Bilayer interactions of ether- and ester-linked phospholipids: dihexadecyl- and dipalmitoylphosphatidylcholines.
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.
Biophysics Institute, Housman Medical Research Center, Boston University School of Medicine, Massachusetts 02118.
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Kim
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Mattai
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Shipley
D015060:1,2-Dipalmitoylphosphatidylcholine; D002152:Calorimetry, Differential Scanning; D004952:Esters; D004987:Ethers; D008051:Lipid Bilayers; D008954:Models, Biological; D008968:Molecular Conformation; D010742:Phospholipid Ethers; D014961:X-Ray Diffraction
C052298:1,2-dihexadecyl-sn-glycero-3-phosphocholine; D004952:Esters; D004987:Ethers; D008051:Lipid Bilayers; D010742:Phospholipid Ethers; D015060:1,2-Dipalmitoylphosphatidylcholine