Standard apparent reduction potentials for biochemical half reactions as a function of pH and ionic strength

Standard apparent reduction potentials are important because they give a more global view of the driving forces for redox reactions than do the standard transformed Gibbs energies of formation of the reactants. This paper emphasizes the effects of pH on biochemical half reactions in the range pH 5 to 9, but it also shows the effect of ionic strength. These effects can be calculated if the pKs of acid groups in the reactants are known in the range pH 4 to 10. Raising the pH decreases the standard apparent reduction potentials of half reactions when it has an effect, and the slope is proportional to minus one times the ratio of the change in binding of hydrogen ions in the half reaction to the number of electrons transferred. These effects are discussed for 19 biochemical reactions. This effect is most striking for the nitrogenase reaction, where the apparent equilibrium constant is proportional to 10(-10 pH) and is unfavorable for nitrogen fixation above pH 8.