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In a correctly oriented Fischer projection of an alpha-amino acid, the amino group on the left means the L series; on the right means the D series. First check the orientation: put the carboxyl group at the top and the side chain below the alpha carbon. The shortcut does not apply to a drawing in an arbitrary orientation.
How to read the standard Fischer projection
- Find the alpha carbon. It is the carbon bonded to the amino group, carboxyl group, hydrogen, and side chain.
- Orient the carbon chain vertically. Place the carboxyl group at the top and the side chain below the alpha carbon.
- Check the amino group’s side. If it is on the left, the amino acid is L; if it is on the right, it is D.
This rule depends on Fischer projection geometry: horizontal bonds point toward the viewer, while vertical bonds point away. IUPAC describes Fischer projections and other structural drawings, including wedge-and-dash diagrams, as ways to represent configuration. IUPAC Blue Book: stereochemical representation
What if the structure is rotated or drawn another way?
Do not classify an amino acid by which side the amino group happens to occupy in a wedge-and-dash drawing or an unverified projection. Establish the three-dimensional arrangement, then redraw it in the standard Fischer orientation before using the left/right shortcut. Rotating a drawing can change what appears to be on the left without changing the molecule’s configuration.
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These labels describe different things and are not interchangeable:
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| Notation | What it describes | How it is determined |
|---|---|---|
| D or L | Configuration in relation to a reference stereochemical series | For the alpha-amino-acid Fischer projection described above, read the amino group’s left/right position after orienting the chain correctly |
| R or S | Absolute configuration at a stereocenter | Assign priorities using the Cahn–Ingold–Prelog (CIP) rules |
| (+) or (−) | The direction of optical rotation | Determine experimentally by measuring rotation of plane-polarized light |
An L label does not mean that a compound is levorotatory. IUPAC says the L configuration of chiral alpha-amino acids found in proteins “nearly always corresponds to S in the RS system”; that correspondence is not universal. IUPAC Blue Book: amino-acid configuration and IUPAC biochemical nomenclature
Exceptions and additional stereocenters
Glycine
Glycine’s alpha carbon is attached to two hydrogen atoms, so it is not a chiral center. It has no L/D pair at that position.
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Cysteine and cystine
Most L alpha-amino acids have S configuration at the alpha carbon under CIP rules. L-cysteine and L-cystine are exceptions: their sulfur-containing side chains alter the priority order, giving the alpha carbon R configuration.
Threonine and isoleucine
These amino acids have an additional stereocenter. The L/D designation discussed here identifies the alpha-carbon series; it does not by itself specify every stereocenter in the molecule.
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Do D-amino acids occur in nature?
L amino-acid residues predominate in proteins, but D residues are not absent from living systems. They occur in some generally small peptides, including bacterial cell-wall peptides and certain peptide antibiotics. Biochemistry textbook excerpt hosted by the USPTO
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