Amino Acid Stereochemistry
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Except for one AA, all standard AAs have an asymmetric or chiral a-carbon.  Thus, stereochemical isomers exist for all but one of the standard amino acids. 

The stereochemistry of  Ca is based on the stereochemical convention based on the enantiomers of glyceraldehyde.

Glyceraldehyde is a three carbon structure with a central chiral carbon giving rise to two enantiomers, designated "D" and "L" in reference to their unique optical activities.   Namely, the plane defined by a beam of plane-polarized light rotates in opposite directions when passing through pure solutions of one or the other stereoisomer.   A pure solution of L-glyceraldehyde rotates the light plane to the left (when viewed emerging from the solution) and hence is Levorotatory.  Conversely a pure solution of D-glyceraldehyde rotates the light plane to the right and hence is Dextrorotatory. Equal molar mixtures of D- and L-glyceraldehyde do not rotate plane-polarized light because the left and right rotational effects of the two enantiomers cancel out.

An example of the stereochemistry of amino acids is found by viewing the two enantiomers of alanine seen in the windows below.  The standard amino acid, alanine, corresponds to the L-stereoisomer L-Ala and its mirror image chemical isomer is D-Ala, the D-stereoisomer. 

The L- and D-amino acid convention is defined by matching their structures to the structures of  L-glyceraldehyde and D-glyceraldehyde.   First the asymmetric alpha-carbon (Ca) of an alpha-amino acid is the aligned with the assymetric carbon 2 of glyceraldehyde.  Then, chemically similar groups in the structures are oriented similarly.  Namely, the alpha-carboxyl group (-COO-) of the amino acid is aligned parallel to the aldehyde group (-CHO) of glyceraldehyde.  The alpha-amino group (-NH3+) of the amino acid is aligned parallel to the hydroxyl group (-HO) linked to the middle carbon of glyceraldehyde.  Finally, the variable R-group of the amino acid is aligned parallel to the methanol group (-CH2OH) of glyceraldehyde.  In this configuration, the alpha-NH3+ group of every  L-amino acid is  located on the left side and spacially above of the alpha-carbon just like the  -OH group linked to the second (asymmetric) carbon of L-glyceraldehyde. This is one way of defining the stereochemistry of  L-amino acids but it is somewhat intuitive based on the similarities of the chemical groups in the two types of molecules.  Another system -- the R-S convention -- is much more rigorous and is recommended for a detailed analysis of chemical stereochemistry. 

Like the isoforms of glyceraldehyde, amino acids are also optically active. Specially, pure solutions of all but one the standard amino acids will rotate plane-polarized light, like the stereoisomers of glyceraldehyde.   However, not all L-amino acids are levorotatory and the actual direction of light rotation with any given amino acid depends on its particular structure in ways that are hard to predict.   In other words, some amino acids are levorotatory while others are dextrorotatory for complicated structural reasons.  In either case, the standard amino acids are still referred to as L-amino acids independent of their particular optical active properties but consistent with their basic structural homology to L-glyceraldehyde as compared to D-glyceraldehyde as discussed above.

Two of the standard amino acids contain 2 asymmetric carbons, namely the beta-carbons of isoleucine and threonine.   Thus, the stereochemistry of Ile and Thr are defined by both enantiomers and diastereoisomers and illustrated with the four stereoisomeric structures of Thr.

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© Duane W. Sears
Revised: December 09, 1998