STRUCTURAL AND THERMOTROPIC PROPERTIES OF SYNTHETIC C16-0 (PALMITOYL)CERAMIDE - EFFECT OF HYDRATION

Differential scanning calorimetry (DSC) and X-ray diffraction techniqu es have been used to investigate the structure and thermotropic proper ties of synthetic, non-hydroxy fatty acid (16:0) ceramide (NFA(C16)CER ) as a function df hydration. Anhydrous NFA(C16)CER shows a single, br oad endothermic transition at 95.4 degrees C (Delta H = 10.4 kcal/mol) . On hydration, a broad exothermic transition appears at approximately 50-70 degrees C while the main endothermic transition decreases to 90 .0 degrees C (Delta H = 13.8 kcal/mol). The enthalpy of the exothermic transition increases with hydration to a maximum value, Delta H = 4.8 kcal/mol. This polymorphic phase behavior depends on the low temperat ure incubation time and prior cooling rate. X-ray diffraction of fully hydrated NEA(C16)CER at 26 degrees C, shows a well-ordered lamellar p hase with a bilayer periodicity d = 46.9 Angstrom. At 68 degrees C, ab ove the first exothermic transition, X-ray diffraction shows again a l amellar phase with reduced bilayer periodicity d = 41.8 Angstrom and a n increased number of both lamellar and wide-angle reflections indicat ive of enhanced layer and chain packing order, respectively. At 90.0 d egrees C, above the main transition, the diffraction pattern shows a b road, intense reflection at 29.9 Angstrom and a diffuse reflection at 4.6 Angstrom, indicative of a melted chain phase. On cooling, NFA(C16) CER exhibits polymorphic phase behavior involving the conversion of th e melted chain phase to a metastable bilayer phase. On heating, this m etastable phase undergoes an exothermic transition to a stable bilayer phase; on further heating, NFA(C16)CER converts endothermically to th e melted-chain phase. In addition, low temperature incubation facilita tes the conversion of the metastable bilayer phase to the stable bilay er phase. Thus, the chemically simple NFA(C16)CER lacking a polar grou p exhibits on hydration a complex polymorphic behavior similar to that of more polar sphingolipids such as sphingomyelin, cerebrosides, and sulfatides.