Mid-point potential

Figure 1. Electron transfer in Photosystem I. The values plotted in the vertical direction are the mid-point potentials, and the numbers next to each arrow are the half-lives for electron transfer. P700 is the primary reaction center PsaA and PsaB are transmembrane proteins and PsaC is a peripheral cytoplasmic component. Courtesy of Professor Parag Chitnis, Iowa State University.

FIGURE 7.7. Cyclic voltanunograms of the transfer of dodecanesulfonate ions at 0.5 mmol dm in W in the absence (solid line) and in the presence (dotted line) of sorbitan monooleate. Scan rate 50 mV s . The location of the mid-point potential is shown as the thick vertical line. Arrows a, b, c and d indicates the potentials stepped in chronoamperometric measurements (Figure 7.8). 

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. 

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... 

Table 1 lists some reduction potentials of the drugs mentioned above (or simpler models), including the potentials for oxygen. Two values of the latter are listed. The first is the standard potential, °(02(1 atm)/02 ), where oxygen is at its thermodynamic standard state of unit fugacity (essentially one atmosphere partial pressure) the second is the mid-point potential 7(02(1 mol dm" )/02 l, where both oxygen and superoxide are at unit concentration/activity [17]. 

Chemical properties of SIF Although concentrated samples of purified SIF have a yellowish color, SIF has only a broad nondescript absorption in the blue end of the spectrum. In its oxidized form, SIF has a shoulder at about 224 nm and a peak at about 264 nm (Fig. 3). Reduction of SIF by the addition of DTT (Fig. 3), NaBHif, or 2-mercaptoethanol, shifts the maximum to about 240 nm with a shoulder at about 288 nm, the Isobestic point. The reduced-oxldlzed difference spectrum shows an almost symmetrical peak at about 240 nm (Fig. 3 Insert). Because SIF cannot be reduced by ascorbate (not shown), we estimate that the mid-point potential for SIF is between 100 to 200 mV. 

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. 

Table 2.2 Mid-point potential (E versus Ag/AgCI (1 M KCI)), peak-to-peak separation ( Ep), reduction peak current (Ip) normalized to v and apparent heterogeneous electron transfer rate constant (kP pp) values estimated on the basis of Nicholson s treatment of DC cyclic voltammetric data obtained with a 3 mm diameter CC electrode for reduction of 2 mM [RufNH ii P in aqueous 1 M H2SO4.

The electrochemical reduction of C o results in the formation of a series of fulleride ions, [C o]" where n = 1-6. The mid-point potentials (obtained using cyclic voltammetry and measured with respect to the ferrocenium/... 

Gallagher and co-workers have characterized the reductase component by EPR and fluorescence spectroscopy. They showed that it contained one FAD and a [IFe-lS] " cluster. The FAD could be reduced in a two-step reaction to the fully reduced flavin. The optical spectrum of the semiquinone species was typical of a neutral flavin radical. The [2Fe-2S] + cluster could also be reduced by one electron equivalent to [2Fe-2S] +. Both paramagnetic species could be detected by EPR. It could also be shown by a combination of mid-point potential measurements and electron-transfer kinetics that this component could supply the energy required for the epoxygenation reaction. 

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

The mid-point potential for IRRS should not overlap the active potential zone of the analyte (ANS) under study. 

The solvent-Fc interaction is also present in ILs. This issue was addressed by studying the effect of ionic liquid structure on the mid-point potentials of Fc and DmFc in 11 different ILs as well as in dichloromethane with added IL as the... 

Table 3.4 Effect of diffusion coefficient inequality on the mid-point potential observed between...

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). 

---