Mid-point redox potential

In the past ten years a large number of novel iron-sulphur proteins have been discovered, largely as a result of ESR measurements. Indeed ESR is the technique of choice for their identification and has made important contributions to knowledge of their structure and function. These proteins are now known to occur widely in animals, plants and bacteria and play important roles in respiration, photosynthesis, nitrogen fixation, hormone synthesis and sulphur and carbon metabolism. They function as electron carriers and most, though not all, have negative mid-point redox potentials at pH 7. 

A simplified representation of the mitochondrial respiratory chain in terms of the oxidation of NADH and succinate by oxygen is illustrated in Figure 4.9 together with Table 4.2 showing the twelve iron sulphur proteins identified, their g values and mid-point redox potentials. The g values found are consistent from a variety of different preparations, though some changes in line share are found. The mid-point potentials are variable, with values from -20 mV to -265 mV for centre N-2. 

Fig. 19.16 Electron flow in phoiosystems I and II ( Z-scheme"). Venical axis gives mid-point redox potential with reducing species (top) and oxidizing species (bottom).

The herbicidal effect of paraquat is attributable to the formation of superoxide anion (02 ). Superoxide anion is very toxic compound and is formed by the reaction of oxygen with paraquat radical (paraquat ). Plants, algae, and cyanobacteria have ferredoxin-NADP reductase to form NADPH for the reduction of carbon dioxide (see below). The chemolithoautotrophs also have NAD(P) (NAD and NADP) reductase to form NAD(P)H for the reduction of carbon dioxide. Paraquat [mid-point redox potential at pH 7.0 (Emj 0) = -0.43 V] radical is produced when paraquat is reduced by the catalysis of ferredoxin-NAD(P) reductase or NAD(P) reductase, which catalyzes the reduction of many compounds with of around -0.4 V. Although the aerobic organisms (and even many anaerobic organisms) have superoxide dismutase (SOD) which detoxifies superoxide anion in cooperation with catalase [ascorbate peroxidase in the case of plants (Asada, 1999)], the anion accumulates in the organisms when it is over-produced beyond the capacity of SOD. 

As one may expect from the peroxidase reaction mechanism described in Eqs. (2-4), the reactivity of the enzyme intermediates towards a particular substrate may be estimated a priori on the basis of the thermodynamic driving force of these two electron-transfer reactions, which is directly related with the difference between the oxidation/reduction potentials of both the enzyme active intermediates (i.e.. Col and Coll) and the substrate radicals. Thus, the thermodynamic driving force for the reaction of Col (or Coll) with the reducing substrates is the difference between the mid-point potentials of the CoI/CoII (or CoII/Felll) and the substrate radical/substrate (R, Hr/RH) redox couples ... 

The mid-point potential of the redox couple is given by the Nemst equation, and is therefore dependent on the relative concentrations of iodide and iodine. The concentrations of these species required for efficient device function are in turn constrained by kinetic requirements of dye regeneration at the working electrode, and iodide regeneration at the counter electrode, as discussed below. Typical concentrations of these species are in the range 0.1-0.7 M iodide and 10-200 mM iodine, constraining the mid-point potential of this electrolyte to -0.4 V vx. NUE. It should... 

In the case of a molecule with multiredox centres, such as cytochrome C3, two types of formal potential (mid-point potential) can be defined the macroscopic formal potential (formal potential of the molecule), where each redox site is indistinguishable by using the given technique and the microscopic formal potential (formal potential of each redox site), where each redox site is distinguishable. A tetra-heme protein, such as cytochrome C3, has five macroscopic oxidation states the fully oxidized (Sq), the one-electron reduced (SJ, two-electron reduced (S2), three-electron reduced (S3), and four-electron or fully reduced (S4) states [123], as shown in Fig. 10. 

For a reversible (electrochemically fast) redox couple, the mid-point between the anodic and cathodic peak potentials Epa and pc is the so-called half-wave potential 1/2, which is related to the formal potential E for the Ox/Red couple by the simple expression... 

Table 3.1 Mid-point potentials of the ferrocene/ferrocenium redox process against different Ag-based reference electrodes...

The mid-point potential for the couple (Eq. 3.6) was found to be considerably solvent dependent and can not always be determined accurately at room temperature, mainly due to the overlap and interaction of this process with the electrochemical response of the organic solvent used [62]. Thereby, only the Cc redox couple (Eq. 3.5) has been extensively used in organic solvents to provide a known and stable reference point. As an extrapolation of the concept, it is also used in IL systems. Surprisingly, little is known about the solvation effect of either different organic solvents with added supporting electrolytes or IL structure on the Cc formal potential. 

The comparison of data reported in Tables 3.2,3.3, and 3.5 is of significant value as they suggest that the mid-point potentials of both Cc and Fc couples are dependent on the solvation properties of organic solvents (with added supporting electrolyte) as well as the solvation properties of ILs, particularly under the assumption that DmFc redox couple is a less solvent dependent process [57]. Consequentiy, potentials reported versus Cc and Fc might need to be corrected using the reported values in case of comparison of experimental data obtained in different organic solvents (Tables 3.2 and 3.5) and/or ILs (Table 3.3). 

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