Effects of poly(ethylene glycol) on neutral lipid bilayers

The effect of poly(ethylene glycol) (PEG) on both unilamellar (ULV) and multilamellar (MLV) vesicles of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylethanolamine (DMPE) was investigated by spin label electron spin resonance (ESR) spectroscopy and by spectrophotometry. On increasing the PEG concentration in the external aqueous phase of the preformed phospholipid bilayers we find: (i) a reduced polarity of the bulk water phase; (ii) an enhanced lateral packing of the phospholipid molecules in both DPPC and DMPE bilayers which proves to be stronger in ULVs than in MLVs; (iii) a progressive increase of the DPPC pretransition temperature and the lack of a pretransition at 25% (wt/v) of PEG; (iv) an upwards shift of the main transition temperature of DPPC and DMPE MLVs. The results are consistent with an indirect action of PEG on the molecular structure of the mesophases investigated.