Galactosylceramides (GalCers) containing nervonoyl (24:1(Δ15(cis)) acyl chains have the capacity to assemble into nanotubular microstuctures in excess water (Kulkarni et al., 1995. Biophys. J. 69:1976-1986). To define the structural parameters that modulate nanotube formation, GalCer derivatives were synthesized that contained cis monounsaturated acyl chains with the formula X:1(X-9). X indicates the total acyl carbon number (24, 22, 20, or 18), and 1 indicates a single cis double bond, the location of which is designated by the superscript (X-9). Deep etching of freeze-fractured 24:1Δ15(cis)) GalCer dispersions followed by replica production and transmission electron microscopic analysis confirmed nanotube morphology (25- 30-nm diameter). Control experiments revealed that tubule formation was promoted by cooling through the main enthalpic phase transition coupled with repetitive freeze-thaw cycling. Imparting a negative charge to the sugar headgroup of 24:1(Δ15)GalCer via sulfate mesomorphology and resulted in myelinic-like, multilamellar structures. Removal of the sugar headgroup (24:1(Δ15Cer) resulted in flattened cylindrical structures with a cochleate appearance. Compared to these large-scale changes in morphology, more subtle changes were induced by structural changes in the acyl chain of 24:1(Δ15)GalCer. 22:1(Δ13)GalCer dispersions consisted of long, smooth tubules (35-40-nm diameters) with a strong tendency to self-align into bundle-like aggregates. In contrast, the microstructures formed by 20:1(Δ11)GalCer resembled helical ribbons with a right-handed twist. Ribbon widths averaged 30-35 nm, with helical pitches of 80-90 nm. 18:1(Δ9)GalCer displayed a variety of morphologies, including large-diameter multilamellar cylinders and liposome-like structures, as well as stacked, plate-like arrays. The results are discussed within the context of current theories of lipid tubule formation.
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The authors gratefully acknowledge the major support provided by U.S. Public Health Service grant GM45928 and the Hormel Foundation.