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 glycosyl ceramides containing an anomeric galactose, a saturated long acyl chain (ce rotoyl, C-26), and a phytosphingosine base, alpha- and beta-GalpCer, was in vestigated by means of differential scanning calorimetry (DSC) and X-ray di ffraction measurements. DSC curves for hydrated alpha-GalpCer showed an end othermic transition from metastable gel to liquid crystalline states at 72. 8 degrees C (Delta H = 12.0 kcal/mol) and exothermic transition of the reve rse change at 66.3 degrees C (Delta H = -10.9 kcal/mol) at the heating/cool ing rate of 1 degrees C/min. The metastable gel of alpha-GalpCer was remark ably stable in contrast with beta-GalpCer or the typical animal beta-galact osylceramide containing a long acyl chain. Annealing at 37 degrees C for 48 h completely converted the metastable gel of alpha-GalpCer into a stable g el, which was directly transformed to a liquid crystal (LC) at 74.9 degrees C with a a-fold larger endothermic peak (Delta H =-23.8 kcal/mol). Cooling of the LC state brought about the transition into the metastable gel. X-ra y diffraction measurements showed that both metastable and stable gels were lamellar structures. The diffraction patterns also suggested that the stab le gel had a bilayer arrangement of higher crystallinity than the metastabl e gel. However, the metastable gel of beta-GalpCer was rapidly converted to the stable gel: Upon heating, the stable gel was directly transformed to t he LC state at 75.8 degrees C(Delta H = 19.0 kcal/mol), and cooling of the LC state led to successive transitions of the LC to the metastable gel at 7 1.0 degrees C and of the metastable gel to the stable gel at 64.8 degrees 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 alpha-GalpCer was interm ixing and the lower fraction of beta-GalpCer was miscible with DPPC in the gel state. The less ordered metastable gel of alpha-GalpCer was assumed to be involved in the better miscibility with DPPC.