Molecular interactions of ether-linked phospholipids.

Earlier studies have shown that ether phospholipids display phase-forming properties distinct from those of their ester phospholipid counterparts. Dihexadecylphosphatidylcholine (DHPC) forms an interdigitated bilayer when fully hydrated, and dihexadecylphosphatidylethanolamine (DHPE) is observed in the inverted hexagonal phase (HII) at elevated temperatures. In contrast, the acyl lipid analogues display these phases only under more extreme conditions. In the present study, we examine fully hydrated mixtures of DHPC and DHPE by X-ray diffraction and differential scanning calorimetry and describe the temperature--composition phase diagram for the binary phospholipid system, DHPC/DHPE. Addition of 7 mol % DHPE to DHPC abolishes the ability of DHPC to form an interdigitated bilayer gel phase (L beta I), whereas 10 mol % DHPC destabilizes the HII phase favored by DHPE by elevating (to > 100 degrees C) or removing the L alpha-->HII transition. Evidence for bilayer gel phase separation occurring in DHPC/DHPE mixtures is obtained. In conclusion, it is found that small amounts of the appropriate phospholipid can seriously compromise the formation of the L beta I and HII phases.