| Abstract
| - The thermotropic behavior of the hydrated bilayers formed by novel glycosylceramides containing ananomeric galactose, a saturated long acyl chain (cerotoyl, C26), and a phytosphingosine base, α- and β-GalpCer,was investigated by means of differential scanning calorimetry (DSC) and X-ray diffraction measurements.DSC curves for hydrated α-GalpCer showed an endothermic transition from metastable gel to liquidcrystalline states at 72.8 °C (ΔH = 12.0 kcal/mol) and exothermic transition of the reverse change at 66.3°C (ΔH = −10.9 kcal/mol) at the heating/cooling rate of 1 °C/min. The metastable gel of α-GalpCer wasremarkably stable in contrast with β-GalpCer or the typical animal β-galactosylceramide containing a longacyl chain. Annealing at 37 °C for 48 h completely converted the metastable gel of α-GalpCer into a stablegel, which was directly transformed to a liquid crystal (LC) at 74.9 °C with a 2-fold larger endothermicpeak (ΔH = 23.8 kcal/mol). Cooling of the LC state brought about the transition into the metastable gel.X-ray diffraction measurements showed that both metastable and stable gels were lamellar structures.The diffraction patterns also suggested that the stable gel had a bilayer arrangement of higher crystallinitythan the metastable gel. However, the metastable gel of β-GalpCer was rapidly converted to the stablegel: Upon heating, the stable gel was directly transformed to the LC state at 75.8 °C (ΔH = 19.0 kcal/mol),and cooling of the LC state led to successive transitions of the LC to the metastable gel at 71.0 °C andof the metastable gel to the stable gel at 64.8 °C. The stable gel gave sharp reflections at 1/4.66 and 1/4.06Å-1. DSC findings also showed that at least 30 mol % of α-GalpCer was intermixing and the lower fractionof β-GalpCer was miscible with DPPC in the gel state. The less ordered metastable gel of α-GalpCer wasassumed to be involved in the better miscibility with DPPC.
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