All amino acids except glycine, have their α- carbon bonded to four different groups: carboxyl, amino, R-, and a hydrogen atom. Thus the α-carbon atom in amino acids becomes a chiral centre and the molecule is optically active. Because of the tetrahedral arrangement of the bonding orbitals around the α-carbon atom, the four different groups can occupy two unique spatial arrangements.
- The molecules with different unique arrangements in space are stereoisomers.
- Since the structures are like non-superimposable mirror images of each other, the two stereoisomers are enantiomers.
- Both of them rotate plane-polarized light in opposite directions i.e. right or left and accordingly are affixed as D (+) or L (-).
D and L system of amino acid
The amino acid residues in protein molecules are exclusively L stereoisomers. D-Amino acid residues have been found only in a few, generally small peptides, including some peptides of bacterial cell walls and certain peptide antibiotics. The absolute configurations of simple sugars and amino acids are specified by the D, L system related to the configuration of D and L glyceraldehyde. Thus, L-alanine has a configuration similar to L-glyceraldehyde.
- By Fischer’s convention, L and D refer only to the absolute configuration of the four substituents around the chiral carbon, not to the optical properties of the molecule.
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