Which amino acid is only zwitterionic at pH 7?

Glutamine is the only amino acid that is zwitterionic at pH 7.

The structure of an amino acid is such that it allows it to behave as both, an acid and a base at a certain pH value. Almost all amino acids exist as zwitterions at a certain pH value, which is different for each amino acid. Glutamine only behaves simultaneously as an acid and as a base at pH 7, which means it is only zwitterionic at pH 7.

The specific pH at which a given amino acid exists in solution as a zwitterion is also known as its isoelectric point. At its isoelectric point of 7, glutamine’s negative and positive charges balance each other out, so the molecule is electrically neutral as a whole.