Which amino acid does not have an isomeric variant?

Glycine is the only amino acid that does not have an isomeric variant because it is the simplest amino acid with the structure of NH₂-CH₂-COOH. Its central carbon is not chiral since it is bonded to two hydrogen atoms, leading to a lack of L and D forms that are characteristic of other amino acids. Chirality in amino acids generally arises from a central carbon atom that is attached to four different substituents, creating enantiomers. In the case of glycine, this unique structure lacking a chiral carbon means it does not have any isomers.

In contrast, tyrosine and serine each have chiral centers and can exist in L and D forms. Proline, while it has a unique cyclic structure, also possesses chirality due to its imino group. Thus, glycine stands out as the only amino acid without isomeric variations.