I'm interested in the binding constants between pyridine and zinc tetraphenylporphyrin (ZnTPP) in dichloromethane. For neutral ZnTPP, the binding constant is around 7000 /M (from UV-vis titration).

For the anion and cation, I derived the binding constants from running cyclic voltammetry on ZnTPP (host H) in presence of increasing amounts of pyridine (guest G). The half-wave potentials then converge from the E_H potentials to the E_HG potentials, and from the difference I can get the anionic binding constant through: E_H - E_HG = RT/nF*ln(K+/K) (and the cationic binding constant in the same way).

Although I now have all the information I wanted to have, I'm puzzled by the graph [G]/[H] versus E_1/2. For the oxidation wave, this one is very steep. For the reduction wave, this one is very shallow. What does this plot tell me? What information is hidden in this plot? If I fit it to a supramolecular model analogously to UV-vis titrations, it does give me some value, but this is neither one of the association constants, nor a ratio thereof...

Anyone??