At physiological pH, the carboxy-terminal of a peptide is

Amino acids are the best-known examples of zwitterions. These compounds contain an ammonium and a carboxylate group and can be viewed as arising a kind of intramolecular acid-basereaction: The amine group deprotonates the carboxylic acid. NH2RCHCO2H NH3+RCHCO2- The zwitterionic structure of glycine in the solid state has been confirmed by neutron diffraction measurements. At least in, some cases, the zwitterionic form of amino acids also persists in the gas phase. In addition to the amino acids, many other compounds that contain both acidic and basic centers tautomerize to the zwitterionic form. Examples, such as bicine and tricine, contain a basic secondary or teiary amine fragment together with the carboxylic acid fragment. Ref: Textbook of Medical Biochemistry, MN Chatterji, 8th Edition, Page no: 81