Amino Acid Stereoisomers
All but one of the standard AAs (Gly) have an asymmetric or chiral α-carbon atom. Thus, ribosomally synthesized proteins are found be covalently assembled from only 1 of 2 possible α-carbon enantiomers, defined as non-superimposable mirror images of chemical isomers composed of identical atoms covalently bonded in the same way in each structure. There are also two standard amino acids that have a second chiral β-carbon atom that define 2 possible diastereoisomers which are non-mirror image stereochemical isomers.
The convention used to define the Cα carbon stereochemistry of amino acids is based on the mirror image enantiomers of glyceraldehyde, which is a three carbon structure with the central chiral carbon covalently bonded to a hydroxyl (OH) group and a hydrogen (H) atom. The two enantiomers of glyceraldehyde are designated "D" and "L" by reference to their unique optical activities. When a plane-polarized light beam passes through a pure solution of D-glyceraldehyde the emergent beam will be rotated the light plane to the right, and hence is the enantiomer is considered to be dextrorotatory ("dextra" is Latin for right) and is designated the "D" enantiomer. However, if beam of plane polarized light passes through a  pure solution of L-glyceraldehyde, the emergent light beam will be rotated in the opposite direction to the left, and hence the enantiomer is considered to be levorotatory (laevus is Latin for left) and is designated the "L" enantiomer.  As expected, an equal molar mixture of D- and L-glyceraldehyde will produce no rotation ofplane-polarized light passing through the solution because the left and right rotational effects of the two pure enantiomers will exactly cancel out.
The stereochemistry of all but one of the standard amino acids is defined by two possible mirror image isomers or enantiomers. For example, consider the two enantiomers of Ser. The standard amino acid itself corresponds to the L-stereoisomer. It's mirror image enantiomer, the D-stereoisomer, D-Ser is no doubt found in nature but it is not found in ribosomally-synthesized polypeptides. The L- and D-amino acid convention is defined by matching the AA chiral structures to the chiral structure of L-glyceraldehyde and D-glyceraldehyde. the asymmetric alpha-carbon (Cα) of an α-amino acid is the visually aligned with the asymetric central carbon-(2) of glyceraldehyde.  First the chemically similar groups in the structures are aligned with a similar orientation. Namely, vertically align the α-carboxyl group (α-COO-) of the amino acidis in parallel (top to bottom) with the chemically similar aldehyde group (-CHO) of glyceraldehyde. Then align the α-amino group (α-NH3+) in parallel to the chemically similar hydroxyl group (-OH) linked to the central carbon of glyceraldehyde.  Finally, align the amino acid's R-group (on the bottom) with the methanol group (-CH2OH) of glyceraldehyde.  In this configuration, the α-NH3+ group of every L-amino acid will be spaceially oriented on the same side of the structure as found with L-glyceraldaldehyde. This is semi-accurate way to identify the two alpha-carbon enantiomers of L-amino acids. Another more rigorous method, the R-S convention, is recommended for acccurate stereochemical analysis .
Like the two stereoisomers of glyceraldehyde, amino acid stereoisomers are also optically active. Specially, pure solutions of all but one the standard amino acids will rotate the plane of plane-polarized light to the left or right. However, not all L-amino acids are levorotatory and the actual direction of light rotation can very with amino acid depending on its particular electronic and chemical structure in ways that are hard to predict. In other words, some amino acids are levorotatory while others are dextrorotatory for complicated structural reasons. However, all standard amino acids are still considered to be L-amino acids, independent of their optical active properties but consistent with their overall structural homology to L-glyceraldehyde in contrast to D-glyceraldehyde as discussed above.
In summary, most of all the standard amino acids (except glycine) in ribosomally synthesized polypeptides exhibit only one or two possible enantiomers while two standard amino acids -- Isoleucine and Threonine with a second asymmetric β-carbon atom exhbit only 1 of 4 possible stereoisomers, i.e., only 1 or 2 possible enantiomers and only 1 or 2 possible diastereoisomers (non-mirror-image stereoisomers). Some or many of these "other" stereoisomers are rounitnely found in many organisms; but they are not found in proteins synthesized by ribosomes.
© Duane W. Sears
Revised: June 25, 2023