MEMBRANE PACKING GEOMETRY OF DIPHYTANOYLPHOSPHATIDYLCHOLINE IS HIGHLYSENSITIVE TO HYDRATION - PHOSPHOLIPID POLYMORPHISM INDUCED BY MOLECULAR REARRANGEMENT IN THE HEADGROUP REGION

Diphytanoylphosphatidylcholine (DPhPC) has often been used in the stud y of protein-lipid interaction and membrane channel activity, because of the general belief that it has high bilayer stability, low ion leak age, and fatty acyl packing comparable to that of phospholipid bilayer s in the liquid-crystalline state. In this solid-state P-31 and H-2 NM R study, we find that the membrane packing geometry and headgroup orie ntation of DPhPC are highly sensitive to the temperature studied and i ts water content, The phosphocholine headgroup of DPhPC starts to chan ge its orientation at a water content as high as similar to 16 water m olecules per lipid, as evidenced by hydration-dependent H-2 NMR study at room temperature. In addition, a temperature-induced structural tra nsition in the headgroup orientation is detected in the temperature ra nge of similar to 20-60 degrees C for lipids with similar to 8-11 wate r molecules per DPhPC. Dehydration of the lipid by one more water mole cule leads to a nonlamellar, presumably cubic, phase formation. The li pid packing becomes a hexagonal phase at similar to 6 water molecules per lipid. A phase diagram of DPhPC in the temperature range of -40 de grees C to 80 degrees C is thus constructed on the basis of NMR result s. The newly observed hydration-dependent DPhPC lipid polymorphism emp hasizes the importance of molecular packing in the headgroup region in modulating membrane structure and protein-induced pore formation of t he DPhPC bilayer.