Interfacial thickness of liposomes containing poly(ethylene glycol)-cholesterol from electrophoresis

The electrophoretic mobilities of liposomes incorporating a polyethylene glycol (PEG) headgroup coupled to cholesterol for PEG of average chain index 3.0, 13.2, and 22.3 have been determined as a function of PEG-cholesterol mole fraction between 5% and 40% and ionic strength between 2 and 200 mM. The liposome compositions were 40 mole % cholesterol plus PEG-cholesterol, 10 mole % 1,2-dipalmitoyl-sn-glyerco-3-phosphoglycerol and 50 mole % egg phosphatidylcholine. The mobilities were fit to a model in which the PEG forms a surface layer of polymer subject to viscous drag arising I from electroosmotic flow within this layer. The model provides estimates of the average layer thickness that are comparable to those determined from contemporary models of surface-attached polymer.