ABSTRACT
The building blocks of peptides are amino acids, which are composed of a carbon atom to which are attached a carboxyl group, an amino group, a hydrogen atom, and a so-called side-chain R
(Figure 1.1). The simplest amino acid is glycine, for which the side-chain is another hydrogen atom, so there are no stereochemical forms of glycine. Glycine is not a chiral compound, but two configurations or arrangements of substituents around the central
α
-carbon atom are possible for all other amino acids, so each exists in two stereochemical forms, known as the
-isomers for the amino acids found in proteins and the
-isomers for those with the opposite configurations. The natural amino acids are so designated because they have the same configuration as that of natural glyceraldehyde, which arbitrarily had been designated the L-form. Two isomers of opposite configuration or chirality (handedness) have the relationship of mirror images and are referred to as enantiomers. Enantiomers are identical in all respects except that solutions of the isomers rotate plane-polarized light in opposite directions. The enantiomer deflecting polarized light to the right is said to be dextrorotatory (+), and the enantiomer deflecting polarized light to the left is levorotatory (–). There is no correlation between the direction of this optical rotation and the configuration of the isomer — the direction cannot be predicted from knowledge of the absolute configuration of the compound. According to the Cahn-Ingold-Prelog system of nomenclature,
-amino acids are of the (
S
)-configuration, except for cysteine and its derivatives. In discussion, when the configuration of an amino acid residue is not indicated, it is assumed to be the
-enantiomer.
