BILAYER NANOTUBES AND HELICAL RIBBONS FORMED BY HYDRATED GALACTOSYLCERAMIDES - ACYL-CHAIN AND HEADGROUP EFFECTS

The molecular basis of bilayer tubule formation in hydrated galactosyl ceramide (GalCer) dispersions has been investigated by synthesizing di fferent chain-pure GalCers and examining their aqueous mesomorphic pha se structure by freeze fracture and negative-stain electron microscopy . Thermotropic characterization of the GalCer species by differential scanning calorimetry provided supplementary information that verified the phase stale under which morphological observations were carried ou t. Under aqueous conditions and at room temperature, N-24:1(Delta 15(c is)) GalSph, the predominant monounsaturated, nonhydroxy acyl species of bovine brain GalCer (NFA-GalCer), formed cylindrical mesomorphic se lf-assemblies consisting almost exclusively of ''nanotubes,'' i.e., li pid bilayer tubules of relatively uniform length and diameter (length, 250-400 nm; diameter, 25-30 nm). In contrast, N-24:0 GalSph, the majo r saturated, nonhydroxy acyl species of bovine brain GalCer, displayed no tendency to form these relatively small ''nanotubes.'' Rather, N-2 4:0 GalSph formed larger, variable-length ribbon-like structures (leng th, 5,000-10,000 nm) that often appeared to undulate and, occasionally , appeared to be helically twisted. Interestingly, bovine brain GalCer , which contains high levels of the N-24:1(Delta 15(cis)) and N-24:0 s pecies as well as 2-hydroxy acyl chains, formed multilamellar liposome s of variable size and showed little tendency to form cylindrical stru ctures. This result suggested that changes to the polar interface/head group region imparted by the 2-hydroxy acyl species strongly influence d bilayer tubule and cylinder formation in GalCer. To define this infl uence more clearly, other sphingoid-based and glycerol-based lipids we re investigated. Morphological characterization of N-24:1(Delta 15(cis )) sphingosylphosphorylcholine (24:1 SM) revealed no evidence of bilay er cylinder or tubule formation. Similar results were obtained with aq ueous dispersions of 1-palmitoyl-2-nervonoyl phosphatidylcholine (16:0 , 24:1 PC). Hence, the bulkier, more hydrated, zwitterionic phosphocho line headgroup inhibited the formation of bilayer nanotubes and cylind ers under physiological saline conditions.