Final answer:
Glycine is the only amino acid without an L/D isomer because its α-carbon is not chiral, making it identical to its mirror image.
Step-by-step explanation:
The amino acid that does not have an L/D Isomer is glycine. Unlike the other amino acids, glycine's α-carbon is not chiral, meaning it does not have a mirror image in the form of enantiomers. Therefore, it cannot exist as D-glycine or L-glycine because both of these forms would be identical. The rest of the proteinogenic amino acids can exist as L- and D-forms with naturally occurring amino acids usually in the L-configurations used to make proteins, except for glycine which is nonchiral.