Phase behavior of two-component lipid membranes: theory and experiments

The structure of the ripple phase of phospholipid membranes remains poorly understood in spite of a large number of theoretical studies, with many experimentally established structural features of this phase unaccounted for. In this article we present a phenomenological theory of phase transitions in single- and two-component achiral lipid membranes in terms of two coupled order parameters -- a scalar order parameter describing {\it lipid chain melting}, and a vector order parameter describing the {\it tilt of the hydrocarbon chains} below the chain-melting transition. This model reproduces all the salient structural features of the ripple phase, providing a unified description of the phase diagram and microstructure. In addition, it predicts a variant of this phase which does not seem to have been experimentally observed. Using this model we have calculated generic phase diagrams of two-component membranes. We have also determined the phase diagram of a two-component lipid membrane from x-ray diffraction studies on aligned multilayers. This phase diagram is found to be in good agreement with that calculated from the model.Comment: 10 pages, 10 figure

Theory of the asymmetric ripple phase in achiral lipid membranes

We present a phenomenological theory of phase transitions in achiral lipid membranes in terms of two coupled order parameters -- a scalar order parameter describing lipid chain melting, and a vector order parameter describing the tilt of the hydrocarbon chains below the chain-melting transition. Existing theoretical models fail to account for all the observed features of the phase diagram, in particular the detailed microstructure of the asymmetric ripple phase lying between the fluid and the tilted gel phase. In contrast, our two-component theory reproduces all the salient structural features of the ripple phase, providing a unified description of the phase diagram and microstructure

Structure of phospholipid-cholesterol membranes: an X-ray diffraction study

We have studied the phase behavior of mixtures of cholesterol with dipalmitoyl phosphatidylcholine (DPPC), dimyristoyl phosphatidylcholine (DMPC), and dilauroyl phosphatidylethanolamine (DLPE), using X-ray diffraction techniques. Phosphatidylcholine (PC)-cholesterol mixtures are found to exhibit a modulated phase for cholesterol concentrations around 15 mol % at temperatures below the chain melting transition. Lowering the relative humidity from 98% to 75% increases the temperature range over which it exists. An electron density map of this phase in DPPC-cholesterol mixtures, calculated from the X-ray diffraction data, shows bilayers with a periodic height modulation, as in the ripple phase observed in many PCs in between the main- and pretransitions. However, these two phases differ in many aspects, such as the dependence of the modulation wavelength on the cholesterol content and thermodynamic stability at reduced humidities. This modulated phase is found to be absent in DLPE-cholesterol mixtures. At higher cholesterol contents the gel phase does not occur in any of these three systems, and the fluid lamellar phase is observed down to the lowest temperature studied (5°C)

Molecular chirality and the “ripple” phase of phosphatidylcholine multibilayers

We present small- and wide-angle X-ray diffraction data of chiral and racemic dimyristoylphosphatidylcholine bilayers in the “ripple” Pβ' phase. In both cases, this phase is found to be characterized by asymmetric ripples. This result differs from that of Lubensky and MacKintosh [Phys. Rev. Lett. 71, 1565 (1993)], whose model predicts symmetric ripples in achiral systems

L<SUB>&#946;'</SUB>&#8594;L<SUB>c'</SUB> phase transition in phosphatidylcholine lipid bilayers: a disorder-order transition in two dimensions

The structure of the Lc' phase exhibited by hydrated dipalmitoylphosphatidylcholine (DPPC) was recently determined by Raghunathan and Katsaras [Phys. Rev. Lett. 74, 4456 (1995)] from x-ray diffraction studies on oriented multibilayers. Here, we reanalyze the powder diffraction data reported in the literature on a number of hydrated lipids possessing the phosphatidylcholine headgroup. As in DPPC, the Lc' phase in all of these systems is found to be characterized by two-dimensional ordering of the lipid molecules on a superlattice of the hydrocarbon chain lattice. We also discuss the influence of headgroup interactions on the structure of this phase

Induction of smectic C phase in binary mixtures of compounds with cyano end groups

We have studied binary mixtures of 4-cyanophenyl-3'-methyl-4'(4"-n-dodecylbenzoyloxy)benzoate (12 CPMBB) and 4-n-dodecylphenyl-3'-methyl-4'(4"-cyanobenzoy1oxy)benzoate (12 PMCBB). The former compound exhibits nematic, smectic A and reentrant nematic phases and the latter one nematic and smectic A phases with a possible AdA2, transition. Over a fairly wide composition range, the mixtures exhibit an induced smectic C phase on cooling the sample from the A phase. The phase diagram, and the temperature variations of the layer spacings for some specific compositions are reported. The possible origin of the C phase is also discussed

Columnar phases exhibited by some polycatenar ligands and a few related metal complexes

The synthesis and characterization of some polycatenar ligands which exhibit hexagonal columnar and cubic phases are reported. A pentacatenar with only four phenyl rings in the core and exhibiting a mesophase is also reported. A few copper (II) and palladium (II) complexes have been synthesized using these ligands and the mesomorphic properties exhibited by them are described. The hexagonal columnar phase exhibited by some of the complexes can be cooled down to room temperature. The mesophases have been characterized using a combination of polarized light microscopy, differential scanning calorimetry and X-ray diffraction methods