3. OPTICAL ISOMERS
D-Amino acids are optical isomers of L-amino acids. These isomers are mirror images of each other and are known as enantiomers. They have identical physical properties, except for the direction of rotation of the plane of polarized light. Although they have very close physico-chemical similarities, enantiomers behave differently to biological bodies. For example, D-tryptophan and D-leucine are sweet whereas these L-forms taste bitter. Monosodium L-glutamic acid (MSG) tastes umami and enhances the flavor of food, but the D-form does not.