OSMOTIC-STRESS INDUCES A PHASE-TRANSITION FROM INTERDIGITATED GEL PHASE TO BILAYER GEL PHASE IN MULTILAMELLAR VESICLES OF DIHEXADECYLPHOSPHATIDYLCHOLINE

We have investigated the effects of poly(ethylene glycol) (PEG) on the structure and phase behavior of multilamellar vesicles of dihexadecy Iphosphatidylcho line (DHPC-MLVs) using an X-ray diffraction method. P it low concentrations of PEG-6K (MW = 7500), DHPC-MLVs were in an inte rdigitated gel (LbetaI) phase, a gel phase with interdigitated hydroca rbon chains. At around 24% (w/v) PEG 6K, a phase transition from the L betaI phase to a bilayer gel phase occurred in the DHPC-MLVs, and abov e this concentration, they were in a bilayer gel phase. On the other h and, ethylene glycol (EG), the monomer of PEG, did not induce this pha se transition in the DHPC-MLVs. A mechanism of this phase transition i s proposed and discussed; a decrease in the repulsive interaction betw een the head groups of the phospholipids in the bilayer gel phase with an increase in PEG concentration, which is due to a decrease in the c ross-sectional area of the head group region by osmotic stress, may be the main reason for this phase transition.