Phase Behavior of an Equimolar Mixture ofN-Palmitoyl-d-erythro-sphingosine, Cholesterol, and Palmitic Acid, a Mixture with Optimized Hydrophobic Matching

Abstract

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.

Hydrophobicity of Nonwetting Soils

“…In contrast, an equimolar mixture of palmitic acid and stigmasterol film formed large aggregates 100-150 nm together with a continuous film coating that itself had a topology consisting of undulations of 3-5 nm, suggestive of a gel-like disordered phase separated from the more structured aggregates. Such phase formation behavior is consistent with the binary solubility reported for these compounds where the mixing of palmitic acid and cholesterol was demonstrated to produce phase separation with 80% forming crystalline phase together with remaining 20% existing as liquid or gel like phase in equilibrium (Brief et al, 2009).…”

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

“…S4B), similar to a neat isotropic solution of OMTS. From the present analysis, we are not able to pinpoint the location of fast isotropic solvent molecules, and we cannot distinguish between cases when the solvent is present in isotropic fluid domains within SC (11,(41)(42)(43)(44) and when it is present in an excess solution. However, we combine the studies of the molecular dynamics of the solvent molecules with an analysis of how the added solvent influences the molecular dynamics in SC lipid and protein components.…”

“…Adding more polar solvents to SC gradually leads to swelling of the corneocytes, and the solvent dynamics approaches that of the neat liquid. The addition of apolar solvents leads to an increased amount of fluid SC lipids, which also shows the characteristics of an isotropic fluid (11,(41)(42)(43). At high concentrations SC will be saturated with the solvent.…”

Effect of dimethyl sulfoxide on the phase behavior of model stratum corneum lipid mixtures

“…The broad nature of these endotherms was associated with the thermal evolution of the lipid mixing properties and was representative of the shape of the co-existing phase regions in the phase diagram. On the basis of the phase behavior previously described by spectroscopic techniques (Brief et al, 2009), the endotherm between 40 and 55 • C corresponds to a transition from solid lipids to a lo phase involving PA, cholesterol and a small fraction of ceramide. The broad variation of heat flow observed at higher temperatures corresponds to the incorporation of CerC16 in this fluid phase, concomitant with the destabilisation of this phase.…”

“…Using this ceramide with a well defined chain length provides an optimized hydrophobic match between the ceramide, cholesterol, and palmitic acid. The behavior of this model mixture has been recently characterized in detail (Brief et al, 2009). Below 42 • C, CerC16 and PA mainly exhibit a crystalline form with orthorhombic chain packing, while between 10% and 20% of these lipids are in a fluid phase.…”