Aggregation, lipid exchange, and metastable phases of dimyristoylphosphatidylethanolamine vesicles

A new fluorescent lipid analogue, bimanephosphatidylcholine, has been synthesized for use in lipid bilayers. This probe is well suited as an energy-transfer donor with N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine as the acceptor. Dimyristoylphosphatidylethanolamine vesicles are prepared by sonication at pH 9 and characterized by electron microscopy and other methods. Resonance energy transfer between separately labeled donor and acceptor vesicles is monitored during HCl-induced aggregation to determine the kinetics of lipid randomization. Light scattering is also monitored to measure the kinetics of aggregation. The light scattering shows a marked reversal with NaOH while the energy transfer does not, indicating lipid exchange during a reversibly aggregated state; the extent of energy transfer suggests that only lipids in the outer monolayers exchange. The gel to liquid-crystalline phase transition temperature in HCl-treated vesicles is found to be 47 degrees C with diphenylhexatriene. The initial sonicated dispersion does not show a sharp phase transition. In vesicles labeled with both donor and acceptor probes, a small, irreversible increase in energy transfer is obtained upon lowering and then restoring the pH. These results suggest a metastable phase in the sonicated vesicles containing a randomized distribution of lipid and probes within the bilayers; the thermodynamically favored phase, whose formation is triggered by the pH shock, contains domains within which the probe lipids are more highly concentrated.