Bilayer Properties of Totally Synthetic C16:0-Lactosyl-Ceramide

Abstract

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°C, fully hydrated C16:0-LacCer shows complex thermal transitions characteristic of polymorphic behavior of exclusively bilayer phases. On heating at 5°C/min, hydrated C16:0-LacCer undergoes a complex two-peak endothermic transition with maxima at 69°C and 74°C and a total enthalpy of 14.6 kcal/mol C16:0-LacCer. At a slower heating rate (1.5°C/min), two endothermic transitions are observed at 66°C and 78°C. After cooling to 0°C, the subsequent heating run shows three overlapping endothermic transitions at 66°C, 69°C, and 71.5°C, followed by a chain-melting endothermic transition at 78°C. Two thermal protocols were used to completely convert C16:0-LacCer to its stable, high melting temperature (78°C) form. As revealed by x-ray diffraction, over the temperature range 20–78°C this stable phase exhibits a bilayer structure, periodicity d ≈ 65 Å with an ordered chain packing mode. At the phase transition (78°C) chain melting occurs, and C16:0-LacCer converts to a liquid crystalline bilayer (L α ) phase of reduced periodicity d ≈ 59 Å. On cooling from the L α phase, C16:0-LacCer converts to metastable bilayer phases undergoing transitions at 66–72°C. These studies allow comparisons to be made with the behavior of the corresponding C16:0-Cer (Shah et al., 1995. J. Lipid Res. 36:1936–1944) and C16:0-GluCer and C16:0-GalCer (Saxena et al., 1999. J. Lipid Res. 40:839–849). Our systematic studies are aimed at understanding the role of oligosaccharide complexity in regulating glycosphingolipid structure and properties.