Biological standard potential

We saw in Section 4.2 that in biochemical work it is common to adopt the biological standard state (pH = 7, corresponding to neutral solution), rather than the thermodynamic standard state (pH = 0). To convert standard potentials to biological standard potentials, , we must first consider the variation of potential with pH. The two potentials differ when hydrogen ions are involved in the half-reaction, as in the fumaric acid/succinic acid couple fum/suc with fum = HOOCCH=CHCOOH and sue = HOOCCH2CH2COOH, which plays a role in the citric acid cycle (Case study 4.3) ... 

Calculate the biological standard potential of the NAD/NADH couple at 25°C Example 5.2) from its thermodynamic value. The reduction half-reaction is... 

Self-test 5.6 ) Calculate the biological standard potential of the half-reaction 02(g) + 4 H+(aq) + 4 e 2 H20(l) at 25 C given its value +1.23 V under thermodynamic standard conditions. 

The biological standard cell potential from biological standard potentials... 

Strategy The aim is to find the values of and v corresponding to the reaction, for then we can use a modified form of eqn 5.16 to calculate the value of K in neutral solution from fd. To do so, we express the equation as the difference of two reduction half-reactions. The stoichiometric number of the electron in these matching half-reactions is the value of v we require. We then look up the biological standard potentials for the couples corresponding to the half-reactions and calculate their difference to find... 

I I 10. Biological standard potentials are measured in neutral solution (pH = 7). 

The biological standard potential of the couple pyruvic acid/ lactic acid is-0.19 V. What is the thermodynamic standard potential of the couple Pyruvic acid is CH3COCOOH and lactic acid is CH3CH(0H)C00H. 

The biological standard potential of the redox couple pyruvic acid/lactic acid is -0.19 V and that of the fumaric acid/succinic acid couple is -tO.03 V at 298 K. What is the equilibrium constant at pH = 7 for the reaction... 

The equihbrium constant for the reaction above is 2.14 X 10" at pH = 7.0 and 298 K. (a) Calculate the biological standard potential of the corresponding galvanic cell and (b) the biological standard potential of the glyoxylate /glycolate couple. 

Table 3c Biological standard potentials at 298.15 K in electrochemical order...

Table 13.3 Potentials E° of some organic redox couples at T = 37°C and pH = 7 (biochemistry conventions biological standard potentials)...

Comparing this relation with (14.6) shows that the difference °(Mn04 /Mn +) — (8/5) 0.06pH plays the part of a standard potential. Such differences are called apparent standard potentials, or normal potentials. They are symbolized by E° (the same symbol than those of biological standard potentials and of formal potentials). In this example, we may set... 

Figure 8.2 Polarization curve of a microbial bioanode. The deviation between biological standard potential and the measured formal potential at open circuit is indicated as well as the different losses during electrode polarization from open circuit to maximum current.

Biological Standard Potentials of Some Biological Electron Donors and Electron Acceptors... 

---