Electron shuttle reactions

Electron shuttle reactions copper(II) complexes, 717 Elongation factor 1 zinc, 1007... 

The second electron shuttle system, called the malate-aspartate shuttle, is shown in Figure 21.34. Oxaloacetate is reduced in the cytosol, acquiring the electrons of NADH (which is oxidized to NAD ). Malate is transported across the inner membrane, where it is reoxidized by malate dehydrogenase, converting NAD to NADH in the matrix. This mitochondrial NADH readily enters the electron transport chain. The oxaloacetate produced in this reaction cannot cross the inner membrane and must be transaminated to form aspartate, which can be transported across the membrane to the cytosolic side. Transamination in the cytosol recycles aspartate back to oxaloacetate. In contrast to the glycerol phosphate shuttle, the malate-aspartate cycle is reversible, and it operates as shown in Figure 21.34 only if the NADH/NAD ratio in the cytosol is higher than the ratio in the matrix. Because this shuttle produces NADH in the matrix, the full 2.5 ATPs per NADH are recovered. 

Ferric iron can act as an electron acceptor under the anaerobic conditions the azo dye is in. Like sulfate, it was found that addition of ferric iron to the reactor stimulates the azo dye reduction. Indeed, the reactions are dealing with the redox couple Fe (III)/Fe (II), which can act as an electron shuttle for transferring electrons from electron donor to the electron accepting azo dye. Meanwhile, reactions of both reduction of Fe (III) to Fe (II) and oxidation of Fe (II) to Fe (III) facilitate the electron transport from the substrate to azo dye, thus acting as an extracellular redox mediator [31]. 

During the last two decades, more studies have been conducted to explore the catalytic effects of different redox mediators on the bio-transformation processes. Redox mediators, also referred to as electron shuttles, have been shown to play an important role not only as final electron acceptor for many recalcitrant organic compounds, but also facilitating electron transfer from an electron donor to an electron acceptor, for example, azo dyes [8, 11, 12], Redox mediators accelerate reactions by lowering the activation energy of the total reaction, and are organic molecules that can reversibly be oxidized and reduced, thereby conferring the capacity to serve as an electron carrier in multiple redox reactions. 

The acceleration mechanism of redox mediators are presumed by van der Zee [15]. Redox mediators as reductase or coenzymes catalyze reactions by lowering the activation energy of the total reaction. Redox mediators, for example, artificial redox mediators such as AQDS, can accelerate both direct enzymatic reduction and mediated/indirect biological azo dye reduction (Fig. 3). In the case of direct enzymatic azo dye reduction, the accelerating effect of redox mediator will be due to redox mediator enzymatic reduction in addition to enzymatic reduction of the azo dye. Possibly, both reactions will be catalyzed by the same nonspecific periplasmic enzymes. In the case of azo dye reduction by reduced enzyme cofactors, the accelerating effect of redox mediator will either be due to an electron shuttle between the reduced enzyme cofactor and redox mediator or be due to redox mediator enzymatic reduction in addition to enzymatic reduction of the coenzymes. In the latter case, the addition of redox mediator simply increases the pool of electron carriers. 

Additionally, it was found that the double reductive alkylation of the 2,5-diester 66 could be achieved under Birch conditions (Li/NH3) to produce the 3-pyrroline 67. On the basis of a mechanistic postulate that such reductions do not involve transfer of a proton from ammonia, the authors discovered that the same reduction could be performed in THF (no ammonia) with lithium metal and catalytic amounts of naphthalene as an electron shuttle, thereby making this reaction more practicable on a large scale <00TL1327>. 

Among the main goals of electrochemical research are the design, characterization and understanding of electrocatalytic systems, (1-2) both in solution and on electrode surfaces. (3.) Of particular importance are the nature and structure of reactive intermediates involved in the electrocatalytic reactions.(A) The nature of an electrocatalytic system can be quite varied and can include activation of the electrode surface by specific pretreatments (5-9) to generate active sites, deposition or adsorption of metallic adlayers (10-111 or transition metal complexes. (12-161 In addition the electrode can act as a simple electron shuttle to an active species in solution such as a metallo-porphyrin or phthalocyanine. 

Immobilizing the catalyst on the electrode surface is useful for both synthetic and sensors applications. Monomolecular coatings do not allow redox catalysis, but multilayered coatings do. The catalytic responses are then functions of three main factors in addition to transport of the reactant from the bulk of the solution to the film surface transport of electrons through the film, transport of the reactant in the reverse direction, and catalytic reaction. The interplay of these factors is described with the help of characteristic currents and kinetic zone diagrams. In several systems the mediator plays the role of an electron shuttle and of a catalyst. More interesting are the systems in which the two roles are assigned to two different molecules chosen to fulfill these two different functions, as illustrated by a typical experimental example. 

The use of titanium and zirconium complexes as electron shuttles to transfer electrons from excess aluminum present in the reaction solution to the fluorinated substrate has been described,210 e.g. formation of perfluoronaphthalene from perfluorodecahydronaphthalene. 

The reaction serves as an illustrative example of the synthetic utilization of the Te(II)—>Te(IV) two-electron shuttle. Another promising application is seen in photodynamic therapy in which tellurapyrylium dyes 119 can function as photosensitizers to produce either singlet oxygen or superoxide radical-anions (via electron transfer), thereby serving as cytotoxic agents. An important useful property of tellurapyrylium dyes is their absorbance in the near-infrared region where biological tissues are most important. 

Figure 16. Optical electron transfer excitation of [2]rotaxane [46]. Shuttling does not take place because the direct and ferrocene-mediated back electron transfer reactions are much faster than the motion of the ring.

Most of the electrons removed from fuels during energy metabolism are transferred via nicotinamide adenine dmucleotide (NAD). NAD collects electrons from many different energy fuels in reactions catalyzed by specific enzymes. These enzymes are dehydrogenases. Reduced NAD, in turn, shuttles the electrons to the respiratory chain. Flavin adenine dinudeohde (FAD) also acts as an electron shuttle. In each reaction involving NAD (or FAD), two electrons are transferred that is, two electrons are carried or shuttled. NAD and PAID are small molecules with molecular weights of 663 and 7S5 and a re manufactured in the body from the vitamins niacin and riboflavin, respectively. These molecules are called N.A.D. and F.A.D., not nad" o r "fad."... 

Although conducting polymers have demonstrated direct electrochemical communication with nitrate reductases, the incorporation of electron relay groups within the polymer matrix provides a more efficient pathway for electron hopping between the enzyme and the electrode surface. Several artificial electron donors can shuttle electrons to the oxidized form of nitrate reductase with methyl viologen being the best choice due to its very negative redox potential [214-216]. The electron transfer reactions can be represented as follows ... 

Synthetic methodology for the preparation of switchable rotaxanes has been primarily developed by Stoddart and coworkers [4]. In collaboration with Stoddart, the author s group reported in 1994 a molecular shuttle that exhibited a novel feature dual control. In this rotaxane the average position of the bead could be controlled by both proton- and electron-transfer reactions... 

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