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Why is glycine the only standard amino acid without a chiral alpha carbon?
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Amino acid
The structure and function of amino acids.
Glycine is the only standard amino acid without a chiral alpha carbon because its side chain consists of a single hydrogen atom.
In all other standard amino acids, the alpha carbon is bonded to four distinct groups: an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R group). This arrangement makes the alpha carbon a stereocenter, resulting in two optically active, mirror-image forms known as enantiomers. Because glycine’s side chain is also a hydrogen atom, its alpha carbon is bonded to two identical hydrogen atoms. This lack of four distinct substituents means the molecule is achiral and does not exist in d or l enantiomeric forms.12