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Different Ligands (different + ligand)
Selected AbstractsStructure of the Glucocorticoid Receptor, a Flexible Protein That Can Adapt to Different LigandsCHEMMEDCHEM, Issue 5 2010Adriana Crystal structures of the glucocorticoid receptor (GR) ligand binding domain in complex with various agonists and antagonists give us an insight on how ligands are recognized by the receptor and how their structure can affect the behavior of the GR. Interestingly, these structural data show how the GR can adapt its binding pocket to accommodate molecules that differ substantially from the natural ligands without loss of function. [source] Developmental changes in neurite outgrowth responses of dorsal root and sympathetic ganglia to GDNF, neurturin, and arteminDEVELOPMENTAL DYNAMICS, Issue 3 2003H. Yan Abstract The ability of glial cell line,derived neurotrophic factor (GDNF), neurturin, and artemin to induce neurite outgrowth from dorsal root, superior cervical, and lumbar sympathetic ganglia from mice at a variety of development stages between embryonic day (E) 11.5 and postnatal day (P) 7 was examined by explanting ganglia onto collagen gels and growing them in the presence of agarose beads impregnated with the different GDNF family ligands. Artemin, GDNF, and neurturin were all capable of influencing neurite outgrowth from dorsal root and sympathetic ganglia, but the responses of each neuron type to the different ligands varied during development. Neurites from dorsal root ganglia responded to artemin at P0 and P7, to GDNF at E15.5 and P0, and to neurturin at E15.5, P0, and P6/7; thus, artemin, GDNF, and neurturin are all capable of influencing neurite outgrowth from dorsal root ganglion neurons. Neurites from superior cervical sympathetic ganglia responded significantly to artemin at E15.5, to GDNF at E15.5 and P0, and to neurturin at E15.5. Neurites from lumbar sympathetic ganglia responded to artemin at all stages from E11.5 to P7, to GDNF at P0 and P7 and to neurturin at E11.5 to P6/7. Combined with the data from previous studies that have examined the expression of GDNF family members, our data suggest that artemin plays a role in inducing neurite outgrowth from young sympathetic neurons in the early stages of sympathetic axon pathfinding, whereas GDNF and neurturin are likely to be important at later stages of sympathetic neuron development in inducing axons to enter particular target tissues once they are in the vicinity or to induce branching within target tissues. Superior cervical and lumbar sympathetic ganglia showed temporal differences in their responsiveness to artemin, GDNF, and neurturin, which probably partly reflects the rostrocaudal development of sympathetic ganglia and the tissues they innervate. Developmental Dynamics 227:395,401, 2003. © 2003 Wiley-Liss, Inc. [source] Mixed-Anion and Mixed-Cation Borohydride KZn(BH4)Cl2: Synthesis, Structure and Thermal DecompositionEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2010Dorthe B. Ravnsbæk Abstract KZn(BH4)Cl2, synthesized for the first time, contains a heteroleptic complex anion [Zn(BH4)Cl2],, extending the structural diversity of metal borohydrides. In-situ synchrotron powder diffraction, NMR and Raman spectroscopy were used to characterize KZn(BH4)Cl2 and to evaluate the mechanism for its thermal decomposition. The title compound decomposes at a significantly lower temperature than KBH4 and may be used for inspiration for the design of novel hydrogen storage materials. Combining different ligands in modified metal borohydrides is proposed as a way to adjust stability with respect to hydrogen desorption. [source] Tuning of Copper(I),Dioxygen Reactivity by Bis(guanidine) Ligands,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 19 2005Sonja Herres-Pawlis Abstract A series of bis(guanidine) ligands designed for use in biomimetic coordination chemistry, namely bis(tetramethylguanidino)-, bis(dipiperidinoguanidino)-, and bis(dimethylpropyleno)propane (btmgp, DPipG2p and DMPG2p, respectively), has been extended to include bis(dimethylethyleneguanidino)propane (DMEG2p), which has both Namine atoms of each guanidine functionality connected by a short ethylene bridge, as a member. From this series, a family of novel bis(guanidine)copper(I) compounds , [Cu2(btmgp)2][PF6]2 (1), [Cu2(DPipG2p)2][PF6]2 (2), [Cu2(DMPG2p)2][PF6]2 (3), and [Cu2(DMEG2p)2][PF6]2·2MeCN (4) , has been synthesised. Single-crystal X-ray analysis of 1,4 demonstrated that these compounds contain dinuclear complex cations that contain twelve-membered heterocyclic Cu2N4C6 rings with the Cu atoms being more than 4 Å apart. Each copper atom is surrounded by a set of two N-donor functions from different ligands, resulting in linear N,Cu,N coordination sites. Depending on their individual substitution patterns, the guanidine moieties deviate from planarity by characteristic propeller-like twists of the amino groups around their N,Cimine bonds. The influence of these groups on the reactivity of the corresponding complexes 1,4 with dioxygen was investigated at low temperatures by means of UV/Vis spectroscopy. The reaction products can be classified into ,-,2:,2 -peroxodicopper(II) or bis(,-oxo)dicopper(III) complex cations that contain the {Cu2O2}2+ core portion as different isomers. The electronic properties of the specific bis(guanidine) ligands are discussed from the viewpoint of their ,-donor and ,-acceptor capabilities, and it is shown that ,-,2:,2 -peroxodicopper(II) complexes are stabilised relative to the bis(,-oxo)dicopper(III) ones if , conjugation within the guanidine moieties is optimised. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Disulfide bond formation through Cys186 facilitates functionally relevant dimerization of trimeric hyaluronan-binding protein 1 (HABP1)/p32/gC1qRFEBS JOURNAL, Issue 1 2002Babal Kant Jha Hyaluronan-binding protein 1 (HABP1), a ubiquitous multifunctional protein, interacts with hyaluronan, globular head of complement component 1q (gC1q), and clustered mannose and has been shown to be involved in cell signalling. In vitro, this recombinant protein isolated from human fibroblast exists in different oligomeric forms, as is evident from the results of various independent techniques in near-physiological conditions. As shown by size-exclusion chromatography under various conditions and glutaraldehyde cross-linking, HABP1 exists as a noncovalently associated trimer in equilibrium with a small fraction of a covalently linked dimer of trimers, i.e. a hexamer. The formation of a covalently-linked hexamer of HABP1 through Cys186 as a dimer of trimers is achieved by thiol group oxidation, which can be blocked by modification of Cys186. The gradual structural transition caused by cysteine-mediated disulfide linkage is evident as the fluorescence intensity increases with increasing Hg2+ concentration until all the HABP1 trimer is converted into hexamer. In order to understand the functional implication of these transitions, we examined the affinity of the hexamer for different ligands. The hexamer shows enhanced affinity for hyaluronan, gC1q, and mannosylated BSA compared with the trimeric form. Our data, analyzed with reference to the HABP1/p32 crystal structure, suggest that the oligomerization state and the compactness of its structure are factors that regulate its function. [source] Gram-negative bacteria and phagocytic cell interaction mediated by complement receptor 3FEMS IMMUNOLOGY & MEDICAL MICROBIOLOGY, Issue 4 2002José Agramonte-Hevia Abstract Complement receptor 3 (CR3) is an integrin that recognizes several different ligands. Binding to CR3 in phagocytic cells activates signaling pathways involved in cytoskeleton rearrangement, regulation of cell motility, alteration of gene expression and phagocytosis of complement-opsonized as well as of some non-opsonized particles and pathogenic bacteria. However, CR3-mediated phagocytosis of some Gram-negative bacteria does not induce bacterial clearance. Pseudomonas aeruginosa, Salmonella and Escherichia coli are eliminated after phagocytic cell,bacteria interaction mediated by CR3. However, Bordetella takes advantage of the CR3 function and uses it to enter into macrophages leading to bacterial survival. The final fate of the pathogen is determined by combinations of host and bacterial factors, in which molecular interactions between CR3 and bacterial ligands are involved. [source] An integrated view of the regulation of NKG2D ligandsIMMUNOLOGY, Issue 1 2009Noam Stern-Ginossar Summary NKG2D is one of the best characterized activating receptors and is expressed on natural killer cells and on various T-cell subsets. This receptor recognizes several different ligands that are induced by cellular stresses. In this review, we described the mechanisms controlling the expression of NKG2D ligands, with the emphasis on post-transcriptional and post-translational regulation. [source] Role of chemokine ligand 2 in the protective response to early murine pulmonary tuberculosisIMMUNOLOGY, Issue 4 2003Andre Kipnis Summary Chemokines play an important role in the development of immunity to tuberculosis. Chemokine ligand 2 (CCL2, JE, monocyte chemoattractant protein-1) is thought to be primarily responsible for recruiting monocytes, dendritic cells, natural killer cells and activated T cells, all of which play critical roles in the effective control of tuberculosis infection in mice. We show here that in mice in which the CCL2 gene was disrupted, low-dose aerosol infection with Mycobacterium tuberculosis resulted in fewer macrophages entering the lungs, but only a minor and transient increase in bacterial load in the lungs; these mice were still able to establish a state of chronic disease. Such animals showed similar numbers of activated T cells as wild-type mice, as determined by their expression of the CD44hi CD62lo phenotype, but a transient reduction in cells secreting interferon-,. These data indicate that the primary deficiency in mice unable to produce CCL2 is a transient failure to focus antigen-specific T lymphocytes into the infected lung, whereas other elements of the acquired host response are compensated for by different ligands interacting with the chemokine receptor CCR2. [source] Cover Picture: Multipotent Polymer Coatings Based on Chemical Vapor Deposition Copolymerization (Adv. Mater.ADVANCED MATERIALS, Issue 12 200612/2006) Abstract The cover shows that chemical vapor deposition can be used to prepare copolymer thin films, on varying substrates, that can bind two different ligands with high selectivity. In work reported by Lahann and co-workers on p.,1521, the actual ligand ratios on the surface can be controlled by varying the copolymer composition. This technology may find applications in biomedical devices, high-throughput screening platforms, microfluidic analysis devices, and diagnostic platforms. [source] Cooperativity and allostery in haemoglobin functionIUBMB LIFE, Issue 2 2008Chiara Ciaccio Abstract Tetrameric haemoglobins display a cooperative ligand binding behaviour, which has been attributed to the functional interrelationship between multiple ligand binding sites. The quantitative description of this feature was initially carried out with a phenomenological approach, which was limited to the functional effect of the occupancy by a ligand molecule of a binding site on further binding steps. However, subsequent development of structural,functional models for the description of the cooperativity in haemoglobin brought about a much deeper information on the interrelationships between ligand binding at the heme and structural variations occurring in the surrounding free subunits. This approach opened the way to the evolution of the concept of allostery, which is intended as the structural,functional effect exerted by the presence of a ligand in a binding site on other binding sites present in the same molecule. This concept can be applied to either sites for the same ligand (homotropic allostery) and for sites of different ligands (heterotropic allostery). Several models trying to take into account the continuous building up of structural and functional information on the physicochemical properties of haemoglobin have been developed along this line. © 2008 IUBMB IUBMB Life, 60(2): 112,123, 2008 [source] Catalytic effect of ferricyanide between myoglobin and luminol and effect of temperatureLUMINESCENCE: THE JOURNAL OF BIOLOGICAL AND CHEMICAL LUMINESCENCE, Issue 2 2007Xin Gao Abstract Specific catalytic oxidation of oxymyoglobin (MbO2) and luminol by ferricyanide was studied in a flow-injection system. MbO2 in different redox states (ferric and ferrous) was oxidized to Mb(FeIII) by ferricyanide, and then specific binding of the ferrocyanide anion to Mb(FeIII) to the His 119 (GH1) region accelerated the electron transfer between Mb(FeIII) and luminol, which produced a chemiluminescence (CL) signal at 425 nm. The increased CL emission was correlated with the myoglobin concentration in the range 0.16,7.5 µg/mL. Thermogravimetry and differential scanning calorimetry were used to investigate the temperature effects on this reaction. The results showed that the CL intensity in the presence of myoglobin changed considerably with heating in the range 15,50°C, and the maximal CL intensity was observed at 40°C, corresponding to the glass transition temperature of myoglobin. The effect of different ligands and interferences were also studied. Copyright © 2006 John Wiley & Sons, Ltd. [source] Thermochemistry, bonding, and reactivity of Ni+ and Ni2+ in the gas phaseMASS SPECTROMETRY REVIEWS, Issue 4 2007Otilia Mó Abstract In this review, we present a general overview on the studies carried out on Ni+- - and Ni2+ -containing systems in the gas phase since 1996. We have focused our attention in the determination of binding energies in parallel with an analysis of the structure and bonding of the complexes formed by the interaction of Ni+ with one ligand, or in clusters where this metal ion binds several identical or different ligands. Solvation of Ni2+ by different ligands is also discussed, together with the theoretical information available of doubly charged Ni-containing species. The final section of this review is devoted to an analysis of the gas-phase uni- and bimolecular reactivity of Ni+ and Ni2+ complexes. © 2007 Wiley Periodicals, Inc., Mass Spectrom. Rev. [source] Potential Role of NKG2D and Its Ligands in Organ Transplantation: New Target for ImmunointerventionAMERICAN JOURNAL OF TRANSPLANTATION, Issue 2 2009B. Suárez-Álvarez NKG2D is one of the best characterized activating receptors on Natural Killer (NK) and CD8+ T cells. This receptor recognizes several different ligands (MICA/MICB and ULBPs) induced by cellular stress and infection. In addition to the role described in cancer surveillance, recent data highlight the importance of NKG2D and its ligands in organ transplantation. Allografts show evidence of MICA and MICB expression in both acute and chronic rejection. The presence of anti-MICA antibodies has been correlated with incidence of graft rejection. Furthermore, NKG2D-ligand engagement activates NK cells, which provides T-cell costimulation, and enhances antigen specific CTL-mediated cytotoxicity. Activated NK cells may function as a bridge between innate and adaptive immunity associated with transplantation. Activated NK cells in response to IL-15 can also trigger organ rejection through NKG2D and affect the maturation of both donor and recipient antigen presenting cells (APCs) and ultimately the T-cell allogeneic response. Regulatory T cells, which modulate T-cell responses in organ transplantation and infections, were reduced in numbers by NK cells exposed to intracellular pathogens, possibly via interaction with one NK2GD receptor. Blockage of NKG2D-NKG2D-L interactions provides a novel pathway for development of inhibitors. These studies have important clinical and therapeutic implications in solid organ transplantation. [source] Heck reactions in monoglyme and diglyme using BmimCl as pre-ligandAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 1 2008Aline A. Bello da Silva Abstract Heck reactions between iodobenzene and methyl acrylate were carried out in monoglyme and diglyme as solvents, using different ligands and palladium sources, and good to high yields were obtained in the production of (E)-substituted olefins. The ionic liquid BmimCl was successfully utilized as pre-ligand to substitute triphenylphosphine. Copyright © 2007 John Wiley & Sons, Ltd. [source] catena -Poly[[bis[,-1,2-bis(1-methyltetrazol-5-yl)ethane-,2N4:N4,]bis[chlorocopper(II)]]-di-,-chloro]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2003Dmitry O. Ivashkevich In the title compound, [Cu2Cl4(C6H10N8)2]n, the ligand has C2 symmetry, and the Cu and Cl atoms lie on a mirror plane. The coordination polyhedron of the Cu atom is a distorted square pyramid, with the basal positions occupied by two N atoms from two different ligands [Cu,N,=,2.0407,(18),Å] and by the two Cl atoms [Cu,Cl,=,2.2705,(8) and 2.2499,(9),Å], and the apical position occupied by a Cl atom [Cu,Cl,=,2.8154,(9),Å] that belongs to the basal plane of a neighbouring Cu atom. The [CuCl2(C6H10N8)]2 units form infinite chains extending along the a axis via the Cl atoms. Intermolecular C,H,Cl contacts [C,Cl,=,3.484,(2),Å] are also present in the chains. The chains are linked together by intermolecular C,H,N interactions [C,N,=,3.314,(3),Å]. [source] Generation and Evaluation of a Homology Model of PfGSK-3ARCHIV DER PHARMAZIE, Issue 6 2009Sebastian Kruggel Abstract Plasmodial GSK-3 is a potential new target for malaria therapy. For a structure-based design project, the three-dimensional information of the designated target is needed. Unfortunately, experimental structure data for plasmodial GSK-3 is not yet available. Homology building can be used to generate such three-dimensional structure data using structure information of a homologous protein. GSK-3 possesses a very flexible ATP-binding site, a fact reflected in the variety of X-ray structures of the human GSK-3, which are deposited in the protein data base and are crystallized with different ligands. We used ten different HsGSK-3, templates for the model building of plasmodial GSK-3 and generated 200 models for each template with different modeling protocols. The quality of the models was evaluated with different tools. The results of these evaluations were used to calculate a rank-by-rank consensus score. The top models of this were used to compile an ensemble of PfGSK-3 models that reflect the flexibility of the ATP-binding site and that will be used for the structure-based design of potential ATP-binding site inhibitors of PfGSK-3. [source] Introduction of a leucine half-zipper engenders multiple high-quality crystals of a recalcitrant tRNA synthetaseACTA CRYSTALLOGRAPHICA SECTION D, Issue 3 2010Min Guo Although Escherichia coli alanyl-tRNA synthetase was among the first tRNA synthetases to be sequenced and extensively studied by functional analysis, it has proved to be recalcitrant to crystallization. This challenge remained even for crystallization of the catalytic fragment. By mutationally introducing three stacked leucines onto the solvent-exposed side of an ,-helix, an engineered catalytic fragment of the synthetase was obtained that yielded multiple high-quality crystals and cocrystals with different ligands. The engineered ,-helix did not form a leucine zipper that interlocked with the same ,-helix from another molecule. Instead, using the created hydrophobic spine, it interacted with other surfaces of the protein as a leucine half-zipper (LHZ) to enhance the crystal lattice interactions. The LHZ made crystal lattice contacts in all crystals of different space groups. These results illustrate the power of introducing an LHZ into helices to facilitate crystallization. The authors propose that the method can be unified with surface-entropy reduction and can be broadly used for protein-surface optimization in crystallization. [source] Molecular structure of the outer bacterial membrane of Pseudomonas aeruginosa via classical simulationBIOPOLYMERS, Issue 6 2002Robert M. Shroll Abstract A detailed structural analysis has been performed of the outer bacterial membrane of Pseudomonas aeruginosa using a parameterized classical simulation model (R. D. Lins and T. P. Straatsma, Biophysical Journal, 2001, Vol. 81, pp. 1037,1046) with modest modifications. The structural analysis of the membrane is presented and newly discovered characteristics of the membrane are discussed. Simulations indicate that the relative contribution of different ligands to calcium ion coordination varies across the membrane, while maintaining a constant average coordination number of 6.1. Water penetrates the surface of the membrane to a depth of about 30 Å. The hydration of ions and phosphate groups is shown to depend on location within the membrane. A measure of saccharide residue orientation is defined and average orientations are presented. Saccharide residues possess varying degrees of motion with a trend of greater mobility at the membrane surface. However, their motion is limited and even in the membrane outer core region the average structure appears fairly rigid over a period of 1 ns. © 2002 Wiley Periodicals, Inc. Biopolymers 65: 395,407, 2002 [source] Role of Capping Ligands on the Nanoparticles in the Modulation of Properties of a Hybrid Matrix of Nanoparticles in a 2D Film and in a Supramolecular OrganogelCHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2009Asish Pal Dr. Abstract We incorporate various gold nanoparticles (AuNPs) capped with different ligands in two-dimensional films and three-dimensional aggregates derived from N -stearoyl- L -alanine and N -lauroyl- L -alanine, respectively. The assemblies of N -stearoyl- L -alanine afforded stable films at the air,water interface. More compact assemblies were formed upon incorporation of AuNPs in the air,water interface of N -stearoyl- L -alanine. We then examined the effects of incorporation of various AuNPs functionalized with different capping ligands in three-dimensional assemblies of N -lauroyl- L -alanine, a compound that formed a gel in hydrocarbons. The profound influence of nanoparticle incorporation into physical gels was evident from evaluation of various microscopic and bulk properties. The interaction of AuNPs with the gelator assembly was found to depend critically on the capping ligands protecting the Au surface of the gold nanoparticles. Transmission electron microscopy (TEM) showed a long-range directional assembly of certain AuNPs along the gel fibers. Scanning electron microscopy (SEM) images of the freeze-dried gels and nanocomposites indicate that the morphological transformation in the composite microstructures depends significantly on the capping agent of the nanoparticles. Differential scanning calorimetry (DSC) showed that gel formation from sol occurred at a lower temperature upon incorporation of AuNPs having capping ligands that were able to align and noncovalently interact with the gel fibers. Rheological studies indicate that the gel,nanoparticle composites exhibit significantly greater viscoelasticity compared to the native gel alone when the capping ligands are able to interact through interdigitation into the gelator assembly. Thus, it was possible to define a clear relationship between the materials and the molecular-level properties by means of manipulation of the information inscribed on the NP surface. [source] Palladium-Catalyzed Cross-Coupling Reactions of Amines with Alkenyl Bromides: A New Method for the Synthesis of Enamines and IminesCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2004José Barluenga Prof. Dr. Abstract The palladium-catalyzed cross-coupling reaction of alkenyl bromides with secondary and primary amines gives rise to enamines and imines, respectively. This new transformation expands the applicability of palladium-catalyzed CN bond forming reactions (the Buchwald,Hartwig amination), which have mostly been applied to aryl halides. After screening of different ligands, bases, and solvents, the catalytic combination [Pd2(dba)3]/BINAP in the presence of NaOtBu in toluene gave the best results in the cross-coupling of secondary amines with 1-bromostyrene (dba=dibenzylideneacetone, BINAP=2,2,-bis(diphenylphosphino)-1,1,-binaphthyl). The corresponding enamines are obtained cleanly and in nearly quantitative yields. However, steric hindrance seems to be a limitation of the reaction, as amines carrying large substituents are not well converted. The same methodology can be applied to the coupling of secondary amines with 2-bromostyrene. Moreover, the reaction with substituted 2-bromopropenes allows regioselective synthesis of isomerizable terminal enamines without isomerization of the double bond. The best catalytic conditions for the cross-coupling of 1-bromostyrene with primary amines include again the use of the Pd0/BINAP/NaOtBu system. The reaction gives rise to the expected imines in very short times and with low catalyst loadings. A set of structurally diverse imines can be prepared by this methodology through variations in the structure of both coupling partners. However, 2-bromostyrene failed to give good results in this coupling reaction, probably due to product inhibition of the catalytic cycle. Competition experiments of vinyl versus aryl amination reveal that the reaction occurs preferentially on vinyl bromides. La reacción de acoplamiento cruzado de bromuros de alquenilo con aminas secundarias y primarias da lugar a enaminas e iminas respectivamente. Esta nueva transformación expande las aplicaciones de la reacción de formación de enlaces CN catalizada por paladio (aminación Buchwald,Hartwig), que se había limitado fundamentalmente a haluros de arilo. Después de un estudio de diferentes ligandos. bases y disolventes, los mejores resultados en la reacción de acoplamiento cruzado de aminas secundarias con 1-bromoestireno, se obtuvieron para el catalizador constituido por la combinación Pd/BINAP en presencia de NaOtBu en tolueno. Las correspondientes enaminas se obtienen limpiamente y con rendimientos prácticamente cuantitativos. Sin embargo, los impedimentos estéricos en la amina parecen ser una limitación de la reacción, puesto que aminas con sustituyentes voluminosos proporcionan conversiones bajas. La misma metodología puede aplicarse al acoplamiento de aminas secundarias con 2-bromoestireno. Además, la reacción con 2-bromopropenos sustituidos permite obtener de forma regioselectiva enaminas terminales isomerizables, sin que la isomerización del doble enlace tenga lugar. Las mejores condiciones catalíticas para el acoplamiento de aminas primarias con 1-bromoestireno incluyen de nuevo la utilización del sistema Pd(0)/BINAP/NaOtBu. La reacción proporciona las iminas esperadas en tiempos de reacción muy cortos y con baja carga del catalizador. Mediante esta metodología pueden prepararse un conjunto de iminas de gran diversidad estructural, permitiendo variaciones en ambos reactivos de acoplamiento. Sin embargo, la reacción con 2-bromoestireno no produce buenos resultados en este acoplamiento, probablemente debido a inhibición del ciclo catalítico por parte del producto de reacción. Finalmente, experimentos de competencia de aminación vinílica frente a aminación arílica, ponen de manifiesto que esta reacción se produce de forma preferente sobre los bromuros de vinilo. [source] Trinuclear Rhodium Complexes and Their Relevance for Asymmetric HydrogenationCHEMISTRY - AN ASIAN JOURNAL, Issue 11 2008Angelika Preetz Abstract Various trinuclear rhodium complexes of the type [Rh3(PP)3(,3 -OH)x(,3 -OMe)2,x]BF4 (where PP=Me-DuPhos, dipamp, dppp, dppe; different ligands and , -bridging anions) are presented, which are formed upon addition of bases such as NEt3 to solvate complexes [Rh(PP)(solvent)2]BF4. They were extensively characterized by X-ray diffraction and NMR spectroscopy (103Rh, 31P, 13C, 1H). Their in,situ formation resulting from basic additives (NEt3) or basic prochiral olefins (without addition of another base) can cause deactivation of the asymmetric hydrogenation. This effect can be reversed by means of acidic additives. [source] Mesoporous Pt,SiO2 and Pt,SiO2,Ta2O5 Catalysts Prepared Using Pt Colloids as TemplatesCHEMPHYSCHEM, Issue 5 2007Vasile I. Pârvulescu Prof. Dr. Abstract Sol-gel synthesis of silica and silica,tantalum oxide embedded platinum nanoparticles is carried out using Pt colloids as templates. These colloids are prepared by reduction with Na[AlEt3H] and stabilized with different ligands (ammonium halide derivatives, non-ionic surfactants with polyether chains, and 2-hydroxy-propionic acid). The aim of the present study is to prepare mesoporous silica embedded Pt colloids combining the "precursor concept" with the model of catalyst preparation using preformed spheres. Nanoparticles of Pt incorporated in high surface area mesoporous materials are formed after calcination. Further, it is observed that calcination of these catalysts causes partial aggregation and oxidation of the parent colloids, a process that is largely dependent on the nature of the stabilizing ligands. Several methods have been used for characterization of these materials: adsorption-desorption isotherms at 77 K, H2 chemisorption, X-ray diffraction(XRD), 29Si and 13C magic angle spinning (MAS) NMR, ammonia diffuse reflectance Fourier transform infrared spectroscopy (NH3 -DRIFT), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). It is found that both metal oxide systems exhibit Brønsted acidity (weaker for silica and quite strong for silica,tantalum oxide). In addition, NH3 -DRIFT experiments demonstrate the oxidative properties of the surface. Part of the adsorbed NH4+ species is oxidized to N2O. Testing these catalysts in the reduction of NO and NO2 with isopentane under lean conditions indicate that the activity of these catalysts is indeed dependent on the size of the platinum particles, with those of size 8,10 nm demonstrating the best results. The support likely contributes to this effect, particularly after Ta incorporation into silica. [source] Nonhanded chirality in octahedral metal complexesCHIRALITY, Issue 8 2001R.B. King Abstract Chiral molecules can either be handed (i.e., "shoes") or nonhanded ("potatoes"). The only chiral ligand partition for tetrahedral metal complexes (or for a tetrahedral carbon atom such as that found in amino acids and other chiral biological molecules) is the fully unsymmetrical degree 6 partition (14), which leads to handed metal complexes of the type MABCD with a lowest-degree chirality polynomial consisting of the product of all six possible linear factors of the type (si,sj) where 1 , i,j , 4. The lowest-degree chiral ligand partitions for octahedral metal complexes are the degree 6 partitions (313) and (23) leading to handed chiral metal complexes of the types fac -MA3BCD and cis -MA2B2C2. The form of the lowest-degree chirality polynomial for the (313) chiral ligand partition of the octahedron resembles that of the (14) chiral ligand partition of the tetrahedron, likewise with four different ligands. However, the form of the lowest-degree chirality polynomial for the (23) chiral ligand partition of the octahedron corresponds to the square of the chirality polynomial of the (13) chiral ligand partition of the polarized triangle, which likewise has three different ligands. Ligand partitions for octahedral metal complexes such as (2212), (214), and (16), which are less symmetrical than the lowest-degree chiral ligand partitions (313) and (23), lead to chiral octahedral metal complexes which are nonhanded. In such complexes, pairs of enantiomers can be interconverted by simple ligand interchanges without ever going through an achiral intermediate. Chirality 13:465,473, 2001. © 2001 Wiley-Liss, Inc. [source] | |||||||||||||||||||||||||||||||