Packing characteristics of a model system mimicking cytoplasmic bacterial membranes

The phase diagram of fully hydrated mixtures of dipalmitoylphosphatidylethanolamine and -phosphatidylglycerol was constructed and the coexistence lines of the solidus and liquidus curve calculated based on regular solution theory using two nonideality parameters for each of the phase to account for nonideal and nonsymmetric mixing. Both lipids show nonideal miscibility in the liquid-crystalline phase, while a region of immiscibility exists in the lamellar-gel phase between the mole fraction x(DPPE)=0.05-0.4. Two lines of three-phase coexistence around 35 and 40 degrees C reflects the presence of lipid domains predominantly composed of phosphatidylglycerol as well as of the mixed lipid system. This is reflected in the positive nonideality parameters of the gel phase obtained from the simulation of the phase diagram. Moreover, segregation of pure phosphatidylethanolamine domains was detected in mixtures x(DPPE)>0.9, which formed multilamellar liposomes, while unilamellarity was observed for the mixed lipid systems owing to the presence of the negatively charged phosphatidylglycerol. The packing constraints of these phospholipids, major components of cytoplasmic bacterial membranes, may be of importance in the interaction with various solutes like antimicrobial peptides, and were explained based on the nature of the headgroups and the molecular geometry of the phospholipids.