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

Recent synchrotron-based X-ray diffraction studies have enabled us to compr ehensively solve the self-assembled structures in mixtures of cationic lipo somes (CLs) complexed with linear X-DNA. In one case the lambda-DNA complex es were found to consist of a higher ordered multilamellar structure (label ed 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 se cond self-assembled structure (labeled H-II(C)) consists of linear DNA stra nds coated by cationic lipid monolayers and arranged on a 2D hexagonal latt ice. In this paper we report on a combined X-ray diffraction and optical mi croscopy study of CLs complexed with functional supercoiled plasmid DNA. We describe the self-assembled structures in eel culture medium for both a hi gh transfectant complex (DOTAP/DOPE, Phi(DOPE) = 0.72) and a low transfecta nt complex (DOTAP/DOPC, Phi(DOPC) = 0.72). Fluorescence optica micros copy shows two distinct interactions between these two types of complexes and mo use fibroblast L-cells, demonstrating the existence of a correlation betwee n structure and transfection efficiency.