Shifts in chain-melting transition temperature of liposomal membranes by polymer-grafted lipids.

The chain-melting transition temperature of dipalmitoyl phosphatidylcholine (DPPC) bilayer membranes containing poly(ethylene glycol)-grafted dipalmitoyl phosphatidylethanolamine (PEG-DPPE) was determined by optical turbidity measurements. The dependence on content, Xp, of PEG-DPPE lipid was studied for different polar headgroup sizes, np, of the polymer lipid, throughout the lamellar phase of the mixtures with DPPC. Mean-field theory for the polymer brush regime predicts that the downward shift in transition temperature should vary with polymer size and content as npXp(5/3) (approximately npXp(11/6) for scaling theory). Any shift induced by the charge on PEG-lipids is independent of polymer size. These predictions are reasonably borne out for the longer polymer lipids (PEG molecular masses 750, 2000 and 5000 Da). Transition temperature shifts in the lamellar phase, before the onset of micellisation, are in the region of -1 to -2 degrees C (+/-0.1-0.2 degrees C) in reasonable accord with theoretical estimates of the lateral pressure exerted by the polymer brush. Shifts of this size are significant to the design of liposomes for controlled release of contents by mild hyperthermia.