Reduced/oxidized redox pairs, changes

Measurements can be done using the technique of redox potentiometry. In experiments of this type, mitochondria are incubated anaerobically in the presence of a reference electrode [for example, a hydrogen electrode (Chap. 10)] and a platinum electrode and with secondary redox mediators. These mediators form redox pairs with Ea values intermediate between the reference electrode and the electron-transport-chain component of interest they permit rapid equilibration of electrons between the electrode and the electron-transport-chain component. The experimental system is allowed to reach equilibrium at a particular E value. This value can then be changed by addition of a reducing agent (such as reduced ascorbate or NADH), and the relationship between E and the levels of oxidized and reduced electron-transport-chain components is measured. The 0 values can then be calculated using the Nernst equation (Chap. 10) ... 

In a complexation reaction, a Lewis base donates a pair of electrons to a Lewis acid. In an oxidation-reduction reaction, also known as a redox reaction, electrons are not shared, but are transferred from one reactant to another. As a result of this electron transfer, some of the elements involved in the reaction undergo a change in oxidation state. Those species experiencing an increase in their oxidation state are oxidized, while those experiencing a decrease in their oxidation state are reduced, for example, in the following redox reaction between fe + and oxalic acid, H2C2O4, iron is reduced since its oxidation state changes from -1-3 to +2. 

The prototype of low M,. TxrRs is the enzyme from E. coli (see Williams " for review). The mechanism of this enzyme involves large conformational changes of the protein. In one conformation, the redox active enzymatic disulfide is near FAD where it can be attacked by the reduced flavin, forming the oxidized flavin-thiolate charge transfer. The two protein domains then rotate 66° into a conformation moving the dithiol pair to the hydrophilic surface of the enzyme where it can reduce the disulfide of thioredoxin. In this conformation, NADPH can also bind and reduce the flavin. " ... 

To understand this phenomenon, we observe that a redox reaction away from equilibrium plus a pair of electrodes constitutes a galvanic cell. The potential difference between the platinum and the reference electrode can cause a current to flow. The current will cause an electrochemical reaction at the platinum electrode, locally changing the ratio of oxidized to reduced ions. The concentration at the electrode now differs from that of the bulk solution, though homogeneity will be restored by stirring and/or diffusion. An erratic signal resulls because the mixing is neither perfect nor instantaneous. 

Photometric redox titrations can be carried out in a so-called self-indicator system i.e. the color of a solution changes during the titrations. The analytical wavelength should be matched to the pair of the oxidant and the reductant, which is the main disadvantage of this method. Another method is to use an indicator, the oxidized form of which has a different color than the reduced one. This can be utilized in the fiber optic sensor. One indicator can be used for the titrations with various oxidants and reductants. 

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