Anomalous mixing of zwitterionic and anionic phospholipids with double-chain cationic amphiphiles in lipid bilayers.

High-sensitivity scanning calorimetry has been used to examine the thermotropic behavior of mixtures combining dipalmitoylphosphatidylcholine (DPPC), phosphatidylethanolamine (DPPE) and O-methylphosphatidic acid (DPPA-OMe) with the double-chain cationic amphiphiles N,N-dihexadecyl-N,N- dimethylammonium chloride (DHDAC), 1,2-dipalmitoyloxy-3-(trimethylammonio)propane (DPTAP) and the corresponding monomethylated tertiary amino compounds (DHMMA-H+ and DPDAP-H+). At physiological ionic strength, mixtures of these cationic amphiphiles with the anionic phospholipid DPPA-OMe can show gel-to-liquid-crystalline phase transitions at considerably higher temperatures than do either of the pure components. Surprisingly, binary mixtures of DPPC and these cationic amphiphiles also show strongly nonideal mixing, with phase diagrams exhibiting pronounced maxima in their solidus and liquidus curves. Similar behavior is not observed for mixtures of DPPC with DPPA-OMe, which closely resembles DPTAP and DPDAP-H+ in backbone configuration and polar headgroup size. The present results suggest that perturbation of the orientation of the phosphatidylcholine headgroup by cationic amphiphiles, as demonstrated previously by Seelig and co-workers (Biochemistry 28 [1989], 7720-7728), can significantly affect the thermotropic behavior of phospholipids such as DPPC. Such effects may exert a generally important (though not always easily recognizable) influence on the organization and thermotropic behavior of systems where zwitterionic phospholipids are combined with charged bilayer-associated molecules.