Topic 7.6 Midpoint Potentials and Redox Reactions
Redox reactions, midpoint potentials, and their relationship to the laws of thermodynamics were discussed in Chapter 2, found on this web site. These concepts are useful for our discussion of electron flow from H2O to NADP+ (see textbook Figure 7.21) and the interactions between the different electron carriers.
The midpoint potential (Em) is a measure of the tendency of a compound to take electrons from other compounds. A large positive midpoint potential means that the compound is a strong oxidant; a large negative value means that the compound is a strong reductant (in both cases relative to the standard hydrogen electrode).
Equilibrium constants can easily be predicted from midpoint potentials, in the same way that free energies were related to equilibrium constants (see textbook Appendix Equation A1.15). Midpoint potentials for many chemical and biochemical reactions have been measured and tabulated. The y-axis on the Z scheme in textbook Figure 7.19 shows midpoint potentials of the electron carriers, with negative values higher than positive ones. This choice makes reactions that are spontaneous (releasing free energy) appear "downhill" on the graph.
Knowledge of the midpoint potentials of the various electron carriers is important in establishing the pathway of electron flow in any biochemical electron transport system, such as those found in chloroplasts or mitochondria. Researchers make this measurement usually by carrying out a redox titration (Dutton 1978). They adjust, or poise, the sample at a particular redox potential, usually by adding small amounts of oxidants or reductants. Redox mediators, small molecules that permit rapid equilibration between the sample and the electrodes of the measurement system, must be included to ensure that the system is at equilibrium when the measurement is made. Several measurements are made at a variety of redox potentials. The sample is stirred in a special cell that contains platinum and reference electrodes, and chemical oxidants and reductants are added to adjust the redox potential, which is read with a voltmeter (Web Figure 7.6.A).
Web Figure 7.6.A
We can measure the extent of the redox reaction by following a particular property of the sample, usually absorbance, at each potential. In the example illustrated in Web Figure 7.6.B, the reduced form of the compound has an absorbance that decreases as the compound is oxidized. The fraction of reduced form at each potential is plotted against redox potential, and the midpoint potential (Em) is determined as the potential at which the compound is half oxidized and half reduced (Web Figure 7.6.C).
Web Figure 7.6.B
Web Figure 7.6.C