bluuebunny/crossref_metadata_2025_split huggingface dataset:
Formation of Dinuclear Titanium and Zirconium Complexes by Olefin Metathesis—Catalytic Preparation of Organometallic Catalyst Systems
Authors: Cano Sierra Jesus , Hüerländer Doris , Hill Michael , Kehr Gerald , Erker Gerhard , Fröhlich Roland 
Published: July 2003

AbstractThe titanium complex [(C5H4allyl)TiCl3] (2) undergoes olefin metathesis coupling when treated with 3 mol % of [Cl2(L1)(L2)RuCHPh] (L1=L2=PCy3, 4 a; L1=PCy3, L2=(H2IMes), 4 b) to yield the dimetallic complex [Cl3Ti(C5H4)‐CH2CHCHCH2‐(C5H4)TiCl3] (5). The allyl‐substituted titanocene complex [Cp(C5H4allyl)TiCl2] (3) analogously yields the dimetallic system 6 when treated with 4. The ansa‐zirconocene complex [Me2Si(C5H4)(C5H3allyl)ZrCl2] (7) cleanly yields the analogous dimetallic coupling product 8 (>95 % isomerically pure), when treated with catalytic amounts of 4 b in toluene. Complex 8 gives an active homogeneous ethene or propene polymerization catalyst, especially at elevated temperatures, when treated with excess methylalumoxane.
Synthesis and Solid‐State Organization of Coil–Ring–Coil Block Copolymers
Authors: Rosselli Silvia , Ramminger Anne‐Désirée , Wagner Thomas , Lieser Günter , Höger Sigurd 
Published: July 2003

AbstractShape‐persistent macrocycles based on the phenyl‐ethynyl‐butadienyl backbone containing two extraannular hydroxyl groups were prepared by the oxidative coupling of the appropriate phenylethynyl oligomers. Carbodiimide‐directed coupling with independently synthesized polystyrene carboxylic acid oligomers led to ABA coil–ring–coil block copolymers in which the central macrocycle serves as rigid and the polystyrene oligomers as flexible elements. Depending on the size of the coil blocks, these structures aggregate in cyclohexane into supramolecular hollow cylindrical brushes in which the rigid core is surrounded by the flexible matrix. However, in the solid state it is not possible to identify a morphology in which isolated channels based on aggregated macrocycles are embedded in a matrix of polystyrene. Detailed X‐ray and electron diffraction studies on samples prepared from a solution in cyclohexane under equilibrium conditions show that the material adopts a lamellar morphology in the solid state in which columns of macrocycles are aggregated into layers which are separated by polystyrene.
The Synthesis of Acid‐ and Base‐Labile Lipopeptides on Solid Support
Authors: Ludolph Björn , Waldmann Herbert 
Published: July 2003

AbstractLipidated peptides, including characteristic partial structures of human Ras proteins, were synthesized by means of a new solid‐phase technique in 22–68 % yield. This technique gives access to farnesylated, palmitoylated, and doubly lipidated peptides as methyl esters or carboxylic acids carrying a fluorescent tag or a maleimide moiety for coupling to proteins. The peptide backbones were built up on the resin by using 9‐fluorenylmethoxycarbonyl chemistry together with the oxidatively cleavable hydrazide linker. As a key step, the acid‐labile farnesyl and basic‐labile palmitoyl lipid groups were introduced onto the resin after the cleavage of appropriate acid‐ or reduction‐sensitive protecting groups from the cysteine residues. Optional introduction of different fluorescent tags or a maleimide group into the peptide was followed by release of the resin‐bound target peptide as the methyl ester or carboxylic acid by very mild copper(II)‐mediated oxidation in slightly acidic or basic media. This new methodology should substantially facilitate the access to lipidated peptides for the study of important biological phenomena like biological signal transduction, localization, and vesicular transport.
Characteristics of Organic Transformations in a Confined Dendritic Core: Studies on the AIBN‐Initiated Reaction of Dendrimer Cobalt(II) Porphyrins with Alkynes
Authors: Uyemura Makoto , Aida Takuzo 
Published: July 2003

AbstractCobalt(II) complexes of poly(aryl ester) dendrimer porphyrins [(m‐[Gn]TPP)CoII] (generation number n=0–4), in the presence of azobisisobutyronitrile (AIBN) at 60 °C, underwent alkenylation with several alkynes at the metal center. A complete inhibition of double‐bond migration (secondary transformation) was observed for [(m‐[Gn]TPP)CoII] (n=3 and 4), which gave [(m‐[Gn]TPP)CoIIIC(CH2)R] (n=3 and 4) exclusively. Overall reaction rates for [(m‐[Gn]TPP)CoII] (n=0–3) were hardly dependent on the size of the dendritic substituents, while a notable retardation was observed for the largest dendrimer, [(m‐[G4]TPP)CoII]. Mechanistic studies on double‐bond migration with pure [(m‐[Gn]TPP)CoIIIC(CH2)Bu] (n=0–4) demonstrated that the secondary transformation involves participation of [(m‐[Gn]TPP)CoIIIH] (n=0–4), derived from [(m‐[Gn]TPP)CoII] and AIBN, rather than [(m‐[Gn]TPP)CoII] alone. Crossover experiments using [(m‐[Gn]TPP)CoIIIC(CH2)Bu] (n=2–4), in combination with nondendritic [(m‐[G0]TPP)CoII] and AIBN, indicated a high level of steric protection of the active center by a robust [G4]‐dendritic cage, as suggested by a 1H NMR pulse relaxation time profile of m‐[G4]TPPH2.
The Role of Cystine Knots in Collagen Folding and Stability, Part I. Conformational Properties of (Pro‐Hyp‐Gly)5 and (Pro‐(4S)‐FPro‐Gly)5 Model Trimers with an Artificial Cystine Knot
Authors: Barth Dirk , Musiol Hans‐Jürgen , Schütt Markus , Fiori Stella , Milbradt Alexander G. , Renner Christian , Moroder Luis 
Published: July 2003

AbstractIn analogy to the cystine knots present in natural collagens, a simplified disulfide cross‐link was used to analyse the conformational effects of a C‐terminal artificial cystine knot on the folding of collagenous peptides consisting of solely (Pro‐Hyp‐Gly) repeating units. Assembly of the α chains into a heterotrimer by previously applied regioselective disulfide‐bridging strategies failed because of the high tendency of (Pro‐Hyp‐Gly)5 peptides to self‐associate and form homotrimers. Only when side‐chain‐protected peptides were used, for example in the Hyp(tBu) form, and a new protection scheme was adopted, selective interchain‐disulfide cross‐linking into the heterotrimer in organic solvents was successful. This unexpected strong effect of the conformational properties on the efficiency of well‐established reactions was further supported by replacing the Hyp residues with (4S)‐fluoroproline, which is known to destabilise triple‐helical structures. With the related [Pro‐(4S)‐FPro‐Gly]5 peptides, assembly of the heterotrimer in aqueous solution proceeded in a satisfactory manner. Both the intermediates and the final fluorinated heterotrimer are fully unfolded in aqueous solution even at 4 °C. Conversely, the disulfide‐crossbridged (Pro‐Hyp‐Gly)5 heterotrimer forms a very stable triple helix. The observation that thermal unfolding leads to scrambling of the disulfide bridges was unexpected. Although NMR experiments support an extension of the triple helix into the cystine knot, thermolysis is not associated with the unfolding process. In fact, the unstructured fluorinated trimer undergoes an equally facile thermodegradation associated with the intrinsic tendency of unsymmetrical disulfides to disproportionate into symmetrical disulfides under favourable conditions. The experimental results obtained with the model peptides fully support the role of triple‐helix nucleation and stabilisation by the artificial cystine knot as previously suggested for the natural cystine knots in collagens.
The Role of Cystine Knots in Collagen Folding and Stability, Part II. Conformational Properties of (Pro‐Hyp‐Gly)n Model Trimers with N‐ and C‐Terminal Collagen Type III Cystine Knots
Authors: Barth Dirk , Kyrieleis Otto , Frank Sabine , Renner Christian , Moroder Luis 
Published: July 2003

AbstractIn mature collagen type III the homotrimer is C‐terminally cross‐linked by an interchain cystine knot consisting of three disulfide bridges of unknown connectivity. This cystine knot with two adjacent cysteine residues on each of the three α chains has recently been used for the synthesis and expression of model homotrimers. To investigate the origin of correct interchain cysteine pairings, (Pro‐Hyp‐Gly)n peptides of increasing triplet number and containing the biscysteinyl sequence C‐ and N‐terminally were synthesised. The possibilities were that this origin may be thermodynamically coupled to the formation of the collagen triple helix as happens in the oxidative folding of proteins, or it could represent a post‐folding event. Only with five triplets, which is known to represent the minimum number for self‐association of collagenous peptides into a triple helix, air‐oxidation produces the homotrimer in good yields (70 %), the rest being intrachain oxidised monomers. Increasing the number of triplets has no effect on yield suggesting the formation of kinetically trapped intermediates, which are not reshuffled by the glutathione redox buffer. N‐terminal incorporation of the cystine knot is significantly less efficient in the homotrimerisation step and also in terms of triple‐helix stabilisation. Compared to an artificial C‐terminal cystine knot consisting of two interchain disulfide bridges, the collagen type III cystine knot produces collagenous homotrimers of remarkably high thermostability, although the concentration‐independent refolding rates are not affected by the type of disulfide bridging. Since the natural cystine knot allows ready access to homotrimeric collagenous peptides of significantly enhanced triple‐helix thermostability it may well represent a promising approach for the preparation of collagen‐like innovative biomaterials. Conversely, the more laborious regioselectively formed artificial cystine knot still represents the only synthetic strategy for heterotrimeric collagenous peptides.
Construction of Artificial Signal Transducers on a Lectin Surface by Post‐Photoaffinity‐Labeling Modification for Fluorescent Saccharide Biosensors
Authors: Nagase Tsuyoshi , Nakata Eiji , Shinkai Seiji , Hamachi Itaru 
Published: July 2003

AbstractA new general method, post‐photoaffinity‐labeling modification (PPALM), for constructing fluorescent saccharide biosensors based on naturally occurring saccharide‐binding proteins, lectins, is described in detail. An active‐site‐directed incorporation of a masked reactive site into a lectin was conducted by using a photoaffinity labeling technique followed by demasking and then chemical modification to yield a fluorescent lectin. Two photoaffinity labeling reagents were designed and synthesized in this study. The labeling reagent with a photoreactive site appended through a disulfide link to a mannoside unit was bound to the saccharide‐binding pocket of the lectin concanavalin A (Con A). After light irradiation, the mannoside unit was cleaved by reduction. The unique thiol group thus produced was site‐specifically modified with various fluorescent groups (dansyl, coumarin, or dimethylaminobenzoate derivatives) to afford fluorescent Con As. The labeling site was characterized by protease‐catalyzed digestion followed by HPLC, MALDI‐TOF MS, and tandem mass–mass spectrometry; these methods indicated that the photolabeling step is remarkably site specific. Strong fluorescence was observed in the engineered Con A with a fluorophore, and the emission changed sensitively upon saccharide complexation. The binding constants for various saccharides were determined by fluorescence titration and demonstrated that the binding selectivity and affinity of the engineered Con As are comparable to those of native Con A. The red shift of the emission maximum, the decrease in the fluorescence anisotropy of the dansyl unit, and the increase in the twisted intramolecular charge transfer emission caused by sugar binding to the engineered Con A explicitly indicate that the microenvironment of the appended fluorophores changes from a restricted and relatively hydrophobic environment into a rather freely mobile and hydrophilic environment.
Intramolecular Dimers: A New Design Strategy for Fluorescence‐Quenched Probes
Authors: Johansson Mary Katherine , Cook Ronald M. 
Published: July 2003

AbstractFluorogenic probes dual‐labeled with reporter and quencher dyes use a change in fluorescence to monitor biochemical events (e.g., substrate binding or enzyme digestion). Such events change the reporter–quencher distance, which affects fluorescence. Recently, it is has been shown that static quenching through intramolecular dimers is an important mechanism that can sometimes be more efficient than Förster resonance energy transfer (FRET).
Cover Picture: Intramolecular Dimers: A New Design Strategy for Fluorescence‐Quenched Probes (Chem. Eur. J. 15/2003)
Authors: Johansson Mary Katherine , Cook Ronald M. 
Published: July 2003

AbstractThe cover picture shows a 96‐well plate illuminated by a UV lamp and viewed through a cutoff filter. All wells contain a Cy3.5‐BHQ1 (reporter quencher) dual‐labeled oligonucleotide probe in buffer. Complementary sequence was added to three columns. The probe is virtually nonfluorescent due to static quenching through formation of an intramolecular dimer. Addition of complement separates the dye and quencher, causing fluorescence. Intramolecular dimers as a new design strategy for fluorescence‐quenched probes is discussed by M. K. Johansson and R. M. Cook in the Concepts article on p. 3466 ff.
Organization of glomeruli in the main olfactory bulb of Xenopus laevis tadpoles
Authors: Nezlin Leonid P. , Heermann Stephan , Schild Detlev , Rössler Wolfgang 
Published: July 2003

AbstractStructural and functional investigations were carried out to study olfactory glomeruli in the main olfactory bulb (OB) in tadpoles of the clawed frog, Xenopus laevis. Calcium imaging of odor response patterns of OB neurons revealed that the synapses within the glomeruli are functional. Tracing axons of individual olfactory receptor neurons (ORNs), dendrites of mitral/tufted (M/T) cells and processes of periglomerular interneurons indicate that the glomerular architecture is solely determined by terminal branches of ORN axons and tufts of M/T primary dendrites. The small population of periglomerular neurons forms wide‐field arborizations that always extend over many glomeruli, enter the glomeruli, but lack any glomerular tufts. Antibodies to synaptophysin indicate a high density of synapses within glomeruli, which was further confirmed at the ultrastructural level and quantified to approximately 0.5 synaptic sites per μm2. Combining immunocytochemistry and ultrastructural investigations, we show that glomeruli in Xenopus laevis tadpoles lack any cellular borders. Glomeruli are surrounded neither by periglomerular somata nor by glial processes. Taken together, our results demonstrate that olfactory glomeruli in Xenopus laevis tadpoles (1) are fully functional, (2) are spheroidal neuropil aggregations of terminal tufts of ORNs and tufts of primary dendrites of M/T cells, and (3) are not enwrapped by a border formed by juxtaglomerular cells. J. Comp. Neurol. 464:257–268, 2003. © 2003 Wiley‐Liss, Inc.
Load redundancy elimination on executable code
Authors: Fernández Manel , Espasa Roger , Debray Saumya 
Published: August 2003

AbstractOptimizations performed at link time or directly applied to final program executables have received increased attention in recent years. This paper discusses the discovery and elimination of redundant load operations in the context of a link‐time optimizer, an optimization that we callLoad Redundancy Elimination (LRE). Our experiments show that between 50% and 75% of a program's memory references can be considered redundant because they are accessing memory locations that have been referenced less than 200–400 instructions away. We then present three profile‐based LRE algorithms targeted at optimizing away these redundancies. Our results show that between 5% and 30% of the redundancy detected can indeed be eliminated, which translates into program speedups of around 8%. We also test our algorithm assuming different cache latencies, and show that, if latencies continue to grow, the load redundancy elimination will become more important. Copyright © 2003 John Wiley & Sons, Ltd.
Perspectives of the high‐dimensional dynamics of neural microcircuits from the point of view of low‐dimensional readouts
Authors: Häusler Stefan , Markram Henry , Maass Wolfgang 
Published: August 2003

AbstractWe investigate generic models for cortical microcircuits, i.e., recurrent circuits of integrate‐and‐fire neurons with dynamic synapses. These complex dynamic systems subserve the amazing information processing capabilities of the cortex, but are at the present time very little understood. We analyze the transient dynamics of models for neural microcircuits from the point of view of one or two readout neurons that collapse the high‐dimensional transient dynamics of a neural circuit into a one‐ or two‐dimensional output stream. This stream may for example represent the information that is projected from such circuit to some particular other brain area or actuators. It is shown that simple local learning rules enable a readout neuron to extract from the high‐dimensional transient dynamics of a recurrent neural circuit quite different low‐dimensional projections, which even may contain “virtual attractors” that are not apparent in the high‐dimensional dynamics of the circuit itself. Furthermore it is demonstrated that the information extraction capabilities of linear readout neurons are boosted by the computational operations of a sufficiently large preceding neural microcircuit. Hence a generic neural microcircuit may play a similar role for information processing as a kernel for support vector machines in machine learning. We demonstrate that the projection of time‐varying inputs into a large recurrent neural circuit enables a linear readout neuron to classify the time‐varying circuit inputs with the same power as complex nonlinear classifiers, such as a pool of perceptrons trained by the p‐delta rule or a feedforward sigmoidal neural net trained by backprop, provided that the size of the recurrent circuit is sufficiently large. At the same time such readout neurons can exploit the stability and speed of learning rules for linear classifiers, thereby overcoming the problems caused by local minima in the error function of nonlinear classifiers. In addition it is demonstrated that pairs of readout neurons can transform the complex trajectory of transient states of a large neural circuit into a simple and clearly structured two‐dimensional trajectory. This two‐dimensional projection of the high‐dimensional trajectory can even exhibit convergence to virtual attractors that are not apparent in the high‐dimensional trajectory. © 2003 Wiley Periodicals, Inc.
Nonequilibrium statistical mechanics of swarms of driven particles
Authors: Ebeling Werner , Erdmann Udo 
Published: August 2003

AbstractAs a rough model for the collective motions of cells and organisms we develop here the statistical mechanics of swarms of self‐propelled particles. Our approach is closely related to the recently developed theory of active Brownian motion and the theory of canonical‐dissipative systems. Free motion and motion of a swarms confined in an external field is studied. Briefly, the case of particles confined on a ring and interacting by repulsive forces is studied. In more detail we investigate self‐confinement by Morse‐type attracting forces. We begin with pairs N = 2; the attractors and distribution functions are discussed, then the case N > 2 is discussed. Simulations for several dynamical modes of swarms of active Brownian particles interacting by Morse forces are presented. In particular we study rotations, drift, fluctuations of shape, and cluster formation. © 2003 Wiley Periodicals, Inc.
Expandable graphite: A fire retardant additive for polyurethane coatings
Authors: Duquesne Sophie , Bras Michel Le , Bourbigot Serge , Delobel René , Vezin Hervé , Camino Giovanni , Eling Berend , Lindsay Chris , Roels Toon 
Published: July 2003

AbstractThe study investigates the mechanism of fire retardancy induced by expandable graphite in a polyurethane coating. A complementary investigation to the spectroscopic study performed previously confirms that little interaction occurs between the matrix and the additive. X‐ray diffraction study demonstrates that the crystalline structure of the graphite is maintained during the intumescent process. The presence of free radicals in the protective shield is then demonstrated but at low concentration compared with classical intumescent systems, such as for example polyurethane/ammonium polyphosphate. Finally, the physical properties of the char are then investigated. Blowing measurement demonstrates the high degree of expansion of the layer which may partially explain the fire retardant performance of expandable graphite in polyurethane. Surface analyses by optical microscopy and optical profilometry also enable a better understanding of the efficiency of expandable graphite. In the absence of a fire retardant additive the coating displays cracks and holes, providing a pathway for oxygen diffusion as well as heat and mass transfer between the flame and the virgin polymer. Both phenomena are limited when expandable graphite is added to polyurethane. Consequently, fire retardancy of expandable graphite results more from a physical than from a chemical action. Copyright © 2003 John Wiley & Sons, Ltd.
Ignition and combustion of low‐density polyimide foam
Authors: Hshieh Fu‐Yu , Hirsch David B. , Beeson Harold D. 
Published: July 2003

AbstractThe ignition, flaming and smoldering combustion of low‐density polyimide foam have been studied using a cone calorimeter. Low‐density polyimide foam exhibits a high ignition resistance. The minimum heat flux for the ignition of flaming combustion ranges from 48 to 54 kW/m2. This minimum heat flux also indicates the heat flux for transition from smoldering to flaming combustion. The flaming combustion results show that the heat release rate of low‐density polyimide foam is very low even at a high incident heat flux of 75 kW/m2. The smoldering combustion results show that the smoldering of low‐density polyimide foam becomes significant when the incident heat flux is greater than 30 kW/m2. The smoldering combustion of low‐density polyimide foam cannot be self‐sustaining when the external heat source is removed. Copyright © 2003 John Wiley & Sons, Ltd.