Bilayer properties of totally synthetic C16:0-lactosyl-ceramide.

Differential scanning calorimetry (DSC) and x-ray diffraction have been used to study the structural and thermal properties of totally synthetic D-erythro-N-palmitoyl-lactosyl-C(18)-sphingosine (C16:0-LacCer). Over the temperature range 0-90 degrees C, fully hydrated C16:0-LacCer shows complex thermal transitions characteristic of polymorphic behavior of exclusively bilayer phases. On heating at 5 degrees C/min, hydrated C16:0-LacCer undergoes a complex two-peak endothermic transition with maxima at 69 degrees C and 74 degrees C and a total enthalpy of 14.6 kcal/mol C16:0-LacCer. At a slower heating rate (1.5 degrees C/min), two endothermic transitions are observed at 66 degrees C and 78 degrees C. After cooling to 0 degrees C, the subsequent heating run shows three overlapping endothermic transitions at 66 degrees C, 69 degrees C, and 71.5 degrees C, followed by a chain-melting endothermic transition at 78 degrees C. Two thermal protocols were used to completely convert C16:0-LacCer to its stable, high melting temperature (78 degrees C) form. As revealed by x-ray diffraction, over the temperature range 20-78 degrees C this stable phase exhibits a bilayer structure, periodicity d approximately 65 A with an ordered chain packing mode. At the phase transition (78 degrees C) chain melting occurs, and C16:0-LacCer converts to a liquid crystalline bilayer (L(alpha)) phase of reduced periodicity d approximately 59 A. On cooling from the L(alpha) phase, C16:0-LacCer converts to metastable bilayer phases undergoing transitions at 66-72 degrees C. These studies allow comparisons to be made with the behavior of the corresponding C16:0-Cer (. J. Lipid Res. 36:1936-1944) and C16:0-GluCer and C16:0-GalCer (. J. Lipid Res. 40:839-849). Our systematic studies are aimed at understanding the role of oligosaccharide complexity in regulating glycosphingolipid structure and properties.