Structure and Permeability of Ceramide Bilayers and Multilayers.

The stratum corneum (SC), which functions as a barrier to many exogenous molecules, contains ceramides (Cer) in both bilayer and multilayer structures. Simple models of the SC using pure Cer lamellae are simulated and studied using CHARMM36 force field. For single bilayers, the chain length is noted to have a significant effect on the thickness and interdigitation, whereas hydroxylation induces more prominent effects in hydrogen bonding. CerEOS, due to its long fatty acid tail (48 carbons), is simulated as a double layer that possesses the potential to transition into an interior-disordered state, which is reminiscent of the sandwich model of the long periodicity phase of the SC. The surface area per lipid of CerEOS is significantly higher than in other Cer bilayers, which affects lateral properties such as clustering. Compared to Cer bilayers, CerEOS chains are similarly ordered except in the disordered linoleate, and interdigitation decreases in disordered CerEOS. Umbrella sampling simulations are performed on bilayers of sphingosine Cer to calculate the potential of mean force (PMF) for ethanol penetration as well as the permeability. The chain length is noted to both cause an outward shift of the PMF as well as an order-of-magnitude decrease in permeability, although the bilayer center is still disordered and favorable for permeation. The structures probed in this work will guide and serve as control comparisons to more complex simulations of accurate SC mixtures.