Reduction six-electron

Controlled-potential coulometry also can be applied to the quantitative analysis of organic compounds, although the number of applications is significantly less than that for inorganic analytes. One example is the six-electron reduction of a nitro group, -NO2, to a primary amine, -NH2, at a mercury electrode. Solutions of picric acid, for instance, can be analyzed by reducing to triaminophenol. 

Sulfite reductase catalyzes the six-electron reduction of sulfite to sulfide, m essential enzymatic reaction in the dissimilatory sulfate reduction process. Several different types of dissimilatory sulfite reductases were already isolated from sulfate reducers, namely desul-foviridin (148-150), desulforubidin (151, 152), P-582 (153, 154), and desulfofuscidin (155). In addition to these four enzymes, an assimila-tory-type sulfite reductase was also isolated from D. vulgaris. Although all these enzymes have significantly different subunit composition and amino acid sequences, it is interesting to note that, as will be discussed later, all of them share a unique type of cofactor. 

Run(Hedta)(NO+)]° and [Fen(Hedta)(NO )] have been shown to be effective electrocatalysts for the reduction of N02 in acidic aqueous media, to yield N20, N2, NH3OH+, or NH4 339,340 An element of selectivity is available by control of pH and applied potential. Steps involved in the typical six-electron reduction of nitrite to ammonia catalyzed by [Run(Hedta)(NO+)]° are summarized in Equations (67)-(69). The mechanisms by which nitrite is reduced appeared to be similar to those identified for Fe-porphyrin331 and Ru or Os-polypyridyl complexes.337... 

These types of switchable electrode surfaces have been used to selectively pattern two different cell populations onto a surface [151] and additionally these surfaces can selectively release different cells at different applied potentials [152]. However, it is important to recognize that electrochemically switching a surface from inactive to conjugation and active to conjugation has been well explored with nitro-terminated aryl diazonium salts. In such studies, the application where very anodic potential resulted in a six-electron reduction to an amine [139], to which proteins could be attached [153-155]. The key difference is that the interaction of the biological medium with the surface is controlled by the presence of the antifouling layer. In many ways these electrode surfaces developed by Mrksich and coworkers [150-152, 156] are very similar to the antifouling surfaces with molecular wires discussed in Section 1.4.2 [131, 132, 138, 142]. In both cases the electrode is... 

The biological nitrogen fixation process is Introduced. Discussion focusses on the Dominant Hypothesis of nitrogenase composition and functioning. The enzyme system catalyzes the six-electron reduction of N2 to 2 NH3 concomitant with the evolution of H2. ATP hydrolysis drives the process. The two protein components of the enzyme,... 

The nitrogenases are a class of enzymes that catalyze the six-electron reduction of N2 to ammonia (eq.l). 

Sulfite reductase catalyzes the six-electron reduction by NADPH of sol" to and NO2 to NH3. In E. coli this enzyme is a complex structure with subunit composition 0 8)84 (Siegel et al, 1982). The enzyme active site is on the /3 subunit, which contains both a 4Fe 4S cluster and a siroheme prophyrin. Substrates and ligands have been found to bind to the siroheme. The a subunit binds NADPH and serves to shuttle electrons to the active site through bound FAD and FMN groups. Isolated )8 subunits can catalyze sulfite reduction in the presence of a suitable electron donor. 

Indeed, other electrocatalytic processes were studied, including the pioneering work on chlorate reduction by the six-electron reduction product of 12-molybdophosphate in water-dioxane solutions [169]. This process, extended to bromate, has become a classical test of the electrocatalytic abilities of several POMs [see Table 13]. 

The formation of azomethane as a two-electron oxidation product of l,2-Me2Hz, and the exclusive label on 15NH3 suggest that the full six-electron reduction of NO+ to NH3 is accomplished through three successive two-electron processes. The plausible intermediates must be [Fen(CN)515HNO]3- and [Fen(CN)515NH2OH]3-. The proposed mechanism involves an initial adduct formation, similar as for other hydrazines (the DFT calculations show that the adduct formation is not sterically hindered), as shown by Eq. (16), followed by a two-electron transfer from l,2-Me2Hz to the N-atom in nitrosyl, Eq. (17). 

JV-Nitraminopyridines are reducible both in acid and alkali. In hydrochloric acid the main product from 2-nitraminopyridine was the hydrazino-pyridine, formed in a six-electron reduction, but 2-aminopyridine and 2-chloropyridine were side products, the latter possibly through reaction by an intermediate diazonium compound with chloride. Contrary to nitramines of most primary amines, 2-nitraminopyridine431 is reducible in alkaline solution uptake of the first two electrons forms the 2-pyridyl-N-nitrosamine, which is further reduced to 2-aminopyridine. 

Spinach nitrite reductase,313 which is considered further in Chapter 24, utilizes reduced ferredoxin to carry out a six-electron reduction of N02 to NH3 or of SO-2 to S2. The 61-kDa monomeric enzyme contains one siroheme and one Fe4S4 cluster. A sulfite reductase from E. coli utilizes NADPH as the reductant. It is a large (38a4 oligomer.312 The 66-kDa a chains contain bound flavin... 

Reduction of nitrite denitrification. The nitrite formed in Eq. 18-25 is usually reduced further to ammonium ions (Eq. 18-27). Tire reaction may not be important to the energy metabolism of the bacteria, but it provides NH4+ for biosynthesis. This six-electron reduction is catalyzed by a hexaheme protein containing six c-type hemes bound to a single 63-kDa polypeptide chain.336,337... 

Assimilatory nitrite reductases of plants, fungi, and bacteria carry out the six-electron reduction of nitrite to ammonium ions (Eq. 24-13) using electron donors such as reduced ferredoxins or NADPH. 

The enzymes from green plants and fungi are large multifunctional proteins,80 which may resemble assimilatory sulfite reductases (Fig. 16-19). These contain siroheme (Fig. 16-6), which accepts electrons from either reduced ferredoxin (in photosynthetic organisms) or from NADH or NADPH. FAD acts as an intermediate carrier. It seems likely that the nitrite N binds to Fe of the siroheme and remains there during the entire six-electron reduction to NH3. Nitroxyl (NOH) and hydroxylamine (NH2OH) may be bound intermediates as is suggested in steps a-c of Eq. 24-14. 

The six-electron reduction of sulfite to sulfide is catalyzed by sulfite reductase, a multisubunit complex composed of a flavoprotein and a heme iron-sulfur protein. 

Several macrocyclic ligands are shown in Figure 2. The porphyrin and corrin ring systems are well known, the latter for the cobalt-containing vitamin Bi2 coenzymes. Of more recent interest are the hydroporphyrins. Siroheme (an isobacteriochlorin) is the prosthetic group of the sulfite and nitrite reductases which catalyze the six-electron reductions of sulfite and nitrite to H2S and NH3 respectively. The demetallated form of siroheme, sirohydrochlorin, is an intermediate in the biosynthesis of vitamin Bi2, and so links the porphyrin and corrin macrocycles. Factor 430 is a tetrahydroporphyrin, and as its nickel complex is the prosthetic group of methyl coenzyme M reductase. F430 shows structural similarities to both siroheme and corrin. 

These include nitrite and trimethylammonium oxide. Nitrite undergoes a six-electron reduction to give ammonia in a reaction catalyzed by a nitrite reductase. Nitrite reductase activity with lactate and formate has been reported, although other donors may support the reduction of nitrite. It should be noted that at least three pathways in E. coli exist for the reduction of nitrite. 

Green plants, algae, fungi, cyanobacteria and bacteria that assimilate nitrate also produce assimilatory nitrite reductases, which catalyze the six-electron reduction of nitrite to ammonia (equation 89). The formation of heme-nitrosyl intermediates has been detected in several cases,1515 while hydroxylamine is commonly thought to be an intermediate. Added hydroxylamine is rapidly reduced to ammonia. However, no intermediates are released, and ammonia is the only product... 

Even though most of the synthetic work was directed toward four- and six-electron reduction of nitro groups, voltammetry suggests that other products could result.117 In one of the above examples, the first-formed nitroso compound can form azoxy dimers, which are subsequently reduced to the azo compounds. 

The iron complex of sirohydrochlorin (119b) is the prosthetic group of nitrite (81 Mil) and sulfite reductases (75M17). These enzymes catalyze six-electron reductions yielding ammonia (sometimes NO, N20, and N2) and hydrogen sulfide, respectively. 

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