Coupled redox reactions

There has been a resurgence of interest in proton-coupled redox reactions because of their importance in catalysis, molecular electronics and biological systems. For example, thin films of materials that undergo coupled electron and proton transfer reactions are attractive model systems for developing catalysts that function by hydrogen atom and hydride transfer mechanisms [4]. In the field of molecular electronics, protonation provides the possibility that electrons may be trapped in a particular redox site, thus giving rise to molecular switches [5]. In biological systems, the kinetics and thermodynamics of redox reactions are often controlled by enzyme-mediated acid-base reactions. 

Chromium concentrations are generally below detection in anaerobic groundwaters that contain ferrous iron. This may reflect reduction of soluble chromate (CrO ") to insoluble chromic ion (Cr(III)) by Fe in a coupled redox reaction that can be written... 

Recent work indicates that metallic iron emplaced in a permeable barrier or wall can degrade chlorinated solvents in groundwater by a coupled redox reaction in which the solvent is the oxidant and the reduciant is Fe metal or product Fe or H2 (cf. Wilson 1995 Roush 1995). A similar Fe metal barrier has been used to reduce chromate concentrations to values below detection, and has been suggested as a means of cleaning up groundwaters high in nitrate (Wilson 1995). 

Each of the coupled redox reactions in biological electron transport involves the transfer of electrons from one redox couple to another couple of higher reduction potential. Thus, each individual redox reaction in the sequence is exergonic under standard conditions. For electrons entering the respiratory chain as NADH, the overall reaction sequence is given by the following equation ... 

The slower relaxation reflects re-equilibration between the forms of reduced azurin and favour A r. This step is spectrally silent but is reported through the coupled redox reaction which is displaced towards ferricytochrome C55, as Ar is converted to A r. 

In order to monitor SOD activity routinely, assays that require only instrumentation typical of a chemical or biochemical laboratory were set up. The assays consist of a reaction mixture that generates superoxide anion and a coupled redox reaction that scavenges the superoxide ion. The latter reaction is usually followed spectrophoto-metrically. The addition of superoxide dismutase destroys superoxide and inhibits the coupled reaction. The specific activity of the enzyme is determined on the basis of the amount of protein required to slow down to 50% the first-order coupled reaction, i.e., one enzymatic unit is the amount of protein that reduces the rate to 50% and the specific activity corresponds to the number of units per milligram of protein. 

This reaction can be coupled with any one of the electron accepting (reduction) reactions listed in Table 7-11. From the AG° values of these coupled reactions it can be deduced that the amount of free energy that a microorganism can obtain from the coupled redox reactions listed in Table 7-11 is in direct proportion to its pe value. 

For example, it was demonstrated by Barton [32] that doublehelical DNA films on gold surface display a marked sensitivity to the presence of base mismatches within the immobilized duplexes. Moreover, it has been observed that mismatch detection is possible regardless of DNA sequence composition and mismatch identity. The presence of mismatches was elucidated based on the electrochemical characteristics of the redox active intercalators bound to the DNA-modified gold surfaces. Coupled redox reactions were employed to induce an electro-catalj ic current and thus increase the method s sensitivity (Fig. 7.4). 

H. Yamamoto, A. Ori, K. Ueda, C. Dusemund, S. Shinkai, Visual sensing of fluoride ion and saccharides utilizing a coupled redox reaction of fer-rocenylboronic acids and dye molecules, Chemical Communications 1996, 407. 

Note that the redox potential is generally a function of both mediator concentrations and pH [45,46]. This is based on the assumption that the mediator undergoes a proton-coupled redox reaction ... 

The. mode of intervention of RUO2 in the oxygen generation process may be best understood in terms of electrochemical concepts. The two coupled redox reactions occurring on the RUO2 particle are depicted schematically in Figure 27 The particle serves as a microelectrode for which the theory of local elements may again be applied. The cathodic branch indicates the reduction of Fe(bipy) (or Ru(bipy) +) while the anodic branch corresponds to water oxidation. 

Finally, Chung and Anson also showed that thin organic films are very useful in the study of proton-coupled redox reactions such as the reduction of tetrachlo-ro-l,4-benzoquinone in nitrobenzene or benzonitrile [281]. 

Kosjek, B., Stampfer, W., Pogorevc, M., Goessler, W., Faber, K., and Kroutil, W. (2004) Purification and characterization of a chemotolerant alcohol dehydrogenase applicable to coupled redox reactions. Biotechnol. Bioeng., 86, 55-62. 

Copper(ll) acetate performs a double role activation of the triacyl methane group by coordination and production of the reactive triacylmethyl radical by a coupled redox reaction. The proposed mechanism is supported by the fact that 274 is present in the auto-oxidation reaction mixture of hexahydrocolupulone, while compounds 275-279 are absent. Auto-oxidation occurs indeed at the ring carbon atom C-4 (see 13.1.2.). The radicals are in this case produced within the 1,3-dicarbonyl system in the ring, whereby only 274 can be formed in an analogous way as described before (pathway B). 

Photosynthesis may also be considered as a coupled redox reaction, in which it is involved in the gain or loss of electrons as follows ... 

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