Calorimetric and spectroscopic investigations of phytosphingosine ceramide membrane organisation

Calorimetric and infrared spectroscopic data are presented for three phytosphingosine ceramides: ceramide 3, 3b and 7. The current investigation utilises differential scanning calorimetry (DSC) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) to examine the thermotropic phase behaviour and membrane organisation of these ceramides. Thus, from the DSC experiments the phase transition enthalpies and the phase transition temperatures are obtained with high accuracy. Information concerning molecular interactions and arrangements are derived from the temperature dependency of characteristic infrared absorption bands. Special attention is focused on the hydration behaviour, hydrocarbon chain organisation and the formation of the hydrogen-bonding network of these lipid membranes. Analysing the hydroxyl and amide H → D exchange monitors the water penetration in the polar region of the ceramide films. The examined ceramides exhibit very high phase transition temperatures and enthalpies for the gel to liquid crystalline phase transition in contrast to data reported for phospholipids. The re-crystallisation of the melted acyl chains is accompanied by a large hysteresis. When hydrated, all phytosphingosine ceramides exhibit a hexagonal hydrocarbon chain packing. Furthermore, very strong head group hydrogen-bonding interactions are formed in hydrated ceramides compared to the dry, anhydrous ceramides. A nearly water impermeable lipid film is formed by ceramide 3, whereas water penetration is higher for ceramide 3b and 7. The presented data demonstrate that the relative stability, the water penetration and the driving force for the molecular organisation in these phytosphingosine ceramides is determined by either the formation of a very strong hydrogen bonding network and/or hexagonal acyl chain packing.