Structure and structure-function studies of lipid/plasmid DNA complexes

Recent synchrotron-based X-ray diffraction studies have enabled us to comprehensively solve the self-assembled structures in mixtures of cationic liposomes (CLs) complexed with linear X-DNA. In one case the lambda-DNA complexes were found to consist of a higher ordered multilamellar structure (labeled L-alpha(C)) with DNA sandwiched between cationic bilayer membranes. The membrane charge density is found to control the DNA interaxial spacing with high densities leading to high DNA compaction between lipid bilayers. A second self-assembled structure (labeled H-II(C)) consists of linear DNA strands coated by cationic lipid monolayers and arranged on a 2D hexagonal lattice. In this paper we report on a combined X-ray diffraction and optical microscopy study of CLs complexed with functional supercoiled plasmid DNA. We describe the self-assembled structures in eel culture medium for both a high transfectant complex (DOTAP/DOPE, Phi(DOPE) = 0.72) and a low transfectant complex (DOTAP/DOPC, Phi(DOPC) = 0.72). Fluorescence optica micros copy shows two distinct interactions between these two types of complexes and mouse fibroblast L-cells, demonstrating the existence of a correlation between structure and transfection efficiency.