Hydration properties of N-(α-hydroxyacyl)-sphingosine: X-ray powder diffraction and FT–Raman spectroscopic studies

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.

Lysine-based amino-functionalized lipids for gene transfection: 3D phase behaviour and transfection performance

Based on previous work, the influence of the chain composition on the physical-chemical properties of five new transfection lipids (TH10, TT10, OH10, OT10 and OO10) containing the same lysine-based head group has been investigated in aqueous dispersions. For this purpose, the chain composition has been gradually varied from saturated tetradecyl (T, C14:0) and hexadecyl (H, C16:0) chains to longer but unsaturated oleyl (O, C18:1) chains with double bonds in the cis configuration. In this work, the lipid dispersions have been investigated in the absence and presence of the helper lipid DOPE and calf thymus DNA by small-angle and wide-angle X-ray scattering (SAXS/WAXS) supplemented by differential scanning calorimetry (DSC), attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) and Fourier-transform Raman spectroscopy (FTRS). Lamellar and inverted hexagonal mesophases have been observed in single-component systems. In the binary mixtures, the aggregation behaviour changes with an increasing amount of DOPE from lamellar to cubic. The lipid mixtures with DNA show a panoply of mesophases. Interestingly, TT10 and OT10 form cubic lipoplexes, whereas OO10 complexes the DNA sandwich-like between lipid bilayers in a lamellar lipoplex. Surprisingly, the latter is the most effective lipoplex.

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.

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.

Basic nanostructure of stratum corneum lipid matrices based on ceramides [EOS] and [AP]: a neutron diffraction study

The goal of this study was to investigate the nanostructure of SC lipid model membranes comprising the most relevant SC lipids such as the unique-structured omega-acylceramide [EOS] in a near natural ratio with neutron diffraction. In models proposed recently the presence of ceramide [EOS] and FFA are necessary for the formation of one of the two existent crystalline lamellar phases of the SC lipids, the long-periodicity phase as well as for the normal barrier function of the SC. The focus of this study was placed on the influence of the FFA BA on the membrane structure and its localization within the membrane based on the ceramides [EOS] and [AP]. The internal nanostructure of such membranes was obtained by Fourier synthesis from the experimental diffraction patterns. The resulting neutron scattering length density profiles showed that the exceptionally long ceramide [EOS] is arranged in a short-periodicity phase created by ceramide [AP] by spanning through the whole bilayer and extending even further into the adjacent bilayer. Specifically deuterated BA allowed us to determine the exact position of this FFA inside this SC lipid model membrane. Furthermore, hydration experiments showed that the presented SC mimic system shows an extremely small intermembrane hydration of approximately 1 A, consequently the headgroups of the neighboring leaflets are positioned close to each other.

Arrangement of ceramide [EOS] in a stratum corneum lipid model matrix: new aspects revealed by neutron diffraction studies

The lipid matrix in stratum corneum (SC) plays a key role in the barrier function of the mammalian skin. The major lipids are ceramides (CER), cholesterol (CHOL) and free fatty acids (FFA). Especially the unique-structured omega-acylceramide CER[EOS] is regarded to be essential for skin barrier properties by inducing the formation of a long-periodicity phase of 130 angstroms (LPP). In the present study, the arrangement of CER[EOS], either mixed with CER[AP] and CHOL or with CER[AP], CHOL and palmitic acid (PA), inside a SC lipid model membrane has been studied for the first time by neutron diffraction. For a mixed CER[EOS]/CER[AP]/CHOL membrane in a partly dehydrated state, the internal membrane nanostructure, i.e. the neutron scattering length density profile in the direction normal to the surface, was obtained by Fourier synthesis from the experimental diffraction patterns. The membrane repeat distance is equal to that of the formerly used SC lipid model system composed of CER[AP]/CHOL/PA/ChS. By comparing both the neutron scattering length density profiles, a possible arrangement of synthetic long-chain CER[EOS] molecules inside a SC lipid model matrix is suggested. The analysis of the internal membrane nanostructure implies that one CER[EOS] molecule penetrates from one membrane layer into an adjacent layer. A 130 angstroms periodicity phase could not be observed under experimental conditions, either in CER/CHOL mixtures or in CER/CHOL/FFA mixture. CER[EOS] can be arranged inside a phase with a repeat unit of 45.2 angstroms which is predominately formed by short-chain CER[AP] with distinct polarity.

Properties of Ceramides and Their Impact on the Stratum Corneum Structure: A Review

The lipid matrix of the stratum corneum (SC) is the major diffusion-rate-limiting pathway by which most drugs intracellularly pass the SC. The major lipid classes extracted from the SC are ceramides, cholesterol and free fatty acids. Ceramides that comprise nine subclasses play a crucial role in maintaining the barrier function of the skin. A profound knowledge of the physical properties of ceramides is essential for a deeper understanding of the impact of each ceramide species on the barrier function. The review summarizes the thermotropic and/or lyotropic behaviour of sphingosine-type ceramides (CER AS, CER NS) and phytosphingosine-type ceramides (CER AP, CER NP) revealed by differential scanning calorimetry, X-ray diffraction, Fourier transform infrared spectroscopy and Fourier transform Raman spectroscopy in past decades. Polymorphism is a characteristic feature of ceramides. At physiological temperatures, all crystalline phases of ceramides exhibit lamellar structures with highly ordered hydrocarbon chains. The differential behaviour of the head groups of ceramides may be an important determinant for the skin barrier function.

Stratum corneum hydration: phase transformations and mobility in stratum corneum, extracted lipids and isolated corneocytes

The outermost layer of skin, stratum corneum (SC), functions as the major barrier to diffusion. SC has the architecture of dead keratin filled cells embedded in a lipid matrix. This work presents a detailed study of the hydration process in extracted SC lipids, isolated corneocytes and intact SC. Using isothermal sorption microcalorimetry and relaxation and wideline (1)H NMR, we study these systems at varying degrees of hydration/relative humidities (RH) at 25 degrees C. The basic findings are (i) there is a substantial swelling both of SC lipids, the corneocytes and the intact SC at high RH. At low RHs corneocytes take up more water than SC lipids do, while at high RHs swelling of SC lipids is more pronounced than that of corneocytes. (ii) Lipids in a fluid state are present in both extracted SC lipids and in the intact SC. (iii) The fraction of fluid lipids is lower at 1.4% water content than at 15% but remains virtually constant as the water content is further increased. (iv) Three exothermic phase transitions are detected in the SC lipids at RH=91-94%, and we speculate that the lipid re-organization is responsible for the hydration-induced variations in SC permeability. (v) The hydration causes swelling in the corneocytes, while it does not affect the mobility of solid components (keratin filaments).

Biophysics of sphingolipids I. Membrane properties of sphingosine, ceramides and other simple sphingolipids

Some of the simplest sphingolipids, namely sphingosine, ceramide, some closely related molecules (eicosasphingosine, phytosphingosine), and their phosphorylated compounds (sphingosine-1-phosphate, ceramide-1-phosphate), are potent metabolic regulators. Each of these lipids modifies in marked and specific ways the physical properties of the cell membranes, in what can be the basis for some of their physiological actions. This paper reviews the mechanisms by which these sphingolipid signals, sphingosine and ceramide in particular, are able to modify the properties of cell membranes.

Structural organisation and phase behaviour of a stratum corneum lipid analogue: ceramide 3A

The thermotropic phase behaviour and structural organisation of ceramide N-linoeoyl-phytosphingosine (ceramide 3A) is investigated by means of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). Its polymorphism and structural properties are compared with two ceramides of the type III class with various hydrocarbon chain saturation degrees. After hydration the main phase transition temperature of ceramide 3A is found at 76 degrees C with a phase transition enthalpy of +29 kJ mol(-1). Analysing the frequency of methylene stretching vibrations (by infrared spectroscopy) reveals that the fluidity (amount of trans-gauche isomers) is strongly increased for ceramide 3A compared to its stearoyl ceramide type III analogue. After lipid hydration, the acyl chains of all investigated phytosphingosine ceramides of type III adopt a hexagonal-like chain packing. The amide I and amide II vibrations are quite sensitive to the phase transition of the ceramide. The corresponding band analysis reveals strong inter- and intramolecular hydrogen bonds between the amide and hydroxyl groups in the ceramide head groups. The H-bonding network and conformation of the head group of ceramide 3A is only slightly influenced by hydration. The water penetration capacity of ceramide 3A is, however, considerably larger compared to other phytosphingosine derivatives. The structural and organisational properties of ceramides of type III class are discussed with respect to their physiological relevancies for the stratum corneum lipid barrier property of the skin.