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.degree. C; a pretransition, Tp = 36.2.degree. C; and a subtransition, TS = 5.5.degree. C. DPPC shows two reversible transitions: TM = 41.3.degree. C and TP = 36.5.degree. C. TM decreases linearly from 44.2 to 41.3.degree. C as DPPC is incorporated into DHPC bilayers; TP exhibits eutectic behavior, decreasing sharply to reach 23.3.degree. C at 40.4 mol % DPPC and then increasing over the range 40-100 mopl % DPPC; TS remains constant at 4-5.degree. C and is not observed at > 20 mol % DPPC. At 50.degree. C, X-ray diffraction shows a liquid-crystalline bilayer L.alpha. phase at all DHPC:DPPC mole ratios. At 22.degree. C, DHPC shows an interdigitated bilayer gel L.beta. phase (bilayer periodicity d = 47.0 .ANG.) into which .apprx. 30 mol % DPPC can be incorporated. Above 30 mol % DPPC, a noninterdigitated gel L.beta.'' phase (d = 64-66 .ANG.) is observed. Thus, at T > TM, DHPC and DPPC are miscible in all proportions in an L.alpha. bilayer phase. In contrast, a composition-dependent gel .fwdarw. gel transition between interdigitated and noninterdigitated bilayers is observed at T < Tp, and this leads to eutectic behavior of the DHPC/DPPC system.