Redox Potentials Give a Measure of Oxidizing and Reducing Strengths

Redox Potentials Give a Measure of Oxidizing and Reducing Strengths 

We now see that mitochondria contain a variety of molecules—cytochromes, flavins, ubiquinone, and iron-sulfur proteins—all of which can act as electron carriers. To discuss how these carriers cooperate to transport electrons from reduced substrates to 02, it is useful to have a measure of each molecule s tendency to release or accept electrons. The standard redox potential, E°, provides such a measure. Redox potentials are thermodynamic properties that depend on the differences in free energy between the oxidized and reduced forms of a molecule. Like the electric potentials that govern electron flow from one pole of a battery to another, E° values are specified in volts. Because electron-transfer reactions frequently involve protons also, an additional symbol is used to indicate that an E° value applies to a particular pH thus, E° refers to an E° at pH 7. 

Consider the transfer of electrons from a reduced molecule, Dred, to an oxidized acceptor, Aox. AG° for the reaction is proportional to the difference between the E° values of the two molecules (AE°). 

Here n is the number of electrons transferred from the donor to the acceptor, and S is the Faraday constant (23,060 cal volt -1 mol -1 or 96.5 kJ volt -1 mol -1). Note that the overall reaction is spontaneous (AG° negative) if AE° is positive, that is, if electrons move from the molecule with the more negative E° value to that with the more positive value. In other words, negative E° values are associated with strong reductants, and positive values with strong oxidants. Electrons flow spontaneously in the direction of more positive potential. With n = 2, a AE° of 0.1 V corresponds to a AG° of —4.61 kcal/mol. 

Equation (6) defines the difference between two E° values. To set the individual values, we need to choose a particular redox couple as a reference. The reference used most commonly by biochemists is the standard hydrogen half-cell, in which protons at pH 0 are reduced to H2 at a 

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A Bucket Brigade of Molecules Carries Electrons from the TCA Cycle to 02 The Sequence of Electron Carriers Was Deduced from Kinetic Measurements Redox Potentials Give a Measure of Oxidizing and Reducing Strengths... 

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