Interaction of DNA with cationic vesicles: A calorimetric study

The interaction of DNA with vesicles of cationic lipids mixed with varying amounts of a zwitterionic lipid in dilute solutions was studied by isotherm al titration microcalorimetry, differential scanning calorimetry (DSC), and turbidity measurements. Extruded pure dimethyldioctadecylammonium bromide (DODAB) and chloride (DODAC) vesicles and mixed vesicles with various amoun ts of dioleoylphosphatidylethanolamine (DOPE) were investigated at 25 degre es C. The interaction of DNA with cationic vesicles is fast and endothermic . The enthalpy of reaction per mole of lipid added is constant with varying charge ratio between cationic lipid and DNA but decreases with increasing amounts of the zwitterionic lipid in the vesicle. The enthalpy change is co nstant up to a critical charge ratio rho+/- of about 0.8 for the addition o f lipid vesicles to DNA and 1.2 for the addition of DNA to lipid vesicles. The enthalpy of reaction decreases with the replacement of Br- by Cl- ion a nd is independent of cationic vesicle size in the range 50-200 nm. The effe ct of DOPE on the thermotropic behavior of DODAB in aqueous dispersions wit h and without DNA was investigated using DSC. Inclusion of DOPE in the DODA B cationic vesicle increases the bilayer fluidity, e.g., decreases the melt ing temperature, T-m. The status of the hydrocarbon chains, solid-like or f luid, has a pronounced effect on the measured enthalpy changes and most str ongly for the formation of positively charged, lipid-enriched aggregates.