A comparative negative staining study of aqueous suspensions of sphingomyelin

Abstract

Bovine brain sphingomyelin liposomes have been studied by TEM when negatively stained at 4, 22 and 60δC with uranyl acetate,sodium phosphotungstate and amonium molybdate. The liposomes images vary slightly in the different negative stains, yet overall agreement exists once the different permeability and ionic propreties of the stains and the orietation of the liposomes are taken into account. Sodium phosphotungstate possesses an undesirable aggregative action on sphingomylin liposomes, but no aggregation has been encountered with the other stains. The liposomes images at 4 and 22δC are very similar, since both temperatures are beneath the crystalline-liqiud crystalline phase transition temperature ( Tc). At 60δC, wich is above the Tc for sphingomyelin, the liposomal particles appear to be much more flexible and accordingly present a more varied shape than at the lower temperatured. The overall conformation of the sphingomyelin liposome is thought to be a ca 50 nm flattened single bilayer vesicle. Nevertheless, data are presented which suggest that some single bilayer disc-like micelles of sphingomyelin are also present. The larger multi-lamellar particular structures or myelion bodies, which are present when solid sphingomyelin is simply disperesed in water, have been studied by negative staining at 22 and 60δC. At 22δC, sphingomylin myelin bodies contain bilayers which exhibit the 26.5 nm undulatory P⨿β′pre-transition phase. The periodic feature is revealed particularly clearly by uranyl acetate. Considerable image complexity is usually present, because of overlapping information from more than one bilayer. Myelin bodies are occasionally split open during the negative staining and the single bilayer regions then reveal the image periodicity with superior clarity. Ammonium molybdate reveals the Pβ′ pre-transition phase undulations rather faintly owing to the permeation of this across the phospholipid bilayers. Sodium phosphotungstate has not been found to reveal this structural feature of the phospholipid bilayer. Ar 22δC some liposomes from spontaneously from the larger lipid bodies, but at 60δC their vesicularization occurs very much more rapidly.