The influence of hydrophilic spacers on the phase behavior of ether lipids

Oxyethylene (EO) units known from nonionic amphiphiles of the type Cn(EO)m have been used as a spacer between the hydrophobic and hydrophilic parts of phospholipid molecules. Here we describe the synthesis of the compounds and present results of aqueous dispersions of phospholipids with 3 and 6 EO units. The mechanism of the phase transition from ordered gel to a liquid-crystalline Lα-phase depends on the number of the EO units. In the case of DH(EO)3PC, the lipid head groups are interdigitated in the Lβ-phase. On heating, first the head groups de-interdigitate and the melting of the hydrocarbon chains starts only after reaching a stretched head group conformation. While on heating the lamellar repeat distance changes continuously, the cooling process is more complex showing two regimes for the decrease of the lamellar repeat distance. In the case of DH(EO)6PC, the same transition occurs in a single step without any hysteresis between heating and cooling.