Lipid Spontaneous Curvatures Estimated from Temperature-Dependent Changes in Inverse Hexagonal Phase Lattice Parameters: Effects of Metal Cations.
Recently 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.
Division of Chemistry, School of Pharmacy and Biomolecular Sciences, University of Brighton , Brighton BN2 4GJ, U.K.
Marcus K
MK
Dymond
Chemistry, Faculty of Natural & Environmental Sciences, University of Southampton , Southampton SO17 1BJ, U.K.
Richard J
RJ
Gillams
Chemistry, Faculty of Natural & Environmental Sciences, University of Southampton , Southampton SO17 1BJ, U.K.
Duncan J
DJ
Parker
Chemistry, Faculty of Natural & Environmental Sciences, University of Southampton , Southampton SO17 1BJ, U.K.
Jamie
J
Burrell
MAX IV Laboratory, Lund University , PO Box 118, SE-221 00 Lund, Sweden.
Ana
A
Labrador
Physical Chemistry, Lund University , PO Box 124, SE-221 00 Lund, Sweden.
Tommy
T
Nylander
Chemistry, Faculty of Natural & Environmental Sciences, University of Southampton , Southampton SO17 1BJ, U.K.
George S
GS
Attard