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 (H-II) 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 H-II phase favored by DHPE by elevating (to >100 degrees C) or removing the L(alpha) --> H-II 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 H-II phases.