Biophysical and structural properties of DNA.diC(14)-amidine complexes. Influence of the DNA/lipid ratio

Cationic liposomes are used as vectors for gene delivery both in vitro and in vivo. Comprehension of both DNA/liposome interactions on a molecular level and a description of structural modifications involved, are prerequisites to an optimization of the transfection protocol and, thus, successful application in therapy. Formation and stability of a DNA/cationic liposome complex were investigated here at different DNA:lipid molar ratios (rho). Isothermal titration calorimetry (ITC) of cationic liposomes with plasmid DNA was used to characterize the DNA-lipid interaction. Two processes were shown to be involved in the complex formation. A fast exothermic process was attributed to the electrostatic binding of DNA to the liposome surface. A subsequent slower endothermic reaction is likely to be caused by the fusion of the two components and their rearrangement into a new structure. Fluorescence and differential scanning calorimetry confirmed this interpretation. A kinetic model analyzes the ITC profile in terms of DNA/cationic liposome interactions.