Lipid chain dynamics in stratum corneum studied by spin label electron paramagnetic resonance

Chemical enhancer solubility in human stratum corneum lipids and enhancer mechanism of action on stratum corneum lipid domain

Previously, chemical enhancer-induced permeation enhancement on human stratum corneum (SC) lipoidal pathway at enhancer thermodynamic activities approaching unity in the absence of cosolvents (defined as Emax) was determined and hypothesized to be related to the enhancer solubilities in the SC lipid domain. The objectives of the present study were to (a) quantify enhancer uptake into SC lipid domain at saturation, (b) elucidate enhancer mechanism(s) of action, and (c) study the SC lipid phase behavior at Emax. It was concluded that direct quantification of enhancer uptake into SC lipid domain using intact SC was complicated. Therefore a liposomal model of extracted human SC lipids was used. In the liposome study, enhancer uptake into extracted human SC lipid liposomes (EHSCLL) was shown to correlate with Emax. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and differential scanning calorimetry (DSC) were used to evaluate lipid phase alterations in enhancer-treated intact SC. IR spectra demonstrated an increase in the lipid domain fluidity and DSC thermograms indicated a decrease in the phase transition temperature with increasing Emax. These results suggest that the enhancer mechanism of action is through enhancer intercalation into SC intercellular lipids and subsequent lipid lamellae fluidization related to enhancer lipid concentration.

Effects of ethanol/l-menthol on the dynamics and partitioning of spin-labeled lipids in the stratum corneum

The interaction of ethanol as well as ethanol/L-menthol mixtures with the uppermost layer of epidermis, the stratum corneum, was investigated by electron paramagnetic resonance (EPR) spectroscopy utilizing spin-labeled analogs of androstanol (ASL), stearic acid (5-DSA) and methyl stearate (5-DMS). The EPR spectra of these spin probes structured in stratum corneum tissue of neonatal rat are characterized by the coexistence of two spectral components indicating the presence of two classes of spin labels with very different states of mobility. Probably, one class of spin labels is H-bonded to the polar surface of the membrane and another class corresponds to spin labels more deeply inserted in the hydrophobic core. EPR results showed that in the ethanol range 0-70% neither fluidity in stratum corneum membranes nor the relative fractions of these two components changes were observed. Instead, ethanol only caused a selective extraction of spin labels. The removal of the steroid ASL began at 30% ethanol, reaching extraction levels over 50% at ethanol concentrations of 60-70%, whereas the more hydrophobic 5-DMS was partially removed only with 70% ethanol. Addition of 5% L-menthol to the solvent containing 20% ethanol increases both the mobility and the fraction of those spin labels situated in the hydrophobic core (more mobile spectral component). Altogether, these findings suggest that the L-menthol stabilizes mainly in the central region of stratum corneum membranes attracting the membrane lipids and causing hydrogen bond ruptures in the polar membrane interface.

Thermal Behaviour of Human Stratum Corneum

PURPOSE

To use high-speed differential scanning calorimetry (DSC) in the identification of transitions in human stratum corneum (SC). Several scanning rates (100 degrees C/min to 400 degrees C/min) are used.

RESULTS

Eight transitions from 0 to 120 degrees C are detected in a significant number of samples. Most of these transitions have already been identified in previous studies, but have been labeled considering essentially that only four are present. Results also indicate some degree of reversibility for transitions occurring at temperatures above 90 degrees C. Dehydrated SC samples displayed slightly more defined transition peaks and a less frequent presence of the transitions below 50 degrees C. In turn, the delipidised SC matrix showed two major endothermic signals, centered around 55 and 100 degrees C, in conjunction with other much less marked features.

CONCLUSIONS

The interpretation of DSC traces in terms of four main transition temperatures must be complemented having in mind the occurrence of other transitions, some of them at physiological temperatures. This work further suggests that transitions at temperatures above 90 degrees C may to a large degree be associated to lipids, while transition at approx. 55 degrees C is probably related to lipids covalently linked to proteins, as previously suggested.

Study of human stratum corneum and extracted lipids by thermomicroscopy and DSC

A study on the thermal behavior of human stratum corneum and lipids is described. The use of high scanning rate DSC for both SC and extracted lipids allows the consistent determination of transition temperatures, including those of lower energy. Changes are found both at physiological and higher temperatures. There is a clear correspondence between the thermotropic behavior of these two systems. However, one of the transitions found in human SC (approximately 55 degrees C) is absent in extracted lipids and may be ascribed to those covalently-linked to corneocytes. Lipidic thermotropic behavior is clearly found above 100 degrees C, in which proteins do not play an exclusive role. Changes related to most transitions are observed directly by polarized light thermal microscopy in extracted lipids. This technique also allowed for the observation of large segregated domains in the extracted lipids. A drastic change is observed at approximately 60 degrees C, corresponding to the disruption of the lamellar structure.

Dynamics and partitioning of spin-labeled stearates into the lipid domain of stratum corneum

The EPR spectra of the positional isomers n-doxyl stearic acids (n-DSA), with n=5, 12 and 16, and 5-doxyl methyl stearate (5-DMS) structured in the lipid domain of intact stratum corneum (SC), are characterized by the thermodynamic equilibrium of two distinct spectral components provided by two different motional states of the spin-labeled chains. A two-component model used in the EPR spectra simulations provided the relative populations of the components, allowing for the calculation of the thermodynamic profile. Based on a detailed investigation, the more motionally restricted population of spin labels (component 1) is found to arise when the spin label is hydrogen-bonded to the polar surfaces of the membranes, while the less motionally restricted population (component 2) is generated by spin labels nonhydrogen-bonded and more deeply inserted in the hydrophobic core. The 5-DSA is bound tightly to the polar surfaces (DeltaG(o)2 --> 1=-1.75 kcal/mol and DeltaH(o)2 --> 1=-13.8 kcal/mol), whereas the more lipophilic 5-DMS has a major spin population stabilized in the hydrophobic core (DeltaG(o)2 --> 10.57 kcal/mol and DeltaH(o)2 --> 1=-9.1 kcal/mol). Upon lipid-depleting SC increases the interactions of the probe with the polar surfaces, thereby decreasing its rotational diffusion. In contrast, the treatment of SC with oleic acid, a permeation enhancer, drastically increases the mobility of the spin labels, particularly that of component 1, and the thermodynamic equilibrium shifts towards the formation of component 2. A mechanism for water permeation in SC is also proposed.

Dynamics of proteins and lipids in the stratum corneum: Effects of percutaneous permeation enhancers

We have spin labeled the stratum corneum (SC) with a lysine specific reagent, succinimidyl-2,2,5,5-tetramethyl-3-pirroline-1-oxyl-carboxylate spin label (SSL), to assess the dynamics and hydration degree of SC proteins by electron paramagnetic resonance (EPR) spectroscopy taking measurements directly from the intact tissue. Treating the SC with two percutaneous penetration enhancers, 8 M urea or 20% (v/v) 1-methyl-2-pyrrolidone (1 MP), destabilizes the proteins thus promoting more mobile and solvent-exposed protein conformations. Upon SC lipid depletion the nitroxide side chain becomes more solvent exposed, suggesting that the removal of hygroscopic substances in the extraction process favors more hydrated protein conformations. On the other hand, the treatments with 8 M urea or 40% (v/v) 1 MP did not alter significantly the fluidity in the SC lipid domain as assessed by the probe 5-doxyl stearic acid; these permeation enhancers, specially 1 MP, seem to increase the probe solubility in the solvent leading to a considerable fraction of spin label to be removed from the lipid domain.

Hydration effects on the protein dynamics in stratum corneum as evaluated by EPR spectroscopy

The uppermost layer of the epidermis, the stratum corneum (SC), was spin-labeled with a sulfhydryl-specific nitroxide reagent to investigate the water content effects upon the protein dynamics directly in the intact tissue. A two-state model for the nitroxide side chain described the coexistence of two spectral components in the electron paramagnetic resonance (EPR) spectra. The so-called strongly immobilized component, S, is associated with the EPR signal of a motionally restricted nitroxide fraction having its N-O group hydrogen bonded to protein (rigid structure) while the weakly immobilized component, W, corresponds to the signal provided by the spin labels with higher mobility (approximately 10 times greater) exposed to the aqueous environment. The relative populations between these two mobility states, S and W, are in thermodynamic equilibrium. The standard Gibbs free energy, enthalpy and entropy changes for transferring the nitroxide side chain from the state contacting the solvent, W, to the one contacting protein, S, indicated that the reduction of the SC water content to below approximately h 0.69 g H(2)O/g dry SC, stabilizes the protein interacting state, S. Upon decreasing the SC hydration level below approximately h 0.69 the segmental motion of the polypeptide chains and the rotational motion of the spin-labeled side chain were also constrained. This work can also be useful to improve the spectral analysis of site-directed spin labeling, especially for a more quantitative description in terms of thermodynamic parameters.

Stratum corneum protein dynamics as evaluated by a spin-label maleimide derivative: effect of urea

The stratum corneum (SC) protein dynamics in the sulfhydryl group regions was studied by electron paramagnetic resonance (EPR) spectroscopy of a covalently attached maleimide derivative spin label. A two-state model for the nitroxide described the coexistence of two spectral components in the EPR spectra. The so-called strongly immobilized component arises from a spin-label fraction with the nitroxide moiety hydrogen-bonded to protein (rigid structure) and the weakly immobilized component is provided by the spin labels with higher mobility (approximately 10 times greater) exposed to the aqueous environment. The relative populations between these two states are in thermodynamic equilibrium. The apparent energetic gain for the nitroxide to form a hydrogen bond with the backbone rather than to be dissolved in the local environment was approximately 10 kcal/mol in the temperature range of 2-30 degrees C and approximately 6 kcal/mol in the range of 30-70 degrees C. Urea treatment caused a drastic increase in the segmental motion of the polypeptide chains that was completely reversible by its removal. Our analyses also indicated that the urea induced unfolding of the SC proteins opening the thiol group cavities. This work can also be useful to improve the spectral analysis of site-directed spin-labeling, especially for a more quantitative description of the nitroxide side chain mobility.

Direct observation of domains in model stratum corneum lipid mixtures by Raman microspectroscopy

Several studies on intact and model stratum corneum (SC), the top layer of the epidermis, have suggested the presence of crystalline domains. In the present work, we used micro-Raman mapping to detect lipid domains in model lipid mixtures formed by an equimolar mixture of ceramides, cholesterol, and palmitic acid, the three main lipid species of SC. We were able to determine the spatial distribution of the three compounds individually based on the systematic analysis of band areas. As a control, we studied freeze-dried lipid mixtures, and the Raman microspectroscopy reported faithfully the homogeneous distribution of the three compounds. Spectral mapping was then performed on hydrated equimolar mixtures carefully annealed. In this case, clear phase separations were observed. Domains enriched in cholesterol, ceramides, or palmitic acid with a size of a few tens of square microns were detected. These findings constitute the first direct evidence of the formation of heterogeneous domains in the SC lipid models in a bulk phase. Raman microspectroscopy is an innovative approach to characterize the conditions leading to the formation of domains and provides new insights into the understanding of the skin barrier.

Stratum corneum protein mobility as evaluated by a spin label maleimide derivative

The molecular dynamics in the vicinity of sulfhydryl groups of stratum corneum (SC) proteins has been studied by electron paramagnetic resonance (EPR) spectroscopy of maleimide spin labels covalently bound to the proteins. The total amount of bound maleimide was around 4 nmol per mg of SC. We have interpreted the coexistence of two spectral components in the EPR spectra by a two-state model with a fraction of label hydrogen bonded to proteins and another fraction exposed to the aqueous environment. We showed that the relative populations among these two states, determined by spectral simulation, are in thermodynamic equilibrium. The calculated energetic gain for the nitroxide to form hydrogen bond with SC proteins rather than to be dissolved in the buffer was approximately 12 kcal/mol in the temperature range of 2-30 degrees C and approximately 5 kcal/mol in the range of 30-86 degrees C. Temperature profiles of other EPR parameters related to the rotational diffusion of the probe also showed changes in the temperature interval of 26-42 degrees C, suggesting alterations in the vibration modes of SC proteins which are sensitive to higher motional freedom above 26-42 degrees C. We also compared samples of intact and lipid-depleted SC and we found that the delipidization process does not alter significantly the backbone mobility in the SH group regions, but the data suggest that the protein cavity is more open in the case of the delipidized samples. These results contribute to the understanding of the protein participation in the barrier function of SC, and can be useful to improve the spectral analysis of site-directed spin labeling, particularly for a more quantitative description of the dynamic modes of the nitroxide side chains.