Structural studies of lipid fibers formed by sphingosine

Abstract

The natural product d- erythro-sphingosine and synthetic racemic dihydrosphingosines were examined for their abilities to self-assemble into high-axial-ratio microstructures. When precipitated from methanol/water solution, d- erythro-sphingosine formed a viscoelastic gel composed of 50-nm diameter flexible fibers. These are ‘cochleate cylinders’ composed of rolls of lamellae. Compared to the biological sphingosine, the dl- erythro- and dl- threo-dihydrosphingosines are much less soluble in methanol/water mixtures. When recrystallized from methanol/water the dihydrosphingosines tend to form irregular lamellar structures or platelets. When higher proportions of methanol are used in the recrystallization solvent, needle-like structures predominate in the dl- erythro-dihydrosphingosine sample, but not in the dl- threo-dihydrosphingosine samples. The needles are mostly very long and narrow crystal platelets often with fracture defects parallel to the long axes. Some curved fiber-like structures are also seen. These results suggest that in comparison to the threo diastereomer, the erythro diastereomer of dihydrosphingosine displays a large differential in intermolecular bonding strengths between divergent orientations within the lamellar sheet. Energy-minimized molecular models indicate that, compared to the threo isomers, intramolecular bonding could bend the erythro headgroup farther toward the hydrocarbon interface of a lamellar microstructure. Moreover, this work illustrates how the erythro headgroup could support a linear pattern of intermolecular hydrogen bonds while the threo could support a two-dimensional network. d- erythro-sphingosine probably displays a similar intermolecular bonding pattern, but as it is optically pure, the molecular packing results in a consistent twist to the neighboring molecules and this is expressed as the bending of the lamellar sheet into a cochleate. The fiber-forming ability of d- erythro-sphingosine may have implications for the reactive and structural properties of biological sphingolipids as well as the design of novel materials based on synthetic high-axial-ratio lipid superstructures.