Thermodynamics of electron transfer reactions

The thermodynamic parameter of central importance to the characterization of electron transfer processes is the reduction potential, E°. The reduction potential provides a measure of the tendency of an oxidized molecule to become reduced. Values of E° are typically expressed in units of either volts (V) or millivolts (mV). For example, the textbook E° value of cytochrome c (at 298 K) is about -1-250 mV (70). It is unlikely, however, that an actual laboratory measurement of the E° of cytochrome c would give this value. Reduction potentials (in common with all thermodynamic quantities) are constant only for the specific conditions under which they were determined. Changes in the protein and solution environment may alter E° from the values measured under different experimental conditions. These variations provide an opportunity to explore how E° values of a redox group can be modulated by the protein and surrounding solution. 

Qualitatively, the higher the value of E°, the greater is the ability of a molecule to accept electrons (75). While absolute values of E° cannot be determined, the difference in E° values between different molecules can be measured experimentally. Let A and B represent two different molecules, with the oxidized and reduced forms of these molecules indicated by the subscripts o and r, respectively. The individual half-cell reduction reactions may be written 

The reduction potential difference, A °, is related to both the equilibrium constant, eq. and the standard free energy change for the reaction, AG°. eq can be derived from the chemical activities of the various species at equilibrium  

By convention in biochemical systems, standard state conditions are defined in the following fashion all components are at unit activity (1 M), except for (10 M = pH 7) and gases (1 atm pressure). Activities are related to concentrations, c, by the activity coefficient, y  

Although it is common to equate activities to concentrations (i.e., assign y = I), there are many situations in biochemical systems (especially when polyelectrolytes such as proteins are involved) for which this is a poor approximation. Some of the consequences of this nonideal behavior for electron transfer reactions are described later. 

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