Oxidation reductive coupling

This program on the 40th year of plutonium chemistry reminds us anew that plutonium has an exceedingly complicated chemistry. The near identity of its oxidation/reduction couples makes all oxidation states accessible and provides a wealth of chemistry not exceeded in any other element. 

It is interesting to note that industry extracts many metals from sulfide ores by oxidation/reduction coupling (compare living processes)... 

In contrast to their instability free in solution, flavin semiquinones bound to proteins are generally (although there are exceptions) quite stable and can be generated in nearly quantitative yields. This stability results from thermodynamic considerations in which the oxidized/semiquinone oxidation-reduction couple is usually more positive and well separated from the semiquinone/hydroquinone couple. In addition to thermodynamic stabilization, kinetic stabilization of protein-bound semiquinones has also been observed and will be discussed in more detail in subsequent portions of this article. 

Photogalvanic cell An electrochemical ceU in which current or voltage changes result from photochemically generated changes in the relative concentrations of reactants in a solution phase oxidation-reduction couple. Compare photovoltaic cell. 

Methods presently employed for obtaining correctly refolded proteins from inclusion body preparations are often all-or-none propositions. They typically consist of denaturant solubilization, in urea or guanidine, followed by dilution or dialysis (2). Recovery of native activity or structure may be aided by using additives (enzyme inhibitors, co-factors, oxidation-reduction couples, etc.), which act to stabilize the native-state protein conformation. However, because such efforts are time-consuming and tedious, systematic examinations of solution conditions for protein folding/unfolding are rarely performed. 

The. neaning of the equilibrium constants of the oxidation-reduction couples can be made clear by the discussion of some examples. For the couple... 

At a reasonable efficiency of 50%, a AJBo of 0.334 is required. Only one oxidation-reduction couple in the noncyclic sequence shown in Figure 3-6 provides a AES of 0.33 or greater—that between cytochrome 6999 and cytochrome /. Thus, for each 4e excited (by 4 quanta), we obtain ... 

Figure 11.10 The theoretical Eh (mV) of some important oxidation-reduction couples at equal molar ion concentrations except as indicated below, at pH = 7 and 25 C. Cross-hatched area gives Eh s for 02(aq)/H20, where 02(aq) ranges from 8.25 to 0.01 mg/L. Other conditions are NOj/N2(aq) at N2(aq) = 14 mg/L (atmospheric Nj = 0.80 bar), NOj =...

Before proceeding to the next topic, we look at another version of artificial phosphorylation by chloroplasts in the dark, i.e., not driven hyphotoinducedelectron transfer. This new type oftwo stage phosphorylation, called dark oxidation-reduction coupled phosphorylation, was reported by Selman and Psczolla and may be considered as a variant of the postillumination or the acid-base transition types discussed above. The authors found that ATP formation in chloroplasts in the dark can be achieved by an artificial, transmembrane redox reaction using ascorbate as the reductant for ferricyanide trapped inside the chloroplasts, provided it is mediated by a redox carrier such as DAD, DCIP or PMS that liberates protons during its oxidation, as illustrated in the scheme in Figure 13. 

Fig. 12. Model for the dark oxidation-reduction coupled phosphorylation in sonicated chloroplast vesicles. M formedlator Ascfor ascorbate subscripts and for oxidized and reduced states Fe Cy and Fe Cy for ferri- and ferrocyanide, respectively.

The role of the oxidation-reduction couple in the catalytic decomposition of hydrogen peroxide,... 

In dealing with groups of such equilibria it is convenient to break them up into two half-reactions, or oxidation-reduction couples, which indicate the mechanism by which electrons are transferred from the reducing agent to the oxidizing agent. 

The electrons from each couple may be thought of as exerting an electromotive force, which is the oxidation-reduction potential of the couple. When equilibrium is reached in a solution containing a number of oxidation-reduction couples, the potentials of all couples must be equal, otherwise electrons would be transferred from one couple to another and further reaction would take place. Thus, if we can evaluate the dependence of oxidation-reduction potentials on concentration, we can determine equilibrium concentrations in solution of ions of mbced valence. 

Figure 11 Active species and cyclic intermediate involved in the oxidation-reduction coupling method.

More information concerning possible hydroxylating entities can be obtained by investigating the autoxidation of the metal ions. The nature of the reaction involved in the autoxidation depends strongly on the redox potentials, E°, for the various oxidation-reduction couples. It is probably significant that the values for the metal ions used here are as follows ... 

Qtdnhydrone Electrode. The quinhydrone electrode is an important hydrogen-ion electrode, and is perhaps typical of a whole class of such electrodes which function as pH sensors owing to a reversible organic oxidation-reduction pair involving protons. Quinhydrone (an equimolar compound of benzoquinone and hydroquinone) is only slightly soluble in water. The reversible oxidation-reduction couple... 

Films on Au and on A1 show approximately the same IR band evolution, whereas on Cu it is in part more pronounced, in part weaker (Fig. 28.5). Again, this specific behavior of fihns on Cu substrates could be due to a catalytic or an inhibiting effect of diffused Cu ions on the various aging mechanisms. Additionally, copper oxides can act as an effective oxidation-reduction couple, thus accelerating oxidation reactions [12]. 

Schemes Multielectron oxidation/reduction couple of SIR uranium system [104]...

Note that at the completion of hydrogenations with these catalysts, the used eatafysts that are filtered off should be kept moist with solvent or H2O as they may be pyrophoric when dry. TUso, with the catalysts that have been prepared using the formaldehyde (HCHO) procedure, the spent catalysts from reduction reactions in which MeOH is used as solvent are particularly flammable, possible because MeOH/HCHO form an oxidation-reduction couple since there is no guarantee that the catalyst is completely free from HCHO. 

Because electrons are transferred, not created or destroyed, in redox reactions, any such reaction may be considered to be a combination of two half reactions, each involving an oxidation-reduction couple. For example, in the reaction of Fe with Sn", ... 

If oxidation-reduction couples exist in the reaction, elections must be transferred. Electrochemical reactions, in which electrons flow through the solid from anodic to cathodic electrode surface sites, are best represented by anodic and cathodic half-cell reactions ... 

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