Estimated redox potentials

Iron-sulfur proteins can be observed by EPR spectroscopy, either in their oxidized or in their reduced state. As a method of observing iron-sulfur clusters, EPR is discriminating but not particularly sensitive lack of a detectable EPR signal cannot be taken as evidence of absence. However, a positive EPR signal is good evidence for the intactness of an iron-sulfur cluster in a protein. Moreover, EPR can be used to follow reduction of the clusters and, by use of mediated electrochemical titrations, to estimate redox potentials. 

Figure 4. Estimated redox potentials of possible photocorrosion processes in strontium titanate at pH = 0 (V vi. SCE)...

Unfortunately, the redox potential of the Pt4 + /3+ couple is not known in literature. Although some stable Ptm compounds have been isolated and characterized (37), the oxidation state III is reached usually only in unstable intermediates of photoaquation reactions (38-40) and on titania surfaces as detected by time resolved diffuse reflectance spectroscopy (41). To estimate the potential of the reductive surface center one has to recall that the injection of an electron into the conduction band of titania (TH) occurs at pH = 7, as confirmed by photocurrent measurements. Therefore, the redox potential of the surface Pt4 + /3+ couple should be equal or more negative than —0.28 V, i.e., the flatband potential of 4.0% H2[PtClal/ TH at pH = 7. From these results a potential energy diagram can be constructed as summarized in Scheme 2 for 4.0% H2[PtCl6]/TH at pH = 7. It includes the experimentally obtained positions of valence and conduction band edges, estimated redox potentials of the excited state of the surface platinum complex and other relevant potentials taken from literature. An important remark which should be made here is concerned with the error of the estimated potentials. Usually they are measured in simplified systems - for instance in the absence of titania - while adsorption at the surface, presence of various redox couples and other parameters can influence their values. Therefore the presented data may be connected with a rather large error. 

Methods to Measure or Estimate Redox Potential of Heme Peroxidases. 71... 

Figure 4.1 shows the correlation between measured redox potentials of Fe(III)/ Fe(II) presented in Table 4.3 and estimated redox potentials of Compound II/Fe(III) presented in Table 4.4. In spite of the different methodology, there is a clear tendency for the more oxidant enzymes to present more positive Fe(III)/Fe(II) redox potential values. 

Table 4.4 Estimated redox potentials of heme peroxidases...

In general, the redox potential data for Compound I and Compound II is unavailable. Direct methods such as cyclic voltammetry cannot be used due to the short lifetime of the activated intermediates and irreversibility of the reactions to generate them. The methods that may be found in the literature to estimate redox potential of the catalytic species, Compound I and Compound II, are redox titrations [64, 97, 98], spectroelectrochemistry [99], and catalytic-based estimation [53]. 

As smnmarized above, the primary photoprocess generates two sruface redox centers. The oxidative one can be described as a kind of Cl/Cl redox pair weakly coordinated to a metal center. It is reasonable to assiune that this oxidant is weaker than the free Cl/Cl pair (E°cyc =2.6 V, value for aqueous solution (38), but stronger than the couple t Cl2/Cl (E° = 1.3 V (39) and the oxidation potential of 4-CP (1.18 V). Further, the potential has to be lower than 2.4 V, the estimated redox potential of the OH/OH couple, because no hydroxyl radical formation could be observed in the presence ofbenzoic acid and tetranitromethane as OH and electron acceptor, respectively (18). Thus, the potential of the oxidative center should be in the range of 1.3-2.4 V. 

In general, the thermodynamics of free radicals has been, up to now, almost unknown due to their extremely high reactivity and electroactivity. For clarification, several methods were proposed [225,226] from anodic half-wave potentials of carbanions and organo-lithium compounds. Redox catalysis may strongly help to estimate redox potential of a certain number of radicals generated electrochemically. Thus, as seen previously in Scheme 4 and in Scheme 6, in the absence of other side reactions, there are two main different ways... 

Fu et al. [79] have used MP2 and DFT calculations to estimate the gas-phase acidities and DMSO pK, s of a diverse set of 105 organic acids. The gas-phase acidities were estimated with a precision of 2.2-2.3 kcal/mol and the DMSO pK s to within 1.7-1.8 pK units. These workers later developed a protocol that estimated the pK s of more than 250 structurally unrelated compounds in DMSO to within 1.4 pK units [495]. They were able to employ this protocol to estimate redox potentials and bond dissociation enthalpies for the compounds. This team also examined the pK values of C-H bonds in aromatic heterocyclic compounds in DMSO, reaching a precision of 1.1 pK units [496]. 

TABLE 1. Estimated redox potentials for dye ground states and excited states at pH 7. (All values in volts versus NHE). 

TABLE 14.1 The estimated redox potentials of electron-transport carriers in thylakoid membranes... 

The term aggressive is often used to imply some approximately quantitative estimate of the likelihood of corrosion and depends on measuring factors such as soil water (resistivity), pH, redox potential, salt concentrations and bacterial populations in order to establish criteria for the prediction of corrosion rates . Similar measurements for predicting corrosion... 

Thus in order to obtain the value of k it is absolutely necessary to know the standard cathodic redox potential. In the case of MeS02Ph, very fast voltammetric measurements at high sweep rates (1000 Vs 1) permit one to reach the reversible step of the sulphone and give °Meso2ph= — 1.85 V. Hence, the value k =0.9 x 105 s 1 may be estimated for the... 

The midpoint redox potentials were estimated to be +230 mV (pH = 8.6) or +281 mV (pH = 7.0) for the Rd-like centers, and +339 and +246 mV (pH = 7.0) for the diiron-oxo center 38, 43). This is a surprising observation, since the normal redox potential of Rd centers is about 0 mV. All spectroscopic evidence points to the fact that the monomeric iron centers present in Rr are virtually identical to the ones found in Rd. Hence, it is reasonable to assume that the first coordination sphere of these centers cannot be held responsible for the 250 mV difference in the midpoint redox potentials. 

Table IV lists the redox potentials of conjugated ferrocene oligomers (mainly dimers with a single bridge). Potential values are denoted against different reference electrodes as given in the references. The values can be primarily compared using the relationship mentioned in the footnote of the table, although care should be taken with some errors derived from junction potentials which depend on experimental conditions. There have been several reports on the quantitative estimation of structural factors affecting internuclear electron delocalization.

Studies of ferredoxin [152] and a photosynthetic reaction center [151] have analyzed further the protein s dielectric response to electron transfer, and the protein s role in reducing the reorganization free energy so as to accelerate electron transfer [152], Different force fields were compared, including a polarizable and a non-polarizable force field [151]. One very recent study considered the effect of point mutations on the redox potential of the protein azurin [56]. Structural relaxation along the simulated reaction pathway was analyzed in detail. Similar to the Cyt c study above, several slow relaxation channels were found, which limited the ability to obtain very precise free energy estimates. Only semiquantitative values were... 

We now turn to the known or estimate concentrations of elements in the other compartments of eukaryotes that show a different distribution of elements, free and in proteins, from that in the cytoplasm. Most of these compartments, vesicles such as the endoplasmic reticulum, operate at a higher redox potential than the cytoplasm and several at a lower, more acidic, pH. Some of the element distributions there, and their functions are given in Table 7.5. Note, as mentioned before, that... 

According to Hercules 5> a measure of the relationship between direct excitation of the first excited singlet state by radical-ion recombination and triplet-triplet annihilation is the entropy factor FAS, estimated to be on average 0.2 eV. The enthalpy of the radical cation-radical anion recombination can be measured as the difference between the redox potentials 1/2 Ar—Ar (oxidation) and 1/2 Ar—Ar<7> (reduction). This difference has to be corrected by the entropy term. If this corrected radical-ion recombination enthalpy is equal to or larger... 

Using linear regression, it is possible to estimate the protonation constants of the Fe(II) complexes of siderophore complexes where the redox potentials have been measured over a range of pH values (59). This also explains the variation in reversibility of reduction as the pH changes, as the stability of the ferro-siderophore complex is much lower than the ferric complex, and the increased lability of ligand exchange and increased binding site competition from H+ may result in dissociation of the complex before the iron center can be reoxidized. 

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