Characterization of a complex dispersion of multilamellar vesicles

Spherulites ^® are multilamellar vesicles made up of surfactant bilayers. These vesicles would potentially be very useful for the encapsulation and protection of molecules; however, traditional formulations of these vesicles are poor at retaining small hydrophilic molecules (below 1000 g/mol). In this study, we present new systems of Spherulites called complex dispersions. These are prepared by dispersing Spherulites in an oil medium, and then emulsifying this oily dispersion of Spherulites within an aqueous solvent. These new systems provide an additional oil barrier between encapsulated molecules and an external aqueous phase. We have used polarized light optical microscopy, X-ray diffraction and freeze–fracture electron microscopy to study a complex dispersion of Spherulites at all stages of its preparation. We first studied the sheared lamellar phase, followed by the dispersion of the multilamellar vesicles in the oily medium and finally the emulsification of the oily dispersion within the aqueous solvent. We compared our results on lamellar phases with previous results obtained with Spherulites directly dispersible in an aqueous medium. Since the formulation of our lamellar phase included a large percentage of oil as a component, we studied the localization of the oil in the lamellar structure. We also studied the influence of osmotic pressure on complex dispersions, because complex dispersions possess a double structure similar to that of water-in-oil-in-water emulsions and multiple emulsions are known to be sensitive to osmotic pressure. In conclusion, complex dispersions proved to be new potential carriers exhibiting some unique physical properties.     

Twin‐like domains in chiral smectic C liquid crystals

In the chiral smectic C phase of liquid crystals with the phase transition N*–SmC*, texture development depending on the sample thickness is reported. In very thin samples, domains of rectangular‐like shape are observed. As two possible tilts of smectic layers are possible for one anchoring direction, smectic layers inside a domain, called twin‐like domains, are tilted with respect to layers in outer regions, similarly to crystalline planes in solid crystalline twins. An elastic model of such a twin domain is proposed and its energy determined.     

Supramolecular polymorphism of DNA in non-cationic L<Subscript>α</Subscript> lipid phases

The structure of a complex between hydrated DNA and a non-cationic lipid is studied, including its phase diagram. The complex is spontaneously formed by adding DNA fragments (ca. 150 base pairs in length) to non-cationic lipids and water. The self-assembly process often leads to highly ordered structures. The structures were studied by combining X-ray scattering, fluorescence and polarized microscopy, as well as freeze-fracture experiments with transmission electron microscopy. We observe a significant increase of the smectic order as DNA is incorporated into the water layers of the lamellar host phase, and stabilization of single phase domains for large amounts of DNA. The effect of confinement on DNA ordering is investigated by varying the water content, following three dilution lines. A rich polymorphism is found, ranging from weakly correlated DNA-DNA in-plane organizations to highly ordered structures, where transmembrane correlations lead to the formation of columnar rectangular and columnar hexagonal superlattices of nucleotides embedded between lipid lamellae. From these observations, we suggest that addition of DNA to the lamellar phase significantly restricts membrane fluctuations above a certain concentration and helps the formation of the lipoplex. The alteration of membrane steric interactions, together with the appearance of interfacial interactions between membranes and DNA molecules may be a relevant mechanism for the emergence of highly ordered structures in the concentrated regime.     

Sequestration of Proteins by Fatty Acid Coacervates for Their Encapsulation within Vesicles

Encapsulating biological materials in lipid vesicles is of interest for mimicking cells; however, except in some particular cases, such processes do not occur spontaneously. Herein, we developed a simple and robust method for encapsulating proteins in fatty acid vesicles in high yields. Fatty acid based, membrane‐free coacervates spontaneously sequester proteins and can reversibly form membranous vesicles upon varying the pH value, the precrowding feature in coacervates allowing for protein encapsulation within vesicles. We then produced enzyme‐enriched vesicles and show that enzymatic reactions can occur in these micrometric capsules. This work could be of interest in the field of synthetic biology for building microreactors.     

Structure and orientation changes of omega- and gamma-gliadins at the air-water interface: a PM-IRRAS spectroscopy and Brewster angle microscopy study

Microscopic and molecular structures of omega- and gamma-gliadin monolayers at the air-water interface were studied under compression by three complementary techniques: compression isotherms, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and Brewster angle microscopy (BAM). For high molecular areas, gliadin films are homogeneous, and a flat orientation of secondary structures relative to the interface is observed. With increasing compression, the nature and orientation of secondary structures changed to minimize the interfacial area. The gamma-gliadin film is the most stable at the air-water interface; its interfacial volume is constant with increasing compression, contrary to omega-gliadin films whose molecules are forced out of the interface. gamma-Gliadin stability at a high level of compression is interpreted by a stacking model.     

Anisotropic Brownian motion in ordered phases of DNA fragments

Using Fluorescence Recovery After Photobleaching, we investigate the Brownian motion of DNA rod-like fragments in two distinct anisotropic phases with a local nematic symmetry. The height of the measurement volume ensures the averaging of the anisotropy of the in-plane diffusive motion parallel or perpendicular to the local nematic director in aligned domains. Still, as shown in using a model specifically designed to handle such a situation and predicting a non-Gaussian shape for the bleached spot as fluorescence recovery proceeds, the two distinct diffusion coefficients of the DNA particles can be retrieved from data analysis. In the first system investigated (a ternary DNA-lipid lamellar complex), the magnitude and anisotropy of the diffusion coefficient of the DNA fragments confined by the lipid bilayers are obtained for the first time. In the second, binary DNA-solvent system, the magnitude of the diffusion coefficient is found to decrease markedly as DNA concentration is increased from isotropic to cholesteric phase. In addition, the diffusion coefficient anisotropy measured within cholesteric domains in the phase coexistence region increases with concentration, and eventually reaches a high value in the cholesteric phase.     

Confined diffusion of hydrophilic probes inserted in lyotropic lamellar phases

The dynamic behaviour of three hydrophilic probes (two dyes and one fluorescently-labelled protein) inserted in the water layers of lyotropic lamellar phases has been studied by confocal fluorescence recovery experiments. Two different, ionic (AOT/NaCl/ H(2)O and non-ionic ( C(12)E(5) /hexanol/ H(2)O host systems were studied. The confinement effect has been carefully monitored using the swelling properties of the lamellar phases. In all cases, we measure the evolution of the probe diffusion coefficient in the layer plane D ( perpendicular) versus the separation between the membranes d(w). Depending on the composition of the lamellar phase, this distance can be continuously adjusted from 500A to about 20A. For all systems, we observe a first regime, called dilute regime, where the diffusion coefficient decreases almost linearly with 1/d (w) . In this regime, the Faxén theory for the friction coefficient of a spherical particle symmetrically dragged between two rigid walls can largely explain our results. More unexpectedly, when the membranes are non-ionic, and also quite flexible ( C(12)E(5) /hexanol in water), we observe the existence of a second, concentrated (or confined) regime, where the diffusion coefficient is nearly constant and different from zero for membrane separations smaller than the particle size. This new regime can be heuristically explained by simple arguments taking into account the membrane fluidity.     

Novel twist grain boundary smectic-C phases

Several new kinds of smectic-C twist grain boundary phases (TGBC) have been observed during the last few years. These pure compounds or mixtures exhibit unusual textures with polygonal lattices (square or hexagonal grids) in the plane normal to the TGB helix. The structure of these new phases seems to be complex and different from reported and predicted TGBC phases. In this article, we review the main results obtained on these different new phases, and we propose new TGBC structures based on the well-known splayed polarization --twisted director structures adopted by chiral smectic-C's in planar aligned (bookshelf) cells. The observed square or hexagonal lattices are made of superimposed pairs of unwinding lines due to the suppression of the helix within smectic-C blocks by the grain boundaries (unwinding walls). A lattice-free TGBC occurs if the helix within smectic-C blocks is suppressed completely. Received 15 March 2001