Imino acid

Imino acids are a group of compounds that contain both an amide and a carboxyl group, bonded to the alpha carbon molecule. The difference between amino acids and imino acids is in the bonding of the nitrogen in the amide group. In imino acids, the nitrogen forms a double covalent bond to another molecule, or two single bonds to two different 'R' groups.

Proline

Although Proline is often referred to as an amino acid, it has a secondary amide group, making it an imino acid. This is due to its cyclic nature, as the nitrogen in the amide group is bonded to two different 'R' groups in the ring.