The biological standard potential

4 ) Converting a standard potential to a biological standard value 

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 

Strategy Write the Nernst equation for the potential, and express the reaction quotient in terms of the activities of the species. All species except are in their standard states, so their activities are all equal to 1. The remaining task is to express the hydrogen ion activity in terms of the pH, exactly as was done in the text, and set pH = 7. 

Solution The Nernst equation for the half-reaction, with Vg = 2, is 

Whenever possible, avoid replacing activities by concentrations, especially when aiming to relate the electrode potential to pH, for the latter is defined in terms of the activity of hydrogen ions. 

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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. 

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... 

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. 

For biological samples four different contributions to the standard potential Ef can be observed the contribution of the internal standard potential of the reference electrode ERef the diffusion potential over the liquid junction Ej generated between the sample solution and the reference electrode a potential difference (electrical asymmetry) of the ion-selective membrane after preparation and conditioning Eei and a sample-induced asymmetry of the membrane Eas. For measurements in human blood samples directly the adsorption of sample components at the membrane surface creates a drift associated with the affinity of the membrane to lipids as well as proteins. 

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) ... 

The measurement of the potential of an electrochemical cell is a convenient source of thermodynamic information on reactions. In practice the standard values (and the biological standard values) of these quantities are the ones normally determined. 

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

More briefly low reduces high and, equivalently, high oxidizes low. The same arguments apply to the biological standard values of the potentials. 

From the biological standard half-cell potentials 2n(02,H, H20) = -tO.82 V and n(NAD,H NADH) = -0.32 V, calculate the standard potential arising from the reaction in which NADH is oxidized to NAD and the corresponding biological standard reaction Gibbs... 

Molecular orbital calculations may be used to predict trends in the standard potentials of conjugated molecules, such as the quinones and flavins, that are involved in biological electron transfer reactions (Chapter 5). It is commonly assumed that decreasing the energy of the LUMO enhances the ability of a molecule to accept an electron into the LUMO, with an attendant increase in the value of the molecules standard potential. Furthermore, a number of studies indicate that there is a linear correlation between the LUMO energy and the reduction potential of aromatic hydrocarbons. 

The redox potentials for these two steps (referred to the biological standard state) are —0.32 V and 0.816 V, respectively so that overall for... 

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.

The half-cell reactions for electrochemical cell and the corresponding reduction potentials are given below. The symbol E° is used here because these values refer to the biological standard state ... 

Tamplin et. al. (54) observed that V. cholerae and A. hydrophila cell extracts contained substances with TTX-like biological activity in tissue culture assay, counteracting the lethal effect of veratridine on ouabain-treated mouse neuroblastoma cells. Concentrations of TTX-like activity ranged from 5 to 100 ng/L of culture when compared to standard TTX. The same bacterial extracts also displaced radiolabelled STX from rat brain membrane sodium channel receptors and inhibited the compound action potential of frog sciatic nerve. However, the same extracts did not show TTX-like blocking events of sodium current when applied to rat sarcolemmal sodium channels in planar lipid bilayers. 

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