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Structure of an amphiphilic lattice gas, and its relationship to microclustering of methanol in water.

We present a lattice gas model to show that a system of amphiphilic molecules in solution forms clusters without phase separating. This is designed to be the simplest possible model that captures the essence of the so-called microclustering observed both experimentally and by molecular-dynamics computer simulation of methanol in water. The Hamiltonian contains no adjustable parameters, yet exhibits surprisingly complex behavior as a function of temperature and concentration. By studying the specific heat, we reveal three phase boundaries-between a low-temperature phase comprising straight bilayers, intermediate phases exhibiting twisted bilayer and clustering, and a high-temperature phase, which is essentially gas-like. None of the observed phases correspond to the crystalization or phase separation expected of a simple mixture; instead, they correspond to loss of different types of entropy.

Journal Article↗

Characterization of solid dispersions of itraconazole and hydroxypropylmethylcellulose prepared by melt extrusion, Part II.

PURPOSE: This study was done to elucidate the physical and pharmaceutical properties of itraconazole-HPMC dispersions and the influence of water on the phase separation. METHODS: Extrudates were prepared using a corotating twin-screw hot-stage extruder with fixed process parameters. Modulated-temperature differential scanning calorimetry (MTDSC) and DSC 111 were used to examine the mixing behavior of itraconazole and the carrier by evaluation of the glass transition region. High temperature diffuse reflectance infrared transform spectroscopy (HT-DRIFT) was performed to reveal interactions between itraconazole and HPMC. Dissolution was performed to investigate the pharmaceutical performance of the dispersions. RESULTS: Although the dissolution rate of itraconazole significantly increased, we found that the solid dispersions do not form a homogeneous system. A different picture was obtained depending on the way MTDSC analysis was performed, i.e., using open or closed sample pans. Water can evaporate in open pans, which allows itraconazole to interact with HPMC and leads to a partially mixed phase. Analysis in hermetically closed pans revealed a further phase separation as water remains on the sample and impedes the interaction between drug and polymer. CONCLUSION: Solid dispersions of itraconazole and HPMC do not form a homogeneous phase.

Hypromellose Derivatives↗

Differentiation of food vacuolar membranes during endocytosis in Tetrahymena.

The origin and differentiation of Tetrahymena pyriformis food vacuolar membranes has been studied by freeze-fracture electron microscopy. By measuring the temperature needed to induce the onset of lipid phase separation (as inferred by the appearance of particle-free regions in replicas) and calculating the changes in average intramembrane particle distribution, a distinct modification of the vacuolar membrane could be observed from the time of its formation from disk-shaped vesicles to its maturation before egestion of its indigestible contents. Whereas the nascent vacuolar membrane first showed signs of phase separation at 9 degrees C, this temperature rose to 14 degrees C in the completed vacuole and then, after lysosomal fusion, eventually declined to 12 degrees C. The average membrane particle density on the PF face increased from 761 +/- 219 to 1,625 +/- 350 per micron 2 during membrane differentiation. Like other membranes of the cell, the vacuolar membrane underwent adaptive changes in its physical properties in cells maintained for several hours at low temperature. This exposure to low temperature caused an equal effect in vacuoles formed before, during, or after the temperature shift-down. Normal changes in the properties of the vacuolar membrane may have some bearing on its programmed sequence of fusion reactions.

Animals↗

Synthetic nano-fibrillar extracellular matrices with predesigned macroporous architectures.

Scaffolding plays a pivotal role in tissue engineering. To mimic the architecture of a natural extracellular matrix component-collagen, nona-fibrous matrices have been created with synthetic biodegradable polymers in our laboratory using a phase-separation technique. To improve the cell seeding, distribution, mass transport, and new tissue organization, three-dimensional macroporous architectures are built in the nano-fibrous matrices. Water-soluble porogen materials are first fabricated into three-dimensional negative replicas of the desired macroporous architectures. Polymer solutions are then cast over the porogen assemblies in a mold, and are thermally phase-separated to form nano-fibrous matrices. The porogen materials are leached out with water to finally form the synthetic nano-fibrous extracellular matrices with predesigned macroporous architectures. In this way, synthetic polymer matrices are created with architectural features at several levels, including the anatomical shape of the matrix, macroporous elements (100 microm to millimeters), interfiber distance (microns), and the diameter of the fibers (50-500 nm). These scaffolding materials circumvent the concerns of pathogen transmission and immuno-rejection associated with natural collagen. With the flexibility in the design of chemical structure, molecular weight, architecture, degradation rate, and mechanical properties, these novel synthetic matrices may serve as superior scaffolding for tissue engineering.

Artificial Organs↗

Lateral order in binary lipid alloys and its coupling to membrane functions.

Densitometry, Raman spectroscopy and small angle neutron scattering are employed to elucidate the miscibility behavior of lipid mixtures organized as liposomal dispersions. First, temperature-composition-phase diagrams for several binary alloys of dialkyl-lecithins differing in chain lengths by an increasing number of CH2-groups are derived. A mixture of dimyristoyllecithin and distearoyllecithin (delta CH2 = 4) shows a peritectic phase behavior with a miscibility gap in the gel state. In the fluid phase, at high enough temperatures, homogeneous mixtures of the two components are formed at all molar ratios. However, upon approaching a critical point by either lowering the temperature or increasing the hydrostatic pressure, critical concentration fluctuations are observed. If one component of a binary mixture is charged, electrostatic interactions can be used to induce phase separation at constant temperature. This is demonstrated for Ca2(+)-driven demixing in alloys of lecithin and negatively charged phosphatidylglycerol. The influence of the various concepts for the induction of lateral structure formation in lipid membranes on integral functional units like ionophores is demonstrated by analysing the single channel current fluctuations of gramicidin in bimolecular lipid membranes. Ca2+, as well as polyelectrolyte-induced phase separations are shown and discussed as examples for the important (lateral) order-function relationship in biomembranes.

Electrochemistry↗

Physicochemical stability assessments of lipid emulsions of varying oil composition.

BACKGROUND AND AIMS: Intravenous lipid emulsions have been shown to be unstable when the percent fat >5 microm (PFAT >5 microm) exceeds 0.4% by weight of the total fat present. We investigated the physicochemical stability of a standard low amino acid and carbohydrate mixture containing electrolytes when combined with four different commercial intravenous lipid emulsions of varying oil composition. METHODS: The 20% (w/v) lipid emulsions studied were composed of the following oils (by weight): 1) 1 : 1 soybean/safflower (SS); 2) 100% soybean (S); 3) 1 : 1 soybean/MCT (SM) and 4) 4 : 1 olive/soybean (OS). Physicochemical stability was assessed by light obscuration or extinction using a single-particle optical sensing technique to detect growth of fat globules in the large diameter tail (>1 microm) of the droplet size distribution and by visual analyses for evidence of phase separation. RESULTS: The physicochemical stability of SS and S-based all-in-one mixtures significantly deteriorated over time when compared to the mixtures made from SM and OS. In addition, of the four mixtures studied that contained SS (n=2) and S (n=2), only one of each bag studied showed visually obvious destabilization by the presence of free oil from phase separation, despite highly abnormal changes in the globule size distribution of all four preparations. CONCLUSION: The results suggest that all-in-one mixtures composed of either soybean oil alone or in combination with safflower oil are less stable than those mixed with either MCT or olive oil which also contain sodium oleate that can act as co-emulsifying agent.

Chemical Phenomena↗

Studies on the high molecular weight form of polypeptide chain elongation factor-1 from pig liver. I. Purification and subunit structure.

The high molecular weight form of the polypeptide chain elongation factor-1 (EF-1H) from pig liver was purified to an apparently homogeneous state. The procedure included aqueous two-phase separation, ammonium sulfate fractionation, four successive column chromatographies on Sephadex G-200, CM-Sephadex C-50, hydroxylapatite and octyl-Sepharose CL-4B, and glycerol density gradient centrifugation. By this procedure, the specific activity of EF-1H increased about 60-fold with a recovery of the total activity of 6%, as compared to the material obtained by aqueous two-phase separation followed by ammonium sulfate fractionation. Purified EF-1H was analyzed by polyacrylamide gel electrophoresis under non-denaturing conditions and glycerol gradient centrifugation, revealing that it is apparently homogeneous and has both EF-1 alpha and EF-1 beta gamma activities. The subunit structure of EF-1H was investigated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate as well as two-dimensional gel electrophoresis, which indicated that EF-1H consists of three different subunits, i.e., EF-1 alpha, EF-1 beta, and EF-1 gamma, in an equimolar ratio. The Stokes radius and the sedimentation coefficient of EF-1H were estimated to be 5.9 nm and 6.2S, respectively, from which the molecular weight of EF-1H was calculated to be about 157,000. This value is close to the sum of the molecular weights of EF-1 alpha (53,000), EF-1 beta (30,000), and EF-1 gamma (53,000). Thus, we concluded that EF-1H is a monomer of a 1:1:1 complex of EF-1 alpha, EF-1 beta, and EF-1 gamma, or EF-1 alpha beta gamma and its molecular weight is 136,000. This conclusion was further confirmed by measuring the amounts of EF-1 alpha and EF-1 beta gamma in EF-1H by enzymatic assays. The amino acid composition of EF-1H was determined, and the results revealed that there was a close resemblance between the values of EF-1H and those of EF-1 alpha beta gamma, which were calculated from the reported values for EF-1 alpha, EF-1 beta, and EF-1 gamma.

Amino Acids↗

Zeaxanthin ([3R,3'R]-beta, beta-carotene-3-3'diol) as a resonance Raman and visible absorption probe of membrane structure.

When zeaxanthin ([3R,3R']-beta, beta-carotene-3,3'diol) is inserted into phospholipid dispersions and the latter heated through their gel-liquid crystal phase transitions, large changes are noted in the resonance Raman and absorption spectra of the carotenoid molecule. By analogy with the data of Carey and co-workers (J. Raman Spectrosc. 6:282) who studied the aggregation of zeaxanthin in acetone-water solutions, it is suggested that the carotenoid aggregates in the phospholipid gel state while forming a monomer in liquid crystal phases. The alterations in both the visible absorption and resonance Raman data have been used to monitor phospholipid phase behavior in dipalmitoylphosphatidylcholine and distearoylphosphatidylcholine, (DSPC) one-component systems and binary mixtures. The phase diagram obtained for the binary system, as constructed from visible absorption and resonance Raman data, is compared with that of Shimshick and McConnell (Biochemistry. 12:2351) obtained from electron spin resonance (ESR) studies. Although the agreement between absorption and ESR data is generally satisfactory, onset temperatures for phase separation at low DSPC mole fractions deduced from resonance Raman measurements are several degrees lower than those from the other methods. Nevertheless, the use of zeaxanthin as a resonance Raman and visible absorption probe behavior will be useful in some situations where ordinary Raman spectroscopic data cannot be obtained easily. The advantage of the resonance Raman approach is illustrated in a study of the phase behavior of a phospholipid extract of a cel- mutant of Neurospora crassa. A phase separation region is observed with onset and completion temperatures of -19 and -6 degrees C, respectively.

Carotenoids↗

Preparation of poly(DL-lactide-co-glycolide) nanoparticles by modified spontaneous emulsification solvent diffusion method.

PURPOSE: The objectives of this study were to establish a new preparation method for poly(DL-lactide-co-glycolide) (PLGA) nanoparticles by modifying the spontaneous emulsification solvent diffusion (SESD) method and to elucidate the mechanism of nanoparticle formation on the basis of the phase separation principle of PLGA and poly(vinyl alcohol) (PVA) in the preparation system. METHODS: PLGA nanoparticles were prepared by the modified-SESD method using various solvent systems consisting of two water-miscible organic solvents, in which one solvent has more affinity to PLGA than to PVA and the other has more affinity to PVA than to PLGA. The yield, particle size, size distribution and PVA content of the PLGA nanoparticles were evaluated, and the phase separation behaviors of the polymers were elucidated. RESULTS: The modified-SESD method provided a good yield of PLGA nanoparticles over a wide range of composition ratios in the binary mixture of organic solvents. Several process parameters, including the fed amount of PLGA, PLGA concentration and PVA concentration were examined to achieve the optimum preparation conditions. The discrete powder of PLGA nanoparticles was obtained by freeze-drying. No change in the PVA content of PLGA nanoparticles was observed even after several times of washing treatment by ultrafiltration, suggesting a strong surface adsorption. It was found that the appropriate selections of binary solvent mixtures and polymeric concentrations in both organic and aqueous phases could provide excellent yield and favorable physical properties of PLGA nanoparticles. CONCLUSION: The proposed modified-SESD method can be used to provide PLGA nanoparticles of satisfactory quality at an acceptable yield for industrial purposes.

Diffusion↗

Influence of associated lipid on the properties of purified bovine erythrocyte acetylcholinesterase.

Acetylcholinesterase has been isolated from bovine erythrocyte membranes by affinity chromatography using a m-trimethylammonium ligand. The purified enzyme had hydrophobic properties by the criterion of phase partitioning into Triton X-114. The activity of the hydrophobic enzyme was seen as a slow-moving band in nondenaturing polyacrylamide gels. After treatment with phosphatidylinositol-specific phospholipase C, another form of active enzyme was produced that migrated more rapidly toward the anode in these gels. This form of the enzyme partitioned into the aqueous phase in Triton X-114 phase separation experiments and was therefore hydrophilic. The hydrophobic form bound to concanavalin A in the absence of Triton X-100. As this binding was partially prevented by detergent, but not by alpha-methyl mannoside, D-glucose, or myo-inositol, it is in part hydrophobic. Erythrocyte cell membranes showed acetylcholinesterase activity present as a major form, which was hydrophobic by Triton X-114 phase separation and in nondenaturing gel electrophoresis moved at the same rate as the purified enzyme. In the membrane the enzyme was more thermostable than when purified in detergent. The hydrophobic enzyme isolated, therefore, represents a native form of the acetylcholinesterase present in the bovine erythrocyte cell membrane, but in isolation its stability becomes dependent on amphiphile concentration. Its hydrophobic properties and lectin binding are attributable to the association with the protein of a lipid with the characteristics of a phosphatidylinositol.

Acetylcholinesterase↗

The polar nature of 7-ketocholesterol determines its location within membrane domains and the kinetics of membrane microsolubilization by apolipoprotein A-I.

7-Ketocholesterol is an oxidized derivative of cholesterol with numerous physiological effects. In model membranes, 7-ketocholesterol and cholesterol were compared by physical measures of bilayer order and polarity, formation of detergent resistant domains (DRM), phase separation, and membrane microsolubilization by apolipoprotein A-I. In binary mixtures of a saturated phosphatidylcholine (PC), dipalmitoyl-PC (DPPC), and cholesterol or 7-ketocholesterol, the sterols modulate bilayer order and polarity and induce DRM formation to a similar extent. Cholesterol induces formation of ordered lipid domains (rafts) in tertiary mixtures with dioleoyl-PC (DOPC) and DPPC, or DOPC and sphingomyelin (SM). In tertiary mixtures, cholesterol increased lipid order and reduces bilayer polarity more than 7-ketocholesterol. This effect was more pronounced when the mixtures were in a miscible liquid-disordered (L(d)) phase. Substitution of 7-ketocholesterol for cholesterol dramatically reduced the extent of DRM formation in DOPC/DPPC and DOPC/SM bilayers and ordered lipid phase separation in mixtures of a spin-labeled PC with DPPC and with SM. Compared to cholesterol, 7-ketocholesterol decreased the rate for the microsolubilization of dimyristoyl-PC multilamellar vesicles by apolipoprotein A-I. The membrane effects of 7-ketocholesterol were dependent on the phospholipid matrix. In L(d) phase phospholipids, a model for 7-ketocholesterol indicates that the proximity of the 7-keto and 3beta-OH groups puts both polar moieties at the lipid-water interface to tilt the sterol nucleus to the plane of the bilayer. 7-Ketocholesterol was less effective in forming ordered lipid domains, in decreasing the level of bilayer hydration, and in forming phase boundary bilayer defects. Compared to cholesterol, 7-ketocholesterol can differentially modulate membrane properties involved in protein-membrane association and function.

1,2-Dipalmitoylphosphatidylcholine↗

Magic-angle 13C NMR analysis of hard wheat flour and dough.

Samples of hard wheat flour and dough are analyzed by magic-angle spinning 13C NMR spectroscopy. Cross-polarization magic-angle spinning (CPMAS) 13C NMR spectra of the dry flour allow its starch and protein content to be accurately measured. These two components are phase-separated. Spectra of hydrated hard wheat doughs are collected under both CPMAS and single-pulse carbon with low-power 1H decoupling conditions. The former report on the macromolecular components of the dough, while the latter reveal small molecules which are solubilized by the water. Results of the present study are interpreted as indicating that the protein is largely unaffected by the added water and remains phased-separated from the starch, while water causes significant changes in polymer dynamics of the starch component.

Flour↗

Physical stabilization of starch-allylurea blends by EB-grafting: a compositional and structural study.

A structural and compositional study of thermoplastic blends prepared from native potato starch (NPS) and various amounts of allylurea (AU) was performed to gain a better understanding of possible radiochemical routes to physically stable materials. The blends, mixed at ca. 130 degrees C, were studied in the form of 150 microns-thick films. Upon aging at room temperature, the samples obtained from these blends exhibit macroscopic phase separation under the form of allylurea blooming at their surface. The maximal compatibility of allylurea in amorphized potato starch was assessed by gravimetry and compared to that of urea in mixtures with NPS prepared in similar conditions. Physical aging of the unstable blends was monitored for various initial AU contents. Electron beam (EB) processing of fresh films with AU content above the solubility limit was shown to prevent phase separation, essentially as a consequence of radiation grafting onto starch of the unsaturated additive. X-ray diffractometry was performed to control (i) the effective amorphization of starch upon mixing, (ii) the recrystallization of the incompatible AU fraction from untreated blends in the tetragonal form, and (iii) the retardation or the suppression of this phenomenon after EB processing. The physical stability of the blends treated with a sufficient radiation dose (400-800 kGy) was confirmed by dynamic thermomechanical analysis of samples submitted to various hygrometric conditioning.

Crystallography, X-Ray↗

Nano-fibrous scaffolds for tissue engineering.

With the ability to form nano-fibrous structures, a drive to mimic the extracellular matrix (ECM) and form scaffolds that are an artificial extracellular matrix suitable for tissue formation has begun. These nano-fibrous scaffolds attempt to mimic collagen, a natural extracellular matrix component, and could potentially provide a better environment for tissue formation in tissue engineering systems. Three different approaches toward the formation of nano-fibrous materials have emerged: self-assembly, electrospinning and phase separation. Each of these approaches is very different and has a unique set of characteristics, which lends to its development as a scaffolding system. For instance, self-assembly can generate small diameter nano-fibers in the lowest end of the range of natural extracellular matrix collagen, while electrospinning has only generated large diameter nano-fibers on the upper end of the range of natural extracellular matrix collagen. Phase separation, on the other hand, has generated nano-fibers in the same range as natural extracellular matrix collagen and allows for the design of macropore structures. These attempts at an artificial extracellular matrix have the potential to accommodate cells and guide their growth and subsequent tissue regeneration.

Biomimetics↗

Adenylate cyclase is inhibited upon depletion of plasma-membrane cholesterol.

A procedure has been developed that allows for the depletion of rat liver plasma membrane cholesterol by incubation with liposomes at 4 degrees C. Upon cholesterol depletion, adenylate cyclase activity was inhibited and the membranes became more rigid, as determined by the flexibility of an incorporated fatty acid spin probe. Decreasing the cholesterol/phospholipid molar ratio elicited a pronounced drop in the net fold-stimulation of adenylate cyclase activity by glucagon. Two lipid phase separations were detected in cholesterol-depleted membranes at around 25 degrees C and 13 degrees C respectively. Breaks at these temperatures were observed in Arrhenius plots of both the mobility of the spin probe and the glucagon-stimulated adenylate cyclase activity for the range 2-40 degrees C, but only the one at the lower temperature for the fluoride-stimulated activity. It is proposed that the lipid phase separation occurring at 25 degrees C is localized in the external half of the bilayer, whereas that at 13 degrees C is due to lipids in the inner half of the bilayer. Similar structural and functional perturbations were manifest if the cholesterol-complexing polyene antibiotic amphotericin B was added to native membranes. The mechanism of adenylate cyclase inhibition achieved by cholesterol depletion and the domain structure of the plasma membrane in relation to cholesterol distribution are discussed. Native cholesterol/phospholipid ratios appear to optimize the functioning of adenylate cyclase in liver plasma membranes.

Adenylyl Cyclase Inhibitors↗

In vitro evaluation of heparin adsorption during haemoperfusion with Dowex 1 x 2 anion exchange resin.

Haemoperfusion treatments have found clinical applications in various pathologies, even though problems due to the insufficient biocompatibility of sorbents have often limited its employment. Among these problems, there is that of heparin adsorption which can determine coagulation disorders or the necessity of a dosage correction during the extra corporeal circulation. The aim of the study was to test the influence of different methods of coating on heparin adsorption from an anion exchange resin coated with poly-hydroxyethyl-methacrylate. Eight haemoperfusion cartridges were prepared using 8 g of resin for each (4 cartridges contained resin coated by spray-coating technique and 4 coated by phase-separation one). Haemoperfusion was performed by using 600 IU of heparin (25,000 IU) corresponding to 0.12 ml in 500 ml of swine blood and a closed circuit. Blood flow rate was maintained at 15 ml/min and the circuit pressure was 70 torr during haemoperfusion. Blood samples were withdrawn at timed intervals before and after the column in order to evaluate blood heparin levels. The results demonstrated the superiority of the phase-separation technique that does not permit any interactions between adsorbent substance and drug. As a result of this, the adjustment of the dosage may not be necessary during further in vivo experiments.

Adsorption↗

Evidence of a distribution difference between two gangliosides in bilayer membranes.

Freeze-etch electron microscopy, a platinum shadowing technique, has been used to compare the lateral distribution of several gangliosides in bilayer model membranes by directly visualizing bound lectin molecules. In particular, GM1 and GD1a, major components of brain ganglioside, were studied in phase-separated mixtures of dipalmitoyl- and dielaidoylphosphatidylcholines exposed to Ricinus communis agglutinin and wheat germ agglutinin. The distribution of glycolipid showed evidence of microheterogeneity in that bound lectin tended to occur in clusters of several or more molecules. With GD1a as receptor such clusters were small and very uniformly distributed over the membrane surface. Somewhat larger, irregularly spaced clusters of up to a dozen lectin particles were more typical of membranes bearing GM1 and, in addition, there were occasional extensive patches of bound lectin coexisting with areas apparently devoid of glycolipid receptor in phase-separated mixtures of dipalmitoyl- and dielaidoylphosphatidylcholine. Gangliosides in the latter mixtures were not obviously influenced in their lateral distribution by the presence of coexisting fluid and rigid domains. These basic observations seem to extend to bilayer membranes containing mixtures of two gangliosides. The patterns of lectin binding were not grossly affected by incubation time or history of warming and cooling. This study was extended to bilayers of pure dipalmitoylphosphatidylcholine in expectation that the distinctive features characteristic of the P beta' phase of this lipid might accentuate any behavioural differences between GM1 and GD1a. GM1 was found to exist preferentially in the 'trough' regions between P beta' ripples, while GD1a showed no apparent preferential arrangement. Given that bound lectins adequately reflect glycolipid distribution in membranes, it would appear that structurally different glycolipids from the same host membrane can assume different distributions on the basis of interactions with defined lipid host matrices.

Freeze Fracturing↗

Cluster kinetics and dynamics during spinodal decomposition.

Spinodal decomposition (barrierless phase transition) is a spontaneous phase separation caused by conditions that force the system to become thermodynamically unstable. We consider spinodal decomposition to occur under conditions of large supersaturation S and/or small ratio of interfacial to thermal energies omega, such that the computed number of monomers in a critical nucleus xi*=(omega/ln S)3 is less than unity. The small critical nucleus size is consistent with a negligible energy barrier for initiating condensation. Thus, in contrast to conventional opinion, it is suggested that the spinodal decomposition is related to the homogeneous nucleation of metastable fluids. Population balance equations show how clusters aggregate and rapidly lead to phase separation. Different mass dependences of aggregation rate coefficients are proposed to investigate the fundamental features of spinodal decomposition. When the mass dependency is an integer, the equations are solved by the moment technique to obtain analytical solutions. When the mass dependency is a noninteger, the general cases are solved numerically. All solutions predict the two time regimes observed experimentally: the average length scale of condensed-phase domains increases as a power law with an exponent of 1/3 at early times, followed by a linear increase at longer times.

Journal Article↗