Poly(ethylene glycol)-lipid conjugates promote bilayer formation in mixtures of non-bilayer-forming lipids.
The influence of poly(ethylene glycol)-lipid conjugates on phospholipid polymorphism has been examined using 31P-NMR and freeze--fracture electron microscopy. An equimolar mixture of dioleoylphosphatidylethanolamine (DOPE) and cholesterol adopts the hexagonal (HII) phase when hydrated under physiological conditions but can be stabilized in a bilayer conformation when a variety of PEG-lipid conjugates are included in the lipid mixture. These PEG conjugates produced an increase in the bilayer to hexagonal (HII) phase transition temperature and a broadening of the temperature range over which both phases coexisted. Further, the fraction of phospholipid adopting the bilayer phase increased with increasing mole fraction of PEG-lipid such that at 20 mole % DOPE--PEG2000 no HII phase phospholipid was observed up to a least 60 degrees C. Increasing the size of the PEG moiety from 2000 to 5000 Da (while maintaining the PEG--lipid molar ratio constant) increased the proportion of lipid in the bilayer phase. In contrast, varying the acyl chains of the PE anchor had no effect on polymorphic behavior. PEG--lipid conjugates in which ceramide provides the hydrophobic anchor also promoted bilayer formation in DOPE:cholesterol mixtures but at somewhat higher molar ratios compared to the corresponding PEG--PE species. The slightly greater effectiveness of the PE conjugates may result from the fact that these derivatives also possess a net negative charge. Phosphorus NMR spectroscopy indicated that a proportion of the phospholipid in DOPE:cholesterol:PEG--PE mixtures experienced isotropic motional averaging with this proportion being sensitive to both temperature and PEG molecular weight. Surprisingly, little if any isotropic signal was observed when PEG--ceramide was used in place of PEG--PE. Consistent with the 31P-NMR spectra, freeze-fracture electron microscopy showed the presence of small vesicles (diameter <200 nm) and lipidic particles in DOPE:cholesterol mixtures containing PEG--PE. We conclude that the effects of PEG--lipid conjugates on DOPE:cholesterol mixtures are 2-fold. First, the complementary "inverted cone" shape of the conjugate helps to accommodate the "cone-shaped" lipids, DOPE and cholesterol, in the bilayer phase. Second, the steric hindrance caused by the PEG group inhibits close apposition of bilayers, which is a prerequisite for the bilayer to HII phase transition.
Holland
J W
JW
Department of Pharmacology and Therapeutics, University of British Columbia, Vancouver, Canada.
Cullis
P R
PR
Madden
T D
TD
D002784:Cholesterol / Q000737:chemistry; D005614:Freeze Fracturing; D066298:In Vitro Techniques; D008051:Lipid Bilayers; D008081:Liposomes / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008854:Microscopy, Electron; D008968:Molecular Conformation; D010714:Phosphatidylethanolamines / Q000737:chemistry; D010743:Phospholipids; D011092:Polyethylene Glycols / Q000737:chemistry*
D008051:Lipid Bilayers; D008081:Liposomes; D010714:Phosphatidylethanolamines; D010743:Phospholipids; D011092:Polyethylene Glycols; C020888:1,2-dielaidoylphosphatidylethanolamine; D002784:Cholesterol