Destabilization of phosphatidylethanolamine-containing liposomes: hexagonal phase and asymmetric membranes.

We have measured the temperature of the L alpha-HII phase transition, TH, for several types of phosphatidylethanolamine (PE), their binary mixtures, and several PE/cholesteryl hemisuccinate (CHEMS) mixtures. We have shown for liposomes composed of pure PE and in mixtures with CHEMS that there is an aggregation-mediated destabilization which is greatly enhanced at and above TH. We now ask the question: How well can a dioleoylphosphatidylethanolamine/CHEMS liposome, for example, destabilize TPE (transesterified from egg phosphatidylcholine)/CHEMS liposome and vice versa? We use Ca2+ and H+ to induce aggregation and to provide different values of TH: the TH of the PE/CHEMS mixture is much lower at low pH than with Ca2+. We find that if the temperature is above the TH of one lipid mixture, e.g., A, and below the TH of the other lipid mixture, e.g., B, then the destabilization sequence [measured by the fluorescent 1-aminonaphthalene-3,6,8-trisulfonic acid/p-xylylenebis(pyridinium bromide) leakage assay] is AA greater than AB much greater than BB. That is, the bilayer of the lipid A (which on its own would end up in the HII phase) destabilizes itself better than it destabilizes the bilayer of lipid B (which on its own would remain in the L alpha phase). The BB contact is the least unstable. From these experiments, we conclude that the enhanced destabilization of membranes provided by the polymorphism accessible to these lipids above TH is effective even if only one of the apposed outer monolayers is HII phase competent. The surprising result is that if the temperature is above the TH of both lipid mixtures, then the destabilization sequence is AB greater than AA, BB. That is, the mixed bilayers are destabilized more by contact than either of the pure pairs. We believe that this is due to specific differences in the kinetics of aggregation or close approach of the membranes. Similar results were obtained with pure PE liposomes induced to aggregate by Ca2+ at pH 9.5. We also found that the kinetics of low-pH-induced leakage from PE/CHEMS liposomes were initially faster when the CHEMS on both sides of the bilayer is fully protonated. However, in a citrate buffer, which cannot cross intact membranes, the leakage was eventually faster. Flip-flop of the protonated CHEMS to the inner monolayer can explain this observation.