Tetraoleoylpyrophosphatidic acid: a four acyl-chain lipid which forms a hexagonal II phase with high curvature

Tetraoleoylpyrophosphatidic acid (bis phosphatidic acid), when hydrated in aqueous buffer, was shown to form an inverted hexagonal phase using 31P NMR. Low-angle x-ray diffraction provided verification of the formation of this phase in dilute aqueous buffer and in 2 M NaCl and permitted comparison of the tube diameter with that of cardiolipin in 2 M NaCl. By using the water cylinder diameters for tetraoleoylpyrophosphatidic acid, bovine cardiolipin, chloroplast monogalactosyl-diglyceride, and dioleoyl phosphatidylethanolamine as a means of estimating the spontaneous curvatures, tetraoleoylpyro-phosphatidic acid was shown to exhibit the greatest curvature of any of the above lipids, equaled only by the calcium salt of cardiolipin. Inverted micelles of hydrated tetraoleoylpyrophosphatidic acid and of cardiolipin in tetradecane were approximately the diameter of the inverted hexagonal tubes. A rationale is given for the differences.