Lyotropic Phase Behavior and Gel State Polymorphism of Phospholipids with Terminal Diene Groups: Infrared Measurements on Molecular Ordering in Lamellar and Hexagonal Phases

The phase behavior of the diene lipid 1,2-bis(11,13-tetradecadienoyl)-sn-glycero-3-phosphorylethanolamine (DTDPE) and the corresponding phosphatidylcholine analogue (DTDPC) has been studied as a function of temperature (T) and relative humidity (RH) by means of infrared (IR) dichroism measurements. Lamellar/ nonlamellar and solid/fluid phase transitions of lipid films spread on an ATR crystal are determined with high resolution by means of spectral parameters. The nonlamellar inverse hexagonal (HII) phase has been found for DTDPE at T > 45 °C, a lamellar gel at T < 5 °C and the lamellar liquid-crystalline phase in between at RH > 90%. The ranges of the gel and the HII states can be extended into the range of moderate temperatures by dehydrating the lipid where a direct transformation between the gel and HII was found. The appearance of the HII phase is somewhat unexpected in view of the short effective length of the tetradecadienoyl chains. It can be attributed to the influence of the terminal diene groups which remains however unspecified. Similar to other PE lipids, DTDPE tends to transform from the gel into crystalline polymorphs. The PC analogue assembles into liquid-crystalline membranes at nearly all conditions studied. The hydration, conformation, and packing of the polar region of the lipid aggregates has been characterized by IR spectral parameters. This method has been complemented by 31P NMR to assign the nonlamellar phase and by DSC to yield thermodynamic information.