Cluster Disruption Research Articles

Characteristics of Rh-SiO2 Catalyst Prepared Using Microemulsion. An Investigation from the Bahavior of Adsorbed CO Species.

Spectroscopic investigations were conducted to elucidate the difference in surface properties between two kinds of Rh-SiO2 catalysts prepared by using microemulsion and by the conventional impregnation method, by means of FT-IR measurements using CO as a probe molecule. The gem-dicarbonyl species adsorbed on isolated Rh1 sites were observed in the IR spectra of the Rh-SiO2 catalyst prepared by using microemulsion even at a temperature as high as 300°C, while it was not observed at this temperature in the spectra of the Rh-SiO2 catalysts prepared by the impregnation method. This result suggested that the disruption of Rh clusters caused by CO gas, leading to the formation of isolated Rh1 sites, still occurred prominently on the Rh-SiO2 catalysts prepared by using microemulsion even at such a high temperature that agglomeration occurred on the catalyst prepared by the impregnation method. It is quite likely that the disruption occurred prominently because the Rh particles of the catalyst existed in an oxidized state. Therefore, the oxidation state of Rh particles was investigated by XPS measurements, which showed that there were more Rh atoms in an oxidized state in the Rh-SiO2 catalyst prepared by using microemulsion. It is concluded, therefore, that the Rh-SiO2 catalyst prepared by using microemulsion had significant features attributed to a strong interaction between Rh particles and SiO2 support.

Disruption of postsynaptic AMPA receptor GluR2 clusters following induction of long term depression in cultured cerebellar Purkinje cells

Disruption of postsynaptic AMPA receptor GluR2 clusters following induction of long term depression in cultured cerebellar Purkinje cells

Abnormal Thymic Expression of Epithelial Cell Adhesion Molecule (EP-CAM) in New Zealand Black (NZB) Mice

New Zealand Black (NZB) mice have been well documented to have a variety of thymic epithelial cell microenvironmental abnormalities, including disruption of corticoepithelial cell networks and medullary cell clusters. These abnormalities of the thymic stromal network are particularly important because similar observations have been noted in other models of murine lupus. Thymic epithelial cells, a key component of the microenvironment, play an important role in selection of the mature T cell receptor repertoire. Recently, a homotypic calcium-independent human and murine epithelial cell adhesion molecule, Ep-CAM, has been described which is located at the thymocyto-cortical cell junction. The function of Ep-CAM is still unclear but its unique location within the thymus suggests that it is critical in the process of providing maturation signals. Consequently, we examined the thymic expression of Ep-CAM in a series of autoimmune prone mice by thymic distribution of Ep-CAM in NZB, NZW, NZB/W, BXSB-Yaa, MRL- lpr/lpr, C3H- gld/gld and the control strains BALB/c, C57BL6, C3H and MRL+/+, by immunohistology and flow cytometry. Interestingly, NZB mice are similar to control mice from day 4 to 2 weeks of age, having a very low expression of Ep-CAM at the thymocyto-cortical junction. In control strains, there is a marked increased in expression of Ep-CAM beginning at 5 weeks of age. In contrast, NZB mice fail to show significant expression of Ep-CAM even well into adulthood. This abnormality of NZB mice was also noted in NZB/W F1 and BXSB mice, but not MRL-lpr/lpr or C3H- gld/gld mice. Given the potential importance of Ep-CAM in thymic selection, this study provides important evidence that a defective stromal microenvironment is likely to be of etiological significance in the susceptibility of NZB to autoimmune disease.

OPEN CLUSTERS AS A RECORD OF THE PAST

The Galactic open cluster population has long been used as a probe of the structure of the Galactic disk and a timeline for studying its evolution. With ages that range up to 12 billion years and positions that span a large range of Galactocentric distances, the open clusters provide a broad sample with which to investigate issues such as the history of star formation in the Galaxy, the chemical evolution of the disk, and the competing influences of cluster formation and disruption that mold the properties of the current cluster population.

Thiol-mediated disassembly and reassembly of [2Fe-2S] clusters in the redox-regulated transcription factor SoxR.

SoxR, a transcription factor containing [2Fe-2S] clusters, governs the cellular response to oxidative stress in Escherichia coli. The oxidation state of the iron-sulfur clusters regulates the SoxR transcriptional activity. When the reduced iron-sulfur clusters become oxidized ([2Fe-2S]2+ state), SoxR is activated to stimulate transcription of the soxS gene, whose product in turn switches on a group of genes encoding various proteins that defend against oxidative stress and antibiotics. A previous study showed that the oxidized [2Fe-2S] clusters of SoxR are destroyed by a free-radical-dependent process in vitro during aerobic exposure to the biological thiol glutathione. Here, we show that different thiols have differing effects on the SoxR [2Fe-2S] clusters. Like reduced glutathione, N-acetyl-L-cysteine, L-cysteine methyl ester, and L-cysteine ethyl ester disrupted the SoxR [2Fe-2S] clusters in aerobic solution. This disruption was blocked by L-cysteine, which was effective at concentrations 100-fold lower (1-10 microM) than the disrupting thiols (1 mM). In view of a previous observation that superoxide dismutase and catalase block the disruption process, this result suggests that L-cysteine may quench reactive SoxR or thiol intermediates involved in the cluster disruption reaction, the detailed mechanism of which remains unknown. In contrast, bifunctional thiols such as dithiothreitol or dithioerythritol promoted the aerobic assembly of the functional [2Fe-2S] clusters into apo-SoxR in the presence of Fe2+ and inorganic sulfide. The dithiol protein thioredoxin-A of E. coli acted catalytically in vitro in the presence of thioredoxin reductase and NADPH to promote [2Fe-2S] cluster assembly into apo-SoxR. The regulatory activity of SoxR in vivo, assessed by monitoring the paraquat-mediated induction of a soxS'::lacZ reporter fusion, was significantly lower in a strain lacking both thioredoxin-A and glutathione reductase, which maintains reduced glutaredoxins. Thus, cellular monothiols and dithiol proteins may contribute to SoxR regulation by affecting the disassembly and reassembly of the [2Fe-2S] clusters.

Nitric Oxide and Peroxynitrite-Dependent Aconitase Inactivation and Iron-Regulatory Protein-1 Activation in Mammalian Fibroblasts

The reaction of reactive oxygen and nitrogen species with the [4Fe–4S]2+cluster of mitochondrial (m-) and cytosolic (c-) aconitases leads to loss of catalytic activity and, in the case of the c-aconitase, triggers total cluster disruption to yield the iron-regulatory protein-1 (IRP-1). Herein we have studied the relative contribution and interplay of reactive oxygen species (O [equation] and H2O2), nitric oxide (•NO), and peroxynitrite in the modulation of m- and c-aconitase and IRP-1 activities in V79-M8 mammalian fibroblasts, identifying key variables that control the various reactivities at the cellular level. Extracellular production of H2O2led to inactivation of both m- and c-aconitase and IRP-1 activation, while extracellular [equation] had no effect. However, increased intracellular production of [equation] caused a loss in m- and c-aconitase activity and IRP-1 activation. Nitric oxide released from NOC-12 had a more complex effect on aconitase and IRP-1 activities. Mitochondrial aconitase was more sensitive than c-aconitase to•NO-mediated inactivation and minimal activation of IRP-1 was observed during a 30-min exposure to the•NO donor. The action of•NO was down- or upregulated by the presence of extra- or intracelular [equation], respectively. Extracellular [equation] decreased the•NO-mediated inactivation of aconitases, due to the preferential extracellular decomposition and the lower diffusivity of peroxynitrite compared to•NO. On the other hand,•NO exposure concomitant with enhanced intracellular [equation] fluxes lead to intracellular peroxynitrite formation as evidenced by Western blot analysis of nitrated proteins, which increased the effects observed with•NO alone. Peroxynitrite-mediated aconitase inactivation, IRP-1 activation, and cellular protein nitration were more pronounced in cells with low GSH content such as V79-M8 glutathione-depleted cells as well as in pGSOD4 cells which contain 32% of the GSH of the parental strain. Mechanistically, our results imply that the differential actions of the studied reactive species toward cellular aconitases depend on at least three critical factors: (i) their reaction rates with aconitases, (ii) the cellular compartment where they are formed, and (iii) the intracellular status of glutathione.

Evolution of the mass function of the Galactic globular cluster system

In this work we investigate the evolution of the mass function of the Galactic globular cluster system (GCMF) taking into account the effects of stellar evolution, two‐body relaxation, disc shocking and dynamical friction on the evolution of individual globular clusters. We have adopted a lognormal initial GCMF and considered a wide range of initial values for the dispersion, σ, and the mean value, 〈log M〉. We have studied in detail the dependence on the initial conditions of the final values of σ, 〈log M〉, the fraction of the initial number of clusters surviving after one Hubble time and the difference between the properties of the GCMF of clusters closer to the Galactic Centre and those of clusters located in the outer regions of the Galaxy. In most of the cases considered, evolutionary processes alter significantly the initial population of globular clusters and the disruption of a significant number of globular clusters leads to a flattening in the spatial distribution of clusters in the central regions of the Galaxy. The initial lognormal shape of the GCMF is preserved in most cases and if a power‐law in M is adopted for the initial GCMF, evolutionary processes tend to modify it into a lognormal GCMF. The difference between initial and final values of σ and 〈log M〉 as well as the difference between the final values of these parameters for inner and outer clusters can be positive or negative depending on initial conditions. A significant effect of evolutionary processes does not necessarily give rise to a strong trend of 〈log M〉 with the galactocentric distance. The existence of a particular initial GCMF able to keep its initial shape and parameters unaltered during the entire evolution through a subtle balance between disruption of clusters and evolution of the masses of those which survive, suggested by Vesperini, is confirmed.

Role for the Salmonella Flavohemoglobin in Protection from Nitric Oxide

Hemoglobin homologs are being identified in an expanding number of unicellular prokaryotic and eukaryotic organisms. Many of these hemoglobins are twodomain proteins that possess a flavin-containing reductase in their C terminus. Determination of a function for these flavohemoglobins has been elusive. A Salmonella typhimurium strain harboring a deletion in the flavohemoglobin gene shows no difference in growth under oxidative stress conditions but displays an increased sensitivity to acidified nitrite and S-nitrosothiols, both of which produce nitric oxide. The effect is seen aerobically or anaerobically, indicating that oxygen is not required for flavohemoglobin function. These results suggest a role for the bacterial flavohemoglobins that is independent of oxygen metabolism and provide evidence for a bacterial route of protection from nitric oxide that is distinct from oxidative stress responses.

Sustained neurotransmitter release: new molecular clues.

Chemical synapses convey impulses at high frequency by exocytosis of synaptic vesicles. To avoid failure of synaptic transmission, rapid replenishment of synaptic vesicles must occur. Recent molecular perturbation studies have confirmed that the recycling of synaptic vesicles involves clathrin-mediated endocytosis. The rate of exocytosis would thus be limited by the capacity of the synaptic clathrin machinery unless vesicles could be drawn from existing pools. The mobilization of vesicles from the pool clustered at the release sites appears to provide a mechanism by which the rate of exocytosis can intermittently exceed the rate of recycling. Perturbation of synapsins causes disruption of vesicle clusters and impairment of synaptic transmission at high but not at low frequencies. Both clathrin-mediated recycling and mobilization of vesicles from the reserve pool are thus important in the replenishment of synaptic vesicles. The efficacy of each mechanism appears to differ between synapses which operate with different patterns of activity.

Dynamical Constraints on Dark Matter in Compact Objects

Many of the baryons in the universe are dark and at least some of the dark baryons could be in the form of compact objects. Such objects could be in various locations—galactic disks, galactic halos, clusters of galaxies, or intergalactic space—and each of these is associated with a dark matter problem. For each site we consider the various dynamical constraints that can be placed on the fraction of the dark matter in compact objects of different mass. Small compact objects in the Galaxy are constrained byupper limits on their encounter rate with the Earth and solar system since they would resemble meteors or comets. Larger objects are constrained by the disruptive or disturbing effects they would have on various astronomical systems. For disk objects, the most interesting constraints come from the disruption of binary stars or open star clusters. For halo objects, they come from the disruption of globular clusters, the heating of the Galactic disk, and their accumulation in the Galactic nucleus as a result of dynamical friction. For cluster objects, they come from the tidal distortion and disruption of galaxies. For intergalactic objects, they come from the upper limit on the peculiar motions induced in galaxies. We also apply these limits to the situation in which the compact objects are clusters of smaller objects.

Erratum: “Evaporation, Tidal Disruption, and Orbital Decay of Star Clusters in a Galactic Halo” (ApJ, 464, 765 [1996

Erratum: “Evaporation, Tidal Disruption, and Orbital Decay of Star Clusters in a Galactic Halo” (ApJ, 464, 765 [1996

The effect of the Galactic spheroid on globular cluster evolution

We study the combined effects of relaxation, tidal heating and binary heating on globular cluster evolution, exploring the physical consequences of external effects and examining evolutionary trends in the Milky Way population. Our analysis demonstrates that heating on circular and low-eccentricity orbits can dominate cluster evolution. The results also predict rapid evolution on eccentric orbits either due to strong relaxation caused by the high densities needed for tidal limitation or due to efficient bulge shocking of low density clusters. The combination of effects leads to strong evolution of the population as a whole. For example, within the solar circle, tidally-limited 10^5 M_sun clusters lose at least 40% of their mass in 10 Gyr. At high eccentricity most of these clusters evaporate completely. Bulge shocking disrupts clusters within 40 kpc which have less than 80% of their mass within their pericentric inner Lagrange point. Our results are consistent with suggestions that the shape of the cluster luminosity function results from evaporation and disruption of low mass clusters; they further predict that the net velocity dispersion of the cluster system in the inner Galaxy has decreased with time. Preliminary constraints on formation models are also discussed. We conclude that the observed cluster system has largely been shaped by dynamical selection.

On the evolution of the Galactic globular cluster system

In this paper we address the issue of the origin of some observational properties of the galactic globular cluster system. After a preliminary study of some general properties of the main evolutionary processes, we investigate the evolution of systems of globular clusters located in a model of the Milky Way starting from different initial conditions. We study the role of the evolutionary processes in changing the spatial distribution and mass function of the cluster system, in establishing and/or preserving some of the observed correlations and trends between internal properties of globular clusters and between internal properties and location inside the host galaxy and we provide an estimate for the rates of core collapse and disruption of globular clusters. The initial mass function and spatial distribution of the cluster system evolve quite significantly in one Hubble time and the evolution is toward a final state similar to the observed one. If the mass function is initially taken to be a log-normal distribution similar to the one currently observed in our galaxy, its shape is not significantly altered during the entire simulation even though a significant number of clusters are disrupted before one Hubble time, which suggests that the present mass function might represent a sort of 'quasi-equilibrium' distribution.

Flow Properties, Firmness and Stability of Double Cream Cheese Containing Whey Protein Concentrate

ABSTRACTAs estimated on‐line, the viscosity after cooling of double cream cheese curd containing heat‐denatured WPC (DCC +) increased from 1.4 Pa.s to 1.7 Pa.s when cooled to the range of 45°C to 24°C, and then decreased from 1.7 Pa.s to 1.0 Pa.s when cooled from 24°C to 15°C. The viscosity of DCC‐ (without heat‐denatured WPC) increased from 1.5 Pa.s to 2.2 Pa.s at temperature shift from 40°C to 15.5°C. The firmness of stored DCC + and DCC‐, respectively, decreased from 15.1N to 6.5N when cooled to temperatures from 45°C to 15°C, and from 17.9N to 9.9N when cooled from 40°C to 15.5°C, as recorded by cone penetrometry. The structure of DCC+ cooled to 15°C collapsed after penetrometry, and DCC+ cooled to 20°C destabilized during shearing in coaxial cylinder rheometer. A new phase in DCC+ based on milk fat globules liberated by cluster disruption may be the cause of the structural and textural instability.

Modulation by nitric oxide of metalloprotein regulatory activities

In many cells, a nitric oxide (NO) synthase inducible by immunological stimuli produces a sustained flow of NO that lasts a long time. NO is a short-lived molecule but it is a diffusible ligand believed to be capable of reaching distal target sites. Further, several lines of evidence indicate that cysteine-rich motifs of metal-binding proteins, as well as redox-sensitive metal clusters of metalloproteins, are natural sensors of bioradicals like NO. In metalloregulatory proteins, metals are often conveniently located at binding sites and bound to cysteine residues. Accordingly, disruption of the metal-thiolate polymetallic clusters should trigger significant remodelling of the protein structure involved in regulation. We can therefore postulate that the nitrosation reaction occurring at metal centres or cysteine-rich motifs will preclude correct binding to regulatory sites. Several examples are given of metalloregulatory proteins whose metal is bound to thiols and may then become sensitive to NO. Recent observations indicate that in response to NO synthesis, iron regulatory protein, a eukaryotic bifunctional [Fe-S] protein, switches from acting as aconitase to being an RNA-binding regulator, and we suggest that the interplay between NO or a NO-derived molecule and metal clusters at critical allosteric sites may be a crucial component of the cellular response to environmental stress.

Evaporation, Tidal Disruption, and Orbital Decay of Star Clusters in a Galactic Halo

Evaporation, Tidal Disruption, and Orbital Decay of Star Clusters in a Galactic Halo

The kinematics of lithium-rich, active late-type stars: evidence for a low-mass Local Association

An EUV-selected sample of late-type stars has been observed for lithium abundances and radial velocity radiations. It is found that essentially all of the single stars have lithium abundances comparable to or greater than late-type stars in the Pleiades. These observations have confirmed the hypothesis that a high proportion of the most coronally active, late-type stars are Li-rich, because they are young, have not yet spun down, and consequently exhibit enhanced dynamo activity. From these data and other activity-selected samples in the literature, a Li-rich sample of single stars has been chosen, which, by comparison with open cluster Li abundances and rotation rates, is substantially younger than 300 Myr. The kinematics of this sample has been investigated, and from the radial velocities a one-dimensional dispersion of only 5.7 km s−1 is found, about a centroid space motion extremely similar to that of a group of young open clusters and nearby B stars, known as the Local Association. Consideration of the space motions confirms that, if this low-mass Local Association can be represented by a three-dimensional velocity dispersion of about 10 km s−1, then the majority of the Li-rich stars are members. The origin of the Local Association is still obscure, but a number of suggestions are put forward, including ejection of stars from nearby open clusters, the disruption of pre-existing clusters, or the dissolution of the unbound remnants from one or more star formation regions.

Radial velocities of stars in the globular cluster M4 and the cluster distance

The internal stellar velocity distribution of the globular cluster M4 is evaluated from nearly 200 new radial velocity measurements good to 1 km/s and a rederivation of existing proper motions. The mean radial velocity of the cluster is 70.9 +/- 0.6 km/s. The velocity dispersion is 3.5 +/- 0.3 km/s at the core, dropping marginally towards the outskirts. Such a low internal dispersion is somewhat at odds with the cluster's orbit, for which the perigalacticon is sufficiently close to the galactic center that the probability of cluster disruption is high; a tidal radius two-thirds the currently accepted value would eliminate the discrepancy. The cluster mass-to-light ratio is also small, M/L(sub V) = 1.0 +/- 0.4 in solar units. M4 thus joins M22 as a cluster of moderate and concentration with a mass-to-light ratio among the lowest known. The astrometric distance to the cluster is also smaller than expected, 1.72 +/- 0.14 kpc. This is only consistent with conventional estimates of the luminosity of horizontal branch stars provided an extinction law R = A(sub V)/E(B-V) approximately 4 is adopted, as has been suggested recently by several authors.

A Monte Carlo simulation of the growth of Si(001)2×1: datom/SA step interactions and growth mechanisms

Results of a solid-on-solid lattice Monte Carlo simulation of the vapor phase epitaxial growth of Si(001)2×1 are described. Comparison of the simulations with published experimental results imply that adatoms are bound to type A surface steps with an energy of 0.1 ± 0.05 eV. There is apparently no significant barrier to adatom motion across the type A step. It is proposed that the motion of the type A step is nucleation limited while that of the type B step is controlled by the rate of attachment of adatoms to the step. The implications of this distinction are discussed. It is suggested that the effective critical nucleus size increases near both SA and SB steps because of coalescence with the SA steps and cluster disruption by vacancies produced at SB steps, respectively.

Interplay of P-H and E-H (E = S, Se) Bonds in Palladium Derivatives: Synthesis and Disruption of New Mixed-Valence Palladium Triangulo Clusters Mediated by Proton Mobility. Crystal and Molecular Structure of [Pd3(.mu.-PCy2)2(.mu.-SPh)(PCy2H)2(SPh)

An excess of PhEH (E=S, Se) reacts with Pd 2 (PCy 2 H) 2 (μ-PCy 2 )(μ-η 3 -C 3 H 5 ) causing, by protonation of the bridging ligands, the disruption of the dinuclear unit and the formation of the monomers trans-[Pd(EPh) 2 (PCy 2 H) 2 ]. The isolated monomers were reacted with the same π-allyl dimer providing a synthetic route to the clusters [Pd 3 -(μ-PCy 2 ) 2 (μ-EPh) 2 (PCy 2 H) 2 (EPh)]; these mixed-valence triangulo complexes exhibit high stability both in the solid state and in solution, but are reactive toward weak proton donors