Conformational and hydrational properties during the Lβ- to Lα- and Lα- to HII-phase transition in phosphatidylethanolamine

Differential scanning calorimetry (DSC) measurements have been carried out simultaneously with small- and wide-angle X-ray scattering recordings on liposomal dispersions of stearoyl-oleoyl-phosphatidylethanolamine (PE) in a temperature range from 20 to 80 °C. The main transition temperature, Tm, was determined at 30.9 °C with an enthalpy of 28.5 kJ/mol and the lamellar-to-inverse hexagonal phase transition temperature, Thex, at 61.6 °C with an enthalpy of 3.8 kJ/mol. Additionally highly resolved small angle X-ray diffraction experiments performed at equilibrium conditions allowed a reliable decomposition of the lattice spacings into hydrophobic and hydrophilic structure elements as well as the determination of the lipid interface area of the lamellar gel-phase (Lβ), the fluid lamellar phase (Lα) and of the inverse hexagonal phase (HII). The rearrangement of the lipid matrix and the coincident change of free water per lipid is illustrated for both transitions. Last, possible transition mechanisms are discussed on a molecular level.