Accelerating redox processes

M aqueous solutions of iodopentaminecobalt(lll) decompose with first-order kinetics at 45 °C with = 6.0x 10" sec 10" M solutions decompose faster after an initial induction period at the normal rate. Product analysis shows the fast decomposition to be a mixture of a redox process leading to iodine and substitution leading to aquopentaminecobalt(iri) and iodide. Addition of sodium iodide (to 10 M) accelerates the decomposition and... 

Thus, the mechanism of MT antioxidant activity might be connected with the possible antioxidant effect of zinc. Zinc is a nontransition metal and therefore, its participation in redox processes is not really expected. The simplest mechanism of zinc antioxidant activity is the competition with transition metal ions capable of initiating free radical-mediated processes. For example, it has recently been shown [342] that zinc inhibited copper- and iron-initiated liposomal peroxidation but had no effect on peroxidative processes initiated by free radicals and peroxynitrite. These findings contradict the earlier results obtained by Coassin et al. [343] who found no inhibitory effects of zinc on microsomal lipid peroxidation in contrast to the inhibitory effects of manganese and cobalt. Yeomans et al. [344] showed that the zinc-histidine complex is able to inhibit copper-induced LDL oxidation, but the antioxidant effect of this complex obviously depended on histidine and not zinc because zinc sulfate was ineffective. We proposed another mode of possible antioxidant effect of zinc [345], It has been found that Zn and Mg aspartates inhibited oxygen radical production by xanthine oxidase, NADPH oxidase, and human blood leukocytes. The antioxidant effect of these salts supposedly was a consequence of the acceleration of spontaneous superoxide dismutation due to increasing medium acidity. 

It must, however, be taken into account that the concept of electrochemical reversibility or irreversibility of an electron transfer is relative. In fact, to accelerate the redox processes one can act either on the mass transport (by stirring the solution) or on kRed and k0x (by changing the electrode potential, as seen in Section 4.1.1). 

Certain substitutions can be catalyzed by the operation of a redox process. It is most easily detected with inert Cr(III), Co(III) and Pt(IV). Hydrolysis, anation and anion interchange all have been accelerated In complexes of these metals by the presence of the lower oxidation state (which is more labile). 

They may accelerate or retard the process. Additives may act in solution (via com-plexation), but more often adsorb on the oxide and either raise or lower the energy of attachment between the surface ions and those of the interior. In extreme cases, adsorbed additives may inhibit dissolution. pH has a strong influence on the dissolution of iron oxides. At atmospheric pressure, dissolution of well crystalline Fe " oxides requires a pH of <1 even at 70 °C. The high affinity of protons with structural 0 assists the release of iron particularly at low pH. It is the release of the cation, rather than the anions which is likely to be rate limiting. pH also influences the electrochemical surface potential and hence redox processes. The surface potential is determined largely by surface charge, which in turn, depends upon pH (see Chap. 10). 

Examples have recently been reported of NHC-catalyzed internal redox processes that directly convert a-reducible aldehydes to cz-reduced amides [19, 20]. 17 is found to be an efficient species to accelerate C-N bond formation via the nucleophilic intermediate 19, as shown in Scheme 14.4. Very recently, a few examples. 

The Heck reaction has now been reviewed448,449. Evidence for the formation of zerova-lent palladium from (AcO Pd and Ph3P via a redox process has been provided450. This explains the origin of Pd(0) required for certain palladium-catalysed reactions in cases where Pd(II) is added to the reaction as the primary form of the Pd-catalyst. Thallium has been found to accelerate the Heck-type cyclization-carbonylation451. 

In recent years, considerable effort has gone into the development of a new class of electrochemical devices called chemically modified electrodes. While conventional electrodes are typified by generally nonspecific electrochemical behavior, i.e., they serve primarily as sites for heterogeneous electron transfer, the redox (reduction-oxidation) characteristics of chemically modified electrodes may be tailored to enhance desired redox processes over others. Thus, the chemical modification of an electrode surface can lead to a wide variety of effects including the retardation or acceleration of electrochemical reaction rates, protection of electrodes, electro-optical phenomena, and enhancement of electroanalytical specificity and sensitivity. As a result of the importance of these effects, a relatively new field of research has developed in which the... 

The biological catalytic activity of metalloproteins for redox reactions is usually associated with a particular coordination environment of the metal active site [160, 161], In particular, there has been considerable interest in 02-binding and -activation by non-heme metalloenzymes [162-167). A redox-active metal center is often associated with another metal center which can accelerate the redox process of O2... 

The water film on the surface of the ice cubes initiates the strongly exothermic redox reaction between the zinc dust and the nitrate (which supplies oxygen) amrnuuiuiii chloride accelerates the process and itself evaporates (smell of NH3 ). The zinc is oxidised to ZnO. 

Mam heterogeneous processes such as dissolution of minerals, formation of he solid phase (precipitation, nucleation, crystal growth, and biomineraliza-r.on. redox processes at the solid-water interface (including light-induced reactions), and reductive and oxidative dissolutions are rate-controlled at the surface (and not by transport) (10). Because surfaces can adsorb oxidants and reductants and modify redox intensity, the solid-solution interface can catalyze rumv redox reactions. Surfaces can accelerate many organic reactions such as ester hvdrolysis (11). 

In other redox, homogeneous catalytic reactions, palladium ions catalyze propylene oxidation to acetone 306). The Rashig process 307) is based on benzene oxidation with air in the presence of cupric and ferric chlorides. Toluene and xylene oxidize in solution containing organic salts of Co, Mn, and Mo 308,309). It is interesting to note that in some cases, reoxidation of the active metal ion to its original valence is assumed slow, for example, Cu(I) to Cu(II) 310). It is conceivable that such steps could be assisted and accelerated electrochemically. Conventional processes, then, can provide a starting point for the study and development of new electrochemical redox processes. 

Aquatic microorganisms supply electrons through transplasmamembrane reductases to external solutes, enzymatically catalyze a variety of redox and other reactions on the cell surface, and are a source of dissolved extracellular enzymes. Both bound and dissolved extracellular enzymes are probably significant in maintaining a state of disequilibrium for some redox processes in natural waters and in accelerating some thermodynamically favorable reactions. In addition, as described for nickel and nitrogen in the urease example, these enzymes may also render the chemistry of the various components of aquatic systems highly interdependent. 

Pulse radiolysis has been employed successfully to resolve mechanisms of action of redox proteins and of electron transfer within their polypeptide matrix. The limitations on the use of this method, set by the requirement for expensive electron accelerators, are more than compensated for by experimental advantages, as illustrated by the results described in this chapter. Future applications to the study of engineered proteins and other model systems would certainly extend our understanding of both of these aspects of redox processes in biological macromolecules. 

The superoxide dismutation can be spontaneous or can be catalyzed and therefore significantly accelerated by the enzyme superoxide dismutase (SOD). The superoxide not only generates lydrogen peroxide (H2O2) but also stimulates its conversion into OH radicals, which are actually extremely strong oxidizers very effective in sterilization through a chain oxidation mechanism (to be especially discussed in Section 12.1.5). The H2O2 conversion into OH, known as the Fenton reaction, proceeds as a redox process provided by oxidation of metal ions (for example, Fe +) ... 

As discussed before in the case of nucleic acids the authors have also considered the incidence of the interfacial conformation of the hemoproteins on the appearance of the SERRS signals from the chromophores. Although under their Raman conditions no protein vibration can be observed, the possibility of heme loss or protein denatura-tion are envisaged to explain a direct interaction of the heme chromophores with the electrode surface in the case of the adsorl Mb. extensive denaturation of Cytc at the electrode appears unlikely to the authors on the basis of the close correspondence of the surface and solution spectra. Furthermore, the sluggish electron transfer kinetics measured by cyclic voltammetry in the case of Cytc is also an argument in favour of some structural hindrance for the accessibility to the heme chromophore in the adsorbed state of Cytc. This electrochemical aspect of the behaviour of Cytc has very recently incited Cotton et al. and Tanigushi et al. to modify the silver and gold electrode surface in order to accelerate the electron transfer. The authors show that in the presence of 4,4-bipyridine bis (4-pyridyl)disulfide and purine an enhancement of the quasi-reversible redox process is possible. The SERRS spectroscopy has also permitted the characterization of the surface of the modified silver electrode. It has teen thus shown, that in presence of both pyridine derivates the direct adsorption of the heme chromophore is not detected while in presence of purine a coadsorption of Cytc and purine occurs In the case of the Ag-bipyridyl modified electrode the cyclicvoltammetric and SERRS data indicate that the bipyridyl forms an Ag(I) complex on Ag electrodes with the appropriate redox potential to mediate electron transfer between the electrode and cytochrome c. 

Hydrogen peroxide catalyses the reaction between [Cr(H20)6] and edta and the Cr i-edta complex accelerates the decomposition of the substrate. In the absence of edta, the analysis of the kinetic trace suggests that Cr v and/or Cr are catalysts for the decomposition reaction in acidic media. The rate of oxygen evolution decreases markedly in the presence of edta, suggesting that the complexes are less active catalysts for the decomposition process. Any catalytic edta-containing species are considered as having chromium in an oxidation state greater than +3. A detailed reaction scheme for the redox process may be simplified to include the reactions (ox=oxidation, sub=substitution) ... 

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