Phytosphingosine and Sphingosine Ceramide Headgroup Hydrogen Bonding: Structural Insights through Thermotropic Hydrogen/Deuterium Exchange

Molecular Dynamics Investigation into CerENP's Effect on the Lipid Matrix of Stratum Corneum

The extracellular lipid matrix in the stratum corneum (SC) plays a critical role in skin barrier functionality, comprising three primary components: ceramides, cholesterol, and free fatty acids. The diverse ceramides, differentiated by molecular structures such as hydroxylations and varying chain lengths, are essential for the lipid matrix's structural integrity. Recently, a new subclass of ceramide, 1-O-acylceramide NP (CerENP), has been identified; however, its precise role in the lipid matrix of the SC is still elusive. Herein, we investigate the role of CerENP on the structure and permeability of the SC using molecular dynamics simulations. Our findings indicate that CerENP contributes to a compact lipid matrix in the lateral dimension of our SC model with a repeat distance of about 13 nm. Additionally, ethanol permeability assessments show that CerENP effectively reduces molecular penetration through the lipid matrix. This study provides an insight into the role of a new subclass of ceramide in the SC, enhancing our understanding of skin structure and the mechanisms behind barrier dysfunction in skin diseases.

The molecular arrangement of ceramides in the unit cell of the long periodicity phase of stratum corneum models shows a high adaptability to different ceramide head group structures

The stratum corneum (SC) lipid matrix, composed primarily of ceramides (CERs), cholesterol and free fatty acids (FFA), has an important role for the skin barrier function. The presence of the long periodicity phase (LPP), a unique lamellar phase, is characteristic for the SC. Insight into the lipid molecular arrangement within the LPP unit cell is imperative for understanding the relationship between the lipid subclasses and the skin barrier function. In this study, the impact of the CER head group structure on the lipid arrangement and barrier functionality was investigated using lipid models forming the LPP. The results demonstrate that the positions of CER N-(tetracosanoyl)-sphingosine (CER NS) and CER N-(tetracosanoyl)-phytosphingosine (CER NP), two essentials CER subclasses, are not influenced by the addition of another CER subclass (N-(tetracosanoyl)-dihydrosphingosine (CER NdS), N-(2R-hydroxy-tetracosanoyl)-sphingosine (CER AS) or D-(2R-hydroxy-tetracosanoyl)-phytosphingosine (CER AP)). However, differences are observed in the lipid organization and the hydrogen bonding network of the three different models. A similar localization of CER NP and CER NS is also observed in a more complex lipid model, with the CER subclass composition mimicking that of human SC. These studies show the adaptability and insensitivity of the LPP unit cell structure to changes in the lipid head group structures of the CER subclasses.

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.

Biomimetic Stratum Corneum Liposome Models: Lamellar Organization and Permeability Studies

The stratum corneum (SC), the outer layer of the skin, plays a crucial role as a barrier protecting the underlying cells from external stress. The SC comprises three key components: ceramide (CER), free fatty acid (FFA), and cholesterol, along with small fractions of cholesterol sulfate and cholesterol ester. In order to gain a deeper understanding about the interdependence of the two major components, CER and FFA, on the organizational, structural, and functional properties of the SC layer, a library of SC lipid liposome (SCLL) models was developed by mixing CER (phytosphingosine or sphingosine), FFA (oleic acid, palmitic acid, or stearic acid), cholesterol, and cholesterol sulfate. Self-assembly of the SC lipids into lamellar phases was first confirmed by small-angle X-ray scattering. Short periodicity and long periodicity phases were identified for SCLLs containing phytosphingosines and sphingosine CERs, respectively. Furthermore, unsaturation in the CER acyl and FFA chains reduced the lipid conformational ordering and packing density of the liposomal bilayer, which were measured by differential scanning calorimetry and Fourier transform infrared spectroscopy. The introduction of unsaturation in the CER and/or FFA chains also impacted the lamellar integrity and permeability. This extensive library of SCLL models exhibiting physiologically relevant lamellar phases with defined structural and functional properties may potentially be used as a model system for screening pharmaceuticals or cosmetic agents.

Polymorphism, Nanostructures, and Barrier Properties of Ceramide-Based Lipid Films

Ceramides belong to sphingolipids, an important group of cellular and extracellular lipids. Their physiological functions range from cell signaling to participation in the formation of barriers against water evaporation. In the skin, they are essential for the permeability barrier, together with free fatty acids and cholesterol. We examined the periodical structure and permeability of lipid films composed of ceramides (Cer; namely, N-lignoceroyl 6-hydroxysphingosine, CerNH24, and N-lignoceroyl sphingosine, CerNS24), lignoceric acid (LIG; 24:0), and cholesterol (Chol). X-ray diffraction experiments showed that the CerNH24-based samples form either a short lamellar phase (SLP, d ∼ 5.4 nm) or a medium lamellar phase (MLP, d = 10.63-10.78 nm) depending on the annealing conditions. The proposed molecular arrangement of the MLP based on extended Cer molecules also agreed with the relative neutron scattering length density profiles obtained from the neutron diffraction data. The presence of MLP increased the lipid film permeability to the lipophilic model permeant (indomethacin) relative to the CerNS24-based control samples and the samples that had the same lipid composition but formed an SLP. Thus, the arrangement of lipids in various nanostructures is responsive to external conditions during sample preparation. This polymorphic behavior directly affects the barrier properties, which could also be (patho)physiologically relevant.

Phytosphingosine ceramide mainly localizes in the central layer of the unique lamellar phase of skin lipid model systems

Understanding the lipid arrangement within the skin's outermost layer, the stratum corneum (SC), is important for advancing knowledge on the skin barrier function. The SC lipid matrix consists of ceramides (CERs), cholesterol, and free fatty acids, which form unique crystalline lamellar phases, referred to as the long periodicity phase (LPP) and short periodicity phases. As the SC lipid composition is complex, lipid model systems that mimic the properties of native SC are used to study the SC lipid organization and molecular arrangement. In previous studies, such lipid models were used to determine the molecular organization in the trilayer structure of the LPP unit cell. The aim of this study was to examine the location of CER N-(tetracosanoyl)-phytosphingosine (CER NP) in the unit cell of this lamellar phase and compare its position with CER N-(tetracosanoyl)-sphingosine (CER NS). We selected CER NP as it is the most prevalent CER subclass in the human SC, and its location in the LPP is not known. Our neutron diffraction results demonstrate that the acyl chain of CER NP was positioned in the central part of the trilayer structure, with a fraction also present in the outer layers, the same location as determined for the acyl chain of CER NS. In addition, our Fourier transformed infrared spectroscopy results are in agreement with this molecular arrangement, suggesting a linear arrangement for the CER NS and CER NP. These findings provide more detailed insight into the lipid organization in the SC lipid matrix.

Using molecular simulation to understand the skin barrier

Skin's effectiveness as a barrier to permeation of water and other chemicals rests almost entirely in the outermost layer of the epidermis, the stratum corneum (SC), which consists of layers of corneocytes surrounded by highly organized lipid lamellae. As the only continuous path through the SC, transdermal permeation necessarily involves diffusion through these lipid layers. The role of the SC as a protective barrier is supported by its exceptional lipid composition consisting of ceramides (CERs), cholesterol (CHOL), and free fatty acids (FFAs) and the complete absence of phospholipids, which are present in most biological membranes. Molecular simulation, which provides molecular level detail of lipid configurations that can be connected with barrier function, has become a popular tool for studying SC lipid systems. We review this ever-increasing body of literature with the goals of (1) enabling the experimental skin community to understand, interpret and use the information generated from the simulations, (2) providing simulation experts with a solid background in the chemistry of SC lipids including the composition, structure and organization, and barrier function, and (3) presenting a state of the art picture of the field of SC lipid simulations, highlighting the difficulties and best practices for studying these systems, to encourage the generation of robust reproducible studies in the future. This review describes molecular simulation methodology and then critically examines results derived from simulations using atomistic and then coarse-grained models.

Cerosomes as skin repairing agent: Mode of action studies with a model stratum corneum layer at liquid/air and liquid/solid interfaces

The stratum corneum (SC) is the largest physical barrier of the human body. It protects against physical, chemical and biological damages, and avoids evaporation of water from the deepest skin layers. For its correct functioning, the homeostasis of the SC lipid matrix is fundamental. An alteration of the lipid matrix composition and in particular of its ceramide (CER) fraction can lead to the development of pathologies such as atopic dermatitis and psoriasis. Different studies showed that the direct replenishment of SC lipids on damaged skin had positive effects on the recovery of its barrier properties. In this work, cerosomes, i.e. liposomes composed of SC lipids, have been successfully prepared in order to investigate the mechanism of interaction with a model SC lipid matrix. The cerosomes contain CER[NP], D-CER[AP], stearic acid and cholesterol. In addition, hydrogenated soybean phospholipids have been added to one of the formulations leading to an increased stability at neutral pH. For the mode of action studies, monolayer models at the air-water interface and on solid support have been deployed. The results indicated that a strong interaction occurred between SC monolayers and the cerosomes. Since both systems were negatively charged, the driving force for the interaction must be based on the ability of CERs head groups to establish intermolecular hydrogen bonding networks that energetically prevailed against the electrostatic repulsion. This work proved for the first time the mode of action by which cerosomes exploit their function as skin barrier repairing agents on the SC.

Two- and Three-Dimensional Physical-Chemical Characterization of CER[AP]: A Study of Stereochemistry and Chain Symmetry

The stratum corneum represents the first skin barrier against chemical and physical damage. These unique properties are based on its peculiar lipid composition with ceramides (CERs) as the main protagonists. In this study, the structural and chemical properties of the α-OH phytosphingosine [AP] CER class have been investigated. α-OH CERs are present in the stratum corneum in their d-forms; however, in most model systems the diastereomer mixture with the synthetically produced l-form is used. The d-form is well-known to form a hydrogen bonding network that helps to reduce the permeability of the lipid matrix, while the l-form does not show any hydrogen bonding network formation. In this paper, 2D (monolayers) and 3D (aqueous dispersions) models have been used to thoroughly study the physical-chemical behaviors of CER[AP] diastereomers taking into account how the symmetry of the chain pattern influences the behavior of the molecules. The chains of both diastereomers arrange in an oblique unit cell, but only the d-CER[AP] forms a supramolecular lattice (subgel phase) in both model systems. Interestingly, the chain pattern does not play any role in structure formation since the hydrogen bonding network dictates the packing properties. The 1:1 mixture of the diastereomers phase separates into two domains: one is composed of practically pure d-form and the other one is composed of a mixture of the l-form with a certain amount of d-form molecules.

Effects of (R)- and (S)-α-Hydroxylation of Acyl Chains in Sphingosine, Dihydrosphingosine, and Phytosphingosine Ceramides on Phase Behavior and Permeability of Skin Lipid Models

Ceramides (Cers) with α-hydroxylated acyl chains comprise about a third of all extractable skin Cers and are required for permeability barrier homeostasis. We have probed here the effects of Cer hydroxylation on their behavior in lipid models comprising the major SC lipids, Cer/free fatty acids (C ₁₆-C ₂₄)/cholesterol, and a minor component, cholesteryl sulfate. Namely, Cers with (R)-α-hydroxy lignoceroyl chains attached to sphingosine (Cer AS), dihydrosphingosine (Cer AdS), and phytosphingosine (Cer AP) were compared to their unnatural (S)-diastereomers and to Cers with non-hydroxylated lignoceroyl chains attached to sphingosine (Cer NS), dihydrosphingosine (Cer NdS), and phytosphingosine (Cer NP). By comparing several biophysical parameters (lamellar organization by X-ray diffraction, chain order, lateral packing, phase transitions, and lipid mixing by infrared spectroscopy using deuterated lipids) and the permeabilities of these models (water loss and two permeability markers), we conclude that there is no general or common consequence of Cer α-hydroxylation. Instead, we found a rich mix of effects, highly dependent on the sphingoid base chain, configuration at the α-carbon, and permeability marker used. We found that the model membranes with unnatural Cer (S)-AS have fewer orthorhombically packed lipid chains than those based on the (R)-diastereomer. In addition, physiological (R)-configuration decreases the permeability of membranes, with Cer (R)-AdS to theophylline, and increases the lipid chain order in model systems with natural Cer (R)-AP. Thus, each Cer subclass makes a distinct contribution to the structural organization and function of the skin lipid barrier.

Behavior of 1-Deoxy-, 3-Deoxy- and N-Methyl-Ceramides in Skin Barrier Lipid Models

Ceramides (Cer) are essential components of the skin permeability barrier. To probe the role of Cer polar head groups involved in the interfacial hydrogen bonding, the N-lignoceroyl sphingosine polar head was modified by removing the hydroxyls in C-1 (1-deoxy-Cer) or C-3 positions (3-deoxy-Cer) and by N-methylation of amide group (N-Me-Cer). Multilamellar skin lipid models were prepared as equimolar mixtures of Cer, lignoceric acid and cholesterol, with 5 wt% cholesteryl sulfate. In the 1-deoxy-Cer-based models, the lipid species were separated into highly ordered domains (as found by X-ray diffraction and infrared spectroscopy) resulting in similar water loss but 4–5-fold higher permeability to model substances compared to control with natural Cer. In contrast, 3-deoxy-Cer did not change lipid chain order but promoted the formation of a well-organized structure with a 10.8 nm repeat period. Yet both lipid models comprising deoxy-Cer had similar permeabilities to all markers. N-Methylation of Cer decreased lipid chain order, led to phase separation, and improved cholesterol miscibility in the lipid membranes, resulting in 3-fold increased water loss and 10-fold increased permeability to model compounds compared to control. Thus, the C-1 and C-3 hydroxyls and amide group, which are common to all Cer subclasses, considerably affect lipid miscibility and chain order, formation of periodical nanostructures, and permeability of the skin barrier lipid models.

Effects of omega-O-acylceramide structures and concentrations in healthy and diseased skin barrier lipid membrane models

Ceramides (Cers) with ultralong (∼32-carbon) chains and ω-esterified linoleic acid, composing a subclass called omega-O-acylceramides (acylCers), are indispensable components of the skin barrier. Normal barriers typically contain acylCer concentrations of ∼10 mol%; diminished concentrations, along with altered or missing long periodicity lamellar phase (LPP), and increased permeability accompany an array of skin disorders, including atopic dermatitis, psoriasis, and ichthyoses. We developed model membranes to investigate the effects of the acylCer structure and concentration on skin lipid organization and permeability. The model membrane systems contained six to nine Cer subclasses as well as fatty acids, cholesterol, and cholesterol sulfate; acylCer content-namely, acylCers containing sphingosine (Cer EOS), dihydrosphingosine (Cer EOdS), and phytosphingosine (Cer EOP) ranged from zero to 30 mol%. Systems with normal physiologic concentrations of acylCer mixture mimicked the permeability and nanostructure of human skin lipids (with regard to LPP, chain order, and lateral packing). The models also showed that the sphingoid base in acylCer significantly affects the membrane architecture and permeability and that Cer EOP, notably, is a weaker barrier component than Cer EOS and Cer EOdS. Membranes with diminished or missing acylCers displayed some of the hallmarks of diseased skin lipid barriers (i.e., lack of LPP, less ordered lipids, less orthorhombic chain packing, and increased permeability). These results could inform the rational design of new and improved strategies for the barrier-targeted treatment of skin diseases.

Structural and barrier properties of the skin ceramide lipid bilayer: a molecular dynamics simulation study

Skin provides excellent protection against the harsh external environment and foreign substances. The lipid matrix of the stratum corneum, which contains various kinds of ceramides, plays a major role in the barrier function of the skin. Here we report a study of the effects of ceramide type on the structural and transport properties of ceramide bilayers using molecular dynamics (MD) simulations. Specifically, the effects of headgroup chemistry (number and positions of hydroxyl groups) and tail structure (unsaturation of the sphingoid moiety) on the structural and transport properties of various ceramide bilayers at 310 K were analyzed. Theoretical results for structural properties such as area per lipid, bilayer thickness, lateral arrangement, order parameter, and hydrogen bonding are reported here and compared with corresponding experimental data. Our study revealed that the presence of a double bond disrupts the bilayer packing, which leads to a low area compressibility modulus, a large area per lipid, and low bilayer thickness. Furthermore, the effect of structural changes on water permeation was studied using steered MD simulations. Water permeation was found to be influenced by headgroup polarity, chain packing, and the ability of the water to hydrogen bond with the ceramides. The molecular-level information obtained from the current study should aid the design of mixed bilayer systems with desired properties and provide the basis for the development of higher order coarse-grained models.

New insight into phase behavior and permeability of skin lipid models based on sphingosine and phytosphingosine ceramides

The intercellular lipid matrix of the stratum corneum (SC), which consist mainly of ceramides (CERs), free fatty acids and cholesterol, is fundamental to the skin barrier function. These lipids assemble into two lamellar phases, known as the long and short periodicity phases (LPP and SPP respectively). The LPP is unique in the SC and is considered important for the skin barrier function. Alterations in CER composition, as well as impaired skin barrier function, are commonly observed in diseased skin, yet the understanding of this relationship remains insufficient. In this study, we have investigated the influence of non-hydroxy and α-hydroxy sphingosine-based CERs and their phytosphingosine counterparts on the permeability and lipid organization of model membranes, which were adjusted in composition to enhance formation of the LPP. The permeability was compared by diffusion studies using ethyl-p-aminobenzoate as a model drug, and the lipid organization was characterized by X-ray diffraction and infrared spectroscopy. Both the sphingosine- and phytosphingosine-based CER models formed the LPP, while the latter exhibited a longer LPP repeat distance. The ethyl-p-aminobenzoate flux across the sphingosine-based CER models was higher when compared to the phytosphingosine counterparts, contrary to the fact that the α-hydroxy phytosphingosine-based CER model had the lowest chain packing density. The unanticipated low permeability of the α-hydroxy phytosphingosine-based model is probably associated with a stronger headgroup hydrogen bonding network. Our findings indicate that the increased level of sphingosine-based CERs at the expense of phytosphingosine-based CERs, as observed in the diseased skin, may contribute to the barrier function impairment.

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.

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].

Effects of Ceramide and Dihydroceramide Stereochemistry at C-3 on the Phase Behavior and Permeability of Skin Lipid Membranes

Ceramides (Cer) are key components of the skin permeability barrier. Sphingosine-based CerNS and dihydrosphingosine-based CerNdS (dihydroCer) have two chiral centers; however, the importance of the correct stereochemistry in the skin barrier Cer is unknown. We investigated the role of the configuration at C-3 of CerNS and CerNdS in the organization and permeability of model skin lipid membranes. Unnatural l-threo-CerNS and l-threo-CerNdS with 24-C acyl chains were synthesized and, along with their natural d-erythro-isomers, incorporated into membranes composed of major stratum corneum lipids (Cer, free fatty acids, cholesterol, and cholesteryl sulfate). The membrane microstructure was investigated by X-ray powder diffraction and infrared spectroscopy, including deuterated free fatty acids. Inversion of the C-3 configuration in CerNS and CerNdS increased phase transition temperatures, had no significant effects on lamellar phases, but also decreased the proportion of orthorhombic packing and decreased lipid mixing in the model membranes. These changes in membrane organization resulted in membrane permeabilities that ranged from unchanged to 5-fold higher (depending on the permeability markers, namely, water loss, electrical impedance, flux of theophylline, and flux of indomethacin) compared to membranes with natural CerNS/NdS isomers. Thus, the physiological d-erythro stereochemistry of skin Cer and dihydroCer appears to be essential for their correct barrier function.

Determination of the influence of C24 D/(2R)- and L/(2S)-isomers of the CER[AP] on the lamellar structure of stratum corneum model systems using neutron diffraction

This study was able to investigate the different influence of the d- and l-ceramide [AP] on the lamellar as well as molecular nanostructure of stratum corneum simulating lipid model mixtures. In this case, neutron diffraction together with specifically deuterated ceramide was used as an effective tool to investigate the lamellar and the molecular nanostructure of the mixtures. It could clearly be demonstrated, that both isomers show distinctly different characteristics, even though the variation between both is only a single differently arranged OH-group. The l-ceramide [AP] promotes a crystalline like phase behaviour even if mixed with ceramide [NP], cholesterol and free fatty acids. The d-ceramide [AP] only shows crystalline-like features if mixed only with cholesterol and free fatty acids but adopts a native-like behaviour if additionally mixed with ceramide [NP]. It furthermore demonstrates that the l-ceramide [AP] should not be used for any applications concerning ceramide substitution. It could however possibly serve its own purpose, if this crystalline like behaviour has some kind of positive influence on the SC or can be utilized for any practical applications. The results obtained in this study demonstrate that the diastereomers of ceramide [AP] are an attractive target for further research because their influence on the lamellar as well as the nanostructure is exceptionally strong. Additionally, the results furthermore show a very strong influence on hydration of the model membrane. With these properties, the d-ceramide [AP] could be effectively used to simulate native like behaviour even in very simple mixtures and could also have a strong impact on the native stratum corneum as well as high relevance for dermal ceramide substitution. The unnatural l-ceramide [AP] on the other hand should be investigated further, to assess its applicability.

Molecular Dynamics Simulations of Ceramide and Ceramide-Phosphatidylcholine Bilayers

Recent studies in lipid raft formation and stratum corneum permeability have focused on the role of ceramides (CER). In this study, we use the all-atom CHARMM36 (C36) force field to simulate bilayers using N-palmitoylsphingosine (CER16) or α-hydroxy-N-stearoyl phytosphingosine (CER[AP]) in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), which serve as general membrane models. Conditions are replicated from experimental studies for comparison purposes, and concentration (X_CER) is varied to probe the effect of CER on these systems. Comparisons with experiment based on deuterium order parameters and bilayer thickness demonstrate good agreement, thus supporting further use of the C36 force field. CER concentration is shown to have a profound effect on nearly all membrane properties including surface area per lipid, chain order and tilt, area compressibility moduli, bilayer thickness, hydrogen bonding, and lipid clustering. Hydrogen bonding in particular can significantly affect other membrane properties and can even encourage transition to a gel phase. Despite CER's tendency to condense the membrane, an expansion of CER lipids with increasing X_CER is possible depending on how the balance between various hydrogen-bond pairs and lipid clustering is perturbed. Based on gel phase transitions, support is given for phytosphingosine's role as a hydrogen-bond bridge between sphingosine ordered domains in the stratum corneum.

Interaction of DNA with Cationic Lipid Mixtures-Investigation at Langmuir Lipid Monolayers

Four different binary lipid mixtures composed of a cationic lipid and the zwitterionic colipids DOPE or DPPC, which show different DNA transfer activities in cell culture models, were investigated at the soft air/water interface to identify transfection efficiency determining characteristics. Langmuir films are useful models to investigate the interaction between DNA and lipid mixtures in a two-dimensional model system by using different surface sensitive techniques, namely, epifluorescence microscopy and infrared reflection-absorption spectroscopy. Especially, the effect of adsorbed DNA on the properties of the lipid mixtures has been examined. Distinct differences between the lipid composites were found which are caused by the different colipids of the mixtures. DOPE containing lipid mixtures form fluid monolayers with a uniform distribution of the fluorescent probe in the presence and absence of DNA at physiologically relevant surface pressures. Only at high nonphysiological pressures, the lipid monolayer collapses and phase separation was observed if DNA was present in the subphase. In contrast, DPPC containing lipid mixtures show domains in the liquid condensed phase state in the presence and absence of DNA in the subphase. The adsorption of DNA at the positively charged mixed lipid monolayer induces phase separation which is expressed in the morphology and the point of appearance of these domains.

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.

Effects of 6-Hydroxyceramides on the Thermotropic Phase Behavior and Permeability of Model Skin Lipid Membranes

Ceramides (Cer) based on 6-hydroxysphingosine are important components of the human skin barrier, the stratum corneum. Although diminished concentrations of 6-hydroxyCer have been detected in skin diseases such as atopic dermatitis, our knowledge on these unusual sphingolipids, which have only been found in the skin, is limited. In this work, we investigate the biophysical behavior of N-lignoceroyl-6-hydroxysphingosine (Cer NH) in multilamellar lipid membranes composed of Cer/free fatty acids (FFAs) (C16-C24)/cholesterol/cholesteryl sulfate. To probe the Cer structure-activity relationships, we compared Cer NH membranes with membranes containing Cer with sphingosine (Cer NS), dihydrosphingosine, and phytosphingosine (Cer NP), all with the same acyl chain length (C24). Compared with Cer NS, 6-hydroxylation of Cer not only increased membrane water loss and permeability in a lipophilic model compound but also dramatically increased the membrane opposition to electrical current, which is proportional to the flux of ions. Infrared spectroscopy revealed that Cer hydroxylation (in either Cer NH or Cer NP) increased the main transition temperature of the membrane but prevented good Cer mixing with FFAs. X-ray powder diffraction showed not only lamellar phases with shorter periodicity upon Cer hydroxylation but also the formation of an unusually long periodicity phase (d = 10.6 nm) in Cer NH-containing membranes. Thus, 6-hydroxyCer behaves differently from sphingosine- and phytosphingosine-based Cer. In particular, the ability to form a long-periodicity lamellar phase and highly limited permeability to ions indicate the manner in which 6-hydroxylated Cer contribute to the skin barrier function.

Phytosphingosine, sphingosine and dihydrosphingosine ceramides in model skin lipid membranes: permeability and biophysics

Ceramides based on phytosphingosine, sphingosine and dihydrosphingosine are essential constituents of the skin lipid barrier that protects the body from excessive water loss. The roles of the individual ceramide subclasses in regulating skin permeability and the reasons for C4-hydroxylation of these sphingolipids are not completely understood. We investigated the chain length-dependent effects of dihydroceramides, sphingosine ceramides (with C4-unsaturation) and phytoceramides (with C4-hydroxyl) on the permeability, lipid organization and thermotropic behavior of model stratum corneum lipid membranes composed of ceramide/lignoceric acid/cholesterol/cholesteryl sulfate. Phytoceramides with very long C24 acyl chains increased the permeability of the model lipid membranes compared to dihydroceramides or sphingosine ceramides with the same chain lengths. Either unsaturation or C4-hydroxylation of dihydroceramides induced chain length-dependent increases in membrane permeability. Infrared spectroscopy showed that C4-hydroxylation of the sphingoid base decreased the relative ratio of orthorhombic chain packing in the membrane and lowered the miscibility of C24 phytoceramide with lignoceric acid. The phase separation in phytoceramide membranes was confirmed by X-ray diffraction. In contrast, phytoceramides formed strong hydrogen bonds and highly thermostable domains. Thus, the large heterogeneity in ceramide structures and in their aggregation mechanisms may confer resistance towards the heterogeneous external stressors that are constantly faced by the skin barrier.

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.

Influence of Hydroxylation, Chain Length, and Chain Unsaturation on Bilayer Properties of Ceramides

Mammalian ceramides constitute a family of at least a few hundred closely related molecules distinguished by small structural differences, giving rise to individual molecular species that are expressed in distinct cellular compartments, or tissue types, in which they are believed to execute distinct functions. We have examined how specific structural details influence the bilayer properties of a selection of biologically relevant ceramides in mixed bilayers together with sphingomyelin, phosphatidylcholine, and cholesterol. The ceramide structure varied with regard to interfacial hydroxylation, the identity of the headgroup, the length of the N-acyl chain, and the position of cis-double bonds in the acyl chains. The interactions of the ceramides with sphingomyelin, their lateral segregation into ceramide-rich domains in phosphatidylcholine bilayers, and the effect of cholesterol on such domains were studied with DSC and various fluorescence-based approaches. The largest differences arose from the presence and relative position of cis-double bonds, causing destabilization of the ceramide's interactions and lateral packing relative to common saturated and hydroxylated species. Less variation was observed as a consequence of interfacial hydroxylation and the N-acyl chain length, although an additional hydroxyl in the sphingoid long-chain base slightly destabilized the ceramide's interactions and packing relative to a nonhydroxyceramide, whereas an additional hydroxyl in the N-acyl chain had the opposite effect. In conclusion, small structural details conferred variance in the bilayer behavior of ceramides, some causing more dramatic changes in the bilayer properties, whereas others imposed only fine adjustments in the interactions of ceramides with other membrane lipids, reflecting possible functional implications in distinct cell or tissue types.

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].

Ceramides with a pentadecasphingosine chain and short acyls have strong permeabilization effects on skin and model lipid membranes

The composition and organization of stratum corneum lipids play an essential role in skin barrier function. Ceramides represent essential components of this lipid matrix; however, the importance of the individual structural features in ceramides is not fully understood. To probe the structure-permeability relationships in ceramides, we prepared analogs of N-lignoceroylsphingosine with shortened sphingosine (15 and 12 carbons) and acyl chains (2, 4 and 6 carbons) and studied their behavior in skin and in model lipid membranes. Ceramide analogs with pentadecasphingosine (15C) chains were more barrier-perturbing than 12C- and 18C-sphingosine ceramides; the greatest effects were found with 4 to 6C acyls (up to 15 times higher skin permeability compared to an untreated control and up to 79 times higher permeability of model stratum corneum lipid membranes compared to native very long-chain ceramides). Infrared spectroscopy using deuterated lipids and X-ray powder diffraction showed surprisingly similar behavior of the short ceramide membranes in terms of lipid chain order and packing, phase transitions and domain formation. The high- and low-permeability membranes differed in their amide I band shape and lamellar organization. These skin and membrane permeabilization properties of some short ceramides may be explored, for example, for the rational design of permeation enhancers for transdermal drug delivery.

The role of the trans double bond in skin barrier sphingolipids: permeability and infrared spectroscopic study of model ceramide and dihydroceramide membranes

Dihydroceramides (dCer) are members of the sphingolipid family that lack the C4 trans double bond in their sphingoid backbone. In addition to being precursors of ceramides (Cer) and phytoceramides, dCer have also been found in the extracellular lipid membranes of the epidermal barrier, the stratum corneum. However, their role in barrier homeostasis is not known. We studied how the lack of the trans double bond in dCer compared to Cer influences the permeability, lipid chain order, and packing of multilamellar membranes composed of the major skin barrier lipids: (d)Cer, fatty acids, cholesterol, and cholesteryl sulfate. The permeability of the membranes with long-chain dCer was measured using various markers and was either comparable to or only slightly greater than (by up to 35%, not significant) that of the Cer membranes. The dCer were less sensitive to acyl chain shortening than Cer (the short dCer membranes were up to 6-fold less permeable that the corresponding short Cer membranes). Infrared spectroscopy showed that long dCer mixed less with fatty acids but formed more thermally stable ordered domains than Cer. The key parameter explaining the differences in permeability in the short dCer and Cer was the proportion of the orthorhombic phase. Our results suggest that the presence of the trans double bond in Cer is not crucial for the permeability of skin lipid membranes and that dCer may be underappreciated members of the stratum corneum lipid barrier that increase its heterogeneity.

Localization of cholesterol and fatty acid in a model lipid membrane: a neutron diffraction approach

The intercellular lipid matrix of the skin's stratum corneum serves to protect the body against desiccation and simultaneously limits the passage of drugs and other xenobiotics into the body. The matrix is made up of ceramides, free fatty acids, and cholesterol, which are organized as two coexisting crystalline lamellar phases. In studies reported here, we sought to use the technique of neutron diffraction, together with the device of isotopic (H/D) substitution, to determine the molecular architecture of the lamellar phase having a repeat distance of 53.9 ± 0.3 Å. Using hydrogenous samples as well as samples incorporating perdeuterated (C24:0) fatty acids and selectively deuterated cholesterol, the diffraction data obtained were used to construct neutron scattering length density profiles. By this means, the locations within the unit cell were determined for the cholesterol and fatty acids. The cholesterol headgroup was found to lie slightly inward from the unit cell boundary and the tail of the molecule located 6.2 ± 0.2 Å from the unit cell center. The fatty acid headgroups were located at the unit cell boundary with their acyl chains straddling the unit cell center. Based on these results, a molecular model is proposed for the arrangement of the lipids within the unit cell.

Ceramides in the skin lipid membranes: length matters

Ceramides are essential constituents of the skin barrier that allow humans to live on dry land. Reduced levels of ceramides have been associated with skin diseases, e.g., atopic dermatitis. However, the structural requirements and mechanisms of action of ceramides are not fully understood. Here, we report the effects of ceramide acyl chain length on the permeabilities and biophysics of lipid membranes composed of ceramides (or free sphingosine), fatty acids, cholesterol, and cholesterol sulfate. Short-chain ceramides increased the permeability of the lipid membranes compared to a long-chain ceramide with maxima at 4-6 carbons in the acyl. By a combination of differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray diffraction, Langmuir monolayers, and atomic force microscopy, we found that the reason for this effect in short ceramides was a lower proportion of tight orthorhombic packing and phase separation of continuous short ceramide-enriched domains with shorter lamellar periodicity compared to native long ceramides. Thus, long acyl chains in ceramides are essential for the formation of tightly packed impermeable lipid lamellae. Moreover, the model skin lipid membranes are a valuable tool to study the relationships between the lipid structure and composition, lipid organization, and the membrane permeability.

Simulation study of the structure and phase behavior of ceramide bilayers and the role of lipid head group chemistry

Ceramides are known to be a key component of the stratum corneum, the outermost protective layer of the skin that controls barrier function. In this work, molecular dynamics simulations are used to examine the behavior of ceramide bilayers, focusing on non-hydroxy sphingosine (NS) and non-hydroxy phytosphingosine (NP) ceramides. Here, we propose a modified version of the CHARMM force field for ceramide simulation, which is directly compared to the more commonly used GROMOS-based force field of Berger (Biophys. J. 1997, 72); while both force fields are shown to closely match experiment from a structural standpoint at the physiological temperature of skin, the modified CHARMM force field is better able to capture the thermotropic phase transitions observed in experiment. The role of ceramide chemistry and its impact on structural ordering is examined by comparing ceramide NS to NP, using the validated CHARMM-based force field. These simulations demonstrate that changing from ceramide NS to NP results in changes to the orientation of the OH groups in the lipid headgroups. The arrangement of OH groups perpendicular to the bilayer normal for ceramide NP, verse parallel for NS, results in the formation of a distinct hydrogen bonding network, that is ultimately responsible for shifting the gel-to-liquid phase transition to higher temperature, in direct agreement with experiment.

Effects of sphingosine 2N- and 3O-methylation on palmitoyl ceramide properties in bilayer membranes

To study the role of the interfacial properties of ceramides in their interlipid interactions, we synthesized palmitoylceramide (PCer) analogs in which a methyl group was introduced to the amide-nitrogen or the C3-oxygen of the sphingosine backbone. A differential scanning calorimetry analysis of equimolar mixtures of palmitoylsphingomyelin (PSM) and PCer showed that these sphingolipids formed a complex gel phase that melted between 67°C and 74°C. The PCer analogs also formed gel phases with PSM, but they melted at lower temperatures compared with the system with PCer. In complex bilayers composed of an unsaturated glycerophospholipid, PSM, and cholesterol, the 3O-methylated ceramide formed a cholesterol-poor ordered phase with PSM. However, the 2N-methylated and doubly methylated (2N and 3O) PCer analogs failed to displace sterol from interactions with PSM. Like PCer, the analogs reduced sterol affinity for the complex bilayers, but this effect was most pronounced for the 3O-methylated ceramide. Taken together, our results show that 2N-methylation weakened the ceramide-PSM interactions, whereas the 3O-methylated ceramide behaved more like PCer in interactions with PSM. Our findings are compatible with the view that interlipid interactions between the amide-nitrogen and neighboring lipids are important for the cohesive properties of sphingolipids in membranes, and this also appears to be a valid model for ceramide.

3,4-Disubstituted oxazolidin-2-ones as constrained ceramide analogs with anticancer activities

Heterocyclic analogs of ceramide as 3-alkanoyl or benzoyl-4-(1-hydroxy-2-enyl)-oxazolidin-2-ones were designed by binding of primary alcohol and amide in sphinogosine backbone as a carbamate. They were synthesized by addition of acyl halide to the common ring 4-(1-t-butyldimethylsilyloxyhexadec-2-enyl)-oxazolidin-2-one which was elaborated from chiral aziridine-2-carboxylate including stereoselective reduction and ring opening reactions as key steps. Other analogs with different carbon frame at C4 position which is corresponding to the sphingoid backbone were prepared from 3-cyclopentanecarbonyl-4-(1-t-butyldimethylsilyloxybut-2-enyl)-oxazolidin-2-one and straight and cyclic alkenes by cross metathesis. All compounds were tested as antileukemic drugs against human leukemia HL-60 cells. Many of them including propionyl, cyclopentanoyl and p-nitrobenzoyl-4-(1-hydroxyhexadec-2-enyl)-oxazolidin-2-ones showed better antileukemic activities than natural C2-ceramide with good correlation between cell death and DNA fragmentation. There is a drastic change of the activities by the carbon chain lengths at C4 position. Cytotoxicity was induced by caspase activation without significant accumulation of endogenous ceramide concentration or any perturbation of ceramide metabolism.

Disposition of ceramide in model lipid membranes determined by neutron diffraction

The lipid matrix present in the uppermost layer of the skin, the stratum corneum, plays a crucial role in the skin barrier function. The lipids are organized into two lamellar phases. To gain more insight into the molecular organization of one of these lamellar phases, we performed neutron diffraction studies. In the diffraction pattern, five diffraction orders were observed attributed to a lamellar phase with a repeat distance of 5.4 nm. Using contrast variation, the scattering length density profile could be calculated showing a typical bilayer arrangement. To obtain information on the arrangement of ceramides in the unit cell, a mixture that included a partly deuterated ceramide was also examined. The scattering length density profile of the 5.4-nm phase containing this deuterated ceramide demonstrated a symmetric arrangement of the ceramides with interdigitating acyl chains in the center of the unit cell.

Investigating the barrier function of skin lipid models with varying compositions

The lipids in the uppermost layer of the skin, the stratum corneum (SC), play an important role in the barrier function. The main lipid classes in stratum corneum are ceramides, cholesterol, and free fatty acids. In previous publications, a lipid model was presented, referred to as the stratum corneum substitute (SCS), that closely mimics the SC lipid organization and SC barrier function. In the present study, we use the SCS to study the effect of changes in lipid organization on the lipid barrier function using benzoic acid as permeation compound. First, in the SCS, we increased the level of one of the three major lipid classes keeping the ratio between the other lipid classes constant. An increased cholesterol level resulted in an increase in phase-separated cholesterol and a reduction in the permeability. An increase in ceramide or free fatty acid level resulted in the formation of additional phases, but had no significant influence on the permeability. We also examined models that mimic selected changes in lipid composition reported for dry or diseased skin. The SCS that mimics the composition in recessive X-linked ichthyosis skin displayed a twofold increase in permeability. This increase is possibly related to the formation of an additional, less ordered phase in this model.

Effect of ceramide acyl chain length on skin permeability and thermotropic phase behavior of model stratum corneum lipid membranes

Stratum corneum ceramides play an essential role in the barrier properties of skin. However, their structure-activity relationships are poorly understood. We investigated the effects of acyl chain length in the non-hydroxy acyl sphingosine type (NS) ceramides on the skin permeability and their thermotropic phase behavior. Neither the long- to medium-chain ceramides (8-24 C) nor free sphingosine produced any changes of the skin barrier function. In contrast, the short-chain ceramides decreased skin electrical impedance and increased skin permeability for two marker drugs, theophylline and indomethacin, with maxima in the 4-6C acyl ceramides. The thermotropic phase behavior of pure ceramides and model stratum corneum lipid membranes composed of ceramide/lignoceric acid/cholesterol/cholesterol sulfate was studied by differential scanning calorimetry and infrared spectroscopy. Differences in thermotropic phase behavior of these lipids were found: those ceramides that had the greatest impact on the skin barrier properties displayed the lowest phase transitions and formed the least dense model stratum corneum lipid membranes at 32°C. In conclusion, the long hydrophobic chains in the NS-type ceramides are essential for maintaining the skin barrier function. However, this ability is not shared by their short-chain counterparts despite their having the same polar head structure and hydrogen bonding ability.

The microstructure of the stratum corneum lipid barrier: mid-infrared spectroscopic studies of hydrated ceramide:palmitic acid:cholesterol model systems

The current mid-infrared spectroscopic study is a systematic investigation of hydrated stratum corneum lipid barrier model systems composed of an equimolar mixture of a ceramide, free palmitic acid and cholesterol. Four different ceramide molecules (CER NS, CER NP, CER NP-18:1, CER AS) were investigated with regard to their microstructure arrangement in a stratum corneum lipid barrier model system. Ceramide molecules were chosen from the sphingosine and phytosphingosine groups. The main differences in the used ceramide molecules result from their polar head group architecture as well as hydrocarbon chain properties. The mixing properties with cholesterol and palmitic acid are considered. This is feasible by using perdeuterated palmitic acid and proteated ceramides. Both molecules can be monitored separately, within the same experiment, using mid-infrared spectroscopy; no external label is necessary. At physiological relevant temperatures, between 30 and 35 degrees C, orthorhombic as well as hexagonal chain packing of the ceramide molecules is observed. The formation of these chain packings are extremely dependent on lipid hydration, with a decrease in ceramide hydration favouring the formation of orthorhombic hydrocarbon chain packing, as well as temperature. The presented data suggest in specific cases phase segregation in ceramide and palmitic acid rich phases. However, other ceramides like CER NP-18:1 show a rather high miscibility with palmitic acid and cholesterol. For all investigated ternary systems, more or less mixing of palmitic acid with cholesterol is observed. The investigated stratum corneum mixtures exhibit a rich polymorphism from crystalline domains with heterogeneous lipid composition to a "fluid" homogeneous phase. Thus, a single gel phase is not evident for the presented stratum corneum model systems. The study shows, that under skin physiological conditions (pH 5.5, hydrated, 30-35 degrees C) ternary systems composed of an equimolar ratio of ceramides, free palmitic acid and cholesterol may form gel-like domains delimitated by a liquid-crystalline phase boundary. The presented results support the microstructural arrangement of the stratum corneum lipids as suggested by the domain mosaic model.