Final answer:
Glycine is the only non-chiral amino acid as it has two identical hydrogen atoms attached to its α-carbon, making the molecule symmetric and without enantiomers. All other amino acids possess chirality and can exist in D- or L-forms, with only L-forms incorporated into proteins naturally.
Step-by-step explanation:
The amino acid that is not chiral is glycine. The reason glycine does not exhibit chirality is because its α-carbon has two hydrogen atoms attached, making it symmetric. As such, the molecule and its mirror image are identical, and glycine does not have enantiomers as do other amino acids, which have four different groups attached to the chiral center.
The common structure of an amino acid includes an amine group (-NH₂) and a carboxylic acid group (-COOH), but in glycine, the α-carbon, which is the central carbon atom to which these groups are attached, also bonds to two hydrogen atoms (H₂NCH₂CO₂H). Other amino acids have a side chain that varies from one to another, providing uniqueness and chirality.
With the exception of glycine, amino acids can be found in D- or L-enantiomeric forms. However, only L-amino acids are utilized in the construction of proteins in nature, with the exception of some D-forms observed in bacterial cell walls. The configuration of all naturally occurring chiral amino acids is S, while cysteine is an exception with an R configuration, and glycine being non-chiral does not conform to this categorization.