At the isoelectric point amino-acids are neutral to the outside. It is however not known to my knowledge if the molecules are indeed in a zwitterionic form or just in its "normal" totally uncharged form, i.e. the dissociation constant for this reaction:
is not known. Does anyone know how they actually look like? What form is more probable, how realistic is the zwitterionic form?
Edit: I did a test calculation for cysteine with DFT, using a PBE functional and implicit water and found that the zwitterionic form is about 7 kcal/mol more stable, which would rather strongly indicate that the zwitterionic form is indeed dominating a lot and you would not at all find the structure on the left as the bars from wikipedia indicate (supposing that I found low lying minima in both calculations and not only local ones). Are there some other studies proving this?
Edit2: Ok the zwitterionic form is definitely more stable, that one can somehow reason from chemical knowledge or pKa's. But how much more stable, can one somehow get a dissociation constant for the reaction above?
Edit3: Since there is much confusion about my question. I want a dissociation constant for the reaction (COOH)(NH2) -> (COO-)(NH3+). Until now I think only the dissociation constants for (COOH)(NH3+) -> (COO-)(NH3+) and (COO-)(NH3+) -> (COO-)(NH2) are known if I am right and I cannot tell how these would help to calculate the constant I want...
Edit4: To clear up my question even a bit more, here is a diagram:
The red arrow and question mark is what I want to know about. The pks1 and pks2 are known, that was clear to me, but how to get any information about the equilibrium in the middle?