Anomeric effects on the stability of bilayers of galactosylphytoceramides and on the interaction with phospholipids

The thermotropic behavior of the hydrated bilayers formed by novel glycosylceramides containing an anomeric galactose, a saturated long acyl chain (cerotoyl, C₂₆), and a phytosphingosine base, α- and β-GapCer, 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 liquid crystalline 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 was remarkably stable in contrast with β-GalpCer or the typical animal β-galactosylceramide containing a long acyl chain. Annealing at 37°C for 48 h completely converted the metastable gel of α-GalpCer into a stable gel, which was directly transformed to a liquid crystal (LC) at 74.9°C with a 2-fold larger endothermic peak (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 crystallinity than the metastable gel. However, the metastable gel of β-GalpCer was rapidly converted to the stable gel: 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 and of the metastable gel to the stable gel at 64.8°C. The stable gel gave sharp reflections at 1/4.66 and 1/4.06 angstrom^-1. DSC findings also showed that at least 30 mol% of α-GalphCer was intermixing and the lower fraction of β-GalpCer was miscible with DPPC in the gel state. The less ordered metastable gel of α-GalpCer was assumed to be involved in the better miscibility with DPPC.