Phase transitions, lateral phase separation and microstructure of model membranes composed of a polymerizable two-chain lipid and dimyristoylphosphatidylcholine

H. Gaub, R. Büschl, H. Ringsdorf, E. Sackmann

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34 Scopus citations

Abstract

The structural and dynamic properties of aqueous dispersions of a cationic two-chain lipid containing one dienoyl group per chain for polymerization as well as of mixtures of this lipid with dimyristoylphosphatidylcholine (DMPC) were studied. A variety of techniques such as calorimetry, densitometry, fluorescence depolarization spectroscopy, light scattering, freeze fracture electron diffraction, and the photobleaching technique were applied. In pure water the lipid forms large thin-walled vesicles above 40°C. These exhibit two phase transitions: a chain melting transition at Tm =35°C and a solid-solid phase change which occurs at Ts+ = 28°C at increasing and at Ts- = 21°C at decreasing temperature. This hysteresis is ionic strength dependent. Evidence is provided that the latter phase change is accompanied by a transition of the chains from a tilted to a non-tilted configuration. At low ionic strengths the vesicles remain stable after repetitive heating and cooling through both phase transitions. At high salt concentrations (≳0.1 M NaCl) the crystallization of the bilayers is accompanied by their aggregation into spherulites or large platelets and the chain melting transition of these multilayers is shifted to high temperatures (about 50°C). A first phase diagram of the mixture is established by freeze fracture electron microscopy and 90°-light-scattering which is characterized by a miscibility gap in the solid state. The vesicles exhibit a domain like lateral organization of the components which may be exploited in order to prepare vesicles composed of patches of cross-linked and monomeric lipid. Photochemical polymerization abolishes the solid-solid transition while the chain-melting transition is preserved and is slightly shifted to higher temperatures (Tm* ≈ 38°C). The lipid chains form a hexagonal lattice at T < Tm. The degree of polymerization, N, has been estimated as to N ≤ 100 on the basis of the freeze fracture results. This rather small degree of polymerization is also suggested by the small reduction of the lateral diffusion coefficients of lipid probes which penetrate the bilayer either partially or completely.

Original languageEnglish
Pages (from-to)19-43
Number of pages25
JournalChemistry and Physics of Lipids
Volume37
Issue number1
DOIs
StatePublished - Apr 1985

Keywords

  • capping
  • polymerizable lipid
  • solid-solid transition

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