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.

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.)

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

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.

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.
Webmaster, Department of Chemistry, University of Maine, Orono, ME 04469