All Amino Acids except Glycine Show Asymmetry

All Amino Acids except Glycine Show Asymmetry 

The chirality of amino acids stems from the chiral, or asymmetric, center, the a-carbon atom. The a-carbon atom is a chiral center if it is connected to four different substituents. Thus glycine has no chiral center. Two of the amino acids, isoleucine and threonine, possess additional chiral centers because each has one additional asymmetric carbon. You should be able to locate these carbons by simple inspection. 

Two structures that constitute a stereoisomeric pair are referred to as enantiomers. The two enantiomers for alanine are illustrated in figure 3.8. These two isomers are called L-alanine and D-alanine, according to the way in which the substituents are arranged about the asymmetric carbon atom. The naming by D and l (for dextrorotatory and levorotatory see chapter 6) refers to a convention established by Emil Fischer many years ago. According to this convention all amino acids found in proteins are of the l form. Some D-amino acids are found in bacterial cell walls and certain antibiotics. 

The covalent structure of alanine, showing the three-dimensional structure of the (l) and stereoisomeric forms. 

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All Amino Acids except Glycine Show Asymmetry Peptides and Polypeptides... 

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