A mechanism of pressure-induced interdigitation of lipid bilayers

The bilayer phase behavior of di-O-hexadecylphosphatidylethanolamine (DHPE), one of ether-linked phosphoethanolamines (PEs), was investigated under atmospheric and high pressure by means of differential scanning calorimetry (DSC) and high-pressure light-transmittance techniques. The gel-to-liquid-crystalline phase transition was observed at 67.1 degrees C under atmospheric pressure. The transition enthalpy was estimated to be 31.7 kJ mol(-1). Also under high pressure, only the same type of phase transition was observed, which means that DHPE cannot form the interdigitated structure even under high pressure. In general, it is known that the PE headgroup has an inhibitory effect against the interdigitation, whereas the presence of the ether linkage is advantageous to that. The result in this study indicates that the strong interaction by the formation of the hydrogen bonding network between the PE headgroups completely abolishes the capability of the phospholipid to form the interdigitated structure. By comparing the result with the previous results for the ether-linked phosphocholine (PC) and the ester-linked PC and PE lipids, we speculate on the mechanism of the transformation into the interdigitated structure induced by pressure in terms of the molecular interactions between the lipid molecules in the bilayer membrane.