Formation of Inverse Topology Lyotropic Phases in
Dioleoylphosphatidylcholine/Oleic Acid and Dioleoylphosphatidylethanolamine/Oleic
Acid Binary Mixtures
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Abstract
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 liquid-crystalline phases with negative
mean curvature. PC/FA mixtures may exhibit inverse bicontinuous cubic
phases (<i>Im</i>3<i>m</i>, <i>Pn</i>3<i>m</i>) or inverse topology hexagonal phases (H<sub>II</sub>), 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 <i>Fd</i>3<i>m</i> 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 H<sub>II</sub> phases to estimate the spontaneous
radii of curvature (<i>R</i><sub>0</sub>) of the binary
mixtures and of the component lipids. Using this method, we calculated
the <i>R</i><sub>0</sub> 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)<sub>2</sub> and −1.1 nm (±0.1 nm) for DOPE(OA)<sub>2</sub>. <i>R</i><sub>0</sub> 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