Morphology of lipid micelles containing lysolecithin

Mixtures of lysolecithin with various phospholipids were studied by electron microscopy using negative staining. Mixtures of dipalmitoyllecithin and lysolecithin produced disc-shaped structures which were stacked in aggregates with a 6.0--6.4 nm repeat. The disc were 10--50 nm in diameter. The disc-shaped structures were best observed in equimolar mixtures of dipalmitoyllecithin and lysolecithin. When dipalmitoyllecithin was replaced by dimyristoyllecithin, the structures were rather different from those observed in the system containing dipalmitoyllecithin; a cylindrical micellar phase was predominant. Equimolar mixtures of egg lecithin and lysolecithin formed the more usual smectic, concentric lamellae (liposomes) and elongated rod-like micelles which might be bimolecular fragments of spherules. The radius of the rod-like micelles was about 6 nm. Structures of rod-like micelles were observed more frequently in the samples after incubation at room temperature and then further incubation at 0 degrees C. Equimolar mixtures of didecanoyllecithin and lysolecithin produced large amounts of elongated rod-like micelles. Beef brain sphingoymyelin showed disc-shaped structures when mixed with lysolecithin. Incorporation of cholesterol into the mixtures of dipalmitoyllecithin and lysolecithin changed the morphological structure; the size of the disc became larger and eventually liposomes were formed with an increase of cholesterol content. The structures observed in mixtures of dipalmitoyllecithin or sphingomyelin and lysolecithin closely resembled those observed in complexes of apolipoprotein and lipid.