D and L Are Outmoded and Wrong
Students who take biochemistry are exposed to an old, confusing, and often incorrect method of specifying configurations at chiral centers as D or L. This scheme was devised in the early years of this century by Emil Fischer, a German organic chemist who worked extensively with carbohydrates. Here's how it is supposed to work.
The compound glyceraldehyde, HOCH2CH(OH)CHO, was chosen as the standard for defining configuration.
- The enantiomer that rotates plane polarized light clockwise (+) was arbitrarily labeled D
- The other enantiomer (-) became L.
As shown, the assignments in modern notation are R and S, respectively. (Note: it will not always work out that D = R and L=S; this is an accident here.)
- The source of the D and L labels was the Latin words dexter (on the right) and laevus (on the left)
- R comes from rectus (right-handed) and S from sinister (left-handed)
Any other molecule containing a single chiral center was to be assigned as D or L by imagining a resemblance between the ligands on its chiral center and those in glyceraldehyde
- The enantiomer having the "same or similar" groups in the same places as D-glyceraldehyde becomes D.
- Thus, for example, the naturally occuring form of the amino acid cysteine was labeled L.
When more than one chiral center is present, similarity is defined only by the arrangement of ligands on the highest numbered chiral center, and the assignment of D or L is made on the basis of that center only.
- The configuration at the other centers usually is specified by giving the diastereomeric molecules entirely different names.
- The student is expected simply to memorize which arrangement of ligands goes with which names.
- For example, look at the two aldotetroses below:
- The D and L were assigned on the basis of the arrangement at the lower of the two chiral centers.
- That the two sugars are diastereomers is specified by giving them different names.
Clearly, this system is impossible to apply widely, requires extensive memorizing of structures, and is also seriously ambiguous.
Consider for example, the reaction outline shown below. Here D-glyceraldehyde is converted into lactic acid by reactions that do not break any bonds to the chiral center! (These conversions actually have been carried out in the laboratory.)
Which structure is D-lactic acid?
For more examples of the inadequacy and ambiguity of this scheme, see an article in the Journal of Chemical Education, 1971, 48, 597. Isn't it about time biochemistry moved into the 20th century?
This page last modified 3:03 PM on Monday June 4th, 2012.
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