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-gl ycero-3-phosphorylethanolamine (DTDPE) and the corresponding phosphatidylch oline analogue (DTDPC) has been studied as a function of temperature (T) an d relative humidity (RH) by means of infrared (IR) dichroism measurements. Lamellar/nonlamellar and solid/fluid phase transitions of lipid films sprea d on an ATR crystal are determined with high resolution by means of spectra l parameters. The nonlamellar inverse hexagonal (H-II) phase has been found for DTDPE at T > 45 degrees C, a lamellar gel at T < 5 degrees C and the l amellar liquid-crystalline phase in between at RH > 90%. The ranges of the gel and the H-II states can be extended into the range of moderate temperat ures by dehydrating the lipid where a direct transformation between the gel and H-II was found. The appearance of the H-II phase is somewhat unexpecte d in view of the short effective length of the tetradecadienoyl chains. It can be attributed to the influence of the terminal diene groups which remai ns however unspecified. Similar to other PE lipids, DTDPE tends to transfor m from the gel into crystalline polymorphs. The PC analogue assembles into liquid-crystalline membranes at nearly all conditions studied. The hydratio n, conformation, and packing of the polar region of the lipid aggregates ha s been characterized by IR spectral parameters. This method has been comple mented by P-31 NMR to assign the nonlamellar phase and by DSC to yield ther modynamic information.