699 research outputs found
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)
L<SUB>β'</SUB>→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
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
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
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
Structures of some surfactant-polyelectrolyte complexes
Structures of complexes formed in aqueous solutions by some anionic polyelectrolytes (double and single stranded (ds and ss) DNA, poly(vinyl sulfonate) (PVS), and poly(styrene sulfonate) (PSS)) with a cationic surfactant system consisting of cetyltrimethylammonium bromide (CTAB) and sodium 3-hydroxy-2-naphthoate (SHN) have been determined using small angle X-ray diffraction. All complexes are found to have a two-dimensional (2-D) hexagonal structure at low SHN concentrations. Analysis of the diffraction data shows that the ds DNA-CTAB complex has an intercalated structure, with each DNA strand surrounded by three cylindrical micelles. On increasing SHN concentration, DNA-CTAB-SHN complexes exhibit a hexagonal-to-lamellar transition, whereas PVS complexes show a hexagonal → centered rectangular → lamellar transition. PSS complexes show yet another sequence of structures. These results indicate the significant influence of the chemical nature of the polyelectrolyte on the structure of the complexes
Reentrant phase transitions of DNA-surfactant complexes
Complexes of double-stranded DNA with the cationic surfactant cetyltrimethylammonium bromide have been studied using small angle X-ray diffraction at varying concentrations of DNA and the cosurfactant hexanol. At low DNA concentrations, an intercalated hexagonal (HIc)→lamellar (Lαc)→inverted hexagonal (HIIc) transformation is found on increasing hexanol content. The HIIc structure is converted into Lαc on adding more DNA. Further increase in hexanol content leads to a phase separation in the surfactant solution, and a reentrant Lαc→HIIc→Lαc transition is observed as DNA concentration is increased. Such structural transformations of DNA-surfactant complexes, driven by DNA concentration, have not been reported until now
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