Liposomes, Disks, and Spherical Micelles: Aggregate Structure in Mixtures of Gel Phase Phosphatidylcholines and Poly(Ethylene Glycol)-Phospholipids

Abstract

Poly(ethylene glycol) (PEG) decorated lipid bilayers are widely used in biomembrane and pharmaceutical research. The success of PEG-lipid stabilized liposomes in drug delivery is one of the key factors for the interest in these polymer/lipid systems. From a more fundamental point of view, it is essential to understand the effect of the surface grafted polymers on the physical-chemical properties of the lipid bilayer. Herein we have used cryo-transmission electron microscopy and dynamic light scattering to characterize the aggregate structure and phase behavior of mixtures of PEG-lipids and distearoylphosphatidylcholine or dipalmitoylphosphatidylcholine. The PEG-lipids contain PEG of molecular weight 2000 or 5000. We show that the transition from a dispersed lamellar phase (liposomes) to a micellar phase consisting of small spherical micelles occurs via the formation of small discoidal micelles. The onset of disk formation already takes place at low PEG-lipid concentrations (<5 mol %) and the size of the disks decreases as more PEG-lipid is added to the lipid mixture. We show that the results from cryo-transmission electron microscopy correlate well with those obtained from dynamic light scattering and that the disks are well described by an ideal disk model. Increasing the temperature, from 25 degrees C to above the gel-to-liquid crystalline phase transition temperature for the respective lipid mixtures, has a relatively small effect on the aggregate structure.

Influence of preparation path on the formation of discs and threadlike micelles in DSPE-PEG2000/lipid systems

“…From previous studies it appears that components, as well as environmental conditions, that increase the monolayer bending modulus and decrease the spontaneous curvature of the mixture promote the formation of discs [12]. As discussed before [10,12] formation of the discoidal micelles requires partial component segregation. For curvature reasons the PEG-lipids accumulate at the hemispherical disc edge whereas the bilayer forming lipids reside preferentially in the flat part of the disc.…”

“…Their presence, however, may seriously influence the aggregate structure. High concentrations of PEG-lipids will, in most phospholipid/PEG-lipid mixtures, induce the formation of mixed spherical micelles [4][5][6][7][8][9][10][11]. Previous studies have shown that two fundamentally different types of mixed micellar aggregates may form at intermediate PEG-lipid to lipid ratios.…”

“…Previous studies have shown that two fundamentally different types of mixed micellar aggregates may form at intermediate PEG-lipid to lipid ratios. Depending on the characteristics of the lipid mixture the PEG-lipids induce formation of either long threadlike or discoidal micelles [7,10,12,13]. Threadlike micelles are, for example, commonly observed in PEG-lipid/lipid samples where the lipid is in the liquid crystalline phase, while discs are observed in such mixtures supplemented with cholesterol [7,12].…”

“…Threadlike micelles are, for example, commonly observed in PEG-lipid/lipid samples where the lipid is in the liquid crystalline phase, while discs are observed in such mixtures supplemented with cholesterol [7,12]. Further, discoidal micelles are formed in cases where the lipid is in the gel phase [10,12,13]. From previous studies it appears that components, as well as environmental conditions, that increase the monolayer bending modulus and decrease the spontaneous curvature of the mixture promote the formation of discs [12].…”

Acoustically-active microbubbles conjugated to liposomes: Characterization of a proposed drug delivery vehicle

“…An increase in the fraction of PEG2kBiotin present resulted in increased binding ( Figure 3B), however PEG2kBiotin at a concentration of 15% reduced the microbubble concentration by 30% as compared to a total PEG lipid concentration of 10%. PEG lipids at a high concentration have the potential to destabilize the microbubble lipid shell [39,40]. As a result, the concentration of PEG2k and PEG2kBiotin was 5% in all experiments.…”