Structure and properties of totally synthetic galacto- and gluco-cerebrosides

The structural and thermal properties of aqueous dispersions of the totally synthetic cerebrosides, D-erythro-N-palmitoyl galactosyl- and glucosyl-C-1 8-sphingosine (C16:0-GalCer and C16:0-GluCer, respectively) have been studi ed using differential scanning calorimetry (DSC) and X-ray diffraction. Ove r the temperature range 0-100 degrees C, both C16:0-GalCer and C16:0-GluCer show complex thermal transitions characteristic of polymorphic behavior of exclusively bilayer phases. On heating, hydrated C16:0-GalCer undergoes an exothermic bilayer-bilayer transition at 59 degrees C to produce a stable bilayer crystal form. X-ray diffraction at 70 degrees C reveals a bilayer s tructure with an ordered hydrocarbon chain-packing arrangement, This ordere d bilayer phase undergoes an endothermic chain-melting transition at 85 deg rees C to the bilayer liquid crystalline state. Similar behavior is exhibit ed by hydrated C16:0-GluCer which undergoes the exothermic transition at 49 degrees C and a chain-melting transition at 87 degrees C, The exothermic t ransitions observed on heating hydrated C16:0-GalCer and C16:0-GluCer are i rreversible and dependent upon previous chain melting, prior cooling rate, and time of incubation at low temperatures. Thus, the structure and propert ies of totally synthetic C16:0-GalCer and C16:0-GluCer with identical sphin gosine (C18:1) and fatty acid (C16:0) chains are quite similar, suggesting that the precise isomeric structure of the linked sugar plays only a minor role in regulating the properties of hydrated cerebrosides, Further, these studies indicate that the complex thermal behavior and bilayer phase format ion exhibited by these single-sugar cerebrosides are intrinsic properties a nd not due to the heterogeneity of the sphingosine base found in natural an d partially synthetic cerebrosides.