Unilamellar vesicle-forming property of N-nervonoylsphingomyelin (C24:1-SM) as studied by differential scanning calorimetry and negative stain electron microscopy

Focusing on an amide-linked fatty acid composition of naturally occurring sphingomyelins (SMs), N-palmitoylSM (C16:0-SM), N-stearoylSM (C18:0-SM), and N-nervonoylSM (C24:1-SM) were partially synthesized, respectively. The effect of C24:1-SM on the thermotropic behavior of respective vesicles of the C16:0-, and C18:0-SMs was investigated by a differential scanning microcalorimetry (DSC), and the structure of these semisynthetic SM vesicles was examined by a negative stain electron microscopy. Two vesicles of the saturated C16:0-SM and C18:0-SM showed sharp transition peaks at temperatures of 39.6 and 43.7 °C, respectively, in contrast with a broad and low-temperature transition peak observed for the unsaturated C24:1-SM vesicle. An electron micrograph of the C24:1-SM showed unilamellar vesicles of relatively small size (100-500 nm in diameter), in contrast to large multilamellar vesicles up to 3.5 μm in diameter observed for the C16:0-, and C18:0-SMs. This result indicated a unilamellar vesicle-forming property for the C24:1-SM. In this accord, a micrograph of sonicated vesicles of the C24:1-SM also showed unilamellar vesicles of regular size (∼100 nm), suggesting that the unilamellar vesicle is a thermodynamically stable state for the unsaturated C24:1-SM. Furthermore, when the C24:1-SM was added to the respective vesicles of C16:0-, and C18:0-SMs, both a broadening and a temperature-lowering for the transition peaks of these vesicles proceeded more and more with increasing the content of C24:1-SM. This was accounted for by a decrease in the multiplicity of the saturated SM vesicles caused by the unilamellar vesicle-forming property of the unsaturated SM.