Fourier transform infrared spectroscopy and differential scanning calorimetry studies of fatty acid homogeneous ceramide 2

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.

A Brief Review of FT-IR Spectroscopy Studies of Sphingolipids in Human Cells

In recent years, sphingolipids have attracted significant attention due to their pivotal role in cellular functions and physiological diseases. A valuable tool for investigating the characteristics of sphingolipids can be represented via FT-IR spectroscopy, generally recognized as a very powerful technique that provides detailed biochemical information on the examined sample with the unique properties of sensitivity and accuracy. In the present paper, some fundamental aspects of sphingolipid components of human cells are summarized, and the most relevant articles devoted to the FT-IR spectroscopic studies of sphingolipids are revised. A short description of different FT-IR experimental approaches adopted for investigating sphingolipids is also given, with details about the most commonly used data analysis procedures. The present overview of FT-IR investigations, although not exhaustive, attests to the relevant role this vibrational technique has played in giving significant insight into many aspects of this fascinating class of lipids.

Molecular interactions of hesperidin with DMPC/cholesterol bilayers

Since cell membranes are complex systems, the use of model lipid bilayers is quite important for the study of their interactions with bioactive molecules. Mammalian cell membranes require cholesterol (CHOL) for their structure and function. For this reason, the mixtures of phospholipid and cholesterol are necessary to use in model membrane studies to better simulate the real systems. In the present study, we investigated the effect of the incorporation of hesperidin in model membranes consisting of dimyristoylphosphatidylcholine (DMPC) and CHOL by using differential scanning calorimetry (DSC), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and atomic force microscopy (AFM). ATR-FTIR results demonstrated that hesperidin increases the fluidity of the DMPC/CHOL binary system. DSC findings indicated that the presence of 5 mol% hesperidin induces a broadening of the main phase transition consisting of three overlapping components. AFM experiments showed that hesperidin increases the thickness of DMPC/CHOL lipid bilayer model membranes. In addition to experimental results, molecular docking studies were conducted with hesperidin and human lanosterol synthase (LS), which is an enzyme found in the final step of cholesterol synthesis, to characterize hesperidin's interactions with its surrounding via its hydroxyl and oxygen groups. Then, hesperidin's ADME/Tox (absorption, distribution, metabolism, excretion and toxicity) profile was computed to see the potential impact on living system. In conclusion, considering the data obtained from experimental studies, this work ensures molecular insights in the interaction between a flavonoid, as an antioxidant drug model, and lipids mimicking those found in mammalian membranes. Moreover, computational studies demonstrated that hesperidin may be a great potential for use as a therapeutic agent for hypercholesterolemia due to its antioxidant property.

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.

Effects of Lactobacillus plantarum CJLP55 on Clinical Improvement, Skin Condition and Urine Bacterial Extracellular Vesicles in Patients with Acne Vulgaris: A Randomized, Double-Blind, Placebo-Controlled Study

Lactobacillus plantarum CJLP55 has anti-pathogenic bacterial and anti-inflammatory activities in vitro. We investigated the dietary effect of CJLP55 supplement in patients with acne vulgaris, a prevalent inflammatory skin condition. Subjects ingested CJLP55 or placebo (n = 14 per group) supplements for 12 weeks in this double-blind, placebo-controlled randomized study. Acne lesion count and grade, skin sebum, hydration, pH and surface lipids were assessed. Metagenomic DNA analysis was performed on urine extracellular vesicles (EV), which indirectly reflect systemic bacterial flora. Compared to the placebo supplement, CJLP55 supplement improved acne lesion count and grade, decreased sebum triglycerides (TG), and increased hydration and ceramide 2, the major ceramide species that maintains the epidermal lipid barrier for hydration. In addition, CJLP55 supplement decreased the prevalence of Proteobacteria and increased Firmicutes, which were correlated with decreased TG, the major skin surface lipid of sebum origin. CJLP55 supplement further decreased the Bacteroidetes:Firmicutes ratio, a relevant marker of bacterial dysbiosis. No differences in skin pH, other skin surface lipids or urine bacterial EV phylum were noted between CJLP55 and placebo supplements. Dietary Lactobacillus plantarum CJLP55 was beneficial to clinical state, skin sebum, and hydration and urine bacterial EV phylum flora in patients with acne vulgaris.

Exploring polar headgroup interactions between sphingomyelin and ceramide with infrared spectroscopy

Ceramide is a major actor in the sphingolipid signaling pathway elicited by various kinds of cell stress. Under those conditions ceramide (Cer) is produced in the plasma membrane as a product of sphingomyelin (SM) hydrolysis, and this may lead to apoptosis. Thus, SM and Cer coexist in the membrane for some time, and they are known to separate laterally from the (more abundant) glycerolipids, giving rise to highly rigid domains or platforms. The properties of these domains/platforms are rather well understood, but the underlying SM:Cer molecular interactions have not been explored in detail. Infrared (IR) spectroscopy is a powerful analytical technique that provides information on all the chemical groupings in a molecule, and that can be applied to membranes and lipid bilayers in aqueous media. IR spectra can be conveniently retrieved as a function of temperature, thus revealing the thermotropic transitions of SM and its mixtures with Cer. Four regions of the IR spectrum of these sphingolipids have been examined, two of them dominated by the hydrophobic regions in the molecules, namely the C–H stretching vibrations (2800–3000 cm⁻¹), and the CH₂ scissoring vibrations (1455–1485 cm⁻¹), and two others arising from chemical groups at the lipid-water interface, the sphingolipid amide I band (1600–1680 cm⁻¹), and the phosphate vibrations in the 1000–1110 cm⁻¹ region. The latter two regions have been rarely studied in the past. The IR data from the hydrophobic components show a gel (or ripple)-fluid transition of SM at 40 °C, that is shifted up to about 70 °C when Cer is added to the bilayers, in agreement with previous studies using a variety of techniques. IR information concerning the polar parts is more interesting. The amide I (carbonyl) band of pure SM exhibits a maximum at 1638 cm⁻¹ at room temperature, and its position is shifted by about 10 cm⁻¹ in the presence of Cer. Cer causes also a change in the overall band shape, but no signs of band splitting are seen, suggesting that SM and Cer carbonyl groups are interacting tightly, presumably through H-bonds. The 1086 cm⁻¹ band, corresponding to PO₂⁻ vibrations, appears more stable in SM than in DPPC, and it is further stabilized by Cer, again suggesting an important role of H-bonds in the formation of SM:Cer clusters. Thus, SM and Cer can interact through their polar headgroups, in a way that is not accessible to other lipid classes.

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.

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.

Probing the interactions among sphingosine and phytosphingosine ceramides with non- and alpha-hydroxylated acyl chains in skin lipid model membranes

Ceramides (Cers) are significant constituents of the stratum corneum (SC), the uppermost skin layer responsible for skin barrier properties. Cers are a heterogeneous group of lipids whose mutual interactions are still unclear. To better understand these interactions, we characterized model membranes containing stearic acid, cholesterol, cholesterol sulfate and one or more of the following ceramides: N-stearoyl-sphingosine (CerNS), N-stearoyl-phytosphingosine (CerNP) and N-(2-hydroxy)stearoyl-phytosphingosine (CerAP). Small angle X-ray scattering and FTIR spectroscopy were used to study lipid arrangement, phase separation and thermotropic behaviour. In the one-Cer systems, the membranes with CerNP showed strong hydrogen bonding and significant phase separation, even after phase transition, while the systems containing CerAP and CerNS had increased lipid miscibility. The multi-Cer systems exhibited different behaviour. In particular, the membrane containing all three Cers was a highly miscible system with narrow one-step phase transition, which, of all the studied samples, occurred at the lowest temperatures. Our results show that even a small variation in Cer structure results in substantially different phase behaviour, which is further affected by the presence of other Cer subclasses. Interestingly, the phase behaviour of the most complex three-Cer system was simpler than that of the others, highlighting the importance of lipid diversity in real SC.

Phase transformation in the C form of myristic-acid crystals and DFT calculations

In this study, the vibrational frequencies of myristic acid (CH₃(CH₂)₁₂COOH) were obtained using density functional theory calculations, and the results were compared with experimental Raman and infrared data. Additionally, Raman spectra of crystalline myristic acid were recorded in the 300-20 K range. Raman spectroscopy gives important insights into the effect of low temperatures on its monoclinic phase. X-ray diffraction was performed from 298 to 133 K to provide additional information about the cryogenic behavior of the crystals. These undergo a phase transformation, which was confirmed by differential scanning calorimetry through an enthalpy anomaly observed at low temperatures. Raman spectra and X-ray diffraction refinement of the cell parameters in combination with differential scanning calorimetry at low temperatures revealed slight modifications, confirming a conformational change in the myristic acid molecules involving rearrangement of dimers within the unit cell.

Clearly Detectable, Kinetically Restricted Solid-Solid Phase Transition in cis-Ceramide Monolayers

Sphingosine [(2 S,3 R,4 E)-2-amino-4-octadecene-1,3-diol] is the most common sphingoid base in mammals. Ceramides are N-acyl sphingosines. Numerous small variations on this canonical structure are known, including the 1-deoxy, the 4,5-dihydro, and many others. However, whenever there is a Δ4 double bond, it adopts the trans (or E) configuration. We synthesized a ceramide containing 4 Z-sphingosine and palmitic acid ( cis-pCer) and studied its behavior in the form of monolayers extended on an air-water interface. cis-pCer acted very differently from the trans isomer in that, upon lateral compression of the monolayer, a solid-solid transition was clearly observed at a mean molecular area ≤44 Ų·molecule^-1, whose characteristics depended on the rate of compression. The solid-solid transition, as well as states of domain coexistence, could be imaged by atomic force microscopy and by Brewster-angle microscopy. Atomistic molecular dynamics simulations provided results compatible with the experimentally observed differences between the cis and trans isomers. The data can help in the exploration of other solid-solid transitions in lipids, both in vitro and in vivo, that have gone up to now undetected because of their less obvious change in surface properties along the transition, as compared to cis-pCer.

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.

The many faces (and phases) of ceramide and sphingomyelin I - single lipids

Ceramides, the simplest kind of two-chained sphingolipids, contain a single hydroxyl group in position 1 of the sphingoid base. Sphingomyelins further contain a phosphocholine group at the OH of position 1 of ceramide. Ceramides and sphingomyelins show a variety of species depending on the fatty acyl chain length, hydroxylation, and unsaturation. Because of the relatively high transition temperature of sphingomyelin compared to lecithin and, particularly, of ceramides with 16:0-18:0 saturated chains, a widespread idea on their functional importance refers to formation of rather solid domains enriched in sphingomyelin and ceramide. Frequently, and especially in the cell biology field, these are generally (and erroneously) assumed to occur irrespective on the type of N-acyl chain in these lipids. This is because most studies indicating such condensed ordered domains employed sphingolipids with acyl chains with 16 carbons while scarce attention has been focused on the influence of the N-acyl chain on their surface properties. However, abundant evidence has shown that variations of the N-acyl chain length in ceramides and sphingomyelins markedly affect their phase state, interfacial elasticity, surface topography, electrostatics and miscibility and that, even the usually conceived "condensed" sphingolipids and many of their mixtures, may exhibit liquid-like expanded states. This review is a summarized overview of our work and of related others on some facts regarding membranes composed of single molecular species of ceramide and sphingomyelin. A second part is dedicated to discuss the miscibility properties between species of sphingolipids that differ in N-acyl and oligosaccharide chains.

Fractionated aliphatic alcohols as synthetic precursors of ultra long-chain monoacylglycerols for cosmetic applications

BACKGROUND

Xerosis is an abnormally dry and flaky skin condition that is associated with a change in the packing behaviour of the lipid matrix in the stratum corneum (SC), the outermost layer of the skin. This skin condition can lead to an increase in transepidermal water loss (TEWL). As ultralong-chain fatty acids have a positive effect on maintaining the packing behaviour of the SC lipid matrix, a moisturizer which contains glycerides of ultralong-chain fatty acids could act as a semi-occlusive layer on the surface of the skin. This will lower the rate of water evaporation through the epidermis and consequently help prevent or improve skin xerosis.

OBJECTIVE

To identify a novel source of ultralong-chain lipids and develop monoacylglycerols with mixed fatty acyl chain lengths that have occlusive properties superior to petrolatum.

METHODS

Initially, Performacol 425, a mixture of very long-chain fatty alcohols, was fractionated using short path distillation to yield a fraction enriched with C22:0-C26:0 fatty alcohols. The fatty alcohol fraction was then oxidized using Jones reagent, and the resulting fatty acids were esterified with glycerol to yield the corresponding monoglycerides using Novozym 435. These were then evaluated using Fourier transform infrared spectroscopy, differential scanning calorimetry and water vapour transmission rate measurements.

RESULTS

The monoacylglycerols enriched with C22:0-C26:0 displayed a melting point of 80°C and orthorhombic packing; packing behaviour mainly present in healthy SC. In addition, a phospholipid-structured emulsion containing 3% of the monoglycerides displayed occlusive properties superior to the vehicle containing 3% petrolatum jelly.

CONCLUSIONS

Performacol 425 can be a potential source of fatty alcohols to synthesize monoacylglycerols that can improve the occlusive behaviour of phospholipid-structured emulsions.

Associations among plasma vitamin C, epidermal ceramide and clinical severity of atopic dermatitis

BACKGROUND/OBJECTIVES

Atopic dermatitis (AD), a chronic inflammatory skin disease, is accompanied by disruption of the epidermal lipid barrier, of which ceramide (Cer) is the major component. Recently it was reported that vitamin C is essential for de novo synthesis of Cer in the epidermis and that the level of vitamin C in plasma is decreased in AD. The objective of this study was to determine the associations among clinical severity, vitamin C in either plasma or epidermis, and Cer in the epidermis of patients with AD.

SUBJECTS/METHODS

A total of 17 patients (11 male and 6 female) aged 20-42 years were enrolled. The clinical severity of AD was assessed according to the SCORAD (SCORing Atopic Dermatitis) system. Levels of vitamin C were determined in plasma and biopsies of lesional epidermis. Levels of epidermal lipids, including Cer, were determined from tape-stripped lesional epidermis.

RESULTS

The clinical severity of patients ranged between 0.1 and 45 (mild to severe AD) based on the SCORAD system. As the SCORAD score increased, the level of vitamin C in the plasma, but not in the epidermis, decreased, and levels of total Cer and Cer2, the major Cer species in the epidermis, also decreased. There was also a positive association between level of vitamin C in the plasma and level of total Cer in the epidermis. However, levels of epidermal total lipids including triglyceride, cholesterol, and free fatty acid (FFA) were not associated with either SCORAD score or level of vitamin C in the plasma of all subjects.

CONCLUSIONS

As the clinical severity of AD increased, level of vitamin C in the plasma and level of epidermal Cer decreased, and there was a positive association between these two parameters, implying associations among plasma vitamin C, epidermal Cer, and the clinical severity of AD.

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.

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.

Thermomechanical transitions of egg-ceramide monolayers

Ceramides have unique biophysical properties. Their high melting temperature and their ability to form lateral domains have converted ceramides into the paradigm of rigid lipids. Here, using shear surface rheology of egg-ceramide Langmuir monolayers, a solid to fluid transition was evidenced as a vanishing shear rigidity at lower temperatures than the lipid melting temperature. Such a mechanical transition, which depends on the lipid lateral pressure, was found in a broad range temperature (40-50 °C). The solid to fluid transition was correlated to a LC to LC+LE phase transition, as confirmed by BAM experiments. Interestingly, together with the softening transition, a supercooling process compatible with a glassy behavior was found upon freezing. A new phase scenario is then depicted that broadens the mechanical behavior of natural ceramides. The phase diversity of ceramides might have important implications in their physiological roles.

Breaking of the phosphodiester bond: a key factor that induces hemolysis

In-depth understanding the toxicity of nanomaterials in red blood cells (RBCs) is of great interest, because of the importance of RBCs in transporting oxygen in blood circulation. Although the toxic effects of nanoparticles in RBCs have been revealed, the conclusions from the literature are conflicting, and in particular, the toxic mechanism is still at the infant stage. Herein, we investigated the size-dependent toxicity of well-known CdTe semiconductor quantum dots (QDs) and revealed the exact toxic mechanism at the molecular level by confocal microscopy and Fourier transform infrared (FT-IR) spectroscopy techniques. We found that smaller mercaptosuccinic acid-capped CdTe QDs (MSA-QDs) with the green-emitting color could cause hemagglutination whereas the middle-size yellow-emitting MSA-QDs induced the formation of stomatocytes and echinocytes and the bigger size red-emitting MSA-QDs induced heavy hemolysis and the formation of lots of ghost cells. The FT-IR data proved that all the MSA-QDs were likely to bond to the RBCs membranes and caused the structural changes of lipid and protein in RBCs. But only the red-emitting MSA-QDs caused the breakage of the phosphodiester bond, which might cause the heavy hemolysis. To some extent, this is the first example that reveals the hemolysis mechanism at the molecular level.

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.

Thermal phase transition behavior of lipid layers on a single human corneocyte cell

We have improved the selected area electron diffraction method to analyze the dynamic structural change in a single corneocyte cell non-invasively stripped off from human skin surface. The improved method made it possible to obtain reliable diffraction images to trace the structural change in the intercellular lipid layers on a single corneocyte cell during heating from 24°C to 100°C. Comparison of the results with those of synchrotron X-ray diffraction experiments on human stratum corneum sheets revealed that the intercellular lipid layers on a corneocyte cell exhibit essentially the same thermal phase transitions as those in a stratum corneum sheet. These results suggest that the structural features of the lipid layers are well preserved after the mechanical stripping of the corneocyte cell. Moreover, electron diffraction analyses of the thermal phase transition behaviors of the corneocyte cells that had the lipid layers with different distributions of orthorhombic and hexagonal domains at 24°C suggested that small orthorhombic domains interconnected with surrounding hexagonal domains transforms in a continuous manner into new hexagonal domains.

The molecular organization of prenylated flavonoid xanthohumol in DPPC multibilayers: X-ray diffraction and FTIR spectroscopic studies

Xanthohumol (XN) is the major prenylated flavonoid found in hop resin. It has attracted considerable attention in recent years due to its wide spectrum of biological activities and the beneficial effect on human health. Since lipid membrane is first target for biologically active compounds, we decided to investigate the influence of XN on the dipalmitoylphosphatidylcholine (DPPC) multibilayers. Interactions of XN with DPPC were investigated as a function of temperature and its concentration by using X-ray diffraction and the ATR-FTIR spectroscopy techniques. The aim of understanding the mechanisms of molecular interactions between XN and DPPC was to indicate the localization of the XN with respect to the membrane and the type of interaction with phospholipids. The results revealed that XN changes the physical properties of the DPPC multibilayers in the form of dry film. A new complex formation between XN and DPPC is reported. The detailed analysis of refraction effect indicates the changes in electron density ratio between hydrophobic and hydrophilic zones of lipid at phase transition. This is in compliance with reported changes in FTIR spectra where at pretransition XN moves from interface region between polar heads to the neighborhood of phosphate groups.

Transdermal Drug Delivery Enhancement by Compounds of Natural Origin

The transdermal route of administration offers an alternative pathway for systemic drug delivery with numerous advantages over conventional routes. Regrettably, the stratum corneum forms a formidable barrier that hinders the percutaneous penetration of most drugs, offering an important protection mechanism to the organism against entrance of possible dangerous exogenous molecules. Different types of penetration enhancers have shown the potential to reversibly overcome this barrier to provide effective delivery of drugs across the skin. Although certain chemical and physical skin penetration enhancers are already employed by the pharmaceutical industry in commercially available transdermal products, some skin penetration enhancers are associated with irritating and toxic effects. This emphasizes the need for the discovery of new, safe and effective skin penetration enhancers. Penetration enhancers from natural origin have become popular as they offer several benefits over their synthetic counterparts such as sustainable mass production from a renewable resource and lower cost depending on the type of extraction used. The aim of this article is to give a comprehensive summary of the results from scientific research conducted on skin penetration enhancers of natural origin. The discussions on these natural penetration enhancers have been organized into the following chemical classes: essential oils, terpenes, fatty acids and polysaccharides.

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.

Computational analysis of water residence on ceramide and sphingomyelin bilayer membranes

Many physical chemical properties of lipid membranes, for example, the thickness, phase state, order parameter, and fluidity, can be understood straightforwardly. Water residence on a membrane is, however, an exception. To tackle this problem, we have performed molecular dynamics simulations of the distribution of water normal to the surface of several lipid membranes and from this deduced the associated water residence time. Our analysis of the results clearly indicates that lipid membranes have hydration shells on their surface, just as a solute in an aqueous solution does, and that the water residence time can be estimated from the potential for the mean force field derived from the distribution function of the water. We have done this atomic-scale analysis for ceramide bilayers and contrasted the calculation results with those for sphingomyelin bilayers, revealing that sphingomyelin bilayers can retain water molecules longer than ceramide bilayers and that the total number of water molecules retained on the membrane surface of sphingomyelin is larger than that for ceramide. In addition, we find that not only polar atoms of lipid molecules, such as oxygen, but also non-polar atoms, such as carbon, influence the motion of water on the membranes.

Phase state and surface topography of palmitoyl-ceramide monolayers

In cell biology (and in many biophysical) studies there is a natural tendency to consider ceramide as a highly condensed, solid-type lipid conferring rigidity and close packing to biomembranes. In the present work we advanced the understanding of the phase behavior of palmitoyl-ceramide restricted to a planar interface using Langmuir monolayers under strictly controlled and known surface packing conditions. Surface pressure-molecular area isotherms were complemented with molecular area-temperature isobars and with observations of the surface topography by Brewster Angle Microscopy. The results described herein indicate that palmitoyl-ceramide can exhibit expanded, as well as condensed phase states. Formation of three phases was found, depending on the surface pressure and temperature: a solid (1.80nm thick), a liquid-condensed (1.73nm thick, likely tilted) and a liquid-expanded (1.54nm thick) phase over the temperature range 5-62 degrees C. A large hysteretic behavior is observed for the S phase monolayer that may indicate high resistance to domain boundary deformation. A second (or higher) order S-->LC phase transition is observed at about room temperature while a first order LC-->LE transition occurs in a range of temperature encompassing the physiological one (observed above 30 degrees C at low surface pressure). This phase behavior broadens the view of ceramide as a type of lipid not-always-rigid but able to exhibit polymorphic properties.

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

The phase behavior and lipid mixing properties of an equimolar mixture of nonhydroxylated palmitoyl ceramide (Cer16), palmitic acid (PA), and cholesterol have been investigated using 2H NMR and vibrational spectroscopy. This mixture is formed by the three main classes of lipids found in the stratum corneum (SC), the top layer of the epidermis, and provides an optimized hydrophobic matching. Therefore, its behavior highlights the role played by hydrophobic matching on the phase behavior of SC lipids. We found that, below 45 degrees C, the mixture is essentially formed of coexisting crystalline domains with a small fraction of lipids (less than 20%) that forms a gel or fluid phase, likely ensuring cohesion between the solid domains. Upon heating, there is the formation of a liquid ordered phase mainly composed of PA and cholesterol, including a small fraction of Cer16. This finding is particularly highlighted by correlation vibrational microspectroscopy that indicates that domains enriched in cholesterol and PA include more disordered Cer16 than those found in the Cer16-rich domains. Solubilization of Cer16 in the fluid phase occurs progressively upon further heating, and this leads to the formation of a nonlamellar self-assembly where the motions are isotropic on the NMR time scale. It is found that the miscibility of Cer16 with cholesterol and PA is more limited than the one previously observed for ceramide III extracted from bovine brain, which is heterogeneous in chain composition and includes, in addition to Cer16, analogous ceramide with longer alkyl chains that are not hydrophobically matched with cholesterol and PA. Therefore, it is inferred that, in SC, the chain heterogeneity is a stronger criteria for lipid miscibility than chain hydrophobic matching.

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

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

Lipid domain morphologies in phosphatidylcholine-ceramide monolayers

In cells, one of the main roles of ceramide-enriched membrane domains is to recruit or exclude intracellular signaling molecules and receptors, thereby facilitating signal transduction cascades. Accordingly, in model membranes, even low contents of ceramide segregate into lateral domains. The impact of the N-acyl chain on this segregation and on the morphology of the domains remains to be explored. Using Langmuir monolayers, we have systematically studied binary mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and ceramide (2:1, molar ratio) and varied the N-acyl chain length of ceramide from 2 to 24 carbon atoms (Cer2 to Cer24). Fluid Cer2, Cer6, and Cer8/DMPC mixtures were miscible at all surface pressures. Longer ceramides, however, formed surface pressure-dependent immiscible mixtures with DMPC. The domain morphology under fluorescence microscopy after including a trace amount of fluorescent NBD-phosphatidylcholine into DMPC/Cer mixtures was found to be very sensitive to the N-acyl chain length. Shorter ceramides (Cer10-Cer14) formed flower-like (seaweed) domains, whereas longer ceramides (N-acyl chain length>14 carbon atoms) formed round and regular domains. We attribute the formation of the flower patterns to diffusive morphological instabilities during domain growth.

Phase behavior of aqueous dispersions of mixtures of N-palmitoyl ceramide and cholesterol: a lipid system with ceramide-cholesterol crystalline lamellar phases

Ceramides are particularly abundant in the stratum corneum lipid matrix, where they determine its unusual mesostructure, are involved in the lateral segregation of lipid domains in biological cell membranes, and are also known to act as signaling agents in cells. The importance attributed to ceramides in several biological processes has heightened in recent years, demanding a better understanding of their interaction with other membrane components, namely, cholesterol. Structural data concerning pure ceramides in water are relatively scarce, and this is even more the case for mixtures of ceramides with other lipids commonly associated with them in biological systems. We have derived the thermotropic binary phase diagram of mixtures of N-palmitoyl- D-erythro-sphingosine, C16:0-ceramide, and cholesterol in excess water, using differential scanning calorimetry and small- and wide-angle X-ray diffraction. These mixtures are self-organized in lamellar mesostructures that, between other particularities, show two ceramide to cholesterol crystalline phases with molar proportions that approach 2:3 and 1:3. The 2:3 phase crystallizes in a tetragonal arrangement with a lamellar repeat distance of 3.50 nm, which indicates an unusual lipid stacking, probably unilamellar. The uncommon mesostructures formed by ceramides with cholesterol should be considered in the rationalization of their singular structural role in biological systems.