Conjugate redox pair

Do not mistake the above reaction for an acid dissociation the ID arises from the removal of a hydrogen atom, H+ + e. ) AH2 and A together constitute a conjugate redox pair (A/AH2), which can reduce another compound B (or redox pair, B/BH2) by transfer of hydrogen atoms ... 

When two conjugate redox pairs are together in solution, electron transfer from the electron donor of one pair to the electron acceptor of the other may proceed spontaneously. The tendency for such a reaction depends on the relative affinity of the electron acceptor of each redox pair for electrons. The standard reduction potential, E°, a measure (in volts) of this affinity, can be determined in an experiment such as that described in Figure 13-14. Electrochemists have chosen as a standard of reference the half-reaction... 

Many half-reactions of interest to biochemists involve protons. As in the definition of AG °, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E °, the standard reduction potential at pH 7. The standard reduction potentials given in Table 13-7 and used throughout this book are values for E ° and are therefore valid only for systems at neutral pH Each value represents the potential difference when the conjugate redox pair, at 1 m concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table 13-7 that when the conjugate pair 2ET/H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell the measured E ° for the 2ET/H2 pair is -0.414 V... 

For each of the three reactions catalyzed by the NADH dehydrogenase complex, identify (a) the electron donor, (b) the electron acceptor, (c) the conjugate redox pair, (d) the reducing agent, and (e) the oxidizing agent. 

NADH is the electron donor (a) and the reducing agent (d) E-FMN is the electron acceptor (b) and the oxidizing agent (e) NAD+/NADH and E-FMN/E-FMNH2 are conjugate redox pairs (c). 

As an example we use mitochondria (Fig. 17.6). These are small corpuscles that exist in large quantities within cells. They possess an exterior and an interior membrane where the enzymes cytochrome by c, Ci, a and a3, ATPase, and NADH are located. The interior membrane, of non-repetitive structure, contains 80 per cent protein and 20 per cent lipid. The Gibbs free energy variation of the conjugated redox pairs is given by the formal potential, according to... 

The electron-transport reaction for the NAD+/NADH conjugate redox pair is ... 

The number of electrons transferred need not be one, and indeed, for many biochemical processes, are frequently two. A conjugate redox pair is analogous to a conjugate acid—base parr (HA and A ) however, unlike the latter, the two half-reactions of a redox reaction, like (2) and (3) above, can be physically separated in an electrochemical cell. In the example shown in Figure 2.5, the half-cell undergoing oxidation passes the electrons... 

This equation indicates that Cu+ is the electron donor. (Together Cu+ and Cu2+ constitute a conjugate redox pair.) As Cu+ loses an electron, Fe3+ gains an electron to form Fe2+ ... 

The tendency for a specific substance to lose or gain electrons is called its redox or reduction potential. The redox potential of a conjugate redox pair is measured in an electrochemical cell against a reference standard, usually a standard hydrogen electrode. The redox potential of the standard hydrogen electrode is 0.0 V at 1 atm, by definition. Substances with a more negative... 

Redox reactions can be studied by using electrochemical cells (Fig. 2-9). In contrast to the components of a conjugate acid-base pair, which cannot exist separately, the components of a conjugate redox pair can exist separately. Such a physical separation is achieved in an electrochemical cell. 

Oxidizing and reducing agents function as conjugate redox pairs, corresponding to the conjugate nature of acid-base pairs ... 

Comparison of the half-wave potentials of the oxidizable and reducible forms of the redox reactants with the potentiometrically determined value of the standard redox potential is the simplest method for proof of reversibility and agreement between these two sets of data provides strong support for reversibility of the process being studied. However, in many polarographic reactions, it is found that one form of the conjugate redox pair is not sufficiently stable to be prepared or to permit determination of E% potentiometrically. In some of these cases, auxiliary methods can be used, particularly where the reactive species is relatively stable when formed at the surface of the mercury electrode. Such methods include the commutator method, an anodic stripping technique which has recently been reviewed by Barendrecht, and oscil-lopolarographic and cyclic voltammetric techniques. 

The mitochondrial electron transport chain (ETC) or respiratory chain comprises a series of membrane-bound redox-aetive intermediates including flavoproteins, quinones, cytochromes, and iron-sulfur elusters [9] (Fig. 1). The latter facilitate the thermodynamically controlled transfer of electrons from conjugate redox pairs of low redox potential (E°) [e.g., —320 mV for reduced nicotinamide-adenine dinueleotide/oxidized nieotinamide-adenine dinucleotide (NADH/ NAD )] to the final electron donor, O2 (with a high redox potential of -1-820... 

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