Simulations of Pure Ceramide and Ternary Lipid Mixtures as Simple Interior Stratum Corneum Models

The barrier function of the stratum corneum (SC) is intimately related to the structure of the lipid matrix, which is composed of ceramides (Cer), cholesterol (Chol), and free fatty acid (FFA). In this study, the all-atom CHARMM36 (C36) force field is used to simulate bilayers of N-palmitoylsphingosine (Cer16), N-lignoceroylsphingosine (Cer24), Chol, and lignoceric acid (LA) as simple models of the SC. Equimolar mixtures of Cer, Chol, and LA are replicated from experiment for comparison and validation of the C36 force field, and the effects of lipid diversity and temperature are studied. The presence of Chol and LA have effects on nearly all membrane properties including surface area per lipid, area compressibility moduli, chain order, Chol tilt, bilayer thickness, interdigitation, hydrogen bonding, and lipid clustering, while temperature has a more moderate effect. In systems containing Cer16, there is a profound difference in interdigitation between pure Cer and mixed systems, while systems containing Cer24 are relatively unaffected. Increasing temperature has the potential to shift hydrogen bonding pairs rather than uniformly decrease bonding, which can lead to greater Cer-Cer bonding at higher temperatures. Comparison with deuterium order parameter experiments demonstrates good agreement, which supports further use of this class of lipids and fatty acids for development of more complex SC models.