Thermotropic phase behavior of cationic lipid-DNA complexes compared to binary lipid mixtures

The thermotropic phase behavior of zwitterionic/cationic binary lipid mixtures is investigated and compared to its corresponding lipidic phase diagram of mixtures complexed with DNA. We focus on isoelectric cationic lipid-DNA condensates where the number of cationic lipids equals the number of phosphate groups on the DNA. Using differential scanning calorimetry, X-ray scattering, freeze fracture electron microscopy, and film balance, we studied mixtures of di-myristoyl-phosphatidyl-choline (DMPC) and the cationic lipid, dimyristoyl-tri-methyl-ammonium-propane (DMTAP). The lipid phase diagram shows the well-known L-alpha, L-beta' and P-beta' ripple phase with peritectic behavior at a low molar fraction of cationic lipid, chi(TAP) < 0.12. Beyond chi(TAP) = 0.8 crystalline phases appear. A systematic variation in the hydrocarbon chain tilt in the prevailing L-beta' phase is measured by wide-angle X-ray scattering. Most importantly, the L-beta' phase shows strong nonideal mixing with an azeotropic point at about 1:1 molar stoichiometry. This finding is related to the reduced headgroup area for equimolar mixtures found in monolayer pressure-area isotherms. The intercalation of DNA in cationic lipid-DNA complexes affects the lipid-phase behavior 2-fold: (i) the chain-melting transition temperature shifts to higher temperatures and (ii) a demixing gap with coexistence of lipid vesicles and lipid-DNA complexes arises at a low cationic fraction, chi(TAP) < 0.25. In agreement with experiments we present a thermodynamic model that describes the shift of the melting transition temperatures by DNA-induced electrostatic screening of the cationic membrane.