34.4.1—Acid / base properties of amino acids

An amino acid contains both a carboxylic acid and an amine. In water it can transfer a proton internally, forming a zwitterion with –NH₃⁺ and –COO⁻ groups.

At low pH the molecule is more positively charged; at high pH it is more negatively charged. The isoelectric point is the pH at which the average net charge is zero.

Glycine is mainly a zwitterion near its isoelectric point, but becomes cationic in acid and anionic in alkaline solution.

A zwitterion is not electrically neutral at every site: it has internal charges whose sum may be zero.