I am learning about zwitterions and am confused about the following:

  1. From what I understand, when amino acids are in their solid form, they exist as zwitterions which is why they have such a high melting point and are so soluble in polar solvents.

  2. Then when placed in a solution with pH lower than their isoelectric point, they act as a base and form positive ions, and when placed in a solution with pH above their isoelectric point, they act as acids and form a negatively charged ion and a water molecule.

  3. If the above is correct, what I am confused about is what happens at the isoelectric point. In my textbook it says that the isoelectric point is 'the pH value at which the amino acid exists as a zwitterion', however I thought it also exists as a zwitterion when not in solution. Are they refering just to the case of the amino acid being in solution and ignoring the solid state?

  4. If this is so, I still do not understand how it remains as a zwitterion at the isoelectric point. Why does it not act as both a base and an acid and accept a H+ ion onto the O- atom in the carbonyl group and donate the extra H+ ion from its amine group? Does this have something to do with the fact that then it would no longer have any charges on it so it would not be soluble in the polar solvent?


1 Answer 1


The accepted definition by IUPAC gold book is: The pH value at which the net electric charge of an elementary entity is zero. pI is a commonly used symbol for this kind-of-quantity. It should be replaced by pH(I) because it is a pH determined under that particular condition.

The solubility of an amino acid is the smallest at its isoelectric point and it easily precipitates.

For question 4 you need to be familiar with dynamic equilibirum. The zwitterion accepts and loses protons all the time, but at the isoelectric point, its concentration is the biggest among all the forms of the amino acid, that is, the species that has a protonated carboxylic group, would easier lose that proton than the zwitterion's carboxylic group would accept a proton.


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