  We present atomistic molecular dynamics results for fully hydrated bilayers
composed of ceramide NS-24:0, free fatty acid 24:0 and cholesterol, to address
the effect of the different components in the stratum corneum (the outermost
layer of skin) lipid matrix on its structural properties. Bilayers containing
ceramide molecules show higher in-plane density and hence lower rate of passive
transport compared to phospholipid bilayers. At physiological temperatures, for
all composition ratios explored, the lipids are in a gel phase with ordered
lipid tails. However, the large asymmetry in the lengths of the two tails of
the ceramide molecule leads to a fluid like environment at the bilayer
mid-plane. The lateral pressure profiles show large local variations across the
bilayer for pure ceramide or any of the two component mixtures. Close to the
skin composition ratio, the lateral pressure fluctuations are greatly
suppressed, the ceramide tails from the two leaflets interdigitate
significantly, the depression in local density at the inter-leaflet region is
lowered, and the bilayer have lowered elastic moduli. This indicates that the
observed composition ratio in the stratum corneum lipid layer is responsible
for both the good barrier properties and the stability of the lipid structure
against mechanical stresses.
