Most amino acids have two optical isomers.
Chirality in Amino Acids Explained
The optical isomerism exhibited by amino acids arises from the presence of a chiral center, typically the alpha-carbon (Cα). This carbon is bonded to four different groups:
- An amino group (-NH2)
- A carboxyl group (-COOH)
- A hydrogen atom (-H)
- A unique side chain or R-group
This tetrahedral arrangement around the alpha-carbon, with four distinct substituents, makes the molecule non-superimposable on its mirror image, thus creating stereoisomers known as enantiomers or optical isomers. These are designated as L- and D- isomers. The L-isomers are the ones found in proteins.
Exception: Glycine
It is important to note that glycine is the only common amino acid that does not have optical isomers. This is because its R-group is a hydrogen atom, making the alpha-carbon bonded to two hydrogen atoms, thus lacking chirality.
Why are Optical Isomers Important?
The different optical isomers can have distinct biological activities. For example, enzymes are stereospecific and typically bind to only one of the two isomers. The importance of chirality is evident in drug design and synthesis where one isomer may have desired therapeutic effects, while the other could be inactive or even harmful.
