DOCSLIB.ORG

Effect of Molybdenum and Tungsten on the Reduction of Nitrate in Nitrate

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Effect of Molybdenum and Tungsten on the Reduction of Nitrate in Nitrate Habib and Hofman Chemistry Central Journal (2017) 11:35 DOI 10.1186/s13065-017-0263-7 RESEARCH ARTICLE Open Access Efect of molybdenum and tungsten on the reduction of nitrate in nitrate reductase, a DFT study Uzma Habib1* and Matthias Hofman2 Abstract The molybdenum and tungsten active site model complexes, derived from the protein X-ray crystal structure of the frst W-containing nitrate reductase isolated from Pyrobaculum aerophilum, were computed for nitrate reduction at the COSMO-B3LYP/SDDp//B3LYP/Lanl2DZ(p) energy level of density functional theory. The molybdenum containing active site model complex (Mo–Nar) has the largest activation energy (34.4 kcal/mol) for the oxygen atom transfer from the nitrate to the metal center as compared to the tungsten containing active site model complex (W–Nar) (12.0 kcal/mol). Oxidation of the educt complex is close to thermoneutral ( 1.9 kcal/mol) for the Mo active site model complex but strongly exothermic ( 34.7 kcal/mol) for the W containing active− site model complex, however, the M­ VI to ­MIV reduction requires equal amount− of reductive power for both metal complexes, Mo–Nar or W–Nar. Keywords: Nitrate reductase, DFT studies, Molybdenum, Tungsten Background and type of reaction they catalyze, these mononuclear Molybdenum and tungsten are the only 4d (Mo) and 5d MPT containing enzymes have been grouped into three (W) transition metals prefer to be essential for biologi- subfamilies (Fig. 1), xanthine oxidase family, sulfte oxi- cal systems. Mononuclear enzymes containing Mo or dase family, and DMSO (dimethylsulfoxide) reductase W at their active sites generally catalyze oxygen atom family [1]. transfer reactions [1, 2]. Despite the high similarity Nitrate reductases (NRs) play key roles in the frst between the chemical properties of Mo and W, W-con- step of biological nitrogen cycles [7–9] i.e., assimilatory taining enzymes are by far less common. Mo-containing ammonifcation (to incorporate nitrogen into biomol- enzymes are found in almost all forms of life [1], whereas ecules), denitrifcation (to generate energy for cellular W-containing enzymes seem to be popular for organisms function) and dissimilatory ammonifcation (to dissipate such as hyperthermophilic archaea that live in extreme extra energy by respiration). Tey always catalyze the environments [2]. However, W-containing enzymes have reduction of nitrate to nitrite, and have been classifed also been found in organisms that do not need extreme into three groups, assimilatory nitrate reductases (Nas), conditions [3–5], suggesting a more important role for respiratory nitrate reductases (Nar) and periplasmic tungsten [6]. nitrate reductases (Nap). Nas belongs to the sulfte oxi- Mononuclear enzymes contain a cofactor that com- dase family and is located in the cytoplasm [10]. It is the prises metallopterin (MPT) or some of its nucleotide frst enzyme of a reduction sequence for nitrogen incor- variants, each of which is coordinated to Mo or W with poration into the biomass that maintains the bioavailabil- an enedithiolene motif. Based on the active site structure ity of nitrate to plants, algae, fungi, archaea and bacteria [11, 12]. Dissimilatory nitrate reductases, Nar and Nap belong to the DMSO reductase family of mononuclear *Correspondence: [email protected] MPT containing molybdo-enzymes. Tey are linked to 1 Research Center for Modeling and Simulation (RCMS), National University of Science and Technology (NUST), H‑12, Islamabad, Pakistan respiratory electron transport systems and are located in Full list of author information is available at the end of the article the membrane and periplasm, respectively. Tey catalyze © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Habib and Hofman Chemistry Central Journal (2017) 11:35 Page 2 of 12 Fig. 1 Active site composition of subfamilies of mononuclear Mo/W enzymes the frst step of the catabolic, anaerobic respiration path- termed as NarGH where NarG hosts the metal (Mo or way in bacteria and archaea [14]. W) catalytic site. Te metal is coordinated by two metal- Nitrate reduction, catalyzed by membrane bound res- lopterin guanine dinucleotide (bis-MGD) ligands, a car- piratory nitrate reductase (Nar), is an important step of boxyl group of Asp222 and a water molecule. Te NarH the denitrifcation in the anaerobic respiratory pathways component possesses an iron–sulfur (FeS) redox active employed by a diverse group of bacteria and archaea [13]. subunit [19]. Nar was found to contain a Mo cofactor in all microbes NarGH reduces nitrate to nitrite, changing the oxida- from which it was isolated and belongs to the DMSO tion state of metal from +IV to +VI. Two electrons and reductase family [14]. In general, Nar becomes inac- two protons are required for the reductive half reaction, 2− tive by the addition of tungstate (WO4 ) to the growth resulting in the formation of a water molecule and a medium [15], although due to similar chemical proper- nitrite ion (Eq. 1). ties W can replace Mo as the active site metal and can- − + − − NO3 + 2H + 2e ⇋ NO2 + H2O. (1) not only retain but increase its catalytic activity in E. coli TMAO reductase [16], the Desulfovibrio alaskensis for- Te active site of dissimilatory nitrate reductase (Des- mate dehydrogenase [17] and the Rhodobactercapsula- ulfovibrio desulfuricans), in the reduced state contains a tus DMSO reductase [18]. However, recently the nitrate Mo atom bound by two metalopterin dithiolene ligands reductase (Nar) from the hyperthermophilic denitrify- and a cysteinate residue. An experimental study on small ing archaeon Pyrobaculum aerophilum has been shown model complexes demonstrates that nitrate reduction by to retain its activity even at a tungsten rich environment primary (direct) oxo transfer [23] is a feasible reaction [19]. pathway (Fig. 2) [24]. Pyrobaculum aerophilum, a hyperthermophilic Here we present a density functional theory (DFT) archaeon, is naturally exposed to high levels of tung- study on model complexes derived from the protein sten, a heavy metal that is abundant in high temperature X-ray crystal structure of P. aerophilum [19] nitrate environments. Tungsten was reported to stimulate the reductase (Nar). Te purpose of the study was to inves- growth of several mesophilic methanogens and some tigate (i) the efect on the reduction of nitrate when W mesophilic and thermophilic bacteria [14]. Te growth of replaces Mo at the active site, (ii) the energy barriers on P. aerophilum also depends on the presence of tungstate the potential energy surface and (iii) the reason for the in the growth medium which suggests the involvement of activity loss of Nars (respiratory nitrate reductase) in the tungstoenzymes in essential metabolic pathways [20]. presence of W. Pyrobaculum aerophilum is the only hyperthermo- philic archaeon isolated that reduces nitrate via a mem- Computational details brane bound respiratory nitrate reductase (Nar) [20]. All geometries were optimized using Gaussian 09 with Nar purifed from P. aerophilum grown in the absence of the hybrid density functional B3LYP [25] and the LAN- 2− added molybdate (MoO4 ) and with 4.5 µM tungstate L2DZ basis set [26–29] augmented by polarization func- 2− (WO4 ) is a tungsten containing enzyme, which is iden- tions on sulfur atoms (ζ = 0.421) [30]. Te starting nitrate tical to Mo-Nar [21] (previously isolated from P. aerophi- complex geometries for transition state searches were lum), indicating that either metal can serve as the active generated by shortening and lengthening of forming and site ion. Te crystal structure is similar to the previously breaking bonds, respectively. Frequency calculations reported Nar from E. coli [22], a heterodimeric enzyme proved transition states to have exactly one imaginary Habib and Hofman Chemistry Central Journal (2017) 11:35 Page 3 of 12 Fig. 2 Schematic description of the proposed mechansim [1] for the nitrate reduction, where M Mo and Y S–Cys. Also the metalopterin dinu- = = cleotide cofactor is shown frequency with the correct transition vector. Single point nitrogen atoms attached to the beta (β) carbon atoms of energies were computed with the B3LYP functional and His546, Asn52, Tyr220 and Asp222 were kept fxed to their the Stuttgart–Dresden efective core potential basis set crystal structure positions to mimic the steric constraints (SDD) [31, 32] augmented by polarization functions for by the protein matrix. Carbon atom C 7 and the nitrogen all atoms except Mo, W and H (ζ = 0.600, 1.154, 0.864, atom attached to carbon atom C5 were kept fxed for resi- and 0.421 for C, O, N, and S, respectively) [30]. Self- due Gly549. Te MPT ligands were truncated at the pyran consistent reaction feld (SCRF) computations were per- rings and oxygen atoms of these pyran rings were also formed on the optimized geometries to model the protein kept fxed (Fig. 3). surrounding the active site by a conductor like polariz- First, hydrogen atoms were geometry optimized apply- able continuum method (CPCM) [33] as implemented ing one negative overall charge (assuming Mo/W at the in Gaussian 09 [34, 35]. Default Gaussian 03 parameters +VI oxidation state), keeping all heavy-atoms fxed at were used for the evaluation of solute–solvent dispersion their positions. Te resulting geometries served to gener- and repulsion interaction energies [36, 37], and solute ate the diferent starting geometries needed for comput- cavitation energy variations [38]. Te molecular cavity ing the mechanism for nitrate reduction. was specifed using a minimum radius (RMin) of 0.5Ǻ Te starting geometries for the substrate and product and an overlap index (OFac) of 0.8 [39].
Load more

Recommended publications
  • Evolution and Understanding of the D-Block Elements in the Periodic Table Cite This: Dalton Trans., 2019, 48, 9408 Edwin C
    Dalton Transactions View Article Online PERSPECTIVE View Journal | View Issue Evolution and understanding of the d-block elements in the periodic table Cite this: Dalton Trans., 2019, 48, 9408 Edwin C. Constable Received 20th February 2019, The d-block elements have played an essential role in the development of our present understanding of Accepted 6th March 2019 chemistry and in the evolution of the periodic table. On the occasion of the sesquicentenniel of the dis- DOI: 10.1039/c9dt00765b covery of the periodic table by Mendeleev, it is appropriate to look at how these metals have influenced rsc.li/dalton our understanding of periodicity and the relationships between elements. Introduction and periodic tables concerning objects as diverse as fruit, veg- etables, beer, cartoon characters, and superheroes abound in In the year 2019 we celebrate the sesquicentennial of the publi- our connected world.7 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. cation of the first modern form of the periodic table by In the commonly encountered medium or long forms of Mendeleev (alternatively transliterated as Mendelejew, the periodic table, the central portion is occupied by the Mendelejeff, Mendeléeff, and Mendeléyev from the Cyrillic d-block elements, commonly known as the transition elements ).1 The periodic table lies at the core of our under- or transition metals. These elements have played a critical rôle standing of the properties of, and the relationships between, in our understanding of modern chemistry and have proved to the 118 elements currently known (Fig. 1).2 A chemist can look be the touchstones for many theories of valence and bonding.
    [Show full text]
  • The Development of the Periodic Table and Its Consequences Citation: J
    Firenze University Press www.fupress.com/substantia The Development of the Periodic Table and its Consequences Citation: J. Emsley (2019) The Devel- opment of the Periodic Table and its Consequences. Substantia 3(2) Suppl. 5: 15-27. doi: 10.13128/Substantia-297 John Emsley Copyright: © 2019 J. Emsley. This is Alameda Lodge, 23a Alameda Road, Ampthill, MK45 2LA, UK an open access, peer-reviewed article E-mail: [email protected] published by Firenze University Press (http://www.fupress.com/substantia) and distributed under the terms of the Abstract. Chemistry is fortunate among the sciences in having an icon that is instant- Creative Commons Attribution License, ly recognisable around the world: the periodic table. The United Nations has deemed which permits unrestricted use, distri- 2019 to be the International Year of the Periodic Table, in commemoration of the 150th bution, and reproduction in any medi- anniversary of the first paper in which it appeared. That had been written by a Russian um, provided the original author and chemist, Dmitri Mendeleev, and was published in May 1869. Since then, there have source are credited. been many versions of the table, but one format has come to be the most widely used Data Availability Statement: All rel- and is to be seen everywhere. The route to this preferred form of the table makes an evant data are within the paper and its interesting story. Supporting Information files. Keywords. Periodic table, Mendeleev, Newlands, Deming, Seaborg. Competing Interests: The Author(s) declare(s) no conflict of interest. INTRODUCTION There are hundreds of periodic tables but the one that is widely repro- duced has the approval of the International Union of Pure and Applied Chemistry (IUPAC) and is shown in Fig.1.
    [Show full text]
  • Diffusion of Carbon in Niobium and Molybdenum
    Materials Transactions, Vol. 55, No. 12 (2014) pp. 1786 to 1791 ©2014 The Japan Institute of Metals and Materials Diffusion of Carbon in Niobium and Molybdenum Jun-ichi Imai1, Osamu Taguchi2,+, Gyanendra Prasad Tiwari3 and Yoshiaki Iijima1 1Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan 2Department of Materials Science and Engineering, Miyagi National College of Technology, Natori 981-1239, Japan 3Department of Information Technology, Ramrao Adik Institute of Technology, Vidya Nagri, Nerul, Navi Mumbai 400709, India Diffusion coefficients of carbon in niobium and molybdenum have been determined by the residual activity method with radioactive tracer 14C in the temperature ranges between 1168 and 1567 K for niobium and between 1271 and 1669 K for molybdenum. The temperature dependences of the diffusion coefficient of carbon in niobium and molybdenum are expressed by D/m2 s¹1 = 2.2 © 10¹6 exp(¹152 kJ mol¹1/ RT) and D/m2 s¹1 = 5.2 © 10¹6 exp(¹163 kJ mol¹1/RT), respectively. Since the solubility of carbon in molybdenum is very small, the diffusion of carbon in molybdenum is strongly influenced by carbide precipitation at lower temperatures. [doi:10.2320/matertrans.M2014277] (Received July 31, 2014; Accepted September 30, 2014; Published November 8, 2014) Keywords: carbon diffusion, niobium, molybdenum, carbon solubility, precipitate effect 1. Introduction 2. Experimental Procedure Iron, nickel and cobalt based superalloys appear to have 2.1 Material achieved full potential in relation to their use as structural Niobium metal rod arc-melted and machined to 12.5 mm in materials for corrosive environments as well as high diameter was supplied by Materials Research Corporation, temperatures.1) The strength of these alloys comes partly USA.
    [Show full text]
  • Periodic Table 1 Periodic Table
    Periodic table 1 Periodic table This article is about the table used in chemistry. For other uses, see Periodic table (disambiguation). The periodic table is a tabular arrangement of the chemical elements, organized on the basis of their atomic numbers (numbers of protons in the nucleus), electron configurations , and recurring chemical properties. Elements are presented in order of increasing atomic number, which is typically listed with the chemical symbol in each box. The standard form of the table consists of a grid of elements laid out in 18 columns and 7 Standard 18-column form of the periodic table. For the color legend, see section Layout, rows, with a double row of elements under the larger table. below that. The table can also be deconstructed into four rectangular blocks: the s-block to the left, the p-block to the right, the d-block in the middle, and the f-block below that. The rows of the table are called periods; the columns are called groups, with some of these having names such as halogens or noble gases. Since, by definition, a periodic table incorporates recurring trends, any such table can be used to derive relationships between the properties of the elements and predict the properties of new, yet to be discovered or synthesized, elements. As a result, a periodic table—whether in the standard form or some other variant—provides a useful framework for analyzing chemical behavior, and such tables are widely used in chemistry and other sciences. Although precursors exist, Dmitri Mendeleev is generally credited with the publication, in 1869, of the first widely recognized periodic table.
    [Show full text]
  • The Disilicides of Tungsten, Molybdenum, Tantalum, Titanium, Cobalt, and Nickel, and Platinum Monosilicide: a Survey of Their Thermodynamic Properties
    The Disilicides of Tungsten, Molybdenum, Tantalum, Titanium, Cobalt, and Nickel, and Platinum Monosilicide: A Survey of Their Thermodynamic Properties Cite as: Journal of Physical and Chemical Reference Data 22, 1459 (1993); https:// doi.org/10.1063/1.555922 Submitted: 21 December 1992 . Published Online: 15 October 2009 M. S. Chandrasekharaiah, J. L. Margrave, and P. A. G. O’Hare ARTICLES YOU MAY BE INTERESTED IN Thermodynamic considerations in refractory metal-silicon-oxygen systems Journal of Applied Physics 56, 147 (1984); https://doi.org/10.1063/1.333738 Thermal and Electrical Conductivity of Graphite and Carbon at Low Temperatures Journal of Applied Physics 15, 452 (1944); https://doi.org/10.1063/1.1707454 Formation of thin films of NiSi: Metastable structure, diffusion mechanisms in intermetallic compounds Journal of Applied Physics 55, 4208 (1984); https://doi.org/10.1063/1.333021 Journal of Physical and Chemical Reference Data 22, 1459 (1993); https://doi.org/10.1063/1.555922 22, 1459 © 1993 American Institute of Physics for the National Institute of Standards and Technology. The Disilicides of Tungsten, Molybdenum, Tantalum, Titanium, Cobalt, and Nickel, and Platinum Monosilicide: A Survey of Their Thermodynamic Properties M. s. Chandrasekharaiah and J. L. Margrave HARC, Materials Science Research Center, 4802 Research Forest Drive, The Woodlands, TX 77381 and P. A. G. O'Hare Chemical Kinetics and Thennodynamics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-0001 Received December 21, 1992; revised manuscript received February 19, 1993 A critical evaluation is presented of the thermodynamic properties of six disiIi­ cides and one monosilicide that are important in the manutacture ot very large scale integrated circuits.
    [Show full text]
  • The Radiochemistry of Molybdenum COMMITTEE on NUCLEAR SCIENCE
    National Academy of Sciences National Research council s NUCLEAR SCIENCE SERIES The Radiochemistry of Molybdenum COMMITTEE ON NUCLEAR SCIENCE L. F. CURTISS, Chairman ROBLEY D. EVANS, ViceCkainna?I NationalBureau ofStandards MassachusettsInstituteofTechnology J.A. DeJUREN, Secretary WestinghouseElectricCorporation H. J. CURTIS G. G. MANOV BrookhavenNationalLaboratory Tracerlah,Inc. SAMUEL EPSTEIN W. WAYNE MEINKE CaliforniaInstituteofTechnology Universityof Michigan HERBERT GOLDSTEIN A. H. SNELL NuclearDevelopmentCorporationof Oak Ridge NationalLaboratory America E. A. UEHLING H. J. GOMBERG UniversityofWashington UniversityofMichigan D. M. VAN PATTER E. D. KLEMA BartolResearch Foundation NorthwesternUniversity ROBERT L. PLATZMAN Argonne NationalLaboratory LIA SON MEMBERS PAUL C. AEBERSOLD W. D. URRY Atomic EnerW Commission U. S.Air Force J.HOWARD McMILLEN WILLIAM E. WRIGHT NationalScienceFoundation OfficeofNavalResearch SUBCOMMITTEE ON RADIOCHEMISTRY W. WAYNE MEINICE, Chai?man EARL HYDE Universityof Mlchlgan UniversityofCalifornia(Berkeley) NATHAN BALLOU HAROLD KIRBY Navy RadiologicalDefenseLaboratory Mound Laboratory GREGORY R. CHOPPIN GEORGE LEDDICOTTE FloridaStateUniver~ity Oak Ridge NationalLaboratory GEORGE A. COWAN ELLIS P. STEINBERG Los Alsmos ScientificLshoratory Argonne NationalLaboratory ARTHUR W. FAIRHALL PETER C. STEVENSON UniversityofWashington UniversityofCalifornia(Llvermore) HARMON FINSTON LEO YAFFE Brookhaven,NationalLaboratory McGillUniversity .. The Radiochernistry d Molybdenum By E. hf.SCADDEN and N. E. BALLOU U. S.
    [Show full text]
  • Molybdenum in Drinking-Water
    WHO/SDE/WSH/03.04/11/Rev/1 Molybdenum in Drinking-water Background document for development of WHO Guidelines for Drinking-water Quality Rev/1: Revisions indicated with a vertical line in the left margin. Molybdenum in Drinking-water Background document for development of WHO Guidelines for Drinking-water Quality © World Health Organization 2011 All rights reserved. Publications of the World Health Organization can be obtained from WHO Press, World Health Organization, 20 Avenue Appia, 1211 Geneva 27, Switzerland (tel.: +41 22791 3264; fax: +41 22 791 4857; e-mail: [email protected]). Requests for permission to reproduce or translate WHO publications—whether for sale or for non-commercial distribution—should be addressed to WHO Press at the above address (fax: +41 22 791 4806; e-mail: [email protected]). The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of the World Health Organization concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. Dotted lines on maps represent approximate border lines for which there may not yet be full agreement. The mention of specific companies or of certain manufacturers’ products does not imply that they are endorsed or recommended by the World Health Organization in preference to others of a similar nature that are not mentioned. Errors and omissions excepted, the names of proprietary products are distinguished by initial capital letters. All reasonable precautions have been taken by the World Health Organization to verify the information contained in this publication.
    [Show full text]
  • Atomic Weights of the Elements 2013 (IUPAC Technical Report)
    Pure Appl. Chem. 2016; 88(3): 265–291 IUPAC Technical Report Juris Meija*, Tyler B. Coplen, Michael Berglund, Willi A. Brand, Paul De Bièvre, Manfred Gröning, Norman E. Holden, Johanna Irrgeher, Robert D. Loss, Thomas Walczyk and Thomas Prohaska Atomic weights of the elements 2013 (IUPAC Technical Report) DOI 10.1515/pac-2015-0305 Received March 26, 2015; accepted December 8, 2015 Abstract: The biennial review of atomic-weight determinations and other cognate data has resulted in changes for the standard atomic weights of 19 elements. The standard atomic weights of four elements have been revised based on recent determinations of isotopic abundances in natural terrestrial materials: cadmium to 112.414(4) from 112.411(8), molybdenum to 95.95(1) from 95.96(2), selenium to 78.971(8) from 78.96(3), and thorium to 232.0377(4) from 232.038 06(2). The Commission on Isotopic Abundances and Atomic Weights (ciaaw.org) also revised the standard atomic weights of fifteen elements based on the 2012 Atomic Mass Evaluation: aluminium (aluminum) to 26.981 5385(7) from 26.981 5386(8), arsenic to 74.921 595(6) from 74.921 60(2), beryllium to 9.012 1831(5) from 9.012 182(3), caesium (cesium) to 132.905 451 96(6) from 132.905 4519(2), cobalt to 58.933 194(4) from 58.933 195(5), fluorine to 18.998 403 163(6) from 18.998 4032(5), gold to 196.966 569(5) from 196.966 569(4), holmium to 164.930 33(2) from 164.930 32(2), manganese to 54.938 044(3) from 54.938 045(5), niobium to 92.906 37(2) from 92.906 38(2), phosphorus to 30.973 761 998(5) from 30.973 762(2), praseodymium to 140.907 66(2) from 140.907 65(2), Article note: Sponsoring body: IUPAC Inorganic Chemistry Division Committee: see more details on page 289.
    [Show full text]
  • Bronze Compounds
    Montclair State University Montclair State University Digital Commons Theses, Dissertations and Culminating Projects 5-2020 A Study and Synthesis : Bronze Compounds Patricia Rose Olsen Follow this and additional works at: https://digitalcommons.montclair.edu/etd Part of the Chemistry Commons ABSTRACT The focus of this thesis is the investigation of ternary and quaternary reduced metal oxide compounds termed “bronzes.” The name bronze originally arose from the metallic-like luster of these compounds. Examples of these compounds range from sodium tungsten oxides to the more complex potassium cesium molybdenum oxides. These compounds are crystalline solids at room temperature. The “bronzes” are compounds that have been studied since their initial synthesis by Wohler in 1824. These compounds belong to a class of compounds that are known as “nonstoichiometric” compounds. The general formula for reduced ternary transition metal oxides is AxMyOz. In this general formula, the value of x is less than one, but greater than 0. The compounds vary in crystal structures due to the radius of the electropositive element, M. The compounds also vary in physical properties because of their crystal structures and energy level occupations. The compounds can range in color from purple to yellow and range in electrical conductivity from metallic behavior to semiconducting behavior. The history of these compounds, including a classification of known bronze phases, will be reviewed with a focus on complex molybdate phases. Experimental methods, including experimental searches for new phases using ceramic synthesis techniques and electrolytic growth, and analysis of resulting products by powder x-ray diffraction and SEM studies will be discussed in this thesis.
    [Show full text]
  • Injection Moulded Tungsten and Molybdenum Copper Alloys for Microelectronic Housings
    AT0100373 44 RM 9 M. Knuwer et al. 15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 Injection Moulded Tungsten and Molybdenum Copper Alloys for Microelectronic Housings M. Knuwer, H. Meinhardf, K.-H. Wichmanrf Fraunhofer-lnstitute for Manufacturing and Advanced Materials (IFAM), Bremen, Germany *H.C. Starck GmbH & Co. KG, Laufenburg, Germany +EADS Deutschland GmbH, Ulm, Germany Summary Molybdenum- and tungsten-copper alloys were developed as promising housing materials for high-frequency- (HF-) microelectronic packaging. Three different powder processing routes (mixing, attritor milling, compound powder), different particle sizes and copper contents were tested to achieve the best possible result in thermal and mechanical properties. Net shape processing of the parts took place via metal-injection-moulding, debindering and sintering. It could be seen that the sintering behaviour (density and shrinking rate) was strongly depending on the copper content and tungsten particle size. Lowest sinter activity was detected for a W90Cu10 powder mixture with coarse (d50=3,5um) tungsten powder, highest for the new developed Mo68Cu32 compound powder. Using compound powder leads to higher density at lower sinter temperatures compared to conventional processed WCu or MoCu powder mixtures. MoCu and WCu housing demonstrators were produced and HF-equipment was integrated. First electrical tests of the integrated demonstrator were performed successfully. Keywords tungsten, molybdenum, copper, thermal management, high frequency, thermal expansion 1 Introduction The current technical development is distinguished by an extensively growing use of mobile-communication and multi-media systems. The increasing number of these systems results in a likewise increasing demand on high 1.
    [Show full text]
  • BNL-79513-2007-CP Standard Atomic Weights Tables 2007 Abridged To
    BNL-79513-2007-CP Standard Atomic Weights Tables 2007 Abridged to Four and Five Significant Figures Norman E. Holden Energy Sciences & Technology Department National Nuclear Data Center Brookhaven National Laboratory P.O. Box 5000 Upton, NY 11973-5000 www.bnl.gov Prepared for the 44th IUPAC General Assembly, in Torino, Italy August 2007 Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. This preprint is intended for publication in a journal or proceedings. Since changes may be made before publication, it may not be cited or reproduced without the author’s permission. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party’s use or the results of such use of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof or its contractors or subcontractors.
    [Show full text]
  • Role of Nitrate Reductase in NO Production in Photosynthetic Eukaryotes
    Review Role of Nitrate Reductase in NO Production in Photosynthetic Eukaryotes Manuel Tejada-Jimenez, Angel Llamas, Aurora Galván and Emilio Fernández * Departamento de Bioquímica y Biología Molecular, Campus de Rabanales y Campus Internacional de Excelencia Agroalimentario (CeiA3), Edif. Severo Ochoa, Universidad de Córdoba, 14071 Córdoba, Spain; [email protected] (M.T.-J.); [email protected] (A.L.); [email protected] (A.G.) * Correspondence: [email protected]; Tel.: +34-957-218-591 Received: 16 January 2019; Accepted: 08 February 2019; Published: 6 March 2019 Abstract: Nitric oxide is a gaseous secondary messenger that is critical for proper cell signaling and plant survival when exposed to stress. Nitric oxide (NO) synthesis in plants, under standard phototrophic oxygenic conditions, has long been a very controversial issue. A few algal strains contain NO synthase (NOS), which appears to be absent in all other algae and land plants. The experimental data have led to the hypothesis that molybdoenzyme nitrate reductase (NR) is the main enzyme responsible for NO production in most plants. Recently, NR was found to be a necessary partner in a dual system that also includes another molybdoenzyme, which was renamed NO-forming nitrite reductase (NOFNiR). This enzyme produces NO independently of the molybdenum center of NR and depends on the NR electron transport chain from NAD(P)H to heme. Under the circumstances in which NR is not present or active, the existence of another NO-forming system that is similar to the NOS system would account for NO production and NO effects. PII protein, which senses and integrates the signals of the C–N balance in the cell, likely has an important role in organizing cell responses.
    [Show full text]