The microstructure of the stratum corneum lipid barrier: mid-infrared spectroscopic studies of hydrated ceramide:palmitic acid:cholesterol model systems.

The current mid-infrared spectroscopic study is a systematic investigation of hydrated stratum corneum lipid barrier model systems composed of an equimolar mixture of a ceramide, free palmitic acid and cholesterol. Four different ceramide molecules (CER NS, CER NP, CER NP-18:1, CER AS) were investigated with regard to their microstructure arrangement in a stratum corneum lipid barrier model system. Ceramide molecules were chosen from the sphingosine and phytosphingosine groups. The main differences in the used ceramide molecules result from their polar head group architecture as well as hydrocarbon chain properties. The mixing properties with cholesterol and palmitic acid are considered. This is feasible by using perdeuterated palmitic acid and proteated ceramides. Both molecules can be monitored separately, within the same experiment, using mid-infrared spectroscopy; no external label is necessary. At physiological relevant temperatures, between 30 and 35 degrees C, orthorhombic as well as hexagonal chain packing of the ceramide molecules is observed. The formation of these chain packings are extremely dependent on lipid hydration, with a decrease in ceramide hydration favouring the formation of orthorhombic hydrocarbon chain packing, as well as temperature. The presented data suggest in specific cases phase segregation in ceramide and palmitic acid rich phases. However, other ceramides like CER NP-18:1 show a rather high miscibility with palmitic acid and cholesterol. For all investigated ternary systems, more or less mixing of palmitic acid with cholesterol is observed. The investigated stratum corneum mixtures exhibit a rich polymorphism from crystalline domains with heterogeneous lipid composition to a "fluid" homogeneous phase. Thus, a single gel phase is not evident for the presented stratum corneum model systems. The study shows, that under skin physiological conditions (pH 5.5, hydrated, 30-35 degrees C) ternary systems composed of an equimolar ratio of ceramides, free palmitic acid and cholesterol may form gel-like domains delimitated by a liquid-crystalline phase boundary. The presented results support the microstructural arrangement of the stratum corneum lipids as suggested by the domain mosaic model.