The influence of redox potentials

The potential health benefits of plant polyphenols, such as the flavonoids, have been addressed over the years in the framework of the oxidative stress concept. Oxidative stress, as initially formulated by Sies (1985), refers to an imbalance in the dynamic equilibrium between oxidants and antioxidants that favors the formers, potentially leading to damage. As several diseases are supposed to mechanistically involve oxidative stress, such as atherosclerosis, ischemia-reperfusion injury, cancer, and neurodegenerative disorders (Halliwell Gutteridge, 1999), an impressive number of studies on the in vitro antioxidant activity of polyphenols have spotted these compounds as putatively useful to counteract the deleterious oxidant imbalance associated with disease. However, as will be discussed later, because of limited bioavailability and recent updated concept of oxidative stress, the putative activity of polyphenols as global antioxidants is a limited and simplistic view of polyphenol activity in vivo that can be scarcely supported by experimental data. 

In spite of the high number of chemical stmctures, in vitro studies suggest that the antioxidant properties of polyphenols, such as the flavonoids (FL-OH), appear to be accounted largely by a mechanism entailing the transfer of radical character with formation of an antioxidant-derived radical (aroxyl radical), thus neutralizing/scavenging the initial radical 

in addition to a suitable reduction potential to annihilate oxidizing radicals, a safe decay of the polyphenol-derived aroxyl radical, which strongly depends on their bimolecular disproportionation reaction and electron delocalization, is a characteristic that affords an adequate antioxidant activity. 

These chemical criteria have been subsequently substantiated in abundance in vitro experiments involving the antioxidant action of polyphenols against lipid peroxidation, protein oxidation, and DNA damage, as well as in models of neurodegeneration (Youdim et al, 2004 Frade et al, 2005). 

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Lissolo, T., Pulvin, S. and Thomas, D. (1984) Reactivation of the hydrogenase from Desulfovibrio gigas by hydrogen. Influence of redox potential. J. Biol. Chem., 259, 11725-9. 

Some authors claim that liposaccharides can depress the content of TNF-a and increase the activity of superoxide dismutase (SOD) and catalase, thus—via mediators—they can affect the immune system (Can et al. 2003). It has been demonstrated that the NF-p transcription factor, (highly sensitive to the redox potential in its environment), which regulates synthesis of many mediators—cytokines, associated with inflammatory condition and the phenomenon of adhesion of cells— becomes deregulated in old age. Defense functions in such cases (and primarily in arthritis and arthritis-related conditions) are said to be performed by antioxidants (including a-lipoic acid), which can modulate the activity of monocytes and inhibit changes caused by deregulating of the transcription factor NF-kB under the influence of redox conditions in elderly people (Lee and Hughes 2002). 

The analysis of redox potential modulation in heme proteins has been undertaken through both experimental and theoretical strategies. In particular, the use of simple models such as microperoxidase (MP) and the design of artificial heme proteins or biomimetics has allowed to single out the effect of different factors on redox potential [17, 18], There are a number of relevant interactions, listed on Table 4.2, related to the thermodynamics terms mentioned above and that have been shown to influence the redox potential of heme proteins and biomimetics. Although they may not entirely explain redox potential modulation, they are the best understood and several examples may be found in the literature. 

In this section, selected studies are presented in which self-assembled monolayers have been used to address topics such as transition-state structures and sequential electron transfer. These studies were selected because they address fundamental mechanistic processes. SAMs have also been used to investigate such basic electrochemical phenomena as the potential profile near an electrode [134, 135], interfacial capacitance [136], the influence of redox [134] or polarizable [137] moieties on double-layer structure and the behavior of ultramicroelectrodes approaching molecular size [138]. These important topics are beyond the scope of this chapter, and the interested reader is directed to the literature for more information. 

The influence of the polypeptide chain on the same active center is notorious, when observing the range of redox potentials available for [2 Fe-2 S] and... 

FIGURE 10.10 Influence of redox potential on reduction of Fe(III) oxide as indicated by the accumulation of Fe(II) in a flooded Crowley silt loam soil. (Redrawn from Patrick and Henderson, 1981.)... 

Figure 4.36 shows the influence of pH on the breakdown potential of nickel in alkaline solutions containing Cl ions, and it is apparent that the breakdown potential becomes more positive as the pH increases, i.e. breakdown is unlikely unless the solution has a very high redox potential. 

The influence of the N-bonded substituents R on the half-wave potentials can be described by a Taft relation, like is found for Mo, W and Au. The small value of p points to the dominance of metal orbitals in the redox orbital (5(5). The phenyl derivates do not fit this relation, probably because of a mesomeric influence. Here, however, the n-butyl and cyclohexyl also show small deviations, probably because of steric effects. 

A few thioether-ligated copper(II) complexes have been reported, however (cf. Section 6.6.3.1.2) (417) (essentially square planar), (418) (two crystalline forms one TBP and other SP),361 (419) (SP),362 (420) (SP),362 (421) (TBP),362 (422) (SP),363 (423) (SP),363 (424) (two independent complexes SP and octahedral),364 (425) (TBP).364 In the complexes (420) and (421), EPR spectra revealed that the interaction between the unpaired electron and the nuclear spin of the halogen atom is dependent on the character of the ligand present. For (424) and (425), spectral and redox properties were also investigated. Rorabacher et al.365 nicely demonstrated the influence of coordination geometry upon CV/Cu1 redox potentials, and reported structures of complexes (426) and (427). Both the Cu1 (Section 6.6.4.5.1) and Cu11 complexes have virtual C3v symmetry. 

We close this section with a note on the influence of pH on reduction potentials. Many redox reactions are pH-dependent, which can be understood with reference to the simple model in Figure 13.4, in which a redox compound in its oxidized state has a pK,t for proton dissociation that is different from (i.e., lower than) the corresponding value for its reduced state the positive charge of Xox is higher than that of Xred, so it is more difficult for Xox to accept a proton (i.e., its pKa is lower). The °(pH) is now... 

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