[Transition processes in Stratum corneum model lipid membranes with a mixture of free fatty acids].

The hydration of model membranes based on ceramide 6 with a mixture of free fatty acids the most commonly encountered the native lipid matrix of Stratum corneum, the outermost layer of the mammalian skin has been studied by neutron diffraction. Membrane hydration with water vapour at a temperature of 25 degrees C is characterized by a small increase in the repeat distance delta d0 = 1.0 A, which is comparable with membrane swelling in the presence of excess water. The kinetics changes in the repeat distance, connected with an increase of the water layer between bilayers during hydration, and water exchange during the processes of hydration and H-D isotopic substitution consists of a fast initial and a subsequent slow stage and is well described by exponential with two characteristic times lying in the range from a few tens of minutes to several hundreds of minutes. During hydration at a temperature of 57 degrees C, the repeat distance increases by delta d0 = 1.6 A, after which the membrane irreversibly separates into two phases. One of the phases is formed mainly by long-chain free fatty acids and is characterized by a large decrease in the repeat distance delta d(ph) = 8.3 A on dehydration. The investigation of the structure of model membranes in the temperature range 20-72 degrees C indicated that the system with 20% (w) of cholesterol in the range of 63-67 degrees C undergoes a structural phase transition caused by the melting of hydrocarbon chains of lipids. In the system with a smaller content of cholesterol, no phase transition was observed up to a temperature of 72 degrees C.