1. D and L are configurational labels defined relative to glyceraldehyde.
2. Draw the sugar as a Fischer projection with the carbonyl (CHO) at the top.
3. Read the bottom-most (highest-numbered) stereocenter to assign the letter.
4. D-glyceraldehyde and L-glyceraldehyde are non-superimposable mirror images.
5. Almost all natural sugars are D, while most natural amino acids are L.
6. D and L is not the same as (+)/(−) rotation or (R)/(S) configuration.
Summary: Reference is glyceraldehyde · CHO at top of the Fischer projection · read the bottom stereocenter · OH right = D, OH left = L · sugars mostly D, amino acids mostly L · D/L ≠ (+)/(−) ≠ R/S.
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Glyceraldehyde. Its two enantiomers, D- and L-glyceraldehyde, define the labels for all other sugars and amino acids.
The bottom-most stereocenter — the highest-numbered chiral carbon, farthest from the carbonyl. OH on the right is D; OH on the left is L.
No. D/L describes only the reference carbon’s configuration; it is independent of (+)/(−) optical rotation and of (R)/(S) descriptors.
L. Most natural amino acids are L-configured, whereas almost all natural sugars are D.
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