Formation of Inverse Topology Lyotropic Phases inDioleoylphosphatidylcholine/Oleic Acid and Dioleoylphosphatidylethanolamine/OleicAcid Binary Mixtures.

The addition of saturated fatty acids(FA) to phosphatidylcholinelipids (PC) that have saturated acyl chains has been shown to promotethe formation of lyotropic liquid-crystalline phases with negativemean curvature. PC/FA mixtures may exhibit inverse bicontinuous cubicphases (Im3m, Pn3m) or inverse topology hexagonal phases (HII), depending on the length of the acyl chains/fatty acid.Here we report a detailed study of the phase behavior of binary mixturesof dioleoylphosphatidylcholine (DOPC)/oleic acid (OA) and dioleoylphosphatidylethanolamine(DOPE)/oleic acid at limiting hydration, constructed using small-angleX-ray diffraction (SAXD) data. The phase diagrams of both systemsshow a succession of phases with increasing negative mean curvaturewith increasing OA content. At high OA concentrations, we have observedthe occurrence of an inverse micellar Fd3mphase in both systems. Hitherto, this phase had not beenreported for phosphatidylethanolamine/fatty acid mixtures, and assuch it highlights an additional route through which fatty acids mayincrease the propensity of bilayer lipid membranes to curve. We alsopropose a method that uses the temperature dependence of the latticeparameters of the HIIphases to estimate the spontaneousradii of curvature (R0) of the binarymixtures and of the component lipids. Using this method, we calculatedthe R0values of the complexes comprisingone phospholipid molecule and two fatty acid molecules, which havebeen postulated to drive the formation of inverse phases in PL/FAmixtures. These are −1.8 nm (±0.4 nm) for DOPC(OA)2and −1.1 nm (±0.1 nm) for DOPE(OA)2. R0values estimated in this way allowthe quantification of the contribution that different lipid speciesmake to membrane curvature elastic properties and hence of their effecton the function of membrane-bound proteins. [ABSTRACT FROM AUTHOR]