Phase behavior and aggregate structure in mixtures of dioleoylphosphatidylethanolamine and poly(ethylene glycol)-lipids.

Cryo-transmission electron microscopy has been used to investigate the phase behavior and aggregate structure in dilute aqueous mixtures of dioleoylphosphatidylethanolamine (DOPE) and poly(ethylene glycol)-phospholipids (PEG-lipids). It is shown that PEG-lipids (micelle-forming lipids) induce a lamellar phase in mixtures with DOPE (inverted hexagonal forming lipid). The amount of PEG-lipid that is needed to induce a pure dispersed lamellar phase, at physiological conditions, depends on the size of the PEG headgroup. In the transition region between the inverted hexagonal phase and the lamellar phase, particles with dense inner textures are formed. It is proposed that these aggregates constitute dispersed cubic phase particles. Above bilayer saturating concentration of PEG-lipid, small disks and spherical micelles are formed. The stability of DOPE/PEG-lipid liposomes, prepared at high pH, against a rapid drop of the pH was also investigated. It is shown that the density of PEG-lipid in the membrane, sufficient to prevent liposome aggregation and subsequent phase transition, depends on the size of the PEG headgroup. Below a certain density of PEG-lipid, aggregation and phase transition occurs, but the processes involved proceed relatively slow, over the time scale of weeks. This allows detailed studies of the aggregate structure during membrane fusion.