In chemistry or biochemistry, optically active molecules are capable of rotating the plane-polarized light to clockwise or anticlockwise. All the chiral molecules are optically active. French physicist Jean-Baptiste Biot in 1815 discovered the optical property of a compound. Louis Pasteur first observed the relationship between the structure of a compound and its effect on plane-polarized light in 1848.
A chiral molecule exists in two stereoisomers or enantiomers. If the enantiomer rotates the plane-polarized light clockwise, the enantiomer is called dextrorotatory (d). If the enantiomer rotates the plane-polarized light anticlockwise, the enantiomer is called levorotatory (l). In Latin, dextrorotatory (to the right) and levorotatory (to the left). Amino acids having an asymmetric α-carbon atom are optically active.
Asymmetry in an amino acid arises if the α-carbon atom is attached to four different groups of atoms. Glycine is the only amino acid with two Hydrogen (H) atoms attached to the α-carbon atom. Due to the absence of four different groups, the amino acid glycine has no asymmetric α-carbon atom. Therefore, glycine does not show optical activity. All others amino acids are optically active.