The structural organization of cationic lipid-DNA complexes

The interaction of cationic liposomes with supercoiled plasmid DNA results in a major rearrangement of each component to form compact multilamellar structures comprised of alternating layers of two-dimensional arrays of DNA sandwiched between lipid bilayers. Fluorescence resonance energy transfer was used to estimate the distance of closest approach of DNA to the lipid bilayers in these complexes. The effect of several compositional variables on this distance, including the ratio of cationic lipid to DNA, and the charge density, intrinsic curvature, and fluidity of the lipid bilayer were examined. Additionally, the effect of ionic strength was studied. For complexes prepared at or above a 3:1 charge ratio (+/-), the observed distance of closest approach was found to be in agreement with the intercalation of DNA between lipid bilayers. As the charge ratio was decreased, a monotonic increase in the distance was observed with a maximum observed at 0.5:1. Correlations between differences in the proximity of DNA to the lipid bilayer and the hydrodynamic size of the complexes were also found. A model based on these observations and previous reports suggests the formation of discrete populations of complexes below a charge ratio of 0.5:1 and above 3:1. The structure of the negatively charged complexes is consistent with DNA extending from the surface of the particles, whereas those possessing excess positive charge were multilamellar aggregates with the DNA effectively condensed between lipid bilayers. Complexes between these two states consist of weighted fractions of these two species.