Complexes formed during interaction of cationic liposomes with poly-nucleotides such as DNA (CLDC) display a variety of polymorphic and metastable structures. These include multilamellar structures of alternating lipid bilayers and DNA monolayers; fibrillar structures, among them spaghetti-liketubules (Figure 1), and map-pin-structures(Figure 2), and, finally, non-bilayer lipid arrangements, such as hexagonal (HII) (Figure 3) and cubic phases.
In order to find out the “active” structure(s) in terms of transfection, we investigated the transfection activity both in vivoand in vitroof CLDC composed of the lipid DDAB (dimethyl-dioctadecylammonium bromide) and Choi (cholesterol) or DOPE (l,2-dioleoyl-sn-glycerol-3- phosphoethanolamine) as helper lipids. In parallel we studied their morphology by freeze-fracture electron microscopy.The in vivostudies were carried out in mice following i.v. injection and therefore the morphology of the CLDC was investigated in mouse serum.