A CRYO-TEM STUDY OF THE COMPLEXATION OF DOTAP LIPOSOMES WITH DIFFERENT POLYELECTROLYTES

Self-assembly is the spontaneous and thermodynamically driven organization of molecules into multimolecular structures, and a key process in forming biological materials. The interaction between amphiphiles and polyelectrolytes has been widely investigated in recent years due to their potential application in industry and medicine, with special focus on gene therapy.

Accordingly, we investigated the interactions of liposomes (vesicles with a lipid bilayer as a model system for biological membranes) with selected polyelectrolytes and proteins. The cationic lipid DOTAP (1,2-dioleoyl-3-trimethylammonium-propane (chloride salt)) was mixed with different anionic polyelectrolytes (PE), such as NaPA (sodium poly-acrylate), CMC (sodium carboxymethyl cellulose) with different degrees of substitution (DS, namely, different charge density) and DNA (deoxyribonucleic acid sodium salt). The goal of this project was to explore the influence of different system parameters, such as the charge ratio CR = [-]/[+] = [PE]/[DOTAP], the charge density of the PE or the type of PE on the morphology of the formed complexes. The investigation of these systems was done by cryo-transmission electron microscopy (cryo-TEM), with additional small-angle X-ray (SAXS) measurements, to support our findings.

In our experiments we obtained a comprehensive picture of the formed complexes, and how their detailed structure depends on the different properties of the employed polyelectrolyte (Figure 1). Although the basic nanostructures of the complexes are always lamellar, their detailed morphology depends strongly on parameters like the persistence length, charge density, or polymer backbone diameter.

These aggregates serve as a model system for lipoplexes as they might be used in gene therapy systems, where stable complexes with tunable properties are needed for optimized delivery.

Figure 1. Cryo-TEM images of 1 mM DOTAP mixed with different polyelectrolytes. Left: CMC1.2, middle: DNA, and right: NaPA; all at CR=2. All samples form multilayered complexes, but the overall size of the complexes, as well as the order and the spacing of the multilayers depend on the polyelectrolyte. Scale bars are 100 nm.