum corneum lipid mixtures composed of the hydroxylated skin ceramides N-lignoceroyl 6-hydroxysphingosine (Cer[NH]) and α-hydroxylignoceroyl phytosphingosine (Cer[AP]). Two model skin lipid mixtures of the composition Cer[NH] or Cer[AP], N-lignoceroyl sphingosine (Cer[NS]), lignoceric acid (C24:0) and cholesterol in a 0.5:0.5:1:1 molar ratio were compared. Model membranes were investigated by differential scanning calorimetry and ²H solid-state NMR spectroscopy at temperatures from 25 °C to 80 °C. Each component of the model mixture was specifically deuterated for selective detection by ²H NMR. Thus, the exact phase composition of the mixture at varying temperatures could be quantified. Moreover, using X-ray powder diffraction we investigated the lamellar phase formation. From the solid-state NMR and DSC studies, we found that both hydroxylated Cer[NH] and Cer[AP] exhibit a similar phase behavior. At physiological skin temperature of 32 °C, the lipids form a crystalline (orthorhombic) phase. With increasing temperature, most of the lipids become fluid and form a liquid-crystalline phase, which converts to the isotropic phase at higher temperatures (65-80 °C). Interestingly, lignoceric acid in the Cer[NH]-containing mixture has a tendency to form two types of fluid phases at 65 °C. This tendency was also observed in Cer[AP]-containing membranes at 80 °C. While Cer[AP]-containing lipid models formed a short periodicity phase featuring a repeat spacing of d = 5.4 nm, in the Cer[NH]-based model skin lipid membranes, the formation of unusual long periodicity phase with a repeat spacing of d = 10.7 nm was observed.

Investigation of a CER[NP]- and [AP]-Based Stratum Corneum Modeling Membrane System: Using Specifically Deuterated CER Together with a Neutron Diffraction Approach

Neutron diffraction was used as a tool to investigate the lamellar as well as molecular nanostructure of ceramide-[NP]/ceramide-[AP]/cholesterol/lignoceric acid model systems with a nativelike 2:1 ratio and a 1:2 ratio to study the influence of the ceramide-[AP]. By using mixtures together with cholesterol and free fatty acids as well as a humidity and temperature chamber while measuring, natural conditions were simulated as closely as possible. Despite its simplicity, the system simulated the native stratum corneum lipid matrix fairly closely, showing a similar lamellar thickness with a repeat distance of 5.45 ± 0.1 nm and a similar arrangement with overlapping long C24 chains. Furthermore, despite the very minor chemical difference between ceramide-[NP] and ceramide-[AP], which is only a single OH group, it was possible to demonstrate substantial differences between the structural influence of the two ceramides. Ceramide-[AP] could be concluded to be arranged in such a way that its C24 chain in both ratios is somehow shorter than that of ceramide-[NP], not overlapping as much with the opposite lamellar leaflet. Furthermore, in the unnatural 1:2 ratio, the higher ceramide-[AP] content causes an increased tilt of the ceramide acyl chains. This leads to even less overlapping within the lamellar midplane, whereas the repeat distance stays the same as for the ceramide-[NP]-rich system. In this nativelike 2:1 ratio, the chains are arranged mostly straight, and the long C24 chains show a broad overlapping region in the lamellar midplane.

Evidence of hydrocarbon nanodrops in highly ordered stratum corneum model membranes

The stratum corneum (SC), the top layer of skin, dictates the rate of both water loss through the skin and absorption of exogenous molecules into the body. The crystalline organization of the lipids in the SC is believed to be a key feature associated with the very limited permeability of the skin. In this work, we characterized the organization of SC lipid models that include, as in native SC, cholesterol, a series of FFAs (saturated with C16-C24 chains), as well as a ceramide bearing an oleate chain-linked to a very long saturated acyl chain [N-melissoyl-oleoyloxy hexacosanoyl-D-erythro-sphingosine (Cer EOS)]. The latter is reported to be essential for the native SC lipid organization. Our ²H-NMR, infrared, and Raman spectroscopy data reveal that Cer EOS leads to the formation of highly disordered liquid domains in a solid/crystalline matrix. The lipid organization imposes steric constraint on Cer EOS oleate chains in such a way that these hydrocarbon nanodroplets remain in the liquid state down to -30°C. These findings modify the structural description of the SC substantially and propose a novel role of Cer EOS, as this lipid is a strong modulator of SC solid/liquid balance.

Influence of a Novel Dimeric Ceramide Molecule on the Nanostructure and Thermotropic Phase Behavior of a Stratum Corneum Model Mixture

The stratum corneum (SC) is the outermost layer of the skin and is composed of a multilayered assembly of mostly ceramids (Cer), free fatty acids, cholesterol (Chol), and cholesterol sulfate (Chol-S). Because of the tight packing of these lipids, the SC features unique barrier properties defending the skin from environmental influences. Under pathological conditions, where the skin barrier function is compromised, topical application of molecules that rigidify the SC may lead to a restored barrier function. To this end, molecules are required that incorporate into the SC and bring back the original rigidity of the skin barrier. Here, we investigated the influence of a novel dimeric ceramide (dim-Cer) molecule designed to feature a long, rigid hydrocarbon chain ideally suited to forming an orthorhombic lipid phase. The influence of this molecules on the thermotropic phase behavior of a SC mixture consisting of Cer[AP18] (55 wt %), cholesterol (Chol, 25 wt %), steric acid (SA, 15 wt %), and cholesterol sulfate (Chol-S, 5 wt %) was studied using a combination of neutron diffraction and ²H NMR spectroscopy. These methods provide detailed insights into the packing properties of the lipids in the SC model mixture. Dim-Cer remains in an all-trans state of the membrane-spanning lipid chain at all investigated temperatures, but the influence on the phase behavior of the other lipids in the mixture is marginal. Biophysical experiments are complemented by permeability measurements in model membranes and human skin. The latter, however, indicates that dim-Cer only partially provides the desired effect on membrane permeability, necessitating further optimization of its structure for medical applications.

Phase separation in ceramide[NP] containing lipid model membranes: neutron diffraction and solid-state NMR

The stratum corneum is the outermost layer of the skin and protects the organism against external influences as well as water loss. It consists of corneocytes embedded in a mixture of ceramides, fatty acids, and cholesterol in a molar ratio of roughly 1 : 1 : 1. The unique structural and compositional arrangement of these stratum corneum lipids is responsible for the skin barrier properties. Many studies investigated the organization of these barrier lipids and, in particular, the exact conformation of ceramides. However, so far no consensus has been reached. In this study, we investigate a model system comprised of N-(non-hydroxy-tetracosanoyl)-phytosphingosine/cholesterol/tetracosanoic acid (CER[NP]-C24/CHOL/TA) at a 1 : 1 : 1 molar ratio using neutron diffraction and ²H solid-state NMR spectroscopy at temperatures from 25 °C to 80 °C. Deuterated variants of all three lipid components of the model system were used to enable their separate investigation in the NMR spectra and quantification of the amount of molecules in each phase. Neutron scattering experiments show the coexistence of two lipid phases at low temperatures with repeat spacings of 54.2 Å and 43.0 Å at a physiological skin temperature of 32 °C. They appear to be indistinguishable in the ²H NMR spectra as both phases are crystalline and ceramide molecules do not rotate around their long axis on a microsecond timescale. The evolution of these phases upon heating is followed and with increasing temperature fluid and even isotropically mobile molecules are observed. A model of the organization of the lamellar phases is proposed in which the thicker phase consists of CER[NP]-C24 in a hairpin conformation mixed with CHOL and TA, while the phase with a repeat spacing of 43.0 Å contains CER[NP]-C24 in a V-shape conformation.

Tracking solvents in the skin through atomically resolved measurements of molecular mobility in intact stratum corneum

Solvents are commonly used in pharmaceutical and cosmetic formulations and sanitary products and cleansers. The uptake of solvent into the skin may change the molecular organization of skin lipids and proteins, which may in turn alter the protective skin barrier function. We herein examine the molecular effects of 10 different solvents on the outermost layer of skin, the stratum corneum (SC), using polarization transfer solid-state NMR on natural abundance ¹³C in intact SC. With this approach it is possible to characterize the molecular dynamics of solvent molecules when present inside intact SC and to simultaneously monitor the effects caused by the added solvent on SC lipids and protein components. All solvents investigated cause an increased fluidity of SC lipids, with the most prominent effects shown for the apolar hydrocarbon solvents and 2-propanol. However, no solvent other than water shows the ability to fluidize amino acids in the keratin filaments. The solvent molecules themselves show reduced molecular mobility when incorporated in the SC matrix. Changes in the molecular properties of the SC, and in particular alternation in the balance between solid and fluid SC components, may have significant influences on the macroscopic SC barrier properties as well as mechanical properties of the skin. Deepened understanding of molecular effects of foreign compounds in SC fluidity can therefore have strong impact on the development of skin products in pharmaceutical, cosmetic, and sanitary applications.

A Coarse-Grained Model of Stratum Corneum Lipids: Free Fatty Acids and Ceramide NS

Ceramide (CER)-based biological membranes are used both experimentally and in simulations as simplified model systems of the skin barrier. Molecular dynamics studies have generally focused on simulating preassembled structures using atomistically detailed models of CERs, which limit the system sizes and time scales that can practically be probed, rendering them ineffective for studying particular phenomena, including self-assembly into bilayer and lamellar superstructures. Here, we report on the development of a coarse-grained (CG) model for CER NS, the most abundant CER in human stratum corneum. Multistate iterative Boltzmann inversion is used to derive the intermolecular pair potentials, resulting in a force field that is applicable over a range of state points and suitable for studying ceramide self-assembly. The chosen CG mapping, which includes explicit interaction sites for hydroxyl groups, captures the directional nature of hydrogen bonding and allows for accurate predictions of several key structural properties of CER NS bilayers. Simulated wetting experiments allow the hydrophobicity of CG beads to be accurately tuned to match atomistic wetting behavior, which affects the whole system, since inaccurate hydrophobic character is found to unphysically alter the lipid packing in hydrated lamellar states. We find that CER NS can self-assemble into multilamellar structures, enabling the study of lipid systems more representative of the multilamellar lipid structures present in the skin barrier. The coarse-grained force field derived herein represents an important step in using molecular dynamics to study the human skin barrier, which gives a resolution not available through experiment alone.

Cholesterol-induced suppression of membrane elastic fluctuations at the atomistic level

Applications of solid-state NMR spectroscopy for investigating the influences of lipid-cholesterol interactions on membrane fluctuations are reviewed in this paper. Emphasis is placed on understanding the energy landscapes and fluctuations at an emergent atomistic level. Solid-state (2)H NMR spectroscopy directly measures residual quadrupolar couplings (RQCs) due to individual C-(2)H labeled segments of the lipid molecules. Moreover, residual dipolar couplings (RDCs) of (13)C-(1)H bonds are obtained in separated local-field NMR spectroscopy. The distributions of RQC or RDC values give nearly complete profiles of the order parameters as a function of acyl segment position. Measured equilibrium properties of glycerophospholipids and sphingolipids including their binary and tertiary mixtures with cholesterol show unequal mixing associated with liquid-ordered domains. The entropic loss upon addition of cholesterol to sphingolipids is less than for glycerophospholipids and may drive the formation of lipid rafts. In addition relaxation time measurements enable one to study the molecular dynamics over a wide time-scale range. For (2)H NMR the experimental spin-lattice (R1Z) relaxation rates follow a theoretical square-law dependence on segmental order parameters (SCD) due to collective slow dynamics over mesoscopic length scales. The functional dependence for the liquid-crystalline lipid membranes is indicative of viscoelastic properties as they emerge from atomistic-level interactions. A striking decrease in square-law slope upon addition of cholesterol denotes stiffening relative to the pure lipid bilayers that is diminished in the case of lanosterol. Measured equilibrium properties and relaxation rates infer opposite influences of cholesterol and detergents on collective dynamics and elasticity at an atomistic scale that potentially affects lipid raft formation in cellular membranes.

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].

Chain Length of Free Fatty Acids Influences the Phase Behavior of Stratum Corneum Model Membranes

The skin, the largest organ of the human body, forms a flexible interface between our internal and external environment that protects our organism from exogenous compounds as well as excessive water loss. The stratum corneum (SC), the outermost layer of mammal epidermis, is mainly responsible for the skin impermeability. The SC is formed by corneocytes embedded in a lipid matrix, which is mostly constituted of ceramides (Cer), free fatty acids (FFA), and cholesterol (Chol), organized in two coexisting crystalline lamellar phases. This arrangement of lipids is crucial to skin barrier function. The aim of this paper is to determine the impact of FFA chain length on the phase behavior of SC model lipid membranes using solid-state deuterium NMR and IR spectroscopy. We studied ternary mixtures of N-lignoceroyl-d-erythro-sphingosine (Cer24), cholesterol, and palmitic (FFA16) or lignoceric (FFA24) acid in an equimolar ratio. This proportion replicates the lipid composition found in the SC lipid matrix. Our studies revealed that the phase behavior of Cer24/FFA/Chol ternary mixtures is strongly affected by the length of the FFA. We found the formation of phase-separated crystalline lipid domains when using palmitic acid whereas the use of lignoceric acid results in a more homogeneous mixture. In addition, it was observed that mixtures with lignoceric acid form a gel phase, a very unusual feature for SC model mixtures.

Impact of particle nanotopology on water transport through hydrophobic soils

The impact of non- and poorly wetting soils has become increasingly important, due to its direct influence on the water-limited potential yield of rain-fed grain crops at a time of enhanced global competition for fresh water. This study investigates the physical and compositional mechanisms underlying the influence of soil organic matter (SOM) on the wetting processes of model systems. These model systems are directly related to two sandy wheat-producing soils that have contrasting hydrophobicities. Atomic force microscopy (AFM), contact angle and Raman micro-spectroscopy measurements on model planar and particulate SOM-containing surfaces demonstrated the role of the hierarchical surface structure on the wetting dynamics of packed particulate beds. It was found that a nanoscale surface topology is superimposed over the microscale roughness of the packed particles, and this controls the extent of water ingress into particulate packed beds of these particles. Using two of the dominant component organic species found in the SOM of the two soils used in this study, it was found that the specific interactions taking place between the SOM components, rather than their absolute quantities, dictated the formation of highly hydrophobic surface nanotopologies. This hydrophobicity was demonstrated, using micro-Raman imaging, to arise from the surface being in a composite Cassie-Baxter wetting state. Raman imaging demonstrated that the particle surface nanotopography influenced the degree of air entrapment in the interstices within the particle bed. The influence of a conventional surfactant on the wetting kinetics of both the model planar surfaces and packed particulate beds was quantified in terms of their respective advancing contact angles and the capillary wetting force vector. The information obtained for all of the planar and particulate surfaces, together with that obtained for the two soils, allowed linear relationships to be obtained in plots of the contact angle data as a function of the wetting liquid surface tensions. These linear relationships were found to reflect the mechanisms underlying the surface energy parameter requirements for wetting.

Probing the role of the ceramide acyl chain length and sphingosine unsaturation in model skin barrier lipid mixtures by (2)H solid-state NMR spectroscopy

We investigated equimolar mixtures of ceramides with lignoceric acid and cholesterol as models for the human stratum corneum by differential scanning calorimetry and (2)H solid-state NMR spectroscopy. Our reference system consisted of lignoceroyl sphingosine (Cer[NS24]), which represents one of the ceramides in the human stratum corneum. Furthermore, the effect of ceramide acyl chain truncation to 16 carbons as in Cer[NS16] and the loss of the C4 trans double bond as in dihydroceramide Cer[NDS24] were studied. Fully relaxed (2)H NMR spectra were acquired for each deuterated component of each mixture separately, allowing the quantitative determination of the individual lipid phases. At skin temperature, the reference system containing Cer[NS24] is characterized by large portions of each component of the mixture in a crystalline phase, which largely restricts the permeability of the skin lipid barrier. The loss of the C4 trans double bond in Cer[NDS24] leads to the replacement of more than 25% of the crystalline phase by an isotropic phase of the dihydroceramide that shows the importance of dihydroceramide desaturation in the formation of the skin lipid barrier. The truncated Cer[NS16] is mostly found in the gel phase at skin temperature, which may explain its negative effect on the transepidermal water loss in atopic dermatitis patients. These significant alterations in the phase behavior of all lipids are further reflected at elevated temperatures. The molecular insights of our study may help us to understand the importance of the structural parameters of ceramides in healthy and compromised skin barriers.

Interaction of fengycin with stratum corneum mimicking model membranes: a calorimetry study

Based on its outstanding antifungal properties, it is reasonable to believe that fengycin might be efficient to topically treat localized dermatomycoses. Since most of the fungi species involved in the formation of those mycotic skin diseases colonize primarily the stratum corneum (SC), studying the interaction between fengycin and SC-mimicking lipid membranes is a primary step to determine the potential of fengycin to overcome the physical barrier of the skin. In this respect, multilamellar lipid vesicles (MLVs), with a lipid composition mimicking that of the SC, were prepared and characterized by differential scanning calorimetry (DSC). The critical micelle concentration (CMC) of fengycin was also assessed under skin conditions and found to be 1.2±0.1μM. The molecular interactions of fengycin with SC-mimicking MLVs were investigated by both DSC and isothermal titration calorimetry (ITC). Results showed that the interactions were considerably affected by changes in lipid phase behaviour. At 40°C and below, fengycin induced exothermic changes in the lipid structures suggesting that less-ordered lipid domains became more-ordered in presence of fengycin. At 60°C, clearly endothermic interaction enthalpies were observed, which could arise from the "melting" of remaining solid domains enriched in high melting lipids that without fengycin melt at higher temperatures.

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.

Characterization of stratum corneum molecular dynamics by natural-abundance ¹³C solid-state NMR

Despite the enormous potential for pharmaceutical applications, there is still a lack of understanding of the molecular details that can contribute to increased permeability of the stratum corneum (SC). To investigate the influence of hydration and heating on the SC, we record the natural-abundance (13)C signal of SC using polarization transfer solid-state NMR methods. Resonance lines from all major SC components are assigned. Comparison of the signal intensities obtained with the INEPT and CP pulse sequences gives information on the molecular dynamics of SC components. The majority of the lipids are rigid at 32°C, and those lipids co-exist with a small pool of mobile lipids. The ratio between mobile and rigid lipids increases with hydration. An abrupt change of keratin filament dynamics occurs at RH = 80-85%, from completely rigid to a structure with rigid backbone and mobile protruding terminals. Heating has a strong effect on the lipid mobility, but only a weak influence on the keratin filaments. The results provide novel molecular insight into how the SC constituents are affected by hydration and heating, and improve the understanding of enhanced SC permeability, which is associated with elevated temperatures and SC hydration.

Insights into sphingolipid miscibility: separate observation of sphingomyelin and ceramide N-acyl chain melting

Ceramide produced from sphingomyelin in the plasma membrane is purported to affect signaling through changes in the membrane's physical properties. Thermal behavior of N-palmitoyl sphingomyelin (PSM) and N-palmitoyl ceramide (PCer) mixtures in excess water has been monitored by ²H NMR spectroscopy and compared to differential scanning calorimetry (DSC) data. The alternate use of either perdeuterated or proton-based N-acyl chain PSM and PCer in our ²H NMR studies has allowed the separate observation of gel-fluid transitions in each lipid in the presence of the other one, and this in turn has provided direct information on the lipids' miscibility over a wide temperature range. The results provide further evidence of the stabilization of the PSM gel state by PCer. Moreover, overlapping NMR and DSC data reveal that the DSC-signals parallel the melting of the major component (PSM) except at intermediate (20 and 30 mol %) fractions of PCer. In such cases, the DSC endotherm reports on the presumably highly cooperative melting of PCer. Up to at least 50 mol % PCer, PSM and PCer mix ideally in the liquid crystalline phase; in the gel phase, PCer becomes incorporated into PSM:PCer membranes with no evidence of pure solid PCer.

Bilayer structure and lipid dynamics in a model stratum corneum with oleic acid

The stratum corneum is the uppermost layer of the skin and acts as a barrier to keep out contaminants and retain moisture. Understanding the molecular structure and behavior of this layer will provide guidance for optimizing its biological function. In this study we use a model mixture comprised of equimolar portions of ceramide NS (24:0), lignoceric acid, and cholesterol to model the effect of the addition of small amounts of oleic acid to the bilayer at 300 and 340 K. Five systems at each temperature have been simulated with concentrations between 0 and 0.1 mol % oleic acid. Our major finding is that subdiffusive behavior over the 200 ns time scale is evident in systems at 340 K, with cholesterol diffusion being enhanced with increased oleic acid. Importantly, cholesterol and other species diffuse faster when radial densities indicate nearest neighbors include more cholesterol. We also find that, with the addition of oleic acid, the bilayer midplane and interfacial densities are reduced and there is a 3% decrease in total thickness occurring mostly near the hydrophilic interface at 300 K with reduced overall density at 340 K. Increased interdigitation occurs independent of oleic acid with a temperature increase. Slight ordering of the long non-hydroxy fatty acid of the ceramide occurs near the hydrophilic interface as a function of the oleic acid concentration, but no significant impact on hydrogen bonding is seen in the chosen oleic acid concentrations.

Vibrational spectroscopy of biomembranes

Vibrational spectroscopy, commonly associated with IR absorption and Raman scattering, has provided a powerful approach for investigating interactions between biomolecules that make up cellular membranes. Because the IR and Raman signals arise from the intrinsic properties of these molecules, vibrational spectroscopy probes the delicate interactions that regulate biomembranes with minimal perturbation. Numerous innovative measurements, including nonlinear optical processes and confined bilayer assemblies, have provided new insights into membrane behavior. In this review, we highlight the use of vibrational spectroscopy to study lipid-lipid interactions. We also examine recent work in which vibrational measurements have been used to investigate the incorporation of peptides and proteins into lipid bilayers, and we discuss the interactions of small molecules and drugs with membrane structures. Emerging techniques and measurements on intact cellular membranes provide a prospective on the future of vibrational spectroscopic studies of biomembranes.

Model membranes prepared with ceramide EOS, cholesterol and free fatty acids form a unique lamellar phase

The lipid matrix present in the human stratum corneum (the thin, uppermost layer of the skin) is considered to play a crucial role in the skin barrier function. The lipid matrix consists of ceramides, cholesterol, and free fatty acids. The 13 nm lamellar phase present in the lipid matrix of the stratum corneum is very characteristic and plays an important role in the skin barrier function. One subclass of ceramides with a linoleic acid linked to a very long acyl (referred to as EOS) plays a crucial role in the formation of the 13 nm lamellar phase. In this article, we focus on the lipid phase behavior of EOS mixed with cholesterol or with cholesterol and free fatty acids. Our studies reveal that an equimolar ratio of EOS, cholesterol, and free fatty acids forms a lamellar phase with a very long repeat distance of approximately 14.7 nm. This phase exhibits exceptional behavior in that in the thermotropic response the fatty acid chains and the ceramide chains undergo an order-disorder transition in different temperature ranges while part of the hydrocarbon chains of ceramides and fatty acids are mixing in the orthorhombic lattice. On the basis of these observations, a molecular model for the 14.7 nm phase has been proposed in which the lipids are organized in a lamellar phase with three different lipid layers in a symmetric unit cell.