HOW REDOX POTENTIALS ARE MEASURED voltmeter salt bridge Areduced and Aoxidized in equimolar amounts 1 M H+ and 1 atmosphere H2 gas examples of redox reactions standard redox potential E0 ° c c e – One beaker (left) contains substance A with an equimolar mixture of the reduced (Areduced) and oxidized (Aoxidized) members of its redox pair. The other beaker contains the hydrogen reference standard (2H+ + 2e – H2), whose redox potential is arbitrarily assigned as zero by international agreement. (A salt bridge formed from a concentrated KCl solution allows K+ and Cl– to move between the beakers, as required to neutralize the charges when electrons fow between the beakers.) The metal wire (dark blue) provides a resistance-free path for electrons, and a voltmeter then measures the redox potential of substance A. If electrons fow from Areduced to H+, as indicated here, the redox pair formed by substance A is said to have a negative redox potential. If they instead fow from H2 to Aoxidized, the redox pair is said to have a positive redox potential. THE STANDARD REDOX POTENTIAL, E˜0 The standard redox potential for a redox pair, defned as E0 , is measured for a standard state where all of the reactants are at a concentration of 1 M, including H+ . Since biological reactions occur at pH 7, biologists instead defne the standard state as Areduced = Aoxidized and H+ = 10–7 M. This standard redox potential is designated by the symbol E0 , in place of E0. ° PANEL 14–1: Redox Potentials 765
CALCULATION OF ˜Go FROM REDOX POTENTIALS E0 ° E0 ° To determine the energy change for an electron transfer, the ˜G o of the reaction (kJ/mole) is calculated as follows: ˜G o = –n(0.096) ˜E0 , where n is the number of electrons transferred across a redox potential change of ˜E0 millivolts (mV), and ˜E0 = (acceptor) – (donor) EXAMPLE: ° ° °
EFFECT OF CONCENTRATION CHANGES As explained in Chapter 2 (see p. 60), the actual free-energy change for a reaction, ˜G, depends on the concentration of the reactants and generally will be different from the standard free-energy change, ˜Go. The standard redox potentials are for a 1:1 mixture of the redox pair. For example, the standard redox potential of –320 mV is for a 1:1 mixture of NADH and NAD+. But when there is an excess of NADH over NAD+, electron transfer from NADH to an electron acceptor becomes more favorable. This is refected by a more negative redox potential and a more negative ˜G for electron transfer.
˜E0 = +30 – (–320) = +350 mV The same calculation reveals that the transfer of one electron from ubiquinone to oxygen has an even more favorable ˜Go of –76 kJ/mole. The ˜Go value for the transfer of one electron from NADH to oxygen is the sum of these two values, –110 kJ/mole. ˜Go = –n(0.096) = –1(0.096)(350) = –34 kJ/mole ° °˜E0 ° ° For the transfer of one electron from NADH to ubiquinone:
Figure 14–15 The structure of the heme group attached covalently to cytochrome c. the porphyrin ring of the heme is shown in red. there are six different cytochromes in the respiratory chain. Because the hemes in different cytochromes have slightly different structures and are kept in different local environments by their respective proteins, each has a different affinity for an electron, and a slightly different spectroscopic signature.
exploited by the membrane protein complexes in the respiratory chain to move electrons both within and between complexes.