Which amino acid is unique for not having a chiral center?

Glycine is the only amino acid that does not have a chiral center. A chiral center is a carbon atom that is attached to four different groups, which allows for the existence of enantiomers—two molecules that are mirror images of each other. In the case of glycine, its structure features a central carbon atom that is bonded to two hydrogen atoms, along with an amino group and a carboxyl group. Because two of the substituents on glycine's central carbon are the same (the two hydrogen atoms), it does not meet the criteria for chirality and therefore lacks chiral properties.

In contrast, other amino acids like alanine, serine, and proline each possess a chiral center due to their unique side chains. Alanine has a methyl side chain, serine has a hydroxymethyl side chain, and proline is characterized by its cyclic structure. All of these provide different substituents attached to the central carbon, resulting in chirality. Thus, glycine stands out among the standard amino acids for its non-chiral nature.