Formation of Inverse Topology Lyotropic Phases in Dioleoylphosphatidylcholine/Oleic Acid and Dioleoylphosphatidylethanolamine/Oleic Acid Binary Mixtures

Summary, in English

The addition of saturated fatty acids (FA) to

phosphatidylcholine lipids (PC) that have saturated acyl chains

has been shown to promote the formation of lyotropic liquidcrystalline

phases with negative mean curvature. PC/FA

mixtures may exhibit inverse bicontinuous cubic phases

(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

mixtures of dioleoylphosphatidylcholine (DOPC)/oleic acid

(OA) and dioleoylphosphatidylethanolamine (DOPE)/oleic

acid at limiting hydration, constructed using small-angle X-ray

diffraction (SAXD) data. The phase diagrams of both systems

show a succession of phases with increasing negative mean

curvature with increasing OA content. At high OA concentrations, we have observed the occurrence of an inverse micellar Fd3m

phase in both systems. Hitherto, this phase had not been reported for phosphatidylethanolamine/fatty acid mixtures, and as such

it highlights an additional route through which fatty acids may increase the propensity of bilayer lipid membranes to curve. We

also propose a method that uses the temperature dependence of the lattice parameters of the HII phases to estimate the

spontaneous radii of curvature (R0) of the binary mixtures and of the component lipids. Using this method, we calculated the R0

values of the complexes comprising one phospholipid molecule and two fatty acid molecules, which have been postulated to drive

the formation of inverse phases in PL/FA mixtures. These are −1.8 nm (±0.4 nm) for DOPC(OA)2 and −1.1 nm (±0.1 nm) for

DOPE(OA)2. R0 values estimated in this way allow the quantification of the contribution that different lipid species make to

membrane curvature elastic properties and hence of their effect on the function of membrane-bound proteins.