A two-state model for the multilamellar structure of a DNA/cationic lipid complex in the bulk.

Polyanionic DNA can bind electrostatically with cationic lipids to form a complex used for gene delivery and nanostructure construction. Here, we reveal two multilamellar phases, L(I) and L(II), characterized by distinct states of lipid packing and DNA conformation in a DNA/cationic lipid complex in the bulk state. The L(II) phase, formed when the lipids are in excess of DNA in terms of overall ionic charge, is composed of B-DNA confined between the bilayers with the lipid tails aligning normal to the lamellar interface. When DNA becomes in excess of the lipids, the L(I) phase in which the DNA is bound with the tilted lipid chains adopting the A conformation is favored because this configuration offers more economical electrostatic binding between these two components.