SMALL-ANGLE X-RAY-SCATTERING - POSSIBILITIES AND LIMITATIONS IN CHARACTERIZATION OF VESICLES

The use of small angle X-ray scattering (SAXS) for characterization of lipid vesicle dispersions is described. The effect of curvature of th e membrane, the presence of proteins in the core and on the surface of the membrane, variations in membrane thickness and distribution in th e number of bilayers of the vesicles in the dispersion on the scatteri ng curve is discussed. Concerning unilamellar vesicles, either the mem brane curvature of vesicles smaller than 50 nm or variations in membra ne thickness result in a disappearance of the first node in the scatte ring curve, even if the bilayer is symmetric with respect to the elect ron density distribution. In the case of dispersion in which unilamell ar as well as multilamellar vesicles are present it is shown that a sm all fraction of multilamellar liposomes changes the scattering curve d ramatically. Liposomes were prepared from various compositions of dipa lmitoylphosphatidylcholine (DPPC) and cholesterol hemisuccinate (CHEMS ) by the film method. The electron density profile of the bilayers and distribution in the number of bilayers of the liposome dispersions we re determined. The average number of bilayers increased as a function of the decrease in CHEMS content. Liposomes with higher CHEMS content than 10 mol % were unilamellar. It seems that increase in charge inter calated in the bilayers resulted in unilamellar vesicles.