Bilayer interactions of ether- and ester-linked phospholipids: dihexadecyl- and dipalmitoylphosphatidylcholines

Abstract

Mixed phospholipid systems of ether-linked 1,2-dihexadecylphosphatidylcholine (DHPC) and ester-linked 1,2-dipalmitoylphosphatidylcholine (DPPC) have been studied by differential scanning calorimetry and X-ray diffraction. At maximum hydration (60 wt % water), DHPC shows three reversible transitions: a main (chain melting) transition, TM = 44.2 degrees C; a pretransition, TP = 36.2 degrees C; and a subtransition, TS = 5.5 degrees C. DPPC shows two reversible transitions: TM = 41.3 degrees C and TP = 36.5 degrees C. TM decreases linearly from 44.2 to 41.3 degrees C as DPPC is incorporated into DHPC bilayers; TP exhibits eutectic behavior, decreasing sharply to reach 23.3 degrees C at 40.4 mol % DPPC and then increasing over the range 40-100 mol % DPPC; TS remains constant at 4-5 degrees C and is not observed at greater than 20 mol % DPPC. At 50 degrees C, X-ray diffraction shows a liquid-crystalline bilayer L alpha phase at all DHPC:DPPC mole ratios. At 22 degrees C, DHPC shows an interdigitated bilayer gel L beta phase (bilayer periodicity d = 47.0 A) into which approximately 30 mol % DPPC can be incorporated. Above 30 mol % DPPC, a noninterdigitated gel L beta' phase (d = 64-66 A) is observed. Thus, at T greater than TM, DHPC and DPPC are miscible in all proportions in an L alpha bilayer phase. In contrast, a composition-dependent gel----gel transition between interdigitated and noninterdigitated bilayers is observed at T less than TP, and this leads to eutectic behavior of the DHPC/DPPC system.

The interdigitated gel phase in mixtures of cationic and zwitterionic phospholipids

“…Unlike DHPC, the ether lipid DHPE does not interdigitate due to intermolecular hydrogen bonding and reduced headgroup repulsion of the phosphatidylethanolamine (PE) head group [20,48]. In other mixtures, such as DPPC/DHPC [49][50][51] and DPPC/EDPPC [25], there is little or no phase coexistence except perhaps at the phase transition regions.…”

Lipid domains in the ram sperm plasma membrane demonstrated by differential scanning calorimetry.

“…There is evidence of lateral phase separations in these membranes from fluorescence depolarization studies using parinaric acid (40). While the thermal properties of the Lp to La transition are similar for ether-and ester-linked phospholipids, ether lipids exhibit polymorphisms in the gel state, including the formation of interdigitated phases (41,42). It has also been suggested from 2H NMR studies that 1-palmitoyl, 2-docosahexaenoyl phosphatidylcholine may form interdigitated phases in the La state (43).…”

Domains and Rafts in Lipid Membranes

“…The mixing of the ester DPPC and ether 1,2‐dihexadecylphosphatidylcholine (DHPC) clearly indicate the presence of different types of phase structures in the solid state. Thus DHPC forms an interdigitated phase,22 where self‐assembly creates a monolayer rather than the usual bilayer.…”