Liposomal Surface Potential and Bilayer Packing As Affected by PEG−Lipid Inclusion
2000; American Chemical Society; Volume: 16; Issue: 8 Linguagem: Inglês
10.1021/la9912646
ISSN1520-5827
AutoresMats Silvander, Per Hansson, Katarina Edwards,
Tópico(s)Electrostatics and Colloid Interactions
ResumoThe surface potentials and the packing properties of phospholipid liposomes containing anionic lipids with covalently attached poly(ethylene glycol) polymers (PEG(2000)−PE) were investigated. The negative surface potential induced by the PEG−lipid in EPC/cholesterol and DSPC/cholesterol liposomes was determined by means of titration of the fluorescent probe 4-heptadecyl-7-hydroxycoumarin (HC). The shift in the apparent pK of the probe corresponds to an induced potential of −13 mV for 5 mol % PEG−lipid in the phospholipid/cholesterol (ionic strength = 150 mM). For EPC liposomes without cholesterol, 5 mol % PEG−lipid did not cause any detectable shift in the pK of the probe. This indicates that the EPC liposome surface in fact carries a net negative charge. The above results were supported by determinations of the ζ potential. The packing order of the outer and inner parts of the phospholipid/cholesterol bilayer membranes was investigated by fluorescence depolarization anisotropy of the probes HC and 1,6-diphenyl-1,3,5-hexatriene (DPH). The PEG−lipid inclusion caused an increase in the anisotropy of HC, located at the water/hydrocarbon interface. In addition, a small increase in anisotropy was observed for DPH upon PEG−lipid inclusion. Since DPH is situated in the hydrocarbon region, these data together with the HC data indicate that the PEG−lipids impose restrictions on the molecular motion in the outer as well as the inner parts of the hydrocarbon region.
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