&;D&;-form of amino acid is derived from

Optical Activity 1. Amino acids having an asymmetric carbon atom exhibit optical activity. Asymmetry arises when 4 different groups are attached to the same carbon atomGlycine is the simplest amino acid and has no asymmetric carbon atom and therefore shows no optical activity. All others are optically active. 3. The mirror image forms produced with reference to the alpha carbon atom, are called D and L isomers. 4. The L-amino acids occur in nature and are therefore called natural amino acids. D-amino acids are seen in small amounts in microorganisms and as constituents of ceain antibiotics such as Gramicidin-S, Polymyxin, Actinomycin-D, and Valinomycin, as well as bacterial cell wall peptidoglycans. 5. Isoleucine and threonine have 2 optically active centers and therefore each has 4 diastereoisomers