The thermotropic phase behaviour of phyto-ceramide 1 as investigated by ATR-FTIR and DSC

Phyto-ceramide 1 {N-(27-stearoyloxy-heptacosanoyl)-phytosphingosine} consists of a C18 phytosphingosine base coupled via an amide link to a very long chain omega-hydroxy fatty acid, which is linked to a further fatty acid. This unusual ceramide found in the lipid barrier of stratum corneum is thought to have important functions for the integrity of the permeability barrier in the stratum corneum. Phyto-ceramide 1 functions as a molecular rivet holding together different bilayers of the lipid barrier. In addition, it is assumed that phyto-ceramide 1 interacts with the corneocyte cell membranes of the stratum corneum and thus fixes these cells to the stratum corneum lipid matrix. The thermotropic phase behaviour of phyto-ceramide 1 was investigated by means of attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) and differential scanning calorimetry (DSC). The aim of this study was the understanding of the organisation of phyto-ceramide 1 bilayers and their thermotropic phase behaviour. Special attention was focused on the intermolecular hydrogen bonding interactions in the polar interface and with water molecules during lipid film hydration. From the calorimetric data the phase transition temperature was determined to 105 °C with a very large phase transition enthalpy ΔH of +146 kJ mol−1. FTIR provided information on the hydrophobic core of the lipids as well as information concerning head group interactions. Analysing the methylene stretching, scissoring and rocking mode indicated a highly ordered gel phase. Hydration of the lipid films had some impact on the reorganisation of the acyl chains as is derived from the band shape and frequencies of the amide I and II bands. Water penetration into the polar lipid region was monitored by the temperature dependence of the amide II and O–H/N–H stretching intensities as a function of H → D exchange. Hydration could only be realized at the onset of the phase transition, which indicated that strong intermolecular interactions between the ceramide head groups exist. Hydration of the lipid film induced the formation of a tighter hydrogen bonding network between the ceramide head groups, as concluded from the position of both amide bands. The analysis of the band of the ester carbonyl group located in the “middle” of the ceramide structure revealed that it is involved in hydrogen bonding. This was concluded from the changes of the ester carbonyl band upon heating and hydration.