Oxido-reduction potential

Relatively far from the present topic and well known, the on-line measurement of the physical and aggregate properties of wastewater does not present any problem. Conductivity, temperature, turbidity and oxido-reduction potential (ORP) are easily measured by well-designed sensors, because these parameters are also used for treatment process control. In practice, turbidity is more used for the treatment of natural water, and ORP for the biological treatment of wastewater. However, conductivity and temperature are often monitored at the same time as the other parameters in this section. 

As the standard oxido-reduction potential of copper is lower than that of the second metals used in this work we suggest that an electronic transfer from copper to M (Ir, Rh, Ru, Pd, Pt, Au) could generate electrophilic copper M - Cu+ able to catalyze the cyclodehydration according to the scheme ... 

The catalytic properties of copper during polyols conversion in aqueous phase may be drastically modified by some additives. Metals having a standard oxido-reduction potential higher than that of copper (Ir, Rh, Ru, Pd, Pt, Au) can be deposited on it by oxido-reduction reaction. The first atoms of second metal deposited exchange with hydroxylated... 

These can be formed in a variety of ways by elimination of water (formation of ester, glucoside or peptide linkages), by removal of hydrogen (formation of disulphide bridges, methylene bridge, etc.), they are influenced by the oxido-reduction potential of the system. 

In addition, it was confirmed that major electron transfer from reduced cytochrome c to oxidized cytochrome a occurred via cytochrome c rather than directly, because reduced cytochrome Cx can reduce oxidized cytochrome c instantaneously. Furthermore, it was ascertained that reduced cytochrome c was rapidly oxidized by a purified preparation of oxidized cytochrome a and that the latter component was reduced under anaerobic conditions. Although it was established that the normal oxido-reduction potentials of the cytochrome components are in the following order a > c > Cj > b, Yakushiji and Okunuki proposed the following electron transferring system, to explain the discrepancy between Eqs. (1) and (2) ... 

Other antioxidant species are synthesized by cells like uric acid, ubiquinol or thiols (cystein, homocystein, etc.). In addition, many compounds found in food display antioxidant properties retinol (vitamin A) and its precursor /(-carotene, and polyphenols (flavonoids, etc.). Figure 8.2 shows the apparent standard potential of some LMWA and ROS explaining the spontaneous oxido-reduction reactions at the origin of the antioxidant protection system. 

The cofactors of both xanthine and aldehyde oxidases belong to the LMoVI(S)(0) subfamily (see Section IV). However, inactive dioxo forms, LMovi(0)2, of both xanthine and aldehyde oxidase are known. These dioxo forms do not catalyze oxidation of the respective substrates of these enzymes. The Mov/Molv redox potential for the inactive bis(oxido) form of xanthine oxidase differs from the oxido-sulfido form by -30 mV (bovine xanthine oxidase) and -lOOmV (chicken liver xanthine oxidase) [91]. Although the difference is small, given the xanthine/uric acid reduction potential (-360 mV), it is possible that the Mov/MoIV couple (-433 mV) of the chicken-liver xanthine oxidase bis(ox-ido) form impedes the effective oxidation of xanthine for redox reasons alone. However, the bis(oxido) form of bovine xanthine oxidase (with a reduction potential of -386 mV) should be able to oxidize xanthine, since the redox potential, and hence the thermodynamic driving force, is sufficient for activity [91,92,99]. As substrate oxidation does not occur, the chemical differences between the bis(oxido) and oxido-sulfido (Movl) forms must be critical to the dramatic difference in activity (see Section VI.E.l). 

For each of the redox couples, the potential can be calculated using the Nernst equation. This equation correlates Gibb s free energy, known as AG, and the electromotive force provided by an oxido-reduction reaction (such a reaction acts as a galvanic cell). Given the following equation due to a chemical reaction ... 

Figure 14.23 Intensity of the N=N peak as a function of the potential and of the number of oxido/reduction cycles with a reverse potential of 800 mV/SCE in a pH 1 solution (a) bvefore cycling (b) after 277 cycles (c) after 1583 cycles.

The analogy of a series of oxido-reduction cells was used, whereby a high electromotive force in one cell could drive a second cell backwards (see above). The situation regarding the action of ATP was seen as exactly analogous to the action of electrons in the oxido-reduction cell system, " in that ATP acted as an energy currency, energetically linking systems of widely different potential. 

The concept of oxido-reduction can be applied in inorganic chemistry as well as in organic chemistry. Otherwise, the majority of cellular synthesis is issued from redox reactions, which are only possible in media endowed with well-determined redox potential values. Furthermore, biological cells exhibit a noteworthy constancy in their redox potential courtesy of the redox buffers present in them. We can affirm that aU biochemical syntheses are governed by the concepts of proton and electron exchanges. 

Furthermore, we have specified the flavin redox states by the terms flavoquinone, flavosemiquinone and flavohydroquinone in view of the polarographic reversibility, i.e. the quinonoid character, of the oxido-reduction. These terms are definitely superior to oxidized flavin, flavin radical, fully reduced flavin which are most widely used in biochemistry. In this context, we have tried to outline chemical evidence for the existence and potential biological relevance of several dihydroflavin isomers. The terms fully reduced and certainly leucoflavin should be dropped definitively in favour of dihydroflavin . Leucoflavin is in fact not leuco (i.e. colourless) but even more deeply (though, of course, less intensely) coloured (79) than oxidized flavin. Hence, this term should be abandoned for 1,5-dihydroflavin , which is identical with the product of the reversible reduction of flavoquinone, namely flavohydroquinone. 

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