Hydration of the dienic lipid dioctadecadienoylphosphatidylcholine in the lamellar phase - An infrared linear dichroism and X-ray study on headgroup orientation, water ordering, and bilayer dimensions

In the phospholipid 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) in each of the fatty acid chains, a rigid diene group is inserted in a position near the polar/apolar boundary that is exceptionally sensitive for membrane stability. DODPC transforms upon gradual dehydration from the liquid-crystalline to a metastable gel state, which rearranges into two subgel phases at low and intermediate degrees of hydration. The molecular dimensions of the respective bilayers were determined by means of x-ray diffraction. Infrared linear dichroism of selected 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 macroscopically oriented samples. The dense packing of the tilted acyl chains in the subgel causes the in-plane orientation of the phosphatidylcholine headgroups with direct interactions between the phosphate and trimethylammonium groups, and a strong orientation of adsorbed water molecules. In the more disordered gel, the thickness of the polar part of the bilayer increases and the lateral interactions between the lipid headgroups weaken. The higher order in the headgroup region of the subgels correlates with shorter decay lengths of the repulsive forces acting between opposite membrane surfaces. This result can be understood if the work to dehydrate the lipid is determined to a certain degree by the work to break up the lipid-water interactions without compensation by adequate lipid-lipid contacts. Almost similar area compressibility moduli are found in the liquid-crystalline and solid phases. Obviously, the lipid avoids lateral stress by the structural rearrangement.