Home About us Contact
|
Home About us Contact | ||
|
|
||
Metal Sites (metal + site)
Selected AbstractsOpen Metal Sites within Isostructural Metal,Organic Frameworks for Differential Recognition of Acetylene and Extraordinarily High Acetylene Storage Capacity at Room Temperature,ANGEWANDTE CHEMIE, Issue 27 2010Shengchang Xiang Dr. Ungesättigte Metallzentren in isostrukturellen Metall-organischen Gerüsten vom Typ [M2(DHTP)] (M=Co2+, Mn2+, Mg2+ und Zn2+; DHTP=2,5-Dihydroxyterephthalat) zeigen eine abgestufte molekulare Erkennung von Acetylen. Die extrem starke Wechselwirkung von Co2+ mit Acetylen (siehe Struktur) macht [Co2(DHTP)] zum bislang besten Acetylenspeichermaterial mit einer Kapazität von 230,cm3,cm,3 bei 295,K und 1,atm. [source] Oxidative Addition of Halogens on Open Metal Sites in a Microporous Spin-Crossover Coordination Polymer,ANGEWANDTE CHEMIE, Issue 47 2009Gloria Agustí Dr. Innige Verbindung: Die Einwirkung von X2 (Cl2, Br2, I2) auf das poröse Spin-Crossover-Material {Fe(pz)[PtII(CN)4]} (pz=Pyrazin) führt durch oxidative Addition zum Halogenid-Einbau in die koordinativ ungesättigten [PtII(CN)4]2, -Einheiten. Eine Serie gemischtvalenter {Fe(pz)[PtII/IV(CN)4(X)]}-Gerüste mit charakteristischem kooperativem Spinübergang wurde erhalten. [source] Heat-Induced Alterations in the Surface Population of Metal Sites in Bimetallic Nanoparticles.CHEMINFORM, Issue 40 2007Bing-Joe Hwang Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Functional characterization of an orphan cupin protein from Burkholderia xenovorans reveals a mononuclear nonheme Fe2+ -dependent oxygenase that cleaves ,-diketonesFEBS JOURNAL, Issue 20 2009Stefan Leitgeb Cupins constitute a large and widespread superfamily of ,-barrel proteins in which a mononuclear metal site is both a conserved feature of the structure and a source of functional diversity. Metal-binding residues are contributed from two core motifs that provide the signature for the superfamily. On the basis of conservation of this two-motif structure, we have identified an ORF in the genome of Burkholderia xenovorans that encodes a novel cupin protein (Bxe_A2876) of unknown function. Recombinant Bxe_A2876, as isolated from Escherichia coli cell extract, was a homotetramer in solution, and showed mixed fractional occupancy of its 16.1 kDa subunit with metal ligands (0.06 copper; 0.11 iron; 0.17 zinc). Our quest for possible catalytic functions of Bxe_A2876 focused on Cu2+ and Fe2+ oxygenase activities known from related cupin enzymes. Fe2+ elicited enzymatic catalysis of O2 -dependent conversion of various ,-diketone substrates via a nucleophilic mechanism of carbon,carbon bond cleavage. Data from X-ray absorption spectroscopy (XAS) support a five-coordinate or six-coordinate Fe2+ center where the metal is bound by three imidazole nitrogen atoms at 1.98 Å. Results of structure modeling studies suggest that His60, His62 and His102 are the coordinating residues. In the ,best-fit' model, one or two oxygens from water and a carboxylate oxygen (presumably from Glu96) are further ligands of Fe2+ at estimated distances of 2.04 Å and 2.08 Å, respectively. The three-histidine Fe2+ site of Bxe_A2876 is compared to the mononuclear nonheme Fe2+ centers of the structurally related cysteine dioxygenase and acireductone dioxygenase, which also use a facial triad of histidines for binding of their metal cofactor but promote entirely different substrate transformations. [source] Unusual Cu(I)/Ag(I) coordination of Escherichia coli CusF as revealed by atomic resolution crystallography and X-ray absorption spectroscopyPROTEIN SCIENCE, Issue 10 2007Isabell R. Loftin Abstract Elevated levels of copper or silver ions in the environment are an immediate threat to many organisms. Escherichia coli is able to resist the toxic effects of these ions through strictly limiting intracellular levels of Cu(I) and Ag(I). The CusCFBA system is one system in E. coli responsible for copper/silver tolerance. A key component of this system is the periplasmic copper/silver-binding protein, CusF. Here the X-ray structure and XAS data on the CusF,Ag(I) and CusF,Cu(I) complexes, respectively, are reported. In the CusF,Ag(I) structure, Ag(I) is coordinated by two methionines and a histidine, with a nearby tryptophan capping the metal site. EXAFS measurements on the CusF,Cu(I) complex show a similar environment for Cu(I). The arrangement of ligands effectively sequesters the metal from its periplasmic environment and thus may play a role in protecting the cell from the toxic ion. [source] Affinity cleavage at the divalent metal site of porcine NAD-specific isocitrate dehydrogenasePROTEIN SCIENCE, Issue 1 2000Yu-Chu Huang Abstract A divalent metal ion, such as Mn2+, is required for the catalytic reaction and allosteric regulation of pig heart NAD-dependent isocitrate dehydrogenase. The enzyme is irreversibly inactivated and cleaved by Fe2+ in the presence of O2 and ascorbate at pH 7.0. Mn2+ prevents both inactivation and cleavage. Nucleotide ligands, such as NAD, NADPH, and ADP, neither prevent nor promote inactivation or cleavage of the enzyme by Fe2+. The NAD-specific isocitrate dehydrogenase is composed of three distinct subunits in the ratio 2,:1 ,:1 ,. The results indicate that the oxidative inactivation and cleavage are specific and involve the 40 kDa , subunit of the enzyme. A pair of major peptides is generated during Fe2+ inactivation: 29.5 + 10.5 kDa, as determined by SDS-PAGE. Amino-terminal sequencing reveals that these peptides arise by cleavage of the Val262-His263 bond of the , subunit. No fragments are produced when enzyme is incubated with Fe2+ and ascorbate under denaturing conditions in the presence of 6 M urea, indicating that the native structure is required for the specific cleavage. These results suggest that His263 of the , subunit may be a ligand of the divalent metal ion needed for the reaction catalyzed by isocitrate dehydrogenase. Isocitrate enhances the inactivation of enzyme caused by Fe2+ in the presence of oxygen, but prevents the cleavage, suggesting that inactivation occurs by a different mechanism when metal ion is bound to the enzyme in the presence of isocitrate: oxidation of cysteine may be responsible for the rapid inactivation in this case. Affinity cleavage caused by Fe2+ implicates , as the catalytic subunit of the multisubunit porcine NAD-dependent isocitrate dehydrogenase. [source] Structures of three diphtheria toxin repressor (DtxR) variants with decreased repressor activityACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2001Ehmke Pohl The diphtheria toxin repressor (DtxR) from Corynebacterium diphtheriae regulates the expression of the gene on corynebacteriophages that encodes diphtheria toxin (DT). Other genes regulated by DtxR include those that encode proteins involved in siderophore-mediated iron uptake. DtxR requires activation by divalent metals and holo-DtxR is a dimeric regulator with two distinct metal-binding sites per three-domain monomer. At site 1, three side chains and a sulfate or phosphate anion are involved in metal coordination. In the DtxR,DNA complex this anion is replaced by the side chain of Glu170 provided by the third domain of the repressor. At site 2 the metal ion is coordinated exclusively by constituents of the polypeptide chain. In this paper, five crystal structures of three DtxR variants focusing on residues Glu20, Arg80 and Cys102 are reported. The resolution of these structures ranges from 2.3 to 2.8,Å. The side chain of Glu20 provided by the DNA-binding domain forms a salt bridge to Arg80, which in turn interacts with the anion. Replacing either of the salt-bridge partners with an alanine reduces repressor activity substantially and it has been inferred that the salt bridge could possibly control the wedge angle between the DNA-binding domain and the dimerization domain, thereby modulating repressor activity. Cys102 is a key residue of metal site 2 and its substitution into a serine abolishes repressor activity. The crystal structures of Zn-Glu20Ala-DtxR, Zn-Arg80Ala-DtxR, Cd-Cys102Ser-DtxR and apo-Cys102Ser-DtxR in two related space groups reveal that none of these substitutions leads to dramatic rearrangements of the DtxR fold. However, the five crystal structures presented here show significant local changes and a considerable degree of flexibility of the DNA-binding domain with respect to the dimerization domain. Furthermore, all five structures deviate significantly from the structure in the DtxR,DNA complex with respect to overall domain orientation. These results confirm the importance of the hinge motion for repressor activity. Since the third domain has often been invisible in previous crystal structures of DtxR, it is also noteworthy that the SH3-like domain could be traced in four of the five crystal structures. [source] Base-Induced Formation of Two Magnesium Metal-Organic Framework Compounds with a Bifunctional Tetratopic LigandEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 23 2008Pascal D. C. Dietzel Abstract Two coordination polymers constructed from magnesium and the tetratopic organic linker 2,5-dihydroxyterephthalic acid are reported, denominated CPO-26-Mg and CPO-27-Mg. The organic component carries two different types of protic functional groups. The degree of deprotonation of the organic component can be regulated by the amount of sodium hydroxide employed in the synthesis, thus determining which of the compounds forms. In CPO-26-Mg, only the carboxylic acid groups of the linker are deprotonated and take part in the construction of the three-dimensional framework. The structure is non-porous, and its topology is based on the PtS net. In CPO-27-Mg, both the carboxylic acid and the hydroxy groups are deprotonated and involved in the construction of a microporous three-dimensional framework which is based on a honeycomb motif containing large solvent-filled channels. The metal atoms are arranged in chiral chains along the intersection of the honeycomb and contain one water molecule in their coordination sphere, which allows for the creation of coordinatively unsaturated metal sites upon dehydration. CPO-27-Mg is a potentially useful lightweight adsorbent with a pore volume of 60,% of the total volume of the structure and an apparent Langmuir surface area of up to 1030 m2,g,1. Its thermal stability was investigated by thermogravimetry and variable-temperature powder X-ray diffraction, which shows framework degradation to commence at 160 °C in air, at 235 °C under nitrogen, and at 430 °C in a dynamic vacuum. Thermogravimetric dehydration and re-hydration experiments at miscellaneous temperatures indicate that it is possible to obtain open metal sites in CPO-27-Mg, but the water is more tightly bound in this material than in the previously reported isostructural nickel compound.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Iminohydroxamato Early and Late Transition Metal Halide Complexes , New Precatalysts for Aluminoxane-Cocatalyzed Olefin Insertion PolymerizationEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2004Alexander Krajete Abstract We report on new families of non-metallocene metal precatalysts for olefin polymerization with titanium, zirconium, vanadium and nickel as the active metal sites. The novel ligand design concept is based on iminohydroxamic acids and their derivatives as the principal chelating units. Various anionic and neutral [N,O] and [N,N] ligand systems are easily accessible by a modular synthetic sequence of imidoyl chlorides with substituted hydroxylamines or hydrazines, respectively. Steric protection of the metal coordination site, a necessary requirement for suppression of chain termination pathways of non-metallocene catalysts, is brought about by bulky aryl substituents on the imino nitrogen atoms. Crystal structures of some of the hydroxamato ligands reveal interesting intermolecular hydrogen-bridged structures, whereas in the solid-state structure of one titanium precatalyst a five-membered chelate was observed, in line with the design principle of these systems. Preliminary ethylene polymerization studies with methylaluminoxane-activated metal complexes (M = Ti, Zr, V, Ni) show that the most active systems are [N,O]NiBr2 catalysts containing neutral O -alkyl iminohydroxamate ligands. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] The SmtB/ArsR family of metalloregulatory transcriptional repressors: structural insights into prokaryotic metal resistanceFEMS MICROBIOLOGY REVIEWS, Issue 2-3 2003Laura S. Busenlehner Abstract The SmtB/ArsR family of prokaryotic metalloregulatory transcriptional repressors represses the expression of operons linked to stress-inducing concentrations of di- and multivalent heavy metal ions. Derepression results from direct binding of metal ions by these homodimeric ,metal sensor' proteins. An evolutionary analysis, coupled with comparative structural and spectroscopic studies of six SmtB/ArsR family members, suggests a unifying ,theme and variations' model, in which individual members have evolved distinct metal selectivity profiles by alteration of one or both of two structurally distinct metal coordination sites. These two metal sites are designated ,3N (or ,3) and ,5 (or ,5C), named for the location of the metal binding ligands within the known or predicted secondary structure of individual family members. The ,3N/,3 sensors, represented by Staphylococcus aureus pI258 CadC, Listeria monocytogenes CadC and Escherichia coli ArsR, form cysteine thiolate-rich coordination complexes (S3 or S4) with thiophilic heavy metal pollutants including Cd(II), Pb(II), Bi(III) and As(III) via inter-subunit coordination by ligands derived from the ,3 helix and the N-terminal ,arm' (CadCs) or from the ,3 helix only (ArsRs). The ,5/,5C sensors Synechococcus SmtB, Synechocystis ZiaR, S. aureus CzrA, and Mycobacterium tuberculosis NmtR form metal complexes with biologically required metal ions Zn(II), Co(II) and Ni(II) characterized by four or more coordination bonds to a mixture of histidine and carboxylate ligands derived from the C-terminal ,5 helices on opposite subunits. Direct binding of metal ions to either the ,3N or ,5 sites leads to strong, negative allosteric regulation of repressor operator/promoter binding affinity, consistent with a simple model for derepression. We hypothesize that distinct allosteric pathways for metal sensing have co-evolved with metal specificities of distinct ,3N and ,5 coordination complexes. [source] Charge-induced modulation of magnetic interactions in a [2 × 2] metal-organic grid complex,INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2006C. Romeike Abstract We investigate the magnetic state of a recently synthesized [2 × 2]-metal-organic grid complex as a function of its redox state. Our analysis of a phenomenological model for the relevant molecular orbitals reveals that additional electrons on the ligands can couple their spins via the bridging metal sites. We find that at certain stages of the reduction of the complex cation, a maximal total spin ground state of the complex (S = 3/2) can be stabilized by the Nagaoka mechanism. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] X-ray absorption spectroscopy to watch catalysis by metalloenzymes: status and perspectives discussed for the water-splitting manganese complex of photosynthesisJOURNAL OF SYNCHROTRON RADIATION, Issue 1 2003Holger Dau Understanding structure,function relations is one of the main interests in the molecular biosciences. X-ray absorption spectroscopy of biological samples (BioXAS) has gained the status of a useful tool for characterization of the structure of protein-bound metal centers with respect to the electronic structure (oxidation states, orbital occupancies) and atomic structure (arrangement of ligand atoms). Owing to progress in the performance characteristics of synchrotron radiation sources and of experimental stations dedicated to the study of (ultra-dilute) biological samples, it is now possible to carry out new types of BioXAS experiments, which have been impracticable in the past. Of particular interest are approaches to follow biological catalysis at metal sites by characterization of functionally relevant structural changes. In this article, the first steps towards the use of BioXAS to `watch' biological catalysis are reviewed for the water-splitting reactions occurring at the manganese complex of photosynthesis. The following aspects are considered: the role of BioXAS in life sciences; methodological aspects of BioXAS; catalysis at the Mn complex of photosynthesis; combination of EXAFS and crystallographic information; the freeze-quench technique to capture semi-stable states; time-resolved BioXAS using a freeze-quench approach; room-temperature experiments and `real-time' BioXAS; tasks and perspectives. [source] Le phosphate de cobalt et de lithium à valence mixte Li4+xCo2,x(P2O7)2 (x = 0,03): étude structurale et analyse de distribution de chargeACTA CRYSTALLOGRAPHICA SECTION C, Issue 1 2010Salah Kouass The title compound, namely lithium cobalt(II/III) bis(diphosphate), Li4.03Co1.97(P2O7)2, is a new mixed-valent lithium/cobalt(II/III) phosphate. Three metal sites out of seven are occupied simultaneously by Li+ and CoII/III ions. This disorder was established both from an analysis of the atomic displacement ellipsoids and Li/Co,O bond distances, and by means of a charge-distribution (CHARDI) model, which provides satisfactory agreement on the computed charges (Q) for all the cations. [source] Flexibility of the Cu,Zn superoxide dismutase structure investigated at 0.57,GPaACTA CRYSTALLOGRAPHICA SECTION D, Issue 6 2010Isabella Ascone The 2,Å resolution crystal structure of bovine erythrocyte Cu,Zn superoxide dismutase (CuZnSOD) has been determined by X-ray diffraction at high pressure (0.57,GPa) and room temperature. At 0.57,GPa the secondary, tertiary and quaternary structures are similar to other previously determined bovine erythrocyte CuZnSOD structures. Nevertheless, pressure has a localized impact on the atomic coordinates of C, atoms and on side chains. The compression of the crystal and of the protein backbone is anisotropic. This anisotropy is discussed, taking into account intermolecular contacts and protein conformation. Pressure perturbation highlights the more flexible zones in the protein such as the electrostatic loop. At 0.57,GPa, a global shift of the dimetallic sites in both subunits and changes in the oxidation state of Cu were observed. The flexibility of the electrostatic loop may be useful for the interaction of different metal carriers in the copper-uptake process, whereas the flexibility of the metal sites involved in the activity of the protein could contribute to explaining the ubiquitous character of CuZnSODs, which are found in organisms living in very different conditions, including the deep-sea environment. This work illustrates the potential of combining X-ray crystallography with high pressure to promote and stabilize higher energy conformational substates. [source] Reaction Mechanisms for Renewable Hydrogen from Liquid Phase Reforming of Sugar CompoundsASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1-2 2006A. Tanksale Hydrogen is anticipated to become a major source of energy in the future. Hydrogen is a clean burning fuel and has been described as a long-term replacement for natural gas. It has been demonstrated here that hydrogen can be produced from biomass in the temperature range of 185-220°C using a single batch reactor pressurised at 25-30 bar. The current work is based on sugars which are considered here as the biomass resource. Glucose, fructose and sucrose solutions were used for the liquid phase reforming using supported platinum catalyst. The sugar molecules might go through reversible dehydrogenation steps to give adsorbed species on metal sites. This adsorption might be either on CC or CO bond cleavage. Platinum is one of the best catalysts for the reforming of hydrocarbons due to its high selectivity for CC bond cleavage. The CC bond cleavage is the limiting factor for the reforming and leads to a high rate of formation of hydrogen. On the other hand CO bond cleavage results in formation of alcohols, acids and other organic groups. [source] Small revisions to predicted distances around metal sites in proteinsACTA CRYSTALLOGRAPHICA SECTION D, Issue 6 2006Marjorie M. Harding A new analysis has been made of distances around metal sites in protein structures in the Protein Data Bank determined with resolution ,1.25,Å and equivalent distances have been extracted from the Cambridge Structural Database. They are for the metals Na, Mg, K, Ca, Mn, Fe, Co, Cu, Zn and the donor atoms O of water, O of Asp and Glu, O of the main-chain carbonyl group, N of His and S of Cys. Some revisions are recommended to the tables of `target distances' previously given [Harding (2001), Acta Cryst. D57, 401,411; Harding (2002), Acta Cryst. D58, 872,874]. As well as small changes in many distances and a large improvement for Mg,Ocarboxylate, the table includes an indication of how reliable each prediction may be. Special attention was given to carboxylate interactions. When the carboxylate group is monodentate, the M,Ocarboxylate distance is well defined, but for bidentate carboxylate groups a wide range of distances is allowable; when the metal is Co, Cu or Zn the M,O1 and M,O2 distances are clearly inversely correlated; for the more purely electrostatic interactions involving Na, K and Ca there is a wider scatter of distances and little correlation. [source] Raman signatures of ligand binding and allosteric conformation change in hexameric insulinBIOPOLYMERS, Issue 5 2001Davide Ferrari Abstract Hexameric insulin is an allosteric protein that undergoes transitions between three conformational states (T6, T3R3, and R6). These allosteric states are stabilized by the binding of ligands to the phenolic pockets and by the coordination of anions to the His B10 metal sites. Raman difference (RD) spectroscopy is utilized to examine the binding of phenolic ligands and the binding of thiocyanate, p -aminobenzoic acid (PABA), or 4-hydroxy-3-nitrobenzoic acid (4H3N) to the allosteric sites of T3R3 and R6. The RD spectroscopic studies show changes in the amide I and III bands for the transition of residues B1,B8 from a meandering coil to an , helix in the T,R transitions and identify the Raman signatures of the structural differences among the T6, T3R3, and R6 states. Evidence of the altered environment caused by the ,30 Å displacement of phenylalanine (Phe) B1 is clearly seen from changes in the Raman bands of the Phe ring. Raman signatures arising from the coordination of PABA or 4H3N to the histidine (His) B10 Zn(II) sites show these carboxylates give distorted, asymmetric coordination to Zn(II). The RD spectra also reveal the importance of the position and the type of substituents for designing aromatic carboxylates with high affinity for the His B10 metal site. © 2001 John Wiley & Sons, Inc. Biopolymers (Biospectroscopy) 62: 249,260, 2001 [source] New Microporous Materials for Acetylene Storage and C2H2/CO2 Separation: Insights from Molecular SimulationsCHEMPHYSCHEM, Issue 10 2010Michael Fischer Abstract Force-field based grand-canonical Monte Carlo simulations are used to investigate the acetylene and carbon dioxide uptake capacity, as well as the C2H2/CO2 adsorption selectivity of three novel microporous materials: Magnesium formate, Cu3(btc)2, and cucurbit[6]uril. Because no comparable computational studies of acetylene adsorption have been reported so far, the study focuses on systems for which experimental data are available to permit a thorough validation of the simulation results. The results for magnesium formate are in excellent agreement with experiment. The simulation predicts a high selectivity for acetylene over CO2, which can be understood from a detailed analysis of the structural features that determine the affinity of Mg-formate towards C2H2. For Cu3(btc)2, preliminary calculations reveal the necessity to include the interaction of the sorbate molecules with the unsaturated metal sites, which is done by means of a parameter adjustment based on ab-initio calculations. In spite of the high C2H2 storage capacity, the C2H2/CO2 selectivity of this material is very modest. The simulation results for the porous organic crystal cucurbit[6]uril show that the adsorption characteristics that have been observed experimentally, particularly the very high isosteric heat of adsorption, cannot be understood when an ideal structure is assumed. It is postulated that structural imperfections play a key role in determining the C2H2 adsorption behavior of this material, and this proposition is supported by additional calculations. [source] | ||||||||||||||||||||||