. Biophysics: concepts and mechanisms. Biophysics. 0,4 2 -m 7 (all reduced) 100% (all oxidized) % oxidized (b) Figure 7-5. Reversible Potential of an Oxidation-Reduction Reaction: (a) as a function of pH,onthe normal hydrogen electrode (NHE) scale; (b) as a function of per cent oxidation. Definition of Em7: potential (on the NHE scale) when pH = 7 and when the redox system is 50 per cent oxidized. As a further clarification and as a summary, Figure 7-6 shows schema- tically the relation between the NHE scale of £"°'s (pH = 0), to which AE values have been traditionally related
. Biophysics: concepts and mechanisms. Biophysics. 0,4 2 -m 7 (all reduced) 100% (all oxidized) % oxidized (b) Figure 7-5. Reversible Potential of an Oxidation-Reduction Reaction: (a) as a function of pH,onthe normal hydrogen electrode (NHE) scale; (b) as a function of per cent oxidation. Definition of Em7: potential (on the NHE scale) when pH = 7 and when the redox system is 50 per cent oxidized. As a further clarification and as a summary, Figure 7-6 shows schema- tically the relation between the NHE scale of £"°'s (pH = 0), to which AE values have been traditionally related through —AE = nEE, and the physio- logical scale, Em7 (pH = 7). The latter is now commonly used as a relative measure of free energy changes in biological reactions. The values in Table 7-5 have been measured simply by putting a platinum wire into a mixture of equal concentrations of sodium succinate and sodium fumarate at pH 7, containing an enzyme and a mediator (discussed later), and measuring its voltage against a hydrogen electrode in the same solution. Such measured values can be used to predict the direction of reaction, or as a basis for com- parison, but not for the determination of AE, because the effective con- centrations (activities) are not known. It is well to be clear on this limitation of the £" -, listing;. Difficulty often arises in this subject because of notation. Different authors use different subscripts and superscripts. In this book we have de- fined, and use, only E, E°, and Em7. One should be aware of the variations which one may find. Further, one should understand clearly that the values given in the table for intermediary processes of oxidation are midpoint values; that although these redox systems are generally poised at their most stable point (Figure 7-5), a tight control must be kept by the living system at all times on the concentration of oxidized and reduced states of each system;. Please note that these images are extracted from
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