Abstract
Diphytanoylphosphatidylcholine (DPhPC) has often been used in the study of protein-lipid interaction and membrane channel activity, because of the general belief that it has high bilayer stability, low ion leakage, and fatty acyl packing comparable to that of phospholipid bilayers in the liquid- crystalline state. In this solid-state 31 P and 2 H NMR study, we find that the membrane packing geometry and headgroup orientation of DPhPC are highly sensitive to the temperature studied and its water content. The phosphocholine headgroup of DPhPC starts to change its orientation at a water content as high as ~16 water molecules per lipid, as evidenced by hydration- dependent 2 H NMR study at room temperature. In addition, a temperature- induced structural transition in the headgroup orientation is detected in the temperature range of ~20-60°C for lipids with ~8-11 water molecules per DPhPC. Dehydration of the lipid by one more water molecule leads to a nonlamellar, presumably cubic, phase formation. The lipid packing becomes a hexagonal phase at ~6 water molecules per lipid. A phase diagram of DPhPC in the temperature range of -40°C to 80°C is thus constructed on the basis of NMR results. The newly observed hydration-dependent DPhPC lipid polymorphism emphasizes the importance of molecular packing in the headgroup region in modulating membrane structure and protein-induced pore formation of the DPhPC bilayer.