Stereochemistry of Amino Acids

Further complications may arise with the larger amino acids such as isoleucine, where the R side-chain itself contains a chiral carbon atom [R = CH3CH2C H(CH)3, where the asterisk denotes the second chiral centre]. This molecule is an example of a diastereomer - a molecule with more than one chiral centre. Diastereomers have different physical and chemical properties, and their interconversion is more complicated, and is termed epimerization. 

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A Neuberger. Stereochemistry of amino acids. Adv Prot Chem 4, 297, 1948. 

A novel technique for dating archaeological samples called amino acid racemiza-tion (AAR) is based on the stereochemistry of amino acids. Over time, the configuration at the a-carbon atom of a protein s amino acids is lost in a reaction that follows first-order kinetics. When the a carbon is the only chirality center, this process corresponds to racemization. For an amino acid with two chirality centers, changing the configuration of the a carbon from L to D gives a diastereomer. In the case of isoleucine, for example, the diastereomer is an amino acid not normally present in proteins, called alloisoleucine. 

Stereochemistry of Amino Acids Naturally occurring amino acids belong to the T-family. That is, when the amino acid is drawn as a Fischer projection, with the carboxyl group at the top and the side chain at the bottom, the amino group is on the left, as shown in Figure 12.45. 

Stereochemistry of Amino Acids 339 Acid-Base Properties of Amino Acids 339 The Isoelectric Point 342 Electrophoresis 343 The Peptide Bond 343 Primary Structure of Proteins 344 Secondary Structure of Proteins 344 Tertiary Structure of Proteins 345 The Folding Problem 346 Denaturing Proteins 346 Enzymes 347... 

The fungal strain Acremonium sp. MST-MF558a was isolated from a Tasmanian estuarine sediment sample, and based on its rDNA sequence was considered to represent a new species. From the culture broth, a novel family of lipodepsipeptides, acremolides A-D (180-183), was obtained, together with known chaetoglobo-sins. The absolute stereochemistry of amino acid residues in 180-183 was determined using a new C3 Marfey s method for amino acid analysis. Compounds 180-183 were neither cytotoxic in their own right nor... 

The stereochemistry of amino acids is also an important concept. The carbon atom marked a in Figure 1 is chiral, so an amino acid can be one of two enantiomers. Note that the structure shown in the illustration has the amine group on the right and the carboxyl group at the top. This configuration is designated the L form, and all naturally occurring amino acids have this form. 

With respect to natural compounds, experimentation concentrated on isolation, purification, and elemental analysis of carbon compounds. By the beginning of the twentieth century all of the major natural products had been isolated, and the stereochemistry of amino acids and carbohydrates had become known in detail. The vital force had lost its reputation in chemistry textbooks. 

Although it is accepted that the method of quasi-racemates (as defined by Fredga and which correlates the absolute stereochemistry of amino acids to the... 

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