HYDRATION OF THE DIENIC LIPID DIOCTADECADIENOYLPHOSPHATIDYLCHOLINE INTHE LAMELLAR PHASE - AN INFRARED LINEAR DICHROISM AND X-RAY STUDY ON HEADGROUP ORIENTATION, WATER ORDERING, AND BILAYER DIMENSIONS

In the phospholipid ,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) in each of the fatty acid chains, a rigid diene group is inse rted in a position near the polar/apolar boundary that is exceptionall y sensitive for membrane stability. DODPC transforms upon gradual dehy dration from the liquid-crystalline to a metastable gel state, which r earranges into two subgel phases at low and intermediate degrees of hy dration. The molecular dimensions of the respective bilayers were dete rmined by means of x-ray diffraction. Infrared linear dichroism of sel ected vibrations of the phosphate and trimethylammonium groups and of the nu(13)(OH) band of water adsorbed onto the lipid was used to study the molecular order in the polar part of the bilayers in macroscopica lly oriented samples. The dense packing of the tilted acyl chains in t he subgel causes the in-plane orientation of the phosphatidylcholine h eadgroups with direct interactions between the phosphate and trimethyl ammonium groups, and a strong orientation of adsorbed water molecules. In the more disordered gel, the thickness of the polar part of the bi layer increases and the lateral interactions between the lipid headgro ups weaken. The higher order in the headgroup region of the subgels co rrelates with shorter decay lengths of the repulsive forces acting bet ween opposite membrane surfaces. This result can be understood if the work to dehydrate the lipid is determined to a certain degree by the w ork to break up the lipid-water interactions without compensation by a dequate lipid-lipid contacts. Almost similar area compressibility modu li are found in the liquid-crystalline and solid phases. Obviously, th e lipid avoids lateral stress by the structural rearrangement.