Structure and metastability of N-lignocerylgalactosylsphingosine (cerebroside) bilayers

Abstract

Differential scanning calorimetry (DSC) and X-ray diffraction have been used to study hydrated N-lignocerylgalactosylsphingosine (NLGS) bilayers. DSC of fully hydrated NLGS shows an endothermic transition at 69–70°C, immediately followed by an exothermic transition at 72–73°C; further heating shows a high-temperature ( T c = 82°C), high-enthalpy ( ΔH = 15.3 kcal/mol NLGS) transition. Heating to 75°C, cooling to 20°C and subsequent reheating shows no transitions at 69–73°C; only the high-temperature (82°C), high-enthalpy (15.3 kcal/mol) transition. Two exothermic transitions are observed on cooling; for the upper transition its temperature (about 65°C) and enthalpy (about 6 kcal/mol NLGS) are essentially independent of cooling rate, whereas the lower transition exhibits marked changes in both temperature (30 → 60°C) and enthalpy (2.2 → 9.5 kcal/mol NLGS) as the cooling rate decreases from 40 to 0.625 Cdeg/min. On reheating, the enthalpy of the 69–70°C transition is dependent on the previous cooling rate. The DSC data provide clear evidence of conversions between metastable and stable forms. X-ray diffraction data recorded at 26, 75 and 93°C show clearly that NLGS bilayer phases are present at all temperatures. The X-ray diffraction pattern at 75°C shows a bilayer periodicity d = 65.4 A ̊ , and a number of sharp reflections in the wide-angle region indicative of a crystalline chain packing mode. This stable bilayer form converts to a liquid-crystal bilayer phase; at 93°C, the bilayer periodicity d = 59.1 A ̊ , and a diffuse reflection at 1 4.6 A ̊ −1 is observed. The diffraction pattern at 22°C represents a combination of the stable and metastable low-temperature bilayer forms. NLGS exhibits a complex pattern of thermotropic changes related to conversions between metastable (gel), stable (crystalline) and liquid-crystalline bilayer phases. The structure and thermotropic properties of NLGS are compared with those of hydrated N-palmitoylgalactosylsphingosine reported previously (Ruocco, M.J., Atkinson, D., Small, D.M., Skarjune, R.P., Oldfield, E. and Shipley, G.G. (1981) Biochemistry 20, 5957–5966).