Final answer:
The non-polar amino acid that does not have separate L and D isomers is glycine. It is achiral because its α-carbon is bonded to two hydrogen atoms and lacks the typical chiral center present in all other proteinogenic amino acids.
Step-by-step explanation:
The non-polar amino acid without separate D and L isomers is glycine. Unlike all other amino acids, the α-carbon of glycine is not chiral, as it has two hydrogen atoms attached. Therefore, it does not have a mirror image or an enantiomer, making it the only proteinogenic amino acid to be achiral. In contrast, other amino acids, which possess a chiral center, can exist as either D- or L-enantiomers. Only the L-isomers of amino acids are used in protein synthesis, with the curious exception that these L-amino acids are dextrorotary (‘d’), and are called d-amino acids when their optical rotation is measured.
In biological contexts, proteinogenic amino acids are typically found in the L configuration, except for glycine which doesn’t have this stereoisomerism due to its lack of chirality. The R group or side chain of each amino acid determines its properties, but in the case of glycine, its R group is simply a hydrogen atom.