Ionization and Brønsted character of N-terminal amino, C-terminal carboxylate, and side chains of amino acid residues
The common natural forms of amino acids have a zwitterionic structure, with −NH+ 3 (−NH+ 2− in the case of proline) and −CO− 2 functional groups attached to the same C atom, and are thus α-amino acids, and are the only ones found in proteins during translation in the ribosome.
In aqueous solution at pH close to neutrality, amino acids exist as zwitterions, i.e. as dipolar ions with both NH+ 3 and CO− 2 in charged states, so the overall structure is NH+ 3−CHR−CO− 2. At physiological pH the so-called "neutral forms" −NH 2−CHR−CO 2H are not present to any measurable degree. Although the two charges in the zwitterion structure add up to zero it is misleading to call a species with a net charge of zero "uncharged".
