Phase behavior in multilamellar vesicles of DPPC containing ganglioside GM3 with a C18:1 sphingoid base and a 24:0 acyl chain (GM3(18,24)) observed by X-ray diffraction
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The phase behavior of mixtures of dimyristoylphosphatidylcholine (DMPC) with N-palmitoylsphingosinephosphorylcholine (C16SHP) has been investigated in both small unilamellar and large multilamellar vesicles. The steady-state fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) has been used to detect temperature-induced structural changes in these membranes. In addition, electron microscopy has revealed vastly different fracture-face morphologies for large multilamellar vesicles "jet-frozen" from different temperatures. These data have been interpreted in terms of proposed phase diagrams for this lipid mixture. The shapes of the proposed phase diagrams have led us to conclude that phosphatidylcholine and sphingomyelin species of similar acyl chain length mix freely in both highly curved and uncurved bilayers, except at temperatures at which both lipids are in low-temperature, ordered phases. In addition, the similarity of these phase diagrams to phase diagrams for analogous mixtures of pure phosphatidylcholines suggested that sphingomyelin and phosphatidylcholine suggested that sphingomyelin and phosphatidylcholine species might substitute for each other in supporting the lamellar phase necessary for each other in supporting the lamellar phase necessary to cell membrane structure. Finally, the anisotropy of DPH fluorescence was found to be essentially invariant with sphingomyelin content at temperatures just above and below the solid--liquid phase separation in small unilamellar vesicles. This demonstrates that the sphingomyelin backbone, per se, does not order the membrane bilayer. These results are discussed in terms of the possible role of sphingomyelin in controlling acyl chain order within mammalian cell membranes.
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Two fatty acids, perfluorononanoic acid (C8F17COOH) and nonanoic acid (C8H17COOH), were mixed with a cationic hydrocarbon surfactant, tetradecyltrimethylammonium hydroxide (TTAOH), in aqueous solutions for comparative investigation. Phase behaviors of the two systems are quite different because of the special properties of the fluorocarbon chains. For the C8H17COOH/TTAOH/H2O system, a single Lα phase region with phase transition from planar lamellar phase (Lαl phase) to vesicle phase (Lαv phase) was observed. For the C8F17COOH/TTAOH/H2O system, two single phases consisting of vesicles were obtained at room temperature. One is a high viscoelastic gel phase consisting of vesicles with crystalline state bialyers at the C8F17COOH-rich side, which was confirmed by freeze-fracture transmission electron microscope (FF-TEM) and differential scanning calorimetry (DSC) measurements. With the increase of TTAOH proportion, another vesicle phase consisting of liquid state bilayers was observed after the two-phase region. The fluorosurfactant systems prefer to form vesicle bilayers than the corresponding hydrocarbon ones because of the rigid structure, the stronger hydrophobicity, and the larger volume of fluorocarbon chains.
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In this thesis we have studied structural changes of a nonionic surfactant self-assembly, as governed by changes in temperature and concentration. The techniques used in experimental investigations were dynamic and static light scattering, nuclear magnetic resonance and cryo-transmission electron microscopy (using a novel specimen preparation protocol). The model systems consist of nonionic surfactants (alkyl oligo poly(oxy)ethylene ethers) C12E4, C12E5, C16E6 and C10E3 in water. These surfactants form micelles in water at lower temperatures, and planar bilayers and inverted phases at elevated temperatures. In a dilute micelle phase, the linear increase in size of wormlike micelles as a function of concentration is investigated and quantified. At higher concentrations, intermicelle interactions start to be significant. Here, micelles start to branch and overlap. When a dilute solution of nonionic wormlike micelles is rapidly heated to the temperatures where a lamellar phase normally exists, vesicles are formed. These vesicles are large and polydisperse and the size distribution depends on the rate of heating. Below a certain temperature, vesicles appear to be stable. Upon rapid increase in temperature (T-jump) to temperatures in the vicinity and above a three-phase line, vesicles fuse. The rate of fusion is determined by the final temperature of the T-jump. At higher concentrations, the behavior of lamellar phase is investigated. An unexpected lamellar phase separation in a centrifugal field is observed and a method for measuring undulation forces is developed. Upon decrease in temperature, an intermediate phase between lamellar and cubic/hexagonal phase was imaged. (Less)
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Phosphatidylcholines and sphingomyelins are among the most abundant lipids in mammalian cell membranes, being major components of platelets or erythrocytes, and of the lipid monolayer of lipoproteins. General efforts have been devoted to the elucidation of the interaction of the ubiquitous membrane component cholesterol with these choline phospholipids, but fewer studies have been reported on the interaction between the phospholipids themselves. A gel to liquid-crystalline phase transition was observed for pure sphingomyelin liposomes at physiological temperature, while palmitoyloleoyl phosphatidylcholine adopts a liquid-crystalline phase in the temperature range 273–323 K. The two phospholipids are miscible at all molar ratios in the liquid-crystalline phase, characterized by very similar lamellar repeat distances for all binary lipid mixtures. The gel phase of pure sphingolipid liposomes exhibited a markedly smaller lamellar repeat distance as compared to mixed lipid vesicles, which increased slightly with temperature for the pure sphingomyelin (66.9–69.2 Å). Concomitantly, altered hydrocarbon chain packing was observed. Similar diffractograms were obtained in the presence of 10 mol% phosphatidylcholine. However, in the composition range between 20 and 60 mol% phosphatidylcholine in the phosphatidylcholine–sphingomyelin admixture, the lamellar repeat distance in the gel phase was markedly increased and remained almost constant (around 75 Å) below the phase transition.
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Time-resolved X-ray diffraction has been used to investigate phase transitions in lipid-water systems. A variety of lamellar and non-lamellar phases have been examined and the transit time of the transitions between phases determined. A comparison is made between the characteristic phase transition transit times for phospholipids, galactolipids and surfactants. Transitions have been measured which span times of less than one second to orders of minutes and longer.
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1,2-Dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) dispersed in 100 mM NaCl at pH 7 forms negatively charged multilamellar vesicles. After addition of divalent cation salts (Mg2+, Ca2+ and Sr2+) in an equimolar cation/lipid ratio, a precipitate of lipid divalent cation (2:1) complexes is formed. These complexes were examined by X-ray diffraction and freeze-fracture electron microscopy to elucidate their phase structure as a function of temperature and incubation time at low temperature. The structural and morphological analysis of the DMPG divalent cation complexes stored for 7 d at 4°C (initial state) revealed the formation of a lamellar phase with a very short lamellar repeat distance of 4.4–4.6 nm. The hydrocarbon chains of the lipid are packed in a subgel state. For all three complexes smooth fracture faces with crystalline appearance were observed in the electron micrographs. Increasing the temperature induces a subgel to liquid-crystalline phase transition at around 58–62°C for the Mg2+ and Sr2+ complexes and at 85°C for the Ca2+ complex. The electron micrographs of the Mg2+ and Sr2+ complexes showed that in the liquid-crystalline phase regular multilamellar vesicles or large multilamellar cylinders are formed, whereas the Ca2+ complex forms lamellar structures of crystalline appearance. At lower temperature (below 35°C), new phases appear: Mg2+ and Sr2+ complexes form a gel-like phase with an increased lamellar repeat distance of ∽1 nm and a phase transition temperature of about 35°C, whereas the Ca2+ complex transforms into a subgel phase which is very similar to the initial state. Our results show that the interaction of Mg2+ and Sr2+ with DMPG is very similar but different from that observed with Ca2+. The results also indicate interactions of Ca2+ with DMPG in the liquid-crystalline phase, whereas this is not observed for the other two ions.
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