Nitrogen Fixation: The Chemical — Biochemical — Genetic Interface PDF
Preview Nitrogen Fixation: The Chemical — Biochemical — Genetic Interface
NITROGEN FIXATION The Chemical-Biochemical-Genetic Interface NITROGEN FIXATION The Chemical-Biochemical-Genetic Interface Edited by Achim MOller University of Bielefefrl Bielefeld. Federal Republic of Germany and William E. Newton Charles F. Kettering Research Laboratory Yellow Springs, Ohio PLENUM PRESS • NEW YORK AND LONDON Library of Congress Cataloglng In Publication Data Maln entry under tide: Nitrogen fixation. "Proceedings of an international meeting'held June 28-July 1, 1981, at the Center for Interdisciplinary Research, University of Bielefeld, in Bielefeld, Federal Republic of Ger many" - T.p. verso. Includes bibliographlcal references and index. 1. Nitrogen - Fixation - Congresses. I. Müller, A. (AchimJ, 1938- . 11. Newton, William E. (William Edward), 1938- QR89.7.N59 1983 589.8'504133 82-24587 ISBN 978-1-4684-8525-7 ISBN 978-1-4684-8523-3 (eBook) DOI 10.1 007/978-1-4684-8523-3 Proceedings of an international meeting held June 28-July 1, 1981, at the Center for Interdisciplinary Research, University of Bielefled, in Bielefeld, Federal Republic of Germany @1983 Plenum Press, New York Softcover reprint of the hardcover 1s t edition 1983 A Division of Plenum Publishing Corporation 233 Spring Street, New York, N.Y. 10013 All rights reserved No part of this book may be reproduced, stored in a retrievaI system, or transmitted in any form or by any means, electronic, mechanical, photocopylng, microfilming, recording, or otherwise, without writt,en permission from the Publisher PREFACE This volume constitutes the proceedings of an international meeting held from June 28 to July 1, 1981, at Zif, Zentrum far Interdisziplin~re Forschung an der Universit~t Bielefeld, West Germany. The meeting was one of a continuing series organized through Zif on topics which transcend the boundaries of the traditional academic disciplines. The almost exponentially growing area of nitrogen fixation research fits this criterion completely. It is, however, so broad that we oriented and focused the meeting on just the chemical, biochemical, and genetic aspects to maximize the interaction among the participants and to produce a volume which would serve as a reference source for the involved researcher, the newly interested scientist and those with a more casual interest. The meeting was generously supported by Zif, the Westf~lisch Lippische Universit~tsgesellschaft, and the Fonds der Chemischen Industrie. We sincerely thank these organizations for their generosity and interest and for recognizing the importance of nitrogen fixation research for the future of mankind. Achim Muller Bielefeld, West Germany William E. Newton Yellow Springs, Ohio, USA CONTENTS Nitrogen Fixation: Its Scope and Importance •••••••.•••••.•.•.•. I W. E. Newton and B. K. Burgess ASPECTS OF BIOCHEMISTRY AND GENETICS Reactions and Physicochemical Properties of the Nitrogenase MoFe Proteins ............................................. 23 B. E. Smith MBssbauer Investigation of Nitrogenase •.•.•.•••.•.•.•.•.•.•••.•• 63 R. Zimmermann and A. X. Trautwein Iron-Molybdenum Cofactor and Its Complementary Protein from Mutant Organisms .......................................... 83 B. K. Burgess and W. E. Newton Genetics of Nitrogen Fixation in Free-Living Organisms .•.••••••• lll A. PUhler and W. Klipp Biochemical Genetics of Nitrogen Fixation in Rhizobium •••.•••.•. 135 H. Hennecke and M. Fuhrmann Regulation and Control of Nitrogenase Activity ••.•.•.•.•••.•.••• 149 D. J. Arp and H. G. Zumft CHEMICAL ASPECTS Thiomolybdates and Thiotungstates: Their Properties and Role as Ligands in Coordination Chemistry •.•••••.•.•.•••.•.•••• 183 A. MUller and E. Diemann The Chemistry of the Fe-M-S Complexes (M=Mo,W) •••.•.•.•••.•.•.•• 211 D. Coucouvanis vii viii CONTENTS Iron-Mo1ybdenum-Su1fur C1usters ••••••••••••••••••••••••••••••••• 245 C. D. Garner, S. R. Acott, G. Christou, D. Collison, F. E. Mabbs, V. Petrou1eas and C. J. Pickett Dinitrogen Complexes and Their Reactions •••••••••••••••••••••••• 275 R. L. Richards Structures of Complexes of Reduced Nitrogen Ligands ••••••••••••• 301 M. Sato and J. H. Enemark Overview of 95Mo NMR ••••••.•••••••••••••.••••••••••••••••••••••• 329 J. H. Enemark Mu1tisu1fur Metal Sites in Enzymes, Complexes, Clusters and Solids: Possible Relevance for Nitrogenase ••••.•••••••.•• 341 E. I. Stiefel and R. R. Chiane11i List of Contributors •••••••••.•.•••••••••.•.•.•.•.•••••.•••••••• 371 Index ................................•.•........................ 373 * NITROGEN FIXATION: ITS SCOPE AND IMPORTANCE William E. Newton and Barbara K. Burgess Charles F. Kettering Research Laboratory Yellow Springs, Ohio 45387, USA INTRODUCTION Although many factors,like climate, plant strains, herbicides, and pesticides,influence agricultural productivity, total depen dence rests on photosynthesis and supply of inorganic nutrients. The essential nutrient most often limiting in crop productivity is combined or "fixed" nitrogen. Because plants do not have the cap ability of "fixing" nitrogen, it must be provided externally for maximal productivity. However, only a very small proportion of the nitrogen on earth (less and 0.001%) is cycling at anyone time between its usable fixed form in terrestrial pools and its inert molecular form in its atmospheric pool. Nitrogen fixation controls the atmosphere-to-terrestrial (land or sea) flow, nitrification and denitrification convert ammonia to nitrate and then to nitrogen gas which is lost to the atmosphere, while leaching and erosion move fixed nitrogen between land and sea. The biological world apparently stays ahead of a nitrogen deficiency because the fixation rate is just above the denitrification rate.l Molecular nitrogen is fixedl-3 either by natural nonbiological and biological processes or by commercial processes. The global biological contribution is estimated at 122 x 106 t/yr, industrial fixation contributes about 50 x 106 t/yr for fertilizer uses while other processes, like lightning and combustion,fix about 30 x 106 t/yr. Thus, the biological process represents the major contributor to the total annual fixation rate. Although how the benefit was *Contribution No. 782 from the Charles F. Kettering Research Laboratory. 2 W. E. NEWTON AND B. K. BURGESS derived was not understood, the Chinese and Greeks used biological nitrogen fixation thousands of years ago in the form of legumes and Azolla as green manures. However, shortly after World War II, com mercially produced nitrogenous fertilizers became widely available and relatively cheap. Together with the trend toward larger farms and mechanization of farming, these sources of fertilizer displaced biologically produced fixed nitrogen fertilezer (in the form of both green manures and crop rotation), as the means to maintain soil fer tility. The importance of these commercial N fertilizers in agri culturally advanced countries is unquestioned. However, attempts to increase these commercial supplies to cultivate evermore intensely the arable land of the earth are running into major problems, such as the increasing cost and declining availability of fossil fuel for feedstock and energy supplies and the enormous ($150,000,000) capital investment required for building new production facilities. In any case, the increasingly intensive cultivation of the arable land of the earth is a short-range solution to the growing problem of global food supply, which has been exacerbated by a continuing population increase. The various natural processes for fixing atmospheric nitrogen hold the key to long-term global food supply. In certain instances, like the legumes (peas, beans, alfalfa, etc), nature has provided a mechanism for biological interaction between the plant and a nitrogen-fixing bacterium. This symbiotic association allows the plant to receive nitrogen fixed by the bacterium directly because the bacterium is harbored in nodules on its roots. Carbohydrate is supplied in return by the plant. Although this approach works well, it is limited and important food crops, like the cereal grains (rice, wheat, and corn), and root and tuber crops, do not harbor symbiotic partners. Hence, for crop productivity to reach com mercially acceptable levels currently, extensive augmentation by commercially produced nitrogen fertilizer is necessary. Such prob lems have encouraged present-day research into all areas of nitro gen fixation. INDUSTRIAL PROCESSES Until the early 19th century, the available fixed nitrogen, stockpiled by natural processes over .millions of years, was enough to sustain the earth's population. But with rapidly increasing populations and the dramatic growth of large cities in industralized nations, the demand for increased supplies led to the beginnings of the nitrogenous fertilizer industry. So, guano (hardened bird droppings) was imported into Europe from Peru, as was saltpeter (sodium nitrate) from Chile. These fertilizer forms were further supplemented by the ammoniacal by-products from coal gas production. Further increasing demand led to the invention of several processes, some of which were commercially successful. The first process, implemented in 1905, was the Birkeland-Eyde NITROGEN FIXATION: ITS SCOPE AND IMPORTANCE 3 process for nitrogen oxidation.4 Air is passed through an electric arc at temperatures above 3000° C to generate nitric oxide (NO), which, on cooling, undergoes further oxidation to nitrogen dioxide (N02). Absorption into water gives a mixture of nitric (HN03) and nitrous (HN02) acids. However, because only ca 2% conversion to NO occurs and large amounts of electricity, ca 60,000 kW'h/t N2 fixed are consumed, this process was only economical in countries like Norway, where it was developed, and where substantial amounts of cheap hydroelectric power are available. At about the same time, the Frank-Caro cyanamide process was commercialized.5 Here, lime stone is heated to produce lime which then reacts with carbon in a highly energy-demanding reaction to give calcium carbide. Reaction with dinitrogen gives calcium cyanamide, which hydrolyzes to ammonia and calcium carbonate. Even though its overall energy requirement CaC03 + CaO + CO2 CaO + 3C + CaC2 + CO CaC2 + N2 + CaCN2 + C CaCN2 + 3H2O + CaCo3 + 2 NH3 is ca 20-25% of the arc process, the Haber-Bosch process, which was developed at about this same time, proved to be more economical. A third process, the Serpak process used a mixture of alumina, coke, and dinitrogen at 1800° C to produce aluminum nitride, which on hydrolysis gave ammonia. It was never exploited to any ~ignificant degree, mainly because of its large energy requirements. A1203 + 3 C + N2 2 AIN + 3 CO 2 A1N + 3 H20 2 NH3 + A1203 Currently, the enormous synthetic ammonia industry employs only the Haber-Bosch process3-6 Discovered in Germany in the years just before World War I, its development was aided by the concomitant development of a simple catalyzed process for the oxidation of am monia to nitrate which was needed at that time for the explosives industry. Nitrogen (N2) and hydrogen (H2) are combined directly under the appropriate operating conditions to reach an equilibrium mixture containing ca.20% ammonia. When the reaction was first discovered, it required ca.1300°C, more than used today. Thus, until Haber discovered the appropriate catalyst, it was not at tractive commercially. Now, it suffers from the requirement of nonrenewable fossil fuels to operate. Although the product,am monia itself, is commonly used as a fertilizer in the United States, elsewhere it is often converted into solid or liquid fertilizers, such as urea, ammonium nitrate or sulfate, and various solutions, before use. A modern ammonia plant performs two distinct functions. The 4 W. E. NEWTON AND B. K. BURGESS more energy-demanding and complex function is the preparation and purification of the synthesis gas, containing N2 and H2 in a 1:3 ratio, from a variety of feedstocks. The second function is the catalytic conversion of synthesis gas to ammonia. In the years since its commercial introduction in 1913, many process changes have been made, particularly with respect to synthesis-gas production, to lower costs and give greater efficiencies. Hydrogen for synthesis gas is produced either by steam reforming of natural gas and other lighter hydrocarbons, such as naphtha, or by the partial oxidation of heavy oils and coal. In both cases, a mixture of H2 with carbon oxides is formed, together with the N2 from the added air. The carbon oxides present are reduced to methane and the gases are com pressed for processing in the catalytic ammonia converter. CH4 + H20 -+ 3H2 + CO or 3 C + H20 + O2 -+ H2 + 3 CO and CO + H20 -+ H2 + CO2 Of the large number 6f catalysts suggested for the ammonia syn thesis reaction, only iron, cobalt, molybdenum, and tungsten have been found to be practical. The addition of certain promoter salts favors ammonia formation. The best and most economical catalyst is metallic iron, produced by reduction of magnetite (Fe304) by H2, with alumina and potassium oxide as promoters. The promoters in crease the catalyst's heat stability and aid in desorption of am monia from the surface. Since ammonia synthesis is an equilibrium, the quantity of am monia produced depends on temperature, pressure, and the H2-to-N2 ratio. At 5000 C and 200 atm, the equilibrium mixture contains 17.6% ammonia. For complete conversion, the unreacted gases must be re cycled after removing the ammonia formed (15-25% yield) from the exit gases by condensation at about -200 C because its presence decreases both the equilibrium yield and the reaction rate by re ducing the partial pressure of the N2-H2 mixture. The mechanism of the synthesis reaction is still not fully clarified.1-6 BIOLOGICAL SYSTEMS Biological nitrogen fixation is confined to microorganisms. Only prokaryotes, such as bacteria, blue~green algae, and action mycetes, can reduce nitrogen to ammonia. Such bacteria can be either free-livers, such as Azotobacter or Clostridium, or can form sym biotic associations with higher plants, like the Rhizobium-legume system. The latter group is 'much more important agriculturally. In exchange for the fixed nitrogen supplied by the bacterium, the legume supplies energy in the form of carbohydrate obtained by photo synthesis. Thus, renewable solar energy powers this fertilizer
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