Polymorphic phase behavior of unsaturated lysophosphatidylethanolamines: A phosphorus-31 NMR and x-ray diffraction study

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Abstract
The polymorphic phase behavior of aqueous dispersions of 1-oleoyl-, 1-linoleoyl-, and 1-linolenoyl-sn-3-glycerophosphoethanolamine (1-C18:1c-PE, 1-C18:2c-PE, and 1-C18:3c-PE, respectively) has been investigated by 31P NMR, small-angle and wide-angle X-ray diffraction, and freeze-fracture techniques in response to changes in temperature and pH. Between -20 and 0.degree. C at pH 7, NMR and X-ray data indicate that 1-C18:1c-PE adopts a lamellar phase. Above 20.degree. C, the X-ray diffraction from 1-C18:1c-PE reveals no long-range lattice order, whereas the NMR data indicate lamellar structure to 90.degree. C. Freeze-fracture electron microscopy shows that 1-C18:1c-PE at pH 8.2 forms closed multilamellar vesicles upon dispersion and also that large unilamellar vesicles produced by extrusion techniques (LUVETs) can be made from 1-C18:1c-PE at pH 7. Such LUVETs can trap [3H]inulin and support a K+ diffusion potential for up to 4 h. At pH 8.5 and above, 1-C18:1c-PE forms optically clear, fluid dispersions with NMR and X-ray characteristics consistent with a micellar (noninverted) phase structure. Attempts to prepare LUVETs from 1-C18:1c-PE at pH 9 result in structures that can neither trap [3H] inulin nor support a membrane potential. At temperatures below -10.degree. C at pH 7, both 1-C18:2c-PE and 1-C18:3c-PE form a lamellar phase whereas at temperatures of 20.degree. C and above, both lipids form optically clear, gellacious phases with NMR and X-ray characteristics consistent with a phase (such as inverted micellar) in which the phospholipid head groups are not in intimate contact with the bulk aqueous phase. For both 1-C18:2c-PE and 1-C18:3c-PE at 0.degree. C, there is X-ray and NMR evidence, respectively, of a hexagonal phase, putatively HII, occurring as an intermediate in the lamellar-inverted phase transition.