Enantiomers  

All  of  the  amino  acids,  except  glycine  (Gly  or  G; ),  have  4 different  groups  arranged  tetrahedrally  around  the  central  C_α   atom  which  is therefore known  as an asymmetric  center  or chiral center  and has the property  of chirality  (Greek;  cheir,  hand) .The  two  non super imposable,  mirror images are termed as enantiomers. Enantiomers are chemically and physically indistinguishable by most techniques, but can be distinguished on the basis of their different optical rotation of plane-polarized light. Molecules are classifieds levorotatory (L; Greek 'levo' = left) or dextrorotatory (D; Greek 'dextro' = right) based on whether they rotate the plane of plane-polarized light anticlockwise or clockwise.  D- and L-amino acids may be distinguished by enzymes also which usually recognize one or other enantiomer only.  The L-amino acids are only found in proteins.  D-Amino acids rarely take place in nature, but are found in bacterial cell walls  and certain antibiotics.