| Stereochemistry: configuration at the α-carbon, d- and l-isomers
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Each amino acid has a central carbon, called the α-carbon, to which four different groups are attached (Fig. 2.1):
- a basic amino group (-NH2)
- an acidic carboxyl group (-COOH)
- a hydrogen atom (-H)
- a distinctive side chain (-R).
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one of the 20 amino acids , proline, is not an α-amino acid but an α-imino acid (see below). Except for glycine, all amino acids contain at least one asymmetric carbon atom (the α-carbon atom), giving two isomers that are optically active, i.e. can rotate plane-polarized light. These isomers, referred to as enantiomers, are said to be chiral, a word derived from the Greek word for hand. Such isomers are non-superimposable mirror images and are analogous to left and right hands, as shown in Figure 2.2. The two amino acid configurations are called d (for dextro or right) and l (for laevo or left). All amino acids in proteins are of the l-configuration, because proteins are biosynthesized by enzymes that insert only l-amino acids into the peptide chains.
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