Fatty acid interdigitation in stratum corneum model membranes: a neutron diffraction study

The influence of the chain length of the free fatty acid (FFA) in a stratum corneum (SC) lipid model membrane composed of N-(alpha-hydroxyoctadecanoyl)-phytosphingosine (CER [AP]), cholesterol (Ch), FFA and cholesterol sulphate (ChS) was investigated by neutron diffraction. The internal nanostructure of the SC lipid membrane in addition to the water distribution function was determined via calculation of the neutron scattering length density profile (Fourier profile). The Fourier profiles of the studied SC model membranes revealed that such membranes have a repeat distance approximately equal to the membrane thickness. Increasing the chain length of the FFA in the CER[AP] based model membrane did not cause an alteration of the internal nanostructure but led to a decrease in the membrane repeat distance from 45.6 A (palmitic acid, C16:0) to 43.7 A (cerotic acid, C26:0) due to a partial interdigitation of the FFA chains. Ceramide [AP] forces the long chain fatty acids to incorporate into the unchanged spacing of the bilayer, thereby obligating the FFA protrude partly through opposing leaflet. Furthermore, the longer chained free fatty acids tend to form a new separate so-called "fatty acid rich phase". Therefore, the elongation of the chain length of the FFA decreases the solubility of the FFA in the SC model membrane based on CER[AP].