Modulation of nanotube formation by structural modifications of sphingolipids

Galactosylceramides (GalCers) containing nervonoyl (24:1(Delta 15(cis))) ac yl chains have the capacity to assemble into nanotubular microstructures in excess water (Kulkarni et al., 1995. Biophys. J. 69:1976-1986). To define the structural parameters that modulate nanotube formation, GalCer derivati ves were synthesized that contained cis monounsaturated acyl chains with th e 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 24:1 (Delta 15(cis)) GalCer dispersions followed by replica production and trans mission electron microscopic analysis confirmed nanotube morphology (25-30- nm diameter). Control experiments revealed that tubule formation was promot ed by cooling through the main enthalpic phase transition coupled with repe titive freeze-thaw cycling. Imparting a negative charge to the sugar headgr oup of 24:1(Delta 15)GalCer via sulfate dramatically altered mesomorpholgy and resulted in myelinic-like, multilamellar structures. Removal of the sug ar headgroup (24:1(Delta 15)Cer) resulted in flattened cylindrical structur es with a cochleate appearance. Compared to these large-scale changes in mo rphology, more subtle changes were induced by structural changes in the acy l chain of 24:1(Delta 15)GalCer. 22:1(Delta 13)GalCer dispersions consisted of long; smooth tubules (35-40-nm diameters) with a strong tendency to sel f-align into bundle-like aggregates. In contrast, the microstructures forme d by 20:1(Delta 11)GalCer resembled helical ribbons with a right-handed twi st. Ribbon widths averaged 30-35 nm, with helical pitches of 80-90 nm. 18:1 (Delta 9)GalCer displayed a variety of morphologies, including large-diamet er multilamellar cylinders and liposome-like structures, as well as stacked , plate-like arrays. The results are discussed within the context of curren t theories of lipid tubule formation.