Reaggregation Process Research Articles

Osmium−Bismuth Complexes from the Reaction of Os3(CO)11(NCMe) with BiPh3

Five new compounds were obtained from the reaction of Os(3)(CO)(11)(NCMe), 1, with BiPh(3) in hexane solution at reflux. These have been identified as Os(2)(CO)(8)(mu-BiPh), 2, Os(CO)(4)Ph(2), 3, Os(4)(CO)(14)(mu-eta(3)-O=CC(6)H(5))(mu(4)-Bi), 4, Os(4)(CO)(15)Ph(mu(4)-Bi), 5, and Os(5)(CO)(19)Ph(mu(4)-Bi), 6. Cleavage of the phenyl groups from the BiPh(3) was the dominant reaction pathway. Fragmentation of the original triosmium cluster was accompanied by reaggregation processes that were facilitated by the naked bismuth to yield the higher nuclearity osmium cluster complexes containing spiro-bridging bismuth ligands. Compound 6 was photo-decarbonylated to yield the compound HOs(5)(CO)(18)(mu-eta(2)-C(6)H(4))(mu(4)-Bi), 7, formed by ortho-CH activation of the sigma-bonded phenyl ring in 6 to form a bridging eta(2)-benzyne ligand. Compounds 2- 7 were each characterized structurally by a single-crystal X-ray diffraction analysis.

Effect of thermal annealing on the electrical conductivity of high-strength bicomponent polymer tapes containing carbon nanofillers

A new concept is described that creates highly oriented multifunctional polymer nanocomposite tapes (or fibres) that combine high stiffness and strength with good electrical properties and a low percolation threshold of conductive nanofillers. The concept is based on a bicomponent construction consisting of a highly oriented polymer core together with conductive polymer composite skins based on a polymer with lower melting temperature than the core. This construction allows for a thermal annealing process that can be applied selectively to the skins to improve their conductivity through a kinetic re-aggregation process while retaining the mechanical properties of the core and hence those of the overall tape or fibre. In the current study this generic concept was applied to bicomponent tapes based on a polypropylene homopolymer core and a multi-wall carbon nanotube or carbon black filled polypropylene copolymer skin. The conductivity of the bicomponent tape containing 5.3 wt.% of MWNTs in its outer skins increased from 1.3E−6 to 1.5 S/cm after annealing while the percolation threshold in the copolymer skins of highly drawn bicomponent tapes could be decreased from 5.3 to 1.1 wt.%. To the best of the authors’ knowledge this is the lowest percolation threshold reported in literature for highly drawn polymer nanocomposites fibres or tapes. In fact, the percolation threshold is as low as 0.1 wt.% when considered on the overall tape as the conductive skins account for only 10% of the total volume of these bicomponent tapes.

Nanocompósitos derivados de dispersões aquosas de poliuretano e argila: influência da argila na morfologia e propriedades mecânicas

Foi sintetizada uma dispersão aquosa de poliuretano (PUD) na qual o poli(propileno glicol) (PPG) foi utilizado como segmento flexível. O segmento rígido foi baseado nos reagentes isoforona diisocianato (IPDI) e a diamina hidrazina (HZ), produzindo um poli(uretano-uréia). A PUD assim obtida foi modificada com Na+-montmorilonita (Na+-MMT) para gerar nanocompósitos (CPUD’s) com 1, 3 e 5 % de argila. Neste processo, o aumento do espaçamento basal das camadas de silicato foi provocado pela água sem necessidade de qualquer tratamento químico da argila. De acordo com as curvas de difração de raios X (XRD), os nanocompósitos obtidos apresentaram partículas de argila intercaladas com o polímero, enquanto a técnica de espalhamento de raios X de baixo ângulo (SAXS) detectou evidências de uma esfoliação parcial da montmorilonita. A esfoliação incompleta das partículas de argila indica re-agregação dos nanocomponentes (inicialmente delaminados na dispersão aquosa) durante a formação do filme. O grau de ligações de hidrogênio, avaliado por espectroscopia no infravermelho (FTIR), foi usado para investigar o efeito da argila no processo de separação de microfases do poliuretano e indicou uma tendência de perturbação da estrutura dos domínios do poliuretano com a presença da argila. A PUD exibiu valores de resistência à tensão na ruptura e alongamento na ruptura na faixa de 30 MPa e 1400% respectivamente. A adição de 1% de argila na PUD promoveu aumentos no módulo e tensão na ruptura de cerca de 230 e 20% respectivamente em relação ao polímero puro, sem perda significativa do alongamento na ruptura.

Tissue surface tensions guide in vitro self‐assembly of rodent pancreatic islet cells

The organization of endocrine cells in pancreatic islets is established through a series of morphogenetic events involving cell sorting, migration, and re-aggregation processes for which intercellular adhesion is thought to play a central role. In animals, these morphogenetic events result in an islet topology in which insulin-secreting cells form the core, while glucagon, somatostatin, and pancreatic polypeptide-secreting cells segregate to the periphery. Isolated pancreatic islet cells self-assemble in vitro into pseudoislets with the same cell type organization as native islets. It is widely held that differential adhesion between cells of the pancreatic islets generates this specific topology. However, this differential adhesion has never been rigorously quantified. In this manuscript, we use tissue surface tensiometry to measure the cohesivity of spherical aggregates from three immortalized mouse pancreatic islet cell lines. We show that, as predicted by the differential adhesion hypothesis, aggregates of the internally segregating INS-1 and MIN6 beta-cell lines are substantially more cohesive than those of the externally segregating alpha-TC line. Furthermore, we show that forced overexpression of P-cadherin by alpha-TC cells significantly perturbs the sorting process. Collectively, the data indicate that differential adhesion can drive the in vitro organization of immortalized rodent pancreatic islet cells.

Reaggregation of Sodium Alginate Microgel Structures After Shear-Induced Deaggregation at Filtering

The time dependence of reaggregation of sodium alginate (SA) after following the shear-induced deaggregation of the aggregates (‘egg-box’ like microgel structures) at filtering through neutral 0.20 μm filters has been found. The aggregation of SA in solution prior to filtering was induced by addition 0.1 mM Ca2+ ions. For SA at concentration of 1mg/ml and room temperature, the reaggregation process is finished after four minutes from the end of the filtration of 3.5 cm3 of the solution with rate of filtration of 1 cm3/min. The reaggregation phenomenon is simply explained by diffusional driven processes of calcium ion – SA electrostatic interactions.

Simulation Study to Model Extremely Slow Reaggregation Processes in Micelle Solutions

Photon correlation measurements on the dodecylpyridinium iodide/water system as nuclei had shown that large micelles are formed after a temperature jump from 80 to 25 °C. The micellar radius exhibits relaxation characteristics with extremely long relaxation times, reaching values up to several hours. Proceeding from a thermodynamic approach of the size distribution of aggregates, model calculations have been performed in which the nonlinearized rate equations of the underlying isodesmic reaction scheme have been used to describe the micelle kinetics. The results of these calculations give valuable indications of the mechanism behind the extremely slow relaxation. In the solutions under consideration, the nucleation of a sufficient number of micellar aggregates obviously is hindered by the extremely unfavorable equilibrium concentration of intermediates of submicellar size. This leads to a strongly deviating nonequilibrium distribution with an excess of extra large micelles during thermal equilibrium. Establishing the final equilibrium is then a very slow process, due to the low monomer concentration, which is maintained by the presence of the extra large micelles. Our model is also capable to verify the slow and fast relaxation process as predicted by the linearized Aniansson−Wall model. It also shows up the ultrafast process of oligomer formation as studied recently by broadband ultrasonic spectrometry.

Extremely Slow Reaggregation Processes in Micelle Solutions. A Dynamic Light Scattering Study

Photon correlation measurements were performed on the dodecylpyridinium iodide/water system to yield the mean radius of micelles as a function of the time interval after a change in the temperature from 80 to 25 °C. Depending on the purification procedure applied to the sample, large aggregates, probably with dust particles as nuclei, may be formed after the temperature jump. Simultaneously, large micelles are also formed, the radius of which may reach four times the equilibrium value of the system. The micellar radius exhibits relaxation characteristics with extremely long relaxation times, reaching values up to several hours. Obviously, the nucleation of a sufficient number of micellar aggregates is hindered by the extremely unfavorable equilibrium concentration of submicellar-size intermediates. This leads to a strongly deviating nonequilibrium distribution with an excess of extra large micelles during the thermal equilibrium. Establishing the final equilibrium is then a very slow process, due to the low monomer concentration, which is maintained by the presence of the extra large micelles.

Talc from Puebla de Lillo, Spain. II. Effect of dry grinding on particle size and shape

The effect of dry grinding on the structure, particle size and shape of talc from Puebla de Lillo (León, Spain), was studied by means of nitrogen adsorption, XRD, DTA-TG and by SEM-EDS methods. Grinding of talc produces an increase of the starting BET surface area value progressively up to a maximum of 110 M 2/g at 30 min. A subsequent decrease of surface area with increasing grinding time was observed. The ground, activated particles with high surface energy start a reaggregation process recovering the original surface area value at 210 min. The values of coherent scattering thickness (crystallite size) indicated a reduction of thickness due to crystalline degradation along [00 l], with initial delamination accompanied by a lesser degree of crystal structure breakdown and amorphization of the structure. The latter increases with increasing grinding time. A profound structural alteration, with progressive smearing of 00 l basal reflections, have been observed. Structural changes have been correlated with particle's size, shape and coefficient of texture. The tendency of particles to preferred orientate decreases with grinding. A breakdown of the original particles due to mechanochemical effect produced more rounded particles, aggregates and agglomerates, as proved by SEM. With prolonged grinding, the rate of size reduction decreases with time, but reduction continues up to about 30 min. At this stage, particle size was reduced to about 0.02 μm in diameter and 0.009 μm in thickness, with aspect ratio 2.6, producing an increased degree of amorphism and a decrease in the surface area by reaggregation of the ground powder. After 30 min, the mechanical reduction of the original particles appears to have a limit. The thermal behavior is strongly dependent on the structural state of the material. Grinding produces a slight shift in the main endothermal DTA peak associated to talc dehydroxylation, as indicated also by the TG and DTG curves. An increase of weight loss on heating at lower temperatures than those observed in unground talc have also been detected. The exothermic DTA effect appears on grinding of talc. It has been detected from 10 min grinding time. This effect increases markedly in intensity during grinding, especially when structural breakdown is very important and the amorphous reactive solid is produced.

Transfection of Reaggregating Embryonic Chicken Retinal Cells with an Antisense 5′‐DNA Butyrylcholinesterase Expression Vector Inhibits Proliferation and Alters Morphogenesis

The function of the enzyme butyrylcholinesterase (BChE) in the developing and mature brain is still unclear. We have inserted 577 bp of the 5' upstream region plus 106 bp of the exon 1 of the rabbit BChE gene in reverse orientation under control of an SV40 early promoter derivative in an expression vector. This vector was introduced by calcium phosphate-mediated transfection into embryonic chicken retina cells during the first days of reaggregation culture. Depending on the retinal origin, the transfected cell population forms histotypic retina-like spheres, so-called rosetted or stratified retinospheroids. We show that antisense 5'-BChE gene expression decreased the steady-state mRNA level of BChE and the translation of the BChE protein, inhibited proliferation, and accelerated histogenesis in both cellular systems. The pronounced effects of antisense 5'-BChE transfection of spheroids document a key role of BChE during the early reaggregation process of retinal cells, most likely by regulating their growth and differentiation.

11 Selective Aggregation Assays for Embryonic Brain Cells and Cell lines

Publisher Summary This chapter illustrates how to assay the role of adhesion mechanisms in selective adhesion of embryonic brain cells and fibroblastic cell lines. The methods described in the chapter include how to dissociate cells while leaving a particular set of adhesion mechanisms intact, how to let them reaggregate, and how to evaluate selective cell adhesion. Cell-cell adhesion mechanisms are complex, and comprise multiple systems, Animals cells are known to exhibit selective adhesiveness. When cells derived from different tissues are mixed, they sort out. This property of cells is thought to play a key role in the organization of complex tissue architecture. Studies suggest that the selective adhesiveness of cells also contributes to regionalization in the embryonic brain. To understand the molecular basis of patterning in the developing brain, therefore, it is critical to accumulate much more information on the adhesion properties of the cells that belong to each compartment. There are many ways to measure and quantify cell-cell adhesiveness. A classic method is to dissociate cell masses into single cells, and observe their reaggregation process. This chapter presents an updated version of classic methods, illustrates how to dissociate cells, how to allow them to reaggregate, and how to assay their selective adhesiveness.

A unique conformational equilibrium of polyacrylic acid at the solid/liquid interface

The adsorption of polyacrylic acid onto an alumina surface is shown using fluorescence spectroscopy to be followed by a rearrangement of its macromolecular segments in the adsorbed state. The adsorption of pyrene-labelled polyacrylic acid has been investigated from its solutions and from solutions mixed with unlabelled acid. The excimer efficiency of the emission of the pyrene-labelled polymer species indicates a pH-independent equilibrium conformation of adsorbed polymer chains. While the slow conformational rearrangement may not affect rapid flocculation processes, it could affect slow flocculation as well as deaggregation and reaggregation processes.

Control of the aggregation factor-aggregation receptor interaction in sponges by protein kinase C

By means of immunobiochemical and immunocytological techniques it was found that the aggregation factor (AF) from the sponge Geodia cydonium is stored in vesicles of spherulous cells. During the reaggregation process of dissociated cells, the AF which is present extracellularly was determined to be bound to the cell-surface-associated aggregation receptor (AR) only during the initial phase (0–5 h after addition of the AF to the single cell suspension). At later stages (20 h), the AF colocalized with extracellular structures, e.g., collagen and glycoconjugates. Immobilized to nitrocellulose, the AR, a molecule with M r of 43.5 kDa, displayed its binding affinity to the AF only if it was isolated from early aggregates (5 h). The transition of the AF-susceptible to the AF-deficient state of the plasma membrane was mimicked in vitro by incubation of plasma membranes from early aggregates with purified protein kinase C. This conversion to the AF-deficient state could be prevented by the protein kinase C inhibitor staurosporine. Together with earlier findings, which revealed that the AR is phosphorylated by protein kinase C, we propose that in the sponge system this enzyme controls intercellular processes involved in morphogenesis.

Factors affecting the reaggregation of rat brain microsomes solubilized with octyl glucoside and their relationship with the base-exchange activity of reaggregates

Rat brain microsomal membranes disaggregated by exposure to octyl glucoside were recovered by centrifugation after dialytic removal of the detergent. The composition of the dialysis medium (divalent cations, pH) was important to this effect; indeed, the reaggregation process which occurred during the dialytic step required the presence of either Ca 2+ or Mg 2+ and a slightly acidic pH. The lipid protein/ratio and choline and ethanolamine base-exchange of recovered particles depended on the conditions of dialysis although their lipid composition did not. The lipid composition of membranes was also varied by adding PE or PC to octyl glucoside-microsome suspensions. This treatment produced reaggregates possessing a low content of cholesterol and varying PC/PE ratios. Both choline and ethanolamine base-exchange activities were related to this parameter.

Xenograft Rejection in Marine Sponges

In sponges there exists a graft rejection mechanism in which an inhibitory aggregation factor is involved. The inhibitory aggregation factor has been isolated from a culture medium containing dissociated cells of the sponge Geodia cydonium. Using ion-exchange and gel fractionation the factor was purified and shown to be electrophoretically pure. The factor has a molecular weight of 27000 and was characterized as a glycoprotein. The activity of the inhibitory aggregation factor was not affected by heat treatment, but treatment with trichloroacetic acid resulted in the irreversible loss of activity. The inhibitory aggregation factor affects the aggregation-factor-mediated reaggregation of dissociated sponge cells by extension of the lag phase preceding the aggregation process; the endpoint of the reaggregation process is not changed. The inhibitory aggregation factor competes with the aggregation receptor for the binding site on the aggregation factor. The inhibitory aggregation factor is not synthesized if homologous aggregation factor is added to the dissociated cells. The results indicate that the inhibitory aggregation factor is synthesized in xenografts of the system Geodia cydonium--Geodia rovinjensis after a grafting period of at least 3 days. Following the synthesis of the factor, the concentration of the extractable aggregation factor decreases.

The effects of surfactants on cell aggregation.

Trypsin-dissociated HeLa and human amnion cells were employed to investigate the effects of an anionic, a cationic, and a non-ionic surfactant on cell reaggregation. All the surfactants inhibited the reaggregation process, the effect increasing with surfactant concentration, and becoming significant at concentrations of about 10(-6) M for the anionic surfactant and about 10(-5) M for the others. The effects of temperature and ionic strength on HeLa cell aggregation, and of surfactants on the electrophoretic mobility of HeLa cells and the release of sialic acid from their surfaces by neuraminidase, were also examined. The results indicate that although electrostatic effects occur and are likely to be of importance in cell reaggregation, the effect of surfactants cannot be accounted for on this basis alone, and it is suggested that interactions involving the cell-surface glycoproteins, possibly indirectly via effects on the fluidity of the cell membrane, are also significant in this respect.

Isolation and characterization of the RNA of membrane-bound ribosomes in Dictyostelium discoideum.

The RNA of membrane-bound ribosomes, isolated from Dictyostelium discoideum, represented 13 to 16% of the total ribosomal RNA (rRNA) present throughout growth and development. Membrane-bound ribosomes were released by treatment with sodium deoxycholate and Brij 58. There were no obvious differences in size and base composition between RNAs derived from membrane-bound or free ribosomes. The 17S membrane-bound rRNA and free rRNAs appeared to have similar methyl contents. However, the 25S membrane-bound rRNA contained about 16 to 20% fewer methyl groups than the 17S membrane-bound rRNA and free rRNAs. Free rRNAs turned over rapidly during early development but not during the disaggregation and reaggregation processes. Membrane-bound rRNAs showed very little turnover during the early stages of morphogenesis, but showed rapid turnover during the late stages of development; this class of rRNAs did not turn over during early stages of reaggregation but turned over rapidly during later stages of reaggregation.

Aggregation of sponge cells: XIV. Possible substitution of calcium ions by polycations

Single cells from the siliceous sponge Geodia cydonium, obtained after chemical dissociation, reaggregate in the presence of the aggregation factor in Ca 2+-containing medium to large aggregates. It was found that polyvalent organic cations (polylysine, spermine, spermidine, putrescine) enhance the Ca 2+-mediated cell aggregation. In Ca 2+-free medium these compounds also cause reaggregation; aggregates of a diameter up to 800 μm are formed within 120 min. Proteins, containing basic groups of amino acid residues have no influence on cell aggregation. Monovalent cations inhibit the reaggregation process. The enhancing effect of polyvalent organic cations on cell aggregation is dependent on the presence of the soluble aggregation factor. From the findings that polycations do not alter the duration of the lag phase (a characteristic of the aggregation factor-mediated Geodia cell reaggregation) and act in cooperation with the aggregation receptor, we assume that the polycations bind between the aggregation factor and the aggregation receptor.

Species-Specific Aggregation Factor in Sponges: Isolation, Purification and Characterization of the Aggregation Factor from Suberites domuncula

An aggregation factor (AF) from the siliceous sponge Suberites domuncula has been isolated and purified by the following steps: Sepharose 2 B gel chromatography, sucrose gradient, Nonidet treatment, Sephadex G-100 gel chromatography and DEAE-Sephadex ion-exchange chromatography. By this procedure the AF was purified 1340-fold with a 63% yield nearly to homogeneity. The AF is originally associated with large particles, characterized by a sedimentation of 2200 S. These particles have been visualized electron microscopically; they are characterized by a filament-like shape of a length of 3400 A and a cross-sectional diameter of 230 A. The purified, low-molecular weight AF has a buoyant density of 1.38 g/cm 3 and an absorbance maximum at 282 nm. The isoelectric pH is approximately 5.75. The molecular weight of the AF has been determined to be 55,000. Chemical analysis revealed that AF consists mainly of protein. The reaggregation process of Suberites cells to large aggregates (>1000 μm), mediated by Suberites AF, is strongly dependent on pH, ionic strength and the presence of calcium ions, and independent of incubation temperature between 0°C and 20°C. Aggregation can be inhibited by trypsin, D-glucosamine and dodecyl sulphate. The Suberites AF is species-specific if tested in a system containing cells from the siliceous sponge Geodia cydonium .

Photophosphorylation and related properties of reaggregated vesicles from spinach Photosystem I particles

The small Photosystem I particles prepared from spinach chloroplasts by the action of Triton X-100 (TSF 1 particles) reaggregate into membrane structures when they are incubated with soybean phospholipids and cholate and then subjected to a slow dialysis. The membranes so formed are vesicular in nature and show the capability of catalyzing phenazine methosulfate-mediated cyclic photophosphorylalation at rates which are usually about 20% of those observed with chloroplasts, but higher rates have been obtained. When coupling factor is removed from the chloroplasts by treatment with EDTA, a requirement for coupling factor can be shown for the subsequent ATP formation. The uncouplers carbonylcyanide 3-chlorophenyl-hydrazone, valinomycin, Triton X-100 and NH + 4 are effective with the reformed vesicles, which do not show the typical light-induced pH gradient observed with chloroplasts. Incubation of the TSF 1 particles with phospholipids alone allows for the formation of membrane vesicles, but such vesicles are only slightly active in ATP formation. In most properties investigated, the reformed membrane vesicles resemble the original chloroplast membrane so far as phenazine methosulfate-mediated cyclic photophosphorylation is concerned, which indicates a high degree of selectivity in the reaggregation process. The major difference between chloroplasts and the reformed vesicles is the failure of the latter to show a light-induced pH gradient.

A possible model for cell-cell recognition via surface macromolecules

Alternative possibilities for the establishment of the proper cell distribution during embryogenesis are summarized at the beginning, followed by an assessment of the examples known so far where cell-cell recognition is known to be mediated via cell surface components. In the second part the species-specific recognition process which occurs during the sorting-out of dissociated sponge cells is analysed since it may serve as a possible model for cell-cell recognition in higher animals. Three possible mechanisms for the establishment of proper cell distribution are considered. These include, first, chemotaxis: secondly, guidance of cell or cell sheet movement by extracellular matrix or by surrounding cells and thirdly, random movement followed by recognition at the final point of destination. Recognition is necessary for both of the two latter processes, i.e. for cell guidance as well as for locking the cells into their final position after random movement. Two basically different recognition mechanisms should be distinguished from each other. On the one hand cells may recognize each other with the help of macromolecules situated in or just outside of the plasmamembrane which fit to each other like enzymes and substrates or antibodies and antigens. On the other hand, cells may exchange information by exchanging cytoplasmatic components via vesicles or gap junctions. The species-specific aggregation of dissociated sponge cells is considered to be a possible model for cell-cell recognition in higher animals. A proteoglycan-like intercellular macromolecule called aggregation factor seems to mediate recognition of a given species of cells in the reaggregation process of dissociated cells. The data available at the present time suggest that a monovalent surface macromolecule (baseplate) may mediate the recognition process probably by recognizing the carbohydrate side chains of the multivalent proteoglycan aggregation factor. A cell-free system was devised to mimic this aggregation process. Addition of aggregation factor to baseplate-coated sepharose beads of approximately the size of the original sponge cells has essentially the same characteristics as the cellular system. Macromolecule-coded surface information for the recognition between cells has not been established during the embryogenesis of higher animals and remains an interesting challenge.