The D and L configurations are different from R and S configurations, even though both describe stereochemistry.
Here’s the difference:
1. Basis of Assignment
- R and S system: Uses the Cahn–Ingold–Prelog (CIP) priority rules (atomic numbers, clockwise vs. counterclockwise). It is an absolute configuration system.
- D and L system: Is based on the molecule’s structural relationship to glyceraldehyde (the simplest chiral sugar). It is a relative configuration system.
2. Reference Standard
- D/L system compares the molecule to D-glyceraldehyde or L-glyceraldehyde.
- If the chiral center farthest from the carbonyl group (in sugars) or the α-carbon (in amino acids) has the same orientation as in D-glyceraldehyde → it is D.
- If it matches L-glyceraldehyde → it is L.
3. No direct link to optical activity
- D does not always mean dextrorotatory (+) (rotates light to the right).
- L does not always mean levorotatory (–) (rotates light to the left).
The sign of optical rotation must be determined experimentally.
4. Where they are used
- D/L system: Commonly used for sugars and amino acids in biochemistry.
- Most naturally occurring amino acids are in the L form.
- Most naturally occurring sugars (like glucose) are in the D form.
- R/S system: Used universally for all types of chiral molecules.
In short:
- R/S → absolute configuration (priority rules, universal).
- D/L → relative configuration (compared to glyceraldehyde, mainly for sugars and amino acids).