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PubMed2004

van der Waals normal form for a one-dimensional hydrodynamic model.

Phase separation in a fluidized granular system is studied. We consider a one-dimensional hydrodynamic model that mimics a two-dimensional fluidized granular system with a vibrating wall and without gravity, which exhibits a phase separation. Close to the critical point, by means of an adiabatic elimination of the temperature, we deduce the van der Waals normal form, which is the equation that describes the slow dynamics of the system and predicts the qualitative behavior in different regions of parameters. This allows us to understand the origin of the effective viscosity and the spatial saturation at the onset of the bifurcation. The hydrodynamic model and van der Waals normal form exhibit a behavior similar to the one observed in molecular dynamics simulations.

Journal Article↗

PubMed2006

Influence of a random field on particle fractionation and solidification in liquid-crystal colloid mixtures.

The influence of a random-anisotropy (RA) type disorder on the phase separation of nematogen-colloid mixtures is studied theoretically by combining the phenomenological Landau-de Gennes, Carnahan-Starling, and hard-sphere crystal theories. We assume that the colloids enforce the RA disorder on the surrounding thermotropic liquid-crystal (LC) molecules. We adopt the Imry-Ma argument according to which the lower-temperature phase exhibits a domain-type pattern. The colloids impose a finite degree of orientational ordering even in the isotropic (paranematic) phase. In the ordered phase they give rise to a domain-type structure, resulting in the distorted nematic (speronematic) phase. The RA field opposes the phase separation tendency. With increasing disorder the difference between the paranematic and speronematic ordering decreases. Consequently there is a critical disorder, above which both phases become identical from the orientation point of view, but have different concentrations of colloids. We have also estimated another characteristic value of disorder above which the isotropic phase can exist only in a liquid state, the crystal phase being suppressed completely.

Anisotropy↗

PubMed2003

Release of decay-accelerating factor into stools of patients with colorectal cancer by means of cleavage at the site of glycosylphosphatidylinositol anchor.

The expression of decay-accelerating factor (DAF), a cell-membrane-complement regulator, is enhanced in colorectal cancer, and DAF is detected in the stools of patients with colorectal cancer. In this study, to elucidate mechanisms whereby DAF is released into the colonic lumen, we analyzed and compared the properties of DAF in stools and colorectal-cancer tissues. Stool specimens taken before surgery and tissue samples from surgically resected colorectal cancers were obtained from 21 patients. We analyzed DAF in stool and tissue specimens using immunoblotting, ultracentrifugation, and phase separation with Triton X-114. We analyzed the expression profile of DAF mRNA in cancer tissues using reverse transcription-polymerase chain reaction to determine whether DAF transcripts for a secretory form of DAF were present. With the use of immunoblotting, stool DAF was detected as a broad band with a molecular weight of around 70,000 kDa that migrated slightly more slowly than cancer-tissue DAF. About 90% of stool DAF was present as a soluble form that remained in the 100,000 g supernatant after ultracentrifugation. On phase separation with Triton X-114, the soluble stool DAF was partitioned mainly into the aqueous phase, indicating its hydrophilic nature and lack of the fatty-acid glycosylphosphatidylinositol anchor component. In colorectal cancer tissues, reverse transcription-polymerase chain reaction experiments revealed a nonspliced DAF messenger RNA that encodes a secretory form of DAF in just 2 of the 21 specimens examined. These data suggest that DAF is released from colorectal cancer cells by way of cleavage of membrane-bound DAF at the site of the glycosylphosphatidylinositol anchor.

Adult↗

PubMed2004

Adsorption of cationic cellulose derivative/anionic surfactant complexes onto solid surfaces. II. Hydrophobized silica surfaces.

The effect of the anionic surfactant SDS (sodium dodecyl sulfate) on the adsorption behavior of cationic hydroxyethyl cellulose (Polymer JR-400) and hydrophobically modified cationic cellulose (Quatrisoft LM-200) at hydrophobized silica has been investigated by null ellipsometry and compared with the previous data for adsorption onto hydrophilic silica surfaces. The adsorbed amount of LM-200 is found to be considerably larger than the adsorbed amount of JR-400 at both surfaces. Both polymers had higher affinity toward hydrophobized silica than to silica. The effect of SDS on polymer adsorption was studied under two different conditions: adsorption of polymer/SDS complexes from premixed solutions and addition of SDS to preadsorbed polymer layers. Association of the surfactant to the polymer seems to control the interfacial behavior, which depends on the surfactant concentration. For the JR-400/SDS complex, the adsorbed amount on hydrophobized silica started to increase progressively from much lower SDS concentrations, while the adsorbed amount on silica increased sharply only slightly below the phase separation region. For the LM-200/SDS complex, the adsorbed amounts increased progressively from very low SDS concentrations at both surfaces, and no large difference in the adsorption behavior was observed between two surfaces below the phase separation region. The complex desorbed from the surface at high SDS concentrations above the critical micelle concentration. The reversibility of the adsorption of polymer/SDS complexes upon rinsing was also investigated. When the premixed polymer/SDS solutions at high SDS concentrations (>5 mM) were diluted by adding water, the adsorbed amount increased due to the precipitation of the complex. The effect of the rinsing process on the adsorbed layer was determined by the hydrophobicity of the polymer and the surface.

Adsorption↗

PubMed1977

[Isolation of viruses from urban sewage: comparison of methods].

A comparison has been made among some methods (concentration on aluminium hydroxide precipitates, polymer two-phase separation, cellulose membrane filtration, direct inoculation) for virus isolation from raw sewage. The concentration on aluminium hydroxide has given significantly better results than the other methods. No significant differences have been noted between the two-phase separation and direct inoculation method. The membrane filtration method has given no positive result. All isolated viruses belonged to the adenovirus group.

Adenoviridae↗

PubMed1984

Permeability of bilayers composed of mixtures of saturated phospholipids: evidence for compositional segregation of lipids at phase boundaries.

Lipid bilayers composed of a binary mixture of saturated phospholipids display broad permeability maxima in the temperature region of phase separation. We have proposed that this enhanced diffusion occurs through defects which develop at boundaries between liquid crystal and gel state domains and that one of the lipid components of the mixture might specifically congregate at these defects. To explore this postulate we have performed the following type of experiment. Liposomes were formed from different mole ratios of the lipid pairs 1,2-dimyristoyl-sn-glycero-3-phosphocholine (dimyristoyl GPC)--1,2-distearoyl-sn-glycero-3-phosphocholine (distearoyl GPC) and dimyristoyl GPC--1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (dimyristoyl GPE) and 22Na effluxes measured over an extensive temperature span (15 to 60 degrees C). For a given mixture we plotted 22Na movement through the boundary regions which develop during phase separation against the fraction of dimyristoyl GPC in the coexisting solid and liquid phases at that temperature. As dimyristoyl GPC is added to either distearoyl GPC or dimyristoyl GPE, boundary efflux remains low until the mole fraction of dimyristoyl GPC in the coexisting solid and liquid phases reaches 0.1 and 0.4 (for dimyristoyl GPC--distearoyl GPC) and 0.3 and 0.7 (for dimyristoyl GPC--dimyristoyl GPE), respectively. As the fraction of dimyristoyl GPC in the coexisting phases exceeds these values, boundary permeability rises and with further increments in this phospholipid, the mixtures begin to assume permeability characteristics similar to that of dimyristoyl GPC alone. These observations suggest that at low mole fractions of dimyristoyl GPC, the boundary regions are populated by the distearoyl GPC or dimyristoyl GPE molecules and hence efflux rates are small.(ABSTRACT TRUNCATED AT 250 WORDS)

Dimyristoylphosphatidylcholine↗

PubMed1979

Temperature dependence of the fluorescence of pyrene labeled crab nerve membranes.

A method, using albumin-pyrene complexes, has been developed for labeling, in a controlled manner, crab leg nerves whose excitability was preserved. The excimer-to-monomer fluorescence intensity ratio of pyrene, embedded in nerve membrane lipids and in their crude lipid extracts, is a fluidity parameter which displayed the following features with temperatures. a--a temperature-dependent increase of fluidity b--three breaks (6 degrees, 19 degrees and 37 degrees C) in the physiological medium c--In Ca++-depleted sea water, the 37 degrees characteristic temperature vanished. These breaks may reflect some lateral phase separations of the lipid components of nerve membranes. The calcium dependent temperature break may involve a segregation of acidic phospholipids while the other two breaks (6 degrees and 19 degrees C) may be due to neutral lipids phase separation. The relationship of these findings to nerve function is discussed.

Animals↗

PubMed2001

Mobile silver ions and glass formation in solid electrolytes.

Solid electrolytes are a class of materials in which the cationic or anionic constituents are not confined to specific lattice sites, but are essentially free to move throughout the structure. The solid electrolytes AgI and Ag2Se (refs 1, 2, 3, 4, 5, 6, 7) are of interest for their use as additives in network glasses, such as chalcogenides and oxides, because the resulting composite glasses can show high electrical conductivities with potential applications for batteries, sensors and displays. Here we show that these composite glasses can exhibit two distinct types of molecular structures-an intrinsic phase-separation that results in a bimodal distribution of glass transition temperatures, and a microscopically homogeneous network displaying a single glass transition temperature. For the first case, the two transition temperatures correspond to the solid-electrolyte glass phase and the main glass phase (the 'base glass'), enabling us to show that the glass transition temperatures for the AgI and Ag2Se phases are respectively 75 and 230 degrees C. Furthermore, we show that the magnitude of the bimodal glass transition temperatures can be quantitatively understood in terms of network connectivity, provided that the Ag+ cations undergo fast-ion motion in the glasses. These results allow us to unambiguously distinguish base glasses in which these additives are homogeneously alloyed from those in which an intrinsic phase separation occurs, and to provide clues to understanding ion-transport behaviour in these superionic conductors.

Journal Article↗

PubMed2004

SNAREs prefer liquid-disordered over "raft" (liquid-ordered) domains when reconstituted into giant unilamellar vesicles.

Membrane domains ("rafts") have received great attention as potential platforms for proteins in signaling and trafficking. Because rafts are believed to form by cooperative lipid interactions but are not directly accessible in vivo, artificial phase-separating lipid bilayers are useful model systems. Giant unilamellar vesicles (GUVs) offer large free-standing bilayers, but suitable methods for incorporating proteins are still scarce. Here we report the reconstitution of two water-insoluble SNARE proteins into GUVs without fusogenic additives. Following reconstitution, protein functionality was assayed by confocal imaging and fluorescence auto- and cross-correlation spectroscopy. Incorporation into GUVs containing phase-separating lipids revealed that, in the absence of other cellular factors, both proteins exhibit an intrinsic preference for the liquid-disordered phase. Although the picture from detergent resistance assays on whole cells is ambiguous, reconstitutions of components of the exocytic machinery into GUVs by this new approach should yield insight into the dynamics of protein complex associations with hypothesized liquid-ordered phase microdomains, the correspondence between detergent-resistant membranes and liquid-ordered phase, and the mechanism of SNARE-mediated membrane fusion.

Exocytosis↗

PubMed1998

Influence of Interfacial Properties of Lipophilic Surfactants on Water-in-Oil Emulsion Stability

The purpose of this study is to investigate interfacial properties of lipophilic nonionic surfactants of the sorbitan fatty acid ester type (Spans 20, 80, 83, and 85) with the aim of selecting appropriate emulsifiers for water-in-oil (W/O) emulsions under different conditions. Interfacial tension and rheology were investigated at a planar mineral oil/double distilled water interface using a Wilhelmy plate method and an oscillatory ring surface rheometer, respectively. The effects of bulk concentration, temperature and the presence of salt in the aqueous phase on the interfacial properties of surfactant films were determined. The relationship between interfacial film properties and W/O emulsion stability in the presence of salt was assessed by visual observation of phase separation on storage. All the surfactants exhibited viscoelastic properties with the exception of Span 85. Interfacial association of inverse micelles and/or surfactant multilayer formation are probably responsible for the observed viscoelasticity. Addition of sodium chloride to the aqueous phase and increase in temperature influenced the viscoelastic properties. Emulsion stability to phase separation decreased in the presence of salt. The interfacial rheology data were predictive of emulsion stability for the systems studied. Copyright 1998 Academic Press. Copyright 1998Academic Press

Journal Article↗

PubMed1984

On the localization of ubiquinone in phosphatidylcholine bilayers.

The location of ubiquinone-10 in phospholipid bilayers was analyzed using a variety of physical techniques. Specifically, we examined the hypothesis that ubiquinone localizes at the geometric center of phospholipid bilayers. Light microscopy of dipalmitoylphosphatidylcholine at room temperature in the presence of 0.05-0.5 mol fraction ubiquinone showed two separate phases, one birefringent lamellar phase and one phase that consisted of isotropic liquid droplets. The isotropic phase had a distinct yellow color, characteristic of melted ubiquinone. [13C]NMR spectroscopy of phosphatidylcholine liposomes containing added ubiquinone indicated a marked effect on the 13C-spin lattice relaxation times of the lipid hydrocarbon chain atoms near the polar head region of the bilayer, but almost no effect on those atoms nearest the center of the bilayer. X-ray diffraction experiments showed that for phosphatidylcholine bilayers, both in the gel and liquid-crystal-line phases, the presence of ubiquinone did not change either the lamellar repeat period or the wide-angle reflections from the lipid hydrocarbon chains. In electron micrographs, the hydrophobic freeze-fracture surfaces of bilayers in the rippled (P beta') phase were also unmodified by the presence of ubiquinone. These results indicate that the ubiquinone which does partition into the bilayer is not localized preferentially between the monolayers, and that an appreciable fraction of the ubiquinone forms a separate phase located outside the lipid bilayer.

Freeze Fracturing↗

PubMed2024

DciA, the Bacterial Replicative Helicase Loader, Promotes LLPS in the Presence of ssDNA.

The loading of the bacterial replicative helicase DnaB is an essential step for genome replication and depends on the assistance of accessory proteins. Several of these proteins have been identified across the bacterial phyla. DciA is the most common loading protein in bacteria, yet the one whose mechanism is the least understood. We have previously shown that DciA from Vibrio cholerae is composed of a globular domain followed by an unfolded extension and demonstrated its strong affinity for DNA. Here, we characterize the condensates formed by VcDciA upon interaction with a short single-stranded DNA substrate. We demonstrate the fluidity of these condensates using light microscopy and address their network organization through electron microscopy, thereby bridging events to conclude on a liquid-liquid phase separation behavior. Additionally, we observe the recruitment of DnaB in the droplets, concomitant with the release of DciA. We show that the well-known helicase loader DnaC from Escherichia coli is also competent to form these phase-separated condensates in the presence of ssDNA. Our phenomenological data are still preliminary as regards the existence of these condensates in vivo, but open the way for exploring the potential involvement of DciA in the formation of non-membrane compartments within the bacterium to facilitate the assembly of replication players on chromosomal DNA.

DNA, Single-Stranded↗

PubMed2000

Mechanisms for cytoplasmic organization: an overview.

One of the basic characteristics of life is the intrinsic organization of cytoplasm, yet we know surprisingly little about the manner in which cytoplasmic macromolecules are arranged. It is clear that cytoplasm is not the homogeneous "soup" it was once envisioned to be, but a comprehensive model for cytoplasmic organization is not available in modern cell biology. The premise of this volume is that phase separation in cytoplasm may play a role in organization at the subcellular level. Other mechanisms for non-membrane-bounded intracellular organization have previously been proposed. Some of these will be reviewed in this chapter. Multiple mechanisms, involving phase separation, specific intracellular targeting, formation of macromolecular complexes, and channeling, all could well contribute to cytoplasmic organization. Temporal and spatial organization, as well as composition, are likely to be important in defining the characteristics of cytoplasm.

Animals↗

PubMed2001

Preparation of liposomes containing Ceramide 3 and their membrane characteristics.

Liposomes composed of Ceramide 3, [2S,3S,4R-2-stearoylamide-1,3,4-octadecanetriol], and L-alpha-dipalmitoylphosphatidylcholine (DPPC) were prepared by varying the amount of Ceramide 3, and the effects of Ceramide 3 on the liposome formation, particle size, dispersibility, microviscosity and phase transition temperature were examined by means of a microscopy, a dynamic light scattering method, a fluorescence polarization method, a differential scanning calorimetry (DSC) and so on. All the DPPC was able to contribute to the formation of liposomes up to 0.130 mol fraction of Ceramide 3. The particle size of liposomes was almost unaffected by the addition of Ceramide 3. The dispersibility of liposomes containing Ceramide 3 was maintained for at least 15 days. The microviscosity of liposomal bilayer membranes in the liquid crystalline state was increased with increasing the mole fraction of Ceramide 3, while that in the gel state was independent of the mole fraction of Ceramide 3. The phase transition temperature from gel to liquid crystalline states of DPPC bilayer membranes was shifted upwards with the addition of Ceramide 3, indicating a cooperative interaction between DPPC and Ceramide 3 molecules. However, a sharp DSC peak became broad and split at higher mole fractions of Ceramide 3, suggesting a phase separation in the mixed DPPC/Ceramide 3 liposomal bilayer membranes. These phenomena were suggested to be related to the previously observed fact for the mixed DPPC/Ceramide 3 monolayers that Ceramide 3 interacts with DPPC in the liquid-expanded phase with consequent phase separation accompanied with domain formation.

Journal Article↗

PubMed2006

Adsorption and mixing behavior of ethers and alkanes at the solid/liquid interface.

The behavior of binary mixtures of linear symmetrical ethers and alkanes adsorbed to a graphite surface from the bulk liquid mixtures is described on the basis of differential scanning calorimetry (DSC) data. Both the ethers and the alkanes are found to form solid monolayers when adsorbed from the liquid. In addition, the monolayer mixing behavior is addressed. The results indicate that there is good, essentially ideal, mixing in the monolayers for ethers and alkanes of the same overall chain length, where the chain length is equal to the total number of carbon and oxygen atoms in the molecule. However, a difference in chain length of more than one atom results in a variation of mixing behavior from nonideal mixing (for long pairs) to phase separation (for short pairs) on the graphite surface. Hence, we conclude that it is the relative chain lengths that control mixing behavior. The results are quantified using a regular solution model with a correction for preferential adsorption. The phase behavior of the mixed monolayers is also compared to the behavior of the bulk. Interestingly, we observe mixtures where the bulk and monolayer behavior are quite different, for example, phase separation in the bulk but essentially ideal mixing in the monolayer for mixtures of ethers and alkanes with the same chain lengths. At present, we attribute this mixing in the monolayer to dilution of the unfavorable ether oxygen-ether oxygen lone pair interactions by the coadsorbed alkanes. In addition, we find evidence for the preferential adsorption of the alkane over the ether. For example, heptane is preferentially adsorbed over dibutyl ether even though it contains two fewer atoms in the molecular chain. This contrasts with the preferential adsorption of alcohols over alkanes reported previously (Messe, L.; Perdigon, A.; Clarke, S. M.; Inaba, A.; Arnold, T. Langmuir 2005, 21, 5085-5093).

Journal Article↗

PubMed1997

A novel biodegradable system based on gelatin nanoparticles and poly(lactic-co-glycolic acid) microspheres for protein and peptide drug delivery.

Gelatin nanoparticle-poly(lactic-co-glycolic acid) (PLGA) microsphere composites were prepared by encapsulating protein-loaded gelatin nanoparticles in PLGA microspheres. This encapsulation was conducted by using a phase separation method and a solvent extraction method. The average diameter of the gelatin nanoparticle-PLGA microsphere composites is between 160 and 175 microm. Protein loading efficiency is 93.2% for the nanoparticle-microsphere composite prepared by the phase separation method, while it is 31.31% for the composite prepared by the solvent extraction method. Protein release experiments indicate that this new composite system possesses sustained release characteristics. This system also demonstrates the capability of preventing the denaturation of protein drugs.

Biotransformation↗

PubMed2025

Accelerated Ostwald ripening by chemical activity.

Phase separation of biomolecular condensates promotes membrane-free compartmentalization in cells. The dynamics of these biocondensates is routinely regulated by energy-consuming processes. Here, we devise a theory pinpointing how active chemical reactions, interconverting molecules between phase-separating and inert forms, can drive faster condensate coarsening. We find that mass conservation limits droplet volume growth to being linear in time regardless of activity, resembling the passive Lifshitz-Slyozov law. However, if reactions are restricted to occur only outside droplets, the rate of Ostwald ripening can be increased by an arbitrarily large factor. Our theory is quantitatively supported by recent experiments on ripening in the presence of fueled interconversion reactions, under precisely the predicted conditions. We posit that the ability to induce rapid biocondensate coarsening can be advantageous in synthetic-biological contexts, e.g., as a regulator of metabolic channeling.

Journal Article↗

PubMed2006

Effect of hydrophobic mismatch on phase behavior of lipid membranes.

We investigate the competing effects of hydrophobic mismatch and chain stretching on the morphology and evolution of domains in lipid membranes via Monte Carlo techniques. We model the membrane as a binary mixture of particles that differ in their preferred lengths, with the shorter particles mimicking unsaturated nonraft lipids and the longer particles mimicking saturated raft lipids. We find that phase separation can be induced upon increasing either the ratio J/kappa of the hydrophobic surface tension J to the compressibility modulus kappa. J/kappa determines the decay length for thickness changes. When this decay length is larger than the system size the membrane remains mixed. Furthermore, increasing the thickness relaxation time can induce transient phase separation.

Computer Simulation↗