Reducing powers of redox couples

Reducing powers of redox couples 300 Reduction potential(s) definition of 300 table, 301... 

Negative values of ° (such as a Na = —2.71 V) indicate that the reduced form of the couple will react with protons to form hydrogen gas, as in Equation (7.35). The more negative the value of e, the more potent the reducing power of the redox state, so for the magnesium couple is —2.36 V, and F K = —2.93. 

The iron-sulfur cluster is often present as the prosthetic group in oxidoreductases where the couple of the substrate is below 0 mV. This reflects the reducing power of the iron-sulfur cluster. There are, however, a small number of iron-sulfur-containing enzymes that function in the positive redox range. These are often associated with bacterial nitrate and nitrite metabolism. The least complicated enzymes contain no additional cofactor, while others contain a number of additional cofactors. 

However, for Pt(0) electrochemically deposited onto p-type Si/SiOx photocathodes in amounts of 10 8 mol/cm2, we find that the output depends on pH such that a lower efficiency is found at the low pH s, Table IV.For the p-type Si/Si0x/Pt(0) photocathodes the pH-efficiency data demand a different mechanism for improvement of efficiency compared to that for p-type Si/Si0x/[(PQ2+ 2Cl- Pt(0))n]8Urf. The key fact is that the efficiency appears to peak at a particular pH for the redox polymer system, consistent with the pH independent reducing power of the redox couple. For the case of Pt(0) on the p-type Si/SiOx the efficiency rises from low to high pH and does not show a peak. The fact that there is a pH dependence at all indicates that the photosensitive interface is not completely buried. The Pt(0) can be regarded as a catalyst for the reactions of the excited electrons and does not completely dominate the behavior of the interface with respect to photovoltage. 

To this aim, a long-lived excited state of P is highly desired in order to slow the rate of eq. 5, and the reducing power of P should be high enough to enable, from a thermodynamic point of view, the process described in eq. 4. Moreover, (iii) the oxidized sensitizer P must have a suitable potential to allow the reactions in eqs. 8-11 to occur. In other words, the oxidation potential of the redox couple P /P (Box (P /P) ) must be more positive than the one of the redox couple (which of course... 

The more negative the potential, the greater the electron-donating power of the oxidation half-reaction and therefore the more strongly reducing the redox couple (that is, the stronger the tendency for the half-reaction to occur as an oxidation). 

The characteristics of redox reactions in non-aqueous solutions were discussed in Chapter 4. Potentiometry is a powerful tool for studying redox reactions, although polarography and voltammetry are more popular. The indicator electrode is a platinum wire or other inert electrode. We can accurately determine the standard potential of a redox couple by measuring the electrode potential in the solution containing both the reduced and the oxidized forms of known concentrations. Poten-tiometric redox titrations are also useful to elucidate redox reaction mechanisms and to obtain standard redox potentials. In some solvents, the measurable potential range is much wider than in aqueous solutions and various redox reactions that are impossible in aqueous solutions are possible. 

All parasitic flatworms capable of anaerobic metabolism favour malate as the primary mitochondrial substrate and the oxidative decarboxylations of first malate and then pyruvate generate intramitochondrial reducing power in the form of NADH (Fig. 20.1). In contrast, the pathways used to reoxidize intramitochondrial NADH are quite diverse and depend on the stage or species of parasite under examination, but in all cases, redox balance is maintained and electron-transport associated ATP is generated by the NADH-reduction of fumarate to succinate. In the cestode, hi. diminuta, succinate and acetate are the major end products of anaerobic malate dismutation and are excreted in the predicted 2 1 ratio. In the trematode F. hepatica, succinate is then further decarboxylated to propionate with an additional substrate level phosphorylation coupled to the decarboxylation of methylmalonyl CoA. F. hepatica forms primarily propionate and acetate as end products, again in a ratio of 2 1 to maintain redox balance. 

The evolution of photosynthetic oxidation of sulfur compounds permitted the development of the full microbial sulfur cycle in sulfureta. In this cycle, some bacteria and archaea reduce oxidized sulfur compounds, pumping them downward in the microbial mat, while other bacteria reoxidize them photosynthetically. The development of this cycle, coupled with the use of stored sulfur as a redox bank balance that could be exploited either way the redox budget swung during tidal and diurnal cycles, would have greatly expanded the thermodynamic power of the biosphere. 

An illustrative experimental example of a cell exposed to an oxidant was carried out by Kirlin et al. (1999). When they used HT29 cells (colon adenocarcinoma) and exposed them to sodium butyrate, it was found that there was an oxidant reaction that caused a drop from 260 to —200 mV in h. The 60 mV decrease resulted in a 100-fold change in protein dithiols disulfide ratio. A correlation was noted between h, GST, and NADPH quinone reductase Kirlin further indicated that the h provides two additional pieces of information (1) in reactions that use GSH as a reductant to maintain protein thiol/disulfides in their reduced form, it is an indicator of the reducing power quantitatively and (2) if the redox state is controlled by a GSH redox couple, it is an indication of the functional state of the protein (Kirlin et al., 1999). 

The versatility of this mode of operation has made it extremely powerful for fabrication of microstructures. In the feedback mode an ultramicroelectrode is held close above a substrate in a solution containing one form of electroactive species, either reduced or oxidized, that serves as a mediator (Fig. 1). The latter is usually used both as a means of controlling the distance between the UME and the surface and to drive the microelectrochemical process on the surface. This poses a number of requirements that must be taken into account when configuring the system. The basic limitation stems from the requirement that the electrochemical reaction be confined only to the surface. This means that the electroactive species generated at the UME will react with the surface or with other species attached to it. In addition, it is preferable in most cases that the redox couple used should exhibit chemical and electrochemical reversibility, so that it is effectively regenerated on the surface. The regeneration of the redox couple on the surface is required for controlling the UME-substrate distance. Finally, the thermodynamics and kinetics of the electrochemical process on the surface will dictate the choice of the redox couple introduced. 

The more negative the standard electrode potential, of a redox couple M (aq)/M(s), the more powerful a reducing agent is that metal. This means it is a more reactive metal because it loses electrons more easily. We can arrange the metals in order of reducing power, producing an activity series of metals (Table 7.2). The metals at the top of the series are more reactive than those below. 

Oxidizing and reducing power is indicated quantitatively by the redox potential or standard electrode potential, E. Redox potentials are normally expressed as reduction potentials. They are obtained by electrochemical measurements and the values are referred to the H /H2 couple for which E" " is set equal to zero. Thus increasingly negative potentials indicate increasing ease of oxidation or difficulty of reduction. Thus in a redox reaction the half... 

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