Molecular dynamics simulations of stratum corneum lipid models: fatty acids and cholesterol

We report the results of an investigation on stratum corneum lipids, which present the main barrier of the skin. Molecular dynamics simulations, therm al analysis and FTIR measurements were applied. The primary objective of th is work was to study the effect of cholesterol on skin structure and dynami cs. Two molecular models were constructed, a free fatty acid bilayer (stear ic acid, palmitic acid) and a fatty acid/cholesterol mixture at a 1:1 molar ratio. Our simulations were performed at constant pressure and temperature on a nanosecond time scale. The resulting model structures were characteri zed by calculating surface areas per headgroup, conformational properties, atom densities and order parameters of the fatty acids. Analysis of the sim ulations indicates that the free fatty acid fraction of stratum corneum lip ids stays in a highly ordered crystalline state at skin temperatures. The p hase behavior is strongly influenced when cholesterol is added. Cholesterol smoothes the rigid phases of the fatty acids: the order of the hydrocarbon tails (mainly of the last eight bonds) is reduced, the area per molecule b ecomes larger, the fraction of tl ans dihedrals is lower and the hydrophobi c thickness is reduced. The simulation results are in good agreement with o ur experimental data from FTIR analysis and NIR-FT Raman spectroscopy. (C) 2001 Elsevier Science B.V. All rights reserved.