BIOPHYSICAL CHARACTERIZATION OF CATIONIC LIPID-DNA COMPLEXES

To better understand the structures formed by the interaction of catio nic lipids with DNA, we undertook a systematic analysis to determine t he biophysical characteristics of cationic lipid:DNA complexes. Four m odel cationic lipids with different net cationic charge were found to interact in similar ways with DNA when that interaction was compared i n terms of the apparent molar charge ratio of lipid to DNA. When DNA w as present in charge excess over the cationic lipid, the complex carri ed a net negative charge as determined by zeta potential measurements. Under these conditions, some DNA was accessible to ethidium bromide, and free DNA was observed in agarose gels and in dextran density gradi ents. Between a lipid:DNA charge ratio of 1.25 and 1.5:1, all the DNA became complexed to cationic lipid, as evidenced by its inaccessibilit y to EtBr and its complete association with lipid upon agarose gel ele ctrophoresis and density gradient separations. These complexes carried a net positive charge. The transition between negatively and positive ly charged complexes a occurred over a very small range of lipid to DN A ratios. Employing a fluorescent lipid probe, the addition of DNA was shown to induce lipid mixing between cationic lipid-containing vesicl es. The extent of DNA-induced lipid mixing reached a maximum at a char ge ratio of about 1.5:1, the point at which all the DNA was involved i n a complex and the complex became positively charged. Together with f reeze-fracture electron micrographs of the complexes, these biophysica l data have been interpreted in light of the existing models of cation ic lipid:DNA complexes.