Nitrogen-fixation

How is nitrogen from the atmosphere incorporated into biologically useful compounds  

Crop production per acre in the United States is higher than in many areas of the world. In part, this is the result of extensive use of fertilizers, especially those that supply nitrogen in a form that plants can use readily. Both ammonium and nitrate ions are used even ammonia gas can be pumped into the ground, if enough water is available in the soil to dissolve it. 

The genes for the enzymes for nitrogen fixation have been studied extensively. Much research is going on to determine whether these genes can be incorporated into crop plants, which would reduce the amount of nitrogen fertilizer needed for maximal plant growth and crop production. 

Two other sources of nitrogen fixation are often overlooked. The first is the chemical synthesis of ammonia from Hg and Ng, called the Haber process, after its discoverer, the German chem- 

Nitrogen enters the biosphere by the process of nitrogen fixation. Atmospheric nitrogen is converted to ammonia in its conjugate acid form, ammonium ion. 

Nitroaen Fixation An enzyme system of particular importance is that which promotes the fixation of 

The discovery that dinitrogen was capable of forming stable complexes with transition metals (Chapter 13) led to extensive investigation of the possibility of fixation via such complexes. An important development was the discovery that certain phosphine complexes of molybdenum and tungsten containing dinitrogen readily yield ammonia in acidic media 9  

94 Jolly, W. L. The Chemistry of the Non-Metais Prcniice-Hull Englewood Cliffs. NJ. 1966 p 72. 

The reduction of mono-co-ordinaled molecular nitrogen to ammonia in a protic environment Fuel-saving way to make fertiliser Fuel break-through More progress in nitrogen fixation Cheaper nitrogen by 1990 Basic life process created in UK lab 

With each retelling the story grew, until by the lime it reached British Columbia, it appeared that the press was almost able in 12 days to duplicate what is recorded as a 6-day event in Genesis The resultant disappointment when scientists are not able to meet expectations benefits neither them nor the public (but that, too, is good copy for the popular press ). 

Reduction of dinitrogen to ammonia requires only six electrons, while two electrons are used to produce hydrogen. Because of the high energy input required to break the triple bond, the enzyme 

The Nitrogenase enzyme complex is not only specihc to reduction of dinitrogen, but also reduces acetylene (C2FI2) to ethylene (C2FI4). This reaction requires the transfer of two electrons. 

In Vivo Nitrogen There are several bacteria and blue-green algae that can fix molecular nitrogen in vivo. 

Fixation free-living species and symbiotic species are involved. There are the strictly 

Molecular nitrogen fixation by microorganisms has been one of the mysteries in science. Although enzymes, the biological catalysts promoting 

Nitrogen fixation under milder conditions using a transition metal catalyst has been reported. The reaction is a simple one Nitrogen was bubbled into a mixture of transition metal halides and ethyl magnesium halide in ether. A typical reaction is shown in Fig. 6-12. 

A plausible mechanism has been postulated that includes the formation of titanocene dihydride intermediate as identified by ESR, Fig. 6-13. 

Discuss the effects of organic substituents in ferrocene on the entering group. 

Suggest a possible mechanism for the formulation of (Tr-ArCsHJFefTi-C5H5), from the reaction of ferrocene and an aryldiazonium salt. 

20 (a) The structure of the FeMo cofactor FOq cluster in nitrogenase. (b) The structure of the so-called P-cluster (FeyMo) in nitrogenase (the identity of Y is not known). 

FIGURE 16.2 The structure of the FeMo-co of A. vinelandii nitrogenase, as revealed by the crystallographic work of Kim and Rees. Y is probably 

Dlnltrogen and N2 Complexes Dinitrogen is a very inert molecule, and no one has yet been able to reduce it catalytically under the mild conditions employed by nitrogenase. N2 will react with Li and Mg to give nitrides, but the only other nonbiological reaction of N2 under mild conditions is the formation of N2 complexes. More than 100 examples are now known, of 

Common preparative routes are reduction of a phosphine substituted metal halide in the presence of N2, degradation of a nitrogen-containing ligand, and displacement of a labile ligand by N2. 

Only on rare occasions is it possible to synthesize and purify a whole series of N2 complexes with different ligands the Mo, W, and Re systems shown above are perhaps the most versatile in this respect. N2 can often displace ti -H2, as shown in Eq. 16.25 if this were the last step in the catalytic cycle, it would explain why N2 always produces at least one mole of H2 per mole of N2 reduced. 

Some examples of complexes in which the N2 bridges two metals are shown in Eqs. 16.26 and 16.27. In the ruthenium case, the system resembles 16.11, and the p.-N2 is little different in length from the terminal N2 in [Ru(NH3)5(N2)] itself. Some dinitrogen complexes are appreciably basic at N, showing once again the strong polarization of the N2. These can bind Lewis acids at to give adducts, some of which have very low N—N stretch- 

There have been several investigations of nitrogen fixation reactions in RF plasma devices, however, and some patents have been granted protecting processes developed from these investigations. 

Stokes et al. 47) reported similar conversions of oxygen in air to nitrogen oxides in a low-pressure RF plasma. After a run time of 2.5 hr, 0.4 ml of material was collected in liquid nitrogen traps, which on analysis showed a 2% conversion to nitric oxide. LaRoche 36) had previously demonstrated the beneficial effect of quenching the reaction products. In experiments with a low-pressure RF discharge, he obtained an increase in conversion to nitric oxide from 2 to 4% by a rapid quenching technique. 

It would appear from these series of investigations of nitric oxide preparation in plasma devices that, although reasonable conversions of oxygen to nitric oxide can be obtained under conditions of excess nitrogen, the final concentrations and production rates are low. The best conversions quoted in the foregoing are equivalent to approximately 200 kWh/lb nitric oxide produced, which is extremely high. The use of 

Plasma device Reactants Feed rate (hter/m) Power (kW) Percent conversion of Oa isro NO in product (%) Reference 

Three processes are widely used to convert elementary nitrogen to useful compounds (nitrogen fixation)  

Nitric oxide, both from the arc process and the high-temperature oxidation of ammonia (the Ostvvald process) is used to make nitric acid. Calcium cyanamide is of use both as a nitrogen fertilizer and as a source of ammonia, to which is converted on hydrolysis with steam  

Dinitrogen is fixed either by natural processes or by industrial ammonia (qv) production (1,8,9). The estimates for the aimual biological contribution range around 100-200 x 10 t. Industrial fixation contributes about 50 x 10 t/yr for fertilizer uses (see Fertilizers). Other processes, eg, lightning and combustion, are estimated to fix about 30 x 10 t/yr. Thus the biological process represents the majority (ca 65%) of the total aimual fixation rate, contributing about three times as much as the commercial production of fertilizer. 

The first such process was the Birkeland-Eyde process for N2 oxidation, implemented in 1905 (12). In this process, air is passed through an electric arc at temperatures above 3000°C to generate nitric oxide [10102-43-9] NO. 

On cooling the ait stream, further oxidation gives nitrogen dioxide [10102 4-0] NO2, which on absorption into water gives a mixture of nitric, HNO, and nitrous, HNO2, acids (see NlTTUC acid). 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

The low (ca 2%) yield of NO, the tendency to revert to N2 and O2 if the product stream is not quenched rapidly, the consumption of large (ca 60,000 kWh/1N2 fixed) amounts of electricity, and the concomitant expense to sustain the arc all led to the demise of this process. The related Wisconsin process for oxidising N2 at high temperatures in a pebble-bed furnace was developed in the 1950s (13). Although a plant that produced over 40 t/d of nitric acid was built, the product recovery costs were not economically competitive. 

The roles of the copper enzymes in electron transport, oxygen transport, and oxidation reactions have guaranteed continued interest in their study. In addition to studies of the natural compounds, there have been many attempts to design model structures of these enzymes, particularly of the binuclear species. Many of these include both nitrogen and oxygen donors built into macrocyclic ligands, although sulfur has been used as well.  

A very important sequence of reactions converts nitrogen from the atmosphere into ammonia  

The NH3 can then be further converted into nitrate or nitrite or directly used in the synthesis of amino acids and other essential compounds. This reaction takes place at 0.8 atm N2 pressure and ambient temperatures in Rhizobium bacteria in nodules on the roots of legumes such as peas and beans, as well as in other independent bacteria. In contrast to these mild conditions, industrial synthesis of ammonia requires high temperatures and pressures with iron oxide catalysts, and even then yields only 15% to 20% conversion of the nitrogen to ammonia. Intensive efforts to determine the bacterial mechanism and to improve the efficiency of the industrial process have so far been only moderately successful the goal of approaching enzymatic efficiency on an industrial scale is still only a goal. 

There have been many attempts to make model compounds for ammonia production, but none have been successful. How the enzyme manages to carry out the reaction at ambient temperature and less than 1 atm pressure of N2 is still an unanswered question. 

Oxidation of ammonia to nitrite, N02, and nitrate, N03, is called nitrification the reverse reaction is ammonification. Reduction from nitrite to nitrogen is called denitrification. All these reactions, and more, occur in enzyme systems, many of which include transition metals. A molybdenum enzyme, nitrate reductase, reduces nitrate to nitrite. Further reduction to ammonia seems to proceed by 2-electron steps, through an uncertain intermediate with a -fl oxidation state (possibly hyponitrite, N202 ) and hydroxylamine  

Nitrates are necessary for most explosives. As noted, the sodium nitrate exported by Chile during World War I was insufficient to meet the wartime nitrate need. Therefore, nitrogen fixation efforts occurred, as well as experimentation with perchlorates, in addition to the extraction of toluol. 

At cellular level, the overall reduction process may be represented as (11.72), where the nitroge-nase system is an enzyme complex which includes phosphate energy carriers ATP and NADPH and three proteins. Two of these proteins are metalloenzymes, one of which contains iron, and the other both iron and molybdenum. The third protein, ferredoxin, contains iron and sulphur and acts as an electron provider through NADPH. 

The complicated processes occurring within the nitrogenase systan are stiU not fully understood and will not be discussed here, but the overall equation for nitrogen fixation can be written as 

If simple catalysts could be evolved to stimulate the action of the nitrogenase complex in reaction (11.72), it would greatly reduce the cost of synthetic nitrogeneous fertilisers and improve the efficiency of Third World agriculture (Chapter 12.2). 

Very little free ammonia is normally found in the soil, although it might be expected as a result of nitrogen fixation or fertiliser application. This is because of rapid oxidation by soil bacteria to 

Enzyme-catalysed nitrate reduction uses NADPH as an electron source. 

The Mo enzyme consists of two components (1) the Fe protein (molecular weight 57,000 daltons), which contains iron and sulfur (4 atoms of each per protein) and (2) the MoFe protein (220,000 daltons, Uz subunits), which contains both metals (1 atom Mo, 32 atoms Fe). Each also contains S ions (ca. one per iron), which act as bridging ligands for the metals. The protein contains special Fe—S clusters called P clusters that have EPR resonances like those of no other Fe—S cluster. A soluble protein-fiee molybdenum and iron-containing cluster can be separated from the enzyme. This iron-molybdenum cofactor, or FeMo-co, was known to have approximately 1 Mo, 7-8 Fe, 4-6 S , and one molecule of homocitrate ion. As for the P cluster, there was no agreement on the structure of FeMo-co for many years. In purified form FeMo-co does crystallize, and it can restore N2 reducing activity to samples of mutant N2ase that are inactive because they lack FeMo-co. On the other hand, no crystal structure of FeMo-co proved possible, and no synthetic model complex was found that could activate the mutant enzyme. 

Only the most basic N2 complexes, notably the bis-dinitrogen Mo and W complexes, can be protonated. According to the exact conditions, various N2H c complexes are obtained, and even, in some cases, free NH3 and N2H4. As strongly reduced Mo(0) and W(0) complexes, the metal can apparently supply the six electrons required by Eq. 16.21, and so the metals are oxidized during the process. Note, too, that in Eq. 16.28, the loss of the very strong N—N triple bond is compensated by the formation of two N—H bonds and a metal nitrogen multiple bond. 

Unfortunately, neither one of these methods is particularly feasible on the large scale needed. 

The crystal structure of the entire enzyme has been central in clearing up some of the mysteries surrounding the system. FeMo-co proves to be a double cubane linked by three sulfide ions (Fig. 16.3). Tlie Mo 

FIGURE 16.3 Structure of the FeMo-co of Azotobacter vinelandii nitrogenase. 

An early state of the refinement, in which the central point of the cluster was taken to be vacant, suggested that six Fe atoms of the cofactor had the unreahstically low coordination number of 3, but subsequent work has put a carbon atom at the center of the cluster, making it unambiguously organometallic.  

The FeMo-co cluster does not form by self-assembly but requires biosynthesis on an external template prior to incorporation into the MoFe protein. The P-cluster is synthesized by fusion of two [Fe4S4] clusters within the MoFe protein. The organism has thus gone to considerable trouble to make these clusters, otherwise unknown in biology.  

The first recognized dinitrogen complex, [Ru(NH3)5(N2)], was isolated as early as 1965 during the attempted synthesis of [Ru(NH3)g] + from RUCI3 and hydrazine. This illustrates how important it can be to avoid throwing out a reaction that has not worked 

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Many key protein ET processes have become accessible to theoretical analysis recently because of high-resolution x-ray stmctural data. These proteins include the bacterial photosynthetic reaction centre [18], nitrogenase (responsible for nitrogen fixation), and cytochrome c oxidase (the tenninal ET protein in mammals) [19, 20]. Although much is understood about ET in these molecular machines, considerable debate persists about details of the molecular transfonnations. 

Howard J B and Rees D C 1996 Structural basis of biological nitrogen fixation Chem. Rev. 96 2965-82... 

COPPERALLOYS-WROUGHT COPPERAND WROUGHT COPPERALLOYS] (Vol 7) -catalysts for synthesis gas [NITROGEN FIXATION] (Vol 17)... 

D. E. Nichols, P. C. WiUiamson, and D. R. Waggoner, paper presented at The Steenbock-Kettering International Symposium on Nitrogen Fixation, Madison, Wis., June 12-16, 1978. 

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