Artificial Nitrogen Fixation

MoFe-Protein contains a second cluster, the M-cluster. [33] This is covalently bonded to cysteine and histidine of the a-suhunit. A homocitrate anion R)-2-Hydroxyhutane-l,2,4-tricarhoxylic acid) is complexed as a bidentate ligand with molybdenum (Fig. 4.5). On the basis of better resolved X-ray structural analysis and quantum mechanical calculations, initially it was thought, a nitrogen atom is located in the centre of the Fe/Mo complex, [34] but more recent results gave evidence for an interstitial carbido ligand [35,36], which is unprecedent in bioinorganic chemistry. 

In an ATP-dependent reaction, the Fe-protein transfers electrons via the P-cluster to the M-duster, where the nitrogen is finally reduced (Fig. 4.6). How this occurs in particular cases, is stiU subject of intensive research. [37, 38] 

6 The energy for reduction of nitrogen is transferred through different iron-sulfur clusters. 

It is interesting, that the substrate specificity of nitrogenases is low in comparison with other enzymes. They also reduce acetylene (to ethylene), hydrazine [39], cyanide and azide. [40] From this, and from the presumably very high biological age of the enzyme system, W. S. Silver and John R. Postgate [41 ] con-duded that the enzyme s original purpose was not nitrogen fixation, but cyanide detoxification in the biosphere of Precambrian bacteria. 

5 The M-duster complexes molecular nitrogen and reduces it to ammonia. 

---

It is also worth recalling that artificial nitrogen fixation consumes large amounts of nonrenewable energy supplies responsible for CO2 emissions and contributes to the greenhouse effect. The same is true for N2O emissions, a gas approximately 300 times more powerful than CO2 in its... 

Bacterial ADPGPP enzymes, 12 491 Bacterial a-amylases, 10 280 Bacterial artificial chromosome (BAC) vectors, 12 508 Bacterial cellulose, 20 557 Bacterial genera, nitrogen fixation by, 17 295-296... 

Table 1 summarizes several redox transformations that can be accomplished in artificial photosynthetic assemblies including the photolysis of water, carbon dioxide reduction, and nitrogen fixation processes. The endoergicities of these transformations, and the number of electrons involved in the reduction processes, are also indicated in the table. It is evident that the energy per electron to drive the various transformations are met by visible light quanta. 

Shi, D.J., Hall, D.O. and Tang, P.S. 1987a. Photosynthesis, Nitrogen fixation, Ammonia photoproduction and structure of Anabaena azollae immobilized in natural and artificial systems. In Progress in Photosynthesis research (Ed. J. Biggens), vol. II. Martinus NijhoffPubs., Dordrecht, pp.641-644. 

Despite many studies there is available, at this time, no unequivocal report of artificial photocatalytic synthesis of ammonia from nitrogen and water on heterogeneous catalysts. The chemical reaction 1, shown at the beginning of this chapter, remains speculative 17 years after it was initially reported. No rational start has yet been made on determining a mechanism for the reported nitrogen fixation process, despite the fact that a clear demonstration and understanding of any such process would be of great importance. Despite 17 years of research, the reported yields of ammonia from artificial photosynthesis remain at or near the limits of detection by routine analytical methods. As we stated earlier in this chapter, the concept of a major discovery that remains forever on the borderline of detectability is internally contradictory. The current state of the field is one where serious scientific skepticism is appropriate. 

Nitrogen mobilization ALL flows that convert inert nitrogen to reactive nitrogen. Includes natural processes such as nitrogen fixation in soybean, microbial fixation, and artificial processes such as the Haber—Bosch process Related agricultural sectors such as oilseed farming, etc. and fertilizer manufacturing sector... 

---