Effect of cholesterol, diacylglycerol and phosphatidylethanolamine on PEG 6000 induced lipid mixing and surface dielectric constant of phosphatidylcholine vesicle.

Using the NBD/Rh fluorescence assay the PEG 6000 induced phospholipid mixing of phosphatidylcholine liposomes containing cholesterol, diacylglycerol or phosphatidylethanolamine was measured. All 3 components shifted the PEG 6000 concentration necessary to induce 50% of maximal phospholipid mixing to lower concentrations. After the addition of PEG cholesterol containing PC liposomes exhibit different values of the surface dielectric constant as measured by the stokes shift of the fluorophore dansyl-PE compared to pure PC, whereas in DAG- and PE-containing PC liposomes no differences were observed. It is concluded that the incorporation of cholesterol leads to a different surface dielectric constant after PEG addition. The changed surface dielectric properties are a prerequest for the onset of fusion, as shown by Ohki and Arnold (1990). The incorporation of DAG and PE into PC membranes leads to structural instabilities as proposed by Siegel et al. (1989). This additional structurally unstable region created by molecules like PE or DAG may shift the onset of fusion to lower PEG concentration.