la6b03098_si_001.pdf (753.74 kB)
Lipid Spontaneous Curvatures Estimated from Temperature-Dependent Changes in Inverse Hexagonal Phase Lattice Parameters: Effects of Metal Cations
journal contribution
posted on 2016-09-07, 00:00 authored by Marcus K. Dymond, Richard J. Gillams, Duncan
J. Parker, Jamie Burrell, Ana Labrador, Tommy Nylander, George S. AttardRecently we reported
a method for estimating the spontaneous curvatures
of lipids from temperature-dependent changes in the lattice parameter
of inverse hexagonal liquid crystal phases of binary lipid mixtures.
This method makes use of 1,2-dioleoyl-sn-glycerol-3-phosphoethanolamine
(DOPE) as a host lipid, which preferentially forms an inverse hexagonal
phase to which a guest lipid of unknown spontaneous curvature is added.
The lattice parameters of these binary lipid mixtures are determined
by small-angle X-ray diffraction at a range of temperatures and the
spontaneous curvature of the guest lipid is determined from these
data. Here we report the use of this method on a wide range of lipids
under different ionic conditions. We demonstrate that our method provides
spontaneous curvature values for DOPE, cholesterol, and monoolein
that are within the range of values reported in the literature. Anionic
lipids 1,2-dioleoyl-sn-glycerol-3-phosphatidic acid
(DOPA) and 1,2-dioleoyl-sn-glycerol-3-phosphoserine
(DOPS) were found to exhibit spontaneous curvatures that depend on
the concentration of divalent cations present in the mixtures. We
show that the range of curvatures estimated experimentally for DOPA
and DOPS can be explained by a series of equilibria arising from lipid-cation
exchange reactions. Our data indicate a universal relationship between
the spontaneous curvature of a lipid and the extent to which it affects
the lattice parameter of the hexagonal phase of DOPE when it is part
of a binary mixture. This universal relationship affords a rapid way
of estimating the spontaneous curvatures of lipids that are expensive,
only available in small amounts, or are of limited chemical stability.