Poly(ethylene glycol)−Lipid Conjugates Promote Bilayer Formation in Mixtures of Non-Bilayer-Forming Lipids†

John W. Holland, Pieter R. Cullis, Thomas D. Madden
1996 Biochemistry  
The influence of poly(ethylene glycol)-lipid conjugates on phospholipid polymorphism has been examined using 31 P-NMR and freeze-fracture electron microscopy. An equimolar mixture of dioleoylphosphatidylethanolamine (DOPE) and cholesterol adopts the hexagonal (H II ) phase when hydrated under physiological conditions but can be stabilized in a bilayer conformation when a variety of PEGlipid conjugates are included in the lipid mixture. These PEG conjugates produced an increase in the bilayer to
more » ... e in the bilayer to hexagonal (H II ) 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-PEG 2000 no H II phase phospholipid was observed up to at least 60°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 31 P-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 H II phase transition. † This work was supported by
doi:10.1021/bi951999j pmid:8611564 fatcat:dcczjf7kszfwbcqwildskp4fwu