MOLECULAR-INTERACTIONS OF ETHER-LINKED PHOSPHOLIPIDS

Earlier studies have shown that ether phospholipids display phase-form ing properties distinct from those of their ester phospholipid counter parts. Dihexadecylphosphatidylcholine (DHPC) forms an interdigitated b ilayer when fully hydrated, and dihexadecylphosphatidylethanolamine (D HPE) is observed in the inverted hexagonal phase (H-II) at elevated te mperatures. 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 diff erential 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 inte rdigitated bilayer gel phase (L(beta)I), whereas 10 mol % DHPC destabi lizes 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 conc lusion, it is found that small amounts of the appropriate phospholipid can seriously compromise the formation of the L(beta)I and H-II phase s.