Ammonia, atmospheric fixation

The fixation of nitrogen is the most fundamental biochemical process after photosynthesis. It is the process whereby atmospheric nitrogen is reduced to ammonia. Nitrogen fixation can be carried out by blue-green algae, some yeasts, and especially bacteria. The reduction of nitrogen... 

Other PK variations include microwave conditions, solid-phase synthesis, and the fixation of atmospheric nitrogen as the nitrogen source (27—>28). Hexamethyldisilazane (HMDS) is also an excellent ammonia equivalent in the PK synthesis. For example, 2,5-hexanedione and HMDS on alumina gives 2,5-dimethylpyrrole in 81% yield at room temperature. Ammonium formate can be used as a nitrogen source in the PK synthesis of pyrroles from l,4-diaryl-2-butene-l,4-diones under Pd-catalyzed transfer hydrogenation conditions. 

Moreover, the lipo-chitooligosaccharides, also known as nod factors, permit nitrogen fixation by which plants and symbiotic Rhizobia bacteria can reduce atmospheric nitrogen to the ammonia that is utihzed by the plant, thus making available nitrogen compounds to other living organisms. 

Fig. 10-13. The links between the cycling of C, N, and O2 are indicated. Total primary production is composed of two parts. The production driven by new nutrient input to the euphotic zone is called new production (Dugdale and Goering, 1967). New production is mainly in the form of the upward flux of nitrate from below but river and atmospheric input and nitrogen fixation (Karl et al, 1997) are other possible sources. Other forms of nitrogen such as nitrite, ammonia, and urea may also be important under certain situations. The "new" nitrate is used to produce plankton protoplasm and oxygen according to the RKR equation. Some of the plant material produced is respired in the euphotic zone due to the combined efforts...

Many bacteria perform services we find useful and even necessary, although we rarely take note of them. Some bacteria participate in the decomposition of dead plants and animals and so help recycle chemicals that are otherwise locked away from the living world. Some are sources of antibiotics that have revolutionized the treatment of infectious disease in the past fifty years. Others are responsible for nitrogen fixation, converting relatively inert atmospheric nitrogen into biologically useful ammonia—a conversion that is not simple in the chemical laboratory. Still other bacteria are indispensable in the world s kitchens, for without them there would be no cheese or yogurt, nor any of hundreds of other traditional milk products. We could not enjoy pickles or sauerkraut, and... 

In marked contrast to the conditions under which nitrogen is converted to ammonia in the laboratory, in nature atmospheric N2 is converted to ammonia under the mildest of conditions by enzymes called nitrogenases in a process termed nitrogen fixation. This example testifies to the power of enzymes, about which much more follows in chapter 9. 

Only certain prokaryotes can fix atmospheric nitrogen. These include the cyanobacteria of soils and fresh and salt waters, other kinds of free-living soil bacteria such as Azotobacter species, and the nitrogen-fixing bacteria that live as symbionts in the root nodules of leguminous plants. The first important product of nitrogen fixation is ammonia, which can be used by all organisms either directly or after its conversion to other soluble compounds such as nitrites, nitrates, or amino acids. 

Figure 17.14. Some unusual reactor configurations, (a) Flame reactor for making ethylene and acetylene from liquid hydrocarbons [Patton et al., Pet Refin 37(li) 180, (1958)]. (b) Shallow bed reactor for oxidation of ammonia, using Pt-Rh gauze [Gillespie and Kenson, Chemtech, 625 (Oct. 1971)]. (c) Sdioenherr furnace for fixation of atmospheric nitrogen, (d) Production of acetic acid anhydride from acetic acid and gaseous ketene in a mixing pump, (e) Phillips reactor for low pressure polymerization of ethylene (closed loop tubular reactor), (f) Polymerization of ethylene at high pressure.

Soil-borne bacteria of the family Rhizobiaceae and leguminous plants form a symbiotic relationship during which a new organ, the root nodule, is developed. Within these root nodules the bacteria fix atmospheric dinitrogen and the product of nitrogen fixation, ammonia, is exported to the plant [69,70]. Root nodules develop from primordia which are established at specific sites in the root cortex shortly after Rhizobium infection. The peptide enod40 is believed to play a critical role in inducing the de-differentiation and the mitotic division of root cortical cells, i.e. the initial steps in nodule development. This however, is not entirely undisputed [3,4,69-72]. 

Ammonia is oxidized in nature to nitrate via several intermediates in the process of nitrification. Nitrate may be reduced to nitrite by either a dissimilatory or an assimilatory process. Nitrite may be assimilated into the cell via reduction to ammonia, or it may be reduced by microorganisms to N20 and N2 in denitrification. A major part of the total nitrogen in this pathway is lost to the atmosphere. However, in turn, atmospheric dinitrogen is converted to ammonia by various bacteria in nitrogen fixation. 

Nitrogenases (EC 1.18.6.1) are the enzymes responsible for the biological fixation of atmospheric dinitrogen to ammonia. These enzymes are found only in bacteria, although some of the bacteria form symbioses with plants. 

Vanadium (II)/catechol systems can be useful in fixation of atmospheric nitrogen, reducing it to ammonia [244-246]. A detailed EPR study concluded that the active catalyst is a trinuclear V-Cat species 966 [125a,246,247] ... 

---