Chiral Forms and Stereoisomers of α-Amino Acids and the Role of Glycine as an Achiral Amino Acid

Of the standard α-amino acids, all but glycine can exist in either of two_____

optical isomers

chiral forms, called enantiomers or stereoisomers.

Explanation:

α-amino acids are the building blocks of proteins. They contain an amine group (-NH2), a carboxylic acid group (-COOH), and a side chain (also called the R-group) attached to the central carbon atom (called the alpha carbon or α-carbon). The side chain can vary in size, shape, and chemical properties, and it determines the specific identity and function of each amino acid.

The α-carbon atom in amino acids is a chiral center, which means it has four different groups attached to it. Therefore, it can exist in two mirror-image forms, which are non-superimposable, like left and right hands. These forms are called enantiomers or stereoisomers.

In the case of amino acids, the two enantiomers are designated as L (levo-, Latin for left) and D (dextro-, Latin for right), based on the orientation of their functional groups around the chiral carbon atom. The L-enantiomer is the one found in most proteins, while the D-enantiomer is less common in nature and usually has no biological activity or is toxic to cells.

Glycine, on the other hand, is the simplest α-amino acid with a hydrogen atom as its R-group. It is not chiral because the α-carbon atom has two identical groups attached to it (one hydrogen and one carboxylate), so it cannot exist in two enantiomers. Instead, glycine is technically achiral or optically inactive.

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