Characterizing the structural transition of cationic DPPC liposomes from the approach of TEM, SAXS and AFM measurements

The structural transition of l-α-dipalmitoylphosphatidylcholine (DPPC) liposomes, caused by the addition of a small amount of stearylamine (SA), has been characterized. It has been reported that the shape of DPPC liposomes changes from multilamellar vesicles to large-unilamellar vesicles at the molar concentration ratio of DPPC/SA = 9.5/0.5, however, the possible diving factors for this phenomenon have not so far been presented. Flat lipid membranes consisting of DPPC and SA, prepared by the quasi-Bangham method or the Langmuir–Blodgett (LB) technique, are employed in this study when considering the molecular interaction in and between lipid membranes, which should play a key role for determining the liposome shape. The colloid probe atomic force microscopy reveals that the addition of SA results in an inter-film electrosteric repulsion. This repulsive interaction causes a significant increase in the inter-film distance, which is confirmed with freeze-fracture transmission electron microscopy (FF-TEM) and small-angle X-ray scattering (SAXS), and thereby, the large-unilamellar vesicles are formed for reducing the inter- and intra-firm repulsive forces. Taking the molecular structures into consideration, it seems that the shape transition of DPPC liposomes results from such electrostatic interactions as well as packing geometry of the two components.