Liposomes, disks, and spherical micelles: aggregate structure in mixtures of gel phase phosphatidylcholines and poly(ethylene glycol)-phospholipids.
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.
Johnsson
Markus
M
Department of Physical Chemistry, Uppsala University, SE-751 23 Uppsala, Sweden.
Edwards
Katarina
K
D055592:Biophysical Phenomena; D001703:Biophysics; D020285:Cryoelectron Microscopy; D016503:Drug Delivery Systems; D004578:Electron Spin Resonance Spectroscopy; D008027:Light; D008051:Lipid Bilayers; D008055:Lipids / Q000737:chemistry; D008081:Liposomes / Q000737:chemistry* / Q000378:metabolism; D008823:Micelles; D008854:Microscopy, Electron; D008954:Models, Biological; D010713:Phosphatidylcholines / Q000737:chemistry*; D010743:Phospholipids / Q000737:chemistry*; D011092:Polyethylene Glycols / Q000737:chemistry*; D012542:Scattering, Radiation; D013696:Temperature
D008051:Lipid Bilayers; D008055:Lipids; D008081:Liposomes; D008823:Micelles; D010713:Phosphatidylcholines; D010743:Phospholipids; D011092:Polyethylene Glycols