Nascent Clusters Research Articles

Determinants of the Drosophila Odorant Receptor Pattern

In most olfactory systems studied to date, neurons that express the same odorant receptor (Or) gene are scattered across sensory epithelia, intermingled with neurons that express different Or genes. In Drosophila, olfactory sensilla that express the same Or gene are dispersed on the antenna and the maxillary palp. Here we show that Or identity is specified in a spatially stereotyped pattern by the cell-autonomous activity of the transcriptional regulators Engrailed and Dachshund. Olfactory sensilla then become highly motile and disperse beneath the epidermis. Thus, positional information and cell motility underlie the dispersed patterns of Drosophila Or gene expression.

Importance of the initial conditions for star formation - III. Statistical properties of embedded protostellar clusters

We investigate the formation of protostellar clusters during the collapse of dense molecular cloud cores with a focus on the evolution of potential and kinetic energy, the degree of substructure, and the early phase of mass segregation. Our study is based on a series of hydrodynamic simulations of dense cores, where we vary the initial density profile and the initial turbulent velocity. In the three-dimensional adaptive mesh refinement simulations, we follow the dynamical formation of filaments and protostars until a star formation efficiency of 20%. Despite the different initial configurations, the global ensemble of all protostars in a setup shows a similar energy evolution and forms sub-virial clusters with an energy ratio $E_\mathrm{kin}/|E_\mathrm{pot}|\sim0.2$. Concentrating on the innermost central region, the clusters show a roughly virialised energy balance. However, the region of virial balance only covers the innermost $\sim10-30%$ of all the protostars. In all simulations with multiple protostars, the total kinetic energy of the protostars is higher than the kinetic energy of the gas cloud, although the protostars only contain 20% of the total mass. The clusters vary significantly in size, mass, and number of protostars, and show different degrees of substructure and mass segregation. Flat density profiles and compressive turbulent modes produce more subclusters then centrally concentrated profiles and solenoidal turbulence. We find that dynamical relaxation and hence dynamical mass segregation is very efficient in all cases from the very beginning of the nascent cluster, i.e., during a phase when protostars are constantly forming and accreting.

Reaction network governing diphosphine-protected gold nanocluster formation from nascent cationic platforms

We identify the reaction network governing gold monolayer protected cluster (MPC) formation during the reduction of Au(PPh(3))Cl and L(5) (L(5) = 1,5-bis(diphenylphosphino)pentane) in solutions. UV-vis spectroscopy and electrospray ionization mass spectrometry (ESI-MS) monitored the formation of ligated Au(x): 6 ≤ x ≤ 12 clusters, which comprise the reaction intermediates and final products. Initially, predominantly [Au(2)L(5)(2)](2+) complexes form through dissolution of Au(PPh(3))Cl. These complexes control the reduction and nucleation reactions that form nascent phosphine-ligated Au(8) and Au(10) ionic clusters. [Au(10)L(5)(4)](2+) is an observed growth platform for ligated Au(11) and Au(12) clusters. The data for syntheses of Au : L(5) systems evidence that the nascent reaction products (t < 3 days) are less dependent on the chosen reducing agent (borane tert-butylamine complex or NaBH(4)); instead, after reduction ceases, subsequent solution phase processing provides greater control for tuning cluster nuclearity.

Behavior of Pt Atoms on Oxide Supports During Reduction Treatments at Elevated Temperatures, Characterized by Aberration Corrected Stem Imaging

Aberration-corrected scanning transmission electron microscopy at the sub-Angstrom resolution allows imaging the structure of catalytic materials at the single atom level and permits fundamental studies of the behavior of heavy metal catalytic species as a result of elevated temperature gas-treatments. The present study is aimed at understanding the development of clusters and nanoparticles of Pt on γ-alumina during reduction treatments of a pre-oxidized highly dispersed catalyst. A special built ex situ reactor and a specimen holder allowing cyclic anaerobic transfer between the reactor and microscope were used for the study. The number of atoms in a nascent cluster can be determined along with the general shape of the cluster. Reduction experiments without air exposure of the sample showed that although clusters are formed at 500 °C, many Pt atoms are not associated with the cluster and are still dispersed on the catalyst support. After a 700 °C reduction, all of the Pt atoms are associated with the clusters. Movement of the clusters on the catalyst support is different depending upon the catalyst support. Aberration-corrected scanning transmission electron microscopy at the sub-Angstrom resolution allows imaging the structure of catalytic materials at the single atom level and permits fundamental studies of the behavior of heavy metal catalytic species as a result of elevated temperature gas-treatments. The present study is aimed at understanding the development of clusters and nanoparticles of Pt on γ-alumina during reduction treatments of a pre-oxidized highly dispersed catalyst. A special built ex situ reactor and a specimen holder allowing cyclic anaerobic transfer between the reactor and microscope were used for the study. The number of atoms in a nascent cluster can be determined along with the general shape of the cluster. Reduction experiments without air exposure of the sample showed that although clusters are formed at 500 °C, many Pt atoms are not associated with the cluster and are still dispersed on the catalyst support. After a 700 °C reduction, all of the Pt atoms are associated with the clusters. Movement of the clusters on the catalyst support is different depending upon the catalyst support.

THE EMERGENCE OF INNOVATION-BASED WIRELESS CLUSTERS: QUALITY AND TIMING MATTER

This study compares the emergence of four wireless clusters in the 1970s and 1980s. Two of them, Calgary in Canada and Finland, initially pursued rather similar service innovations for not very different markets but with very different outcomes, which raises the question why. One major reason that emerges from the reviewed extant research on cluster emergence and innovation diffusion concerns the differences in timing and quality of the initial innovations, affecting their respective perceived diffusion attributes, and market growth and extent. The initial innovation in Finland was well received, diffused rapidly and eventually globally, and led to a positive spiral spurring the industry on to take a global lead. In the case of Calgary, however, it was un-competitive in the broader international market, forcing the anchor firm to adapt and reorient. The study analyses and compares the characteristics of the respective initial innovations and their impact on the outcome, and concludes with a discussion and some propositions on cluster emergence. Enhanced understanding of nascent clusters, especially regarding the role of globally attractive initial innovations and their diffusion quality and timing, should provide value for both scholars and practitioners.

Isotope scrambling in the formation of cyanopolyynes by laser ablation of carbon particles in liquid acetonitrile

Cyanopolyynes, H(C C) n C N ( n = 3–6), were formed by laser ablation of carbon particles in liquid acetonitrile. The molecules were separated according to the size n and characterized by UV absorption spectroscopy, mass spectroscopy in combination with gas-chromatographic separation, and nuclear magnetic resonance (NMR) spectroscopy. For the study of nascent carbon cluster distribution during the growth of the long carbon chain molecules, isotopomer distribution was analyzed by NMR spectroscopy for the product molecules. Two cyanopolyynes of HC 7N and HC 9N were isolated from solutions after laser ablation of isotope-enriched carbon powder (99% 13C) in liquid acetonitrile, CH 3CN, of natural isotopic abundance (1.1% 13C). With the observed chemical shift, δ, and spin–spin coupling constants, J CH and J CC, spectral simulation was made to determine relative abundance of possible isotopomeric forms for HC 9N. We found that the isotope of 12C, mostly from solvent molecules, contributes substantially for the part of carbon in the cyano group, –C N, in HC 9N. The isotopomer distribution observed for the sequence of H–C C–C C– was fairly explainable by a binomial, random distribution of the two carbon isotopes of 12C and 13C, reducing the concentration of 13C to 76–55%.

Thermodynamic Origin of the Vitreous Transition.

The vitreous transition is characterized by a freezing of atomic degrees of freedom at a temperature Tg depending on the heating and cooling rates. A kinetic origin is generally attributed to this phenomenon instead of a thermodynamic one which we develop here. Completed homogeneous nucleation laws reflecting the energy saving due to Fermi energy equalization of nascent crystals and their melt are used. They are applied to bulk metallic glasses and extended to inorganic glasses and polymers. A transition T*g among various Tg corresponds to a crystal homogeneous nucleation temperature, leading to a preliminary formation of a cluster distribution during the relaxation time preceding the long steady-state nucleation time of crystals in small samples. The thermally-activated energy barrier ΔG*2ls/kBT at T*g for homogeneous nucleation is nearly the same in all glass-forming melts and determined by similar values of viscosity and a thermally-activated diffusion barrier from melt to cluster. The glass transition T*g is a material constant and a linear function of the energy saving associated with charge transfers from nascent clusters to the melt. The vitreous transition and the melting temperatures alone are used to predict the free-volume disappearance temperature equal to the Vogel-Fulcher-Tammann temperature of fragile glass-forming melts, in agreement with many viscosity measurements. The reversible thermodynamic vitreous transition is determined by the disappearance temperature T*g of the fully-relaxed enthalpy Hr that is not time dependent; the observed specific heat jump at T*g is equal to the proportionality coefficient of Hr with (T*g − Ta) for T ≤ T*g as expected from the enthalpy excess stored by a quenched undercooled melt at the annealing temperature Ta and relaxed towards an equilibrium vitreous state. However, the heat flux measurements found in literature over the last 50 years only gave an out-of-equilibrium Tg since the enthalpy is continuous at T*g without visible heat jump.

East London Tech City: Ideas without a strategy?

This article examines the Coalition Government’s ‘Tech City’ proposals for East London. The Government wants to support the nascent tech cluster in East London, encourage inward investment, and develop the post-2012 Olympic Park into a high-tech hub. After examining the initiative in more detail, the article moves on to discuss why, and how, policy should support the development of high-tech industries in East London. It draws on location and cluster theory, the experience of initiatives to support high-tech clusters in other countries, and an examination of London’s existing strengths in order to suggest a realistic and evidence-based way forward.

Gold Cluster Formation with Phosphine Ligands: Etching as a Size-Selective Synthetic Pathway for Small Clusters?

Triphenylphosphine (PPh(3)) is commonly used during syntheses of stable, closed-shell monolayer protected clusters (MPCs). Models of transition metal (TM) cluster and nanoparticle syntheses commonly assign PPh(3) a passive role as a chemical placeholder, electron balancing species, or surfactant. This study provides the first direct evidence that PPh(3) is a proactive etching agent that promotes the formation of specific closed-shell cluster sizes. To observe this effect, we developed a colorimetric tool that simultaneously monitors size distribution and population of PPh(3)-protected clusters as a function of time. The distribution of the clusters is assigned to different bin sizes by chemical conversion with L(3) (L(3) = 1,3-bis(diphenylphosphino)propane): (i) total conversion of PPh(3)-protected Au(8) and Au(9) clusters into [Au(6)L(3)(4)](2+) and (ii) ligand exchange with [Au(x)(PPh(3))(y)](z+) (10 ≤ x ≤ 13) clusters to form L(3)-protected Au(10) and Au(11) clusters. Evolution of the nascent cluster distribution in ethanol and methanol solvent systems was monitored by the colorimetric assay, which revealed a cyclic process of growth and etching reactions around the most stable cluster species to form nearly monodisperse product distributions. We formally define the population growth of specific clusters through cyclic processing of the Au MPCs as "size selective" processing. The current study highlights the need for incorporating bidirectional processing, including relative rate information, into TM kinetic models for ligands with growth and etching efficacy.

Star Formation in Molecular Clouds

Stars and star clusters form by gravoturbulent fragmentation of interstellar gas clouds. The supersonic turbulence ubiquitously observed in Galactic molecular gas generates strong density fluctuations with gravity taking over in the densest and most massive regions. Collapse sets in to build up stars. Turbulence plays a dual role. On global scales it provides support, while at the same time it can promote local collapse. Stellar birth is thus intimately linked to the dynamical behavior of parental gas cloud, which governs when and where protostars form, and how they contract and grow in mass via accretion from the surrounding cloud material. The thermodynamic behavior of the star forming gas plays a crucial part in this process and influences the stellar mass function as well as the dynamic properties of the nascent stellar cluster. This lecture provides a critical review of our current understand- ing of stellar birth and compares observational data with competing theoretical models.

Zebrafish Neural Tube Morphogenesis Requires Scribble-Dependent Oriented Cell Divisions

How control of subcellular events in single cells determines morphogenesis on the scale of the tissue is largely unresolved. The stereotyped cross-midline mitoses of progenitors in the zebrafish neural keel provide a unique experimental paradigm for defining the role and control of single-cell orientation for tissue-level morphogenesis in vivo. We show here that the coordinated orientation of individual progenitor cell division in the neural keel is the cellular determinant required for morphogenesis into a neural tube epithelium with a single straight lumen. We find that Scribble is required for oriented cell division and that its function in this process is independent of canonical apicobasal and planar polarity pathways. We identify a role for Scribble in controlling clustering of α-catenin foci in dividing progenitors. Loss of either Scrib or N-cadherin results in abnormally oriented mitoses, reduced cross-midline cell divisions, and similar neural tube defects. We propose that Scribble-dependent nascent cell-cell adhesion clusters between neuroepithelial progenitors contribute to define orientation of their cell division. Finally, our data demonstrate that while oriented mitoses of individual cells determine neural tube architecture, the tissue can in turn feed back on its constituent cells to define their polarization and cell division orientation to ensure robust tissue morphogenesis.

Three intermediate-mass young stellar objects with different properties emerging from the same natal cloud in IRAS 00117+6412

We observed with the VLA, PdBI, and SMA the centimeter and millimeter continuum, N2H+(1-0), and CO(2-1) emission associated with a dusty cloud harboring a nascent cluster with intermediate-mass protostars. At centimeter wavelengths we found a strong source, tracing a UCHII region, at the eastern edge of the dusty cloud, with a shell-like structure, and with the near-infrared counterpart falling in the center of the shell. This is presumably the most massive source of the forming cluster. About 15'' to the west of the UCHII region and well embedded in the dusty cloud, we detected a strong millimeter source, MM1, associated with centimeter and near-infrared emission. MM1 seems to be driving a prominent high-velocity CO bipolar outflow, and is embedded in a ridge of dense gas traced by N2H+. We estimated that MM1 is an intermediate-mass source in the Class 0/I phase. About 15'' to the south of MM1, and still more deeply embedded in the dusty cloud, we detected a compact millimeter source, MM2, with neither centimeter nor near-infrared emission, but with water maser emission. MM2 is associated with a clump of N2H+, whose kinematics reveal a clear velocity gradient and additionally we found signposts of infall motions. MM2, being deeply embedded within the dusty cloud, with an associated water maser but no hints of CO outflow emission, is an intriguing object, presumably of intermediate mass. In conclusion, the UCHII region is found at the border of a dusty cloud which is currently undergoing active star formation. Two intermediate-mass protostars in the dusty cloud seem to have formed after the UCHII region and have different properties related to the outflow phenomenon.

Government policy, clusters and the 'Dubai model'

This paper looks at how clusters can help to create competitive advantage for firms. The differences between clusters in terms of their structure and internal dynamics are discussed, with reference to a number of typologies. The life cycle dimension of cluster development is also considered. This paper then addresses issues related to public policy in terms of initiating clusters and ensuring their longer-term sustainability. This is considered in the context of clusters as an integral and growing feature of regional policy formulation by governments. The 'Dubai model' of regional development is then introduced, highlighting how the establishment of free zones (nascent clusters) constitute the basis of industry planning in the United Arab Emirates. Some insights are provided from the experiences of two particular zones: Dubai Media City and Dubai Logistics City. Some observations are made as to the efficacy of the 'Dubai model'.

The onset of molecular condensation: hydrogen

The very first steps of condensation as studied experimentally in the simplest molecular system (para-H(2)) are reported. The fast time-space evolution of the nascent clusters have been measured using state-of-the-art Raman spectroscopy implemented on cryogenic supersonic jets. The time-dependent onset of condensation is presented in a non-equilibrium pressure-temperature phase diagram. Dimer and trimer formation are found to obey three-body processes whose rates have been determined.

Modelling the internal structure of nascent soot particles

In this paper we present studies of clusters assembled from polycyclic aromatic hydrocarbon (PAH) molecules similar in size to small soot particles. The clusters studied were comprised of coronene (C 24H 12) or pyrene (C 16H 10) molecules and represent the types of soot precursor molecule typically found in flame environments. A stochastic ‘basin-hopping’ global optimisation scheme was used to locate low-lying local minima on the potential energy surface of the molecular clusters. TEM-style projections of the resulting geometries show similarities with those observed experimentally in TEM images of soot particles. The mass densities of these clusters have also been calculated and are lower than bulk values of the pure crystalline PAH structures. They are also significantly lower than the standard value of 1.8 g/cm 3 used in our soot models. Consequently we have varied the mass density between 1.0 g/cm 3 and 1.8 g/cm 3 to examine the effects of varying soot density on our soot model and observed how the shape of the particle size distribution changes. Based on similarities between nascent soot particles and PAH clusters a more accurate soot density is likely to be significantly lower than 1.8 g/cm 3. As such, for modelling purposes, we recommend that the density of nascent soot should be taken to be the value obtained for our coronene cluster of 1.12 g/cm 3.

The SufBCD Fe−S Scaffold Complex Interacts with SufA for Fe−S Cluster Transfer

Iron-sulfur clusters are key iron cofactors in biological pathways ranging from nitrogen fixation to respiration. Because of the toxicity of ferrous iron and sulfide to the cell, in vivo Fe-S cluster assembly transpires via multiprotein biosynthetic pathways. Fe-S cluster assembly proteins traffic iron and sulfide, assemble nascent Fe-S clusters, and correctly transfer Fe-S clusters to the appropriate target metalloproteins in vivo. The Gram-negative bacterium Escherichia coli contains a stress-responsive Fe-S cluster assembly system, the SufABCDSE pathway, that functions under iron starvation and oxidative stress conditions that compromise Fe-S homeostasis. Using a combination of protein-protein interaction and in vitro Fe-S cluster assembly assays, we have characterized the relative roles of the SufBCD complex and the SufA protein during Suf Fe-S cluster biosynthesis. These studies reveal that SufA interacts with SufBCD to accept Fe-S clusters formed de novo on the SufBCD complex. Our results represent the first biochemical evidence that the SufBCD complex within the Suf pathway functions as a novel Fe-S scaffold system to assemble nascent clusters and transfer them to the SufA Fe-S shuttle.

Cerium Oxyhydroxide Clusters: Formation, Structure, and Reactivity

Cerium oxyhydroxide cluster anions were produced by irradiating ceric oxide particles by using 355 nm laser pulses that were synchronized with pulses of nitrogen gas admitted to the irradiation chamber. The gas pulse stabilized the nascent clusters that are largely anhydrous [Ce(x)O(y)] ions and neutrals. These initially formed species react with water, principally forming oxohydroxy species that are described by the general formula [Ce(x)O(y)(OH)(z)](-) for which all the Ce atoms are in the IV oxidation state. In general, the extent of hydroxylation varies from a value of three OH per Ce atom when x = 1 to a value slightly greater than 1 for x >or= 8. The Ce(3) and Ce(6) species deviate significantly from this trend: the x = 3 cluster accommodates more hydroxyl moieties compared to neighboring congeners at x = 2 and 4. Conversely, the x = 6 cluster is significantly less hydroxylated than its x = 5 and 7 neighbors. Density functional theory (DFT) modeling of the cluster structures shows that the hydrated clusters are hydrolyzed, and contain one-to-multiple hydroxide moieties, but not datively bound water. DFT also predicts an energetic preference for formation of highly symmetric structures as the size of the clusters increases. The calculated structures indicate that the ability of the Ce(3) oxyhydroxide to accommodate more extensive hydroxylation is due to a more open, hexagonal structure in which the Ce atoms can participate in multiple hydrolysis reactions. Conversely the Ce(6) oxyhydroxide has an octahedral structure that is not conducive to hydrolysis. In addition to the fully oxidized (Ce(IV)) oxyhydroxides, reduced oxyhydroxides (containing a Ce(III) center) are also formed. These become more prominent as the size of the clusters increases, suggesting that the larger ceria clusters have an increased ability to accommodate a reduced Ce(III) moiety. In addition, the spectra offer evidence for the formation of superoxide derivatives that may arise from reaction of the reduced oxyhydroxides with dioxygen. The overall intensity of the clusters tends to monotonically decrease as the cluster size increases; however, this trend is interrupted at Ce(13), which is significantly more stable compared to neighboring congeners, suggesting formation of a dehydrated Keggin-type structure.

Argon clusters embedded in helium nanodroplets

Electron impact ionization of argon clusters embedded in helium droplets is investigated. Superior mass resolution makes it possible to distinguish between nominally isobaric cluster ions. An abundance maximum for ArHe(12)(+) is unambiguously confirmed; the spectra also prove the formation of Ar(2)He(n)(+) complexes that had been claimed to fragment into pure Ar(2)(+). Distributions of larger argon cluster ions containing up to 60 atoms closely resemble distributions observed upon electron impact or photoionization of bare argon clusters; caging and evaporative cooling provided by the helium matrix do not suffice to quench fragmentation of the nascent argon cluster ions. Intriguing abundance anomalies are observed in distributions of argon cluster ions that contain water, nitrogen or oxygen impurities. The strong abundance of Ar(55)H(2)O(+), Ar(54)O(2)(+) and Ar(54)N(2)(+) contrasts with the virtual absence of slightly larger cluster ions containing the corresponding impurities. The features are probably related to enhanced cluster ion stability upon closure of the second icosahedral shell but the difference in magic numbers (54 versus 55) and the well-known reactivity of charged argon-nitrogen complexes suggest structural differences.

The asymmetric architecture of 2Fe–2S IscU, a scaffold protein for iron–sulfur cluster biosynthesis

Iron-sulfur (Fe-S) clusters are ubiquitous prosthetic groups that are required to maintain fundamental life processes. The assembly of Fe-S clusters in several bacteria as well as eukaryotic mitochondria is achieved by a multicomponet system, called ISC machinery. In this machinery, IscU serves as a scaffold for assembly of a nascent Fe-S cluster, prior to its delivery to an apo-protein. The Hsp70-type molecular chaperon HscA and the J-type cochaperone HscB comprise a specialized chaperone system that selectively binds IscU and facilitate the cluster transfer process. We have determined the crystal structure of holo-IscU from the hyperthermophilic bacterium Aquifex aeolicus at 2.3 A resolution, using multi-wavelength anomalous diffraction of the [2Fe-2S] cluster. IscU formed an asymmetric homotrimeric structure harboring only one [2Fe-2S] cluster that was coordinated by three cysteines and one histidine (Cys36, Cys63, His106, and Cys107) on the surface of just one of the protomers. The cluster was buried inside the homotrimer by the neighboring protomers. The three protomers were conformationally distinct from each other, and associated around a non-crystallographic pseudo-three-fold axis. This unique trimeric holo-IscU architecture was clearly distinct from other known monomeric apo-IscU/SufU structures, indicating that asymmetric trimer organization would be involved in the scaffolding function of IscU. This oligomeric holoIscU structure provides mechanistic implications concerning the initial event in the assembly of Fe-S cluster, as well as the event in the transfer to target apo-proteins.

Femtoradical events in aqueous molecular environments: the tenuous borderline between direct and indirect radiation damages

The complex links existing between radiation physics and radiobiology concern the complete understanding of spatio-temporal events triggered by an initial energy deposition in confined spaces called spurs. Microscopic radiation effects (photons or relativistic particles) on integrated biological targets such as water ‘the solvent of life’ and biomolecular architectures (DNA, histones, enzymes) cannot be satisfactorily described from an absorbed dose delivery profile or a linear energy transfer (LET) approach. Primary radiation damages on biological targets being dependent on the survival probability of secondary electrons and short-lived radicals inside nascent nanometric clusters of ionisation, a thorough knowledge of these processes require the real-time probing of early events on sub-micrometric scale, in the temporal range 10-15 - 10-10 s. Major strides concern early water damages: primary water cation formation (H2O•+ or positive hole), concerted electron-proton couplings, attachment dynamics of p-like excited prehydrated electron on biomolecule, short-lived radical pairs involving water-bridged radical OH• and hydronium ion H3O+. The deactivation frequency of electron-radical pairs is comparable to an H-OH deactivation of excited water molecules (vH2O* ~ 0.33 × 1013 s-1). These short-lived events take place in the prethermal regime of delocalized secondary electrons and represent a tenuous borderline between direct and indirect molecular damages.