Final answer:
Amino acids are chiral molecules, with the α-carbon being a stereogenic center, leading to non-superimposable mirror images called enantiomers, except for glycine which is achiral.
Step-by-step explanation:
Amino acids are chiral molecules. This characteristic means that, with the exception of glycine, amino acids cannot be superimposed on their mirror image. Each amino acid has a central carbon (known as the α-carbon) to which four different groups are attached: a hydrogen atom, a carboxyl group (-COOH), an amino group (-NH₂), and a variable side chain (R group). Because of these four different groups, the α-carbon is a stereogenic center, making the molecule chiral. Chiral molecules exist in two forms, known as enantiomers, which are mirror images of each other but cannot be placed on top of one another in the same orientation.
In the case of amino acids, these two enantiomeric forms are designated as D and L. While these forms exist, in biological systems, proteins are primarily constructed using the L-forms of amino acids. The D-forms may be present in some biological contexts, like in bacterial cell walls. Thus, glycine, having two hydrogen atoms attached to its α-carbon, is the only amino acid that is not chiral as it is symmetrical and can be superimposed on its mirror image.