The D,L-System

D- and L- are older configurational descriptors used to denote the configuration of chiral molecules, especially carbohydrates and a-amino acids. Fischer projections are used to assign the symbols D- and L-. 

Nowadays, R,S- descriptors are used for all classes of molecule except for the following  

For carbohydrates, in general, the position of the OH group attached to the highest-numbered carbon atom in the chain determines the assignment of d- and l-. For instance, in D-glncose the OH at position 5 is on the right-hand side of the Fischer projection. 

Organic Chemist s Desk Reference, Second Edition 

Amino acids. These retain the d,l- system because all of the protein amino acids belong to the L- series, but not all of them are S- according to the sequence rnle. See Chapter 5. Biochemists often use the d,l- system for synthetic compounds derived from amino acids where most organic chemists would use R.S-. In a-amino acids, the L-componnds are those in which the NH2 gronp is on the left-hand side of the Fischer projection in which the COOH group is at the top. Conversely, the o-compounds are those in which the NH2 group is on the right-hand side. 

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We mentioned in Section 7 6 that the d l system of stereochemical notation while outdated for most purposes is still widely used for carbohydrates and amino acids Likewise Fischer projections find their major application m these same two families of compounds... 

The convention that is used to designate the configurations of chiral carbons of naturally occurring compounds is called the d,l system. To use it, we view the molecule of interest according to the following rules ... 

Use the d,l system to designate the configuration at each chiral center in Structures a-e in Exercise 5-33. 

Exercise 15-15 How can D-glucose, D-fructose, and D-ribose be considered products of the addition of an alcohol to the carbonyl group of an aldehyde or ketone Name each of the carbonyl compounds by the IUPAC system. For the ribose carbonyl structure, determine the configuration at each chiral center, using the D,L system. 

Most amino acids, with the notable exception of glycine, are optically active. Their configurations are usually indicated by the D/L system (Figure 1.4) rather than the R/S system. Most naturally occuring amino acids have an L configuration but there are some important exceptions. For example, some bacteria also possess D-amino acids. This is important in the development of some antibacterial drugs. 

The system has nothing to do with optical rotation and should not be confused with the d/l system of nomenclature. 

The D,L- system shonld no longer be used except for compounds that are closely and unambiguously related to either carbohydrates or amino acids. This is to avoid ambiguity. By suitably modifying the groups on, e.g., an amino acid in different ways, it is possible to arrive at compounds that can be described either as d- or l-, depending on the route used, whereas the R,S- system is unambiguous. 

Note also that the D,L system of carbohydrate nomenclature describ the configuration at only one chirality center and says nothing about i configuration of other chirality centers that may be present. The advanti of the system, though, is that it allows us to relate one sugar to anotl rapidly and visually. 

Two conventions are in use to describe the configuration of a stereogenic centre. The older system uses the symbols D and l, and is still employed for two important classes of compound, namely amino acids and sugars otherwise it has been superseded. The d/l system will be considered first. The use of capital letters here serves to distinguish this convention from dll (page 22), which relates only to the sense in which the plane of polarized light is rotated. 

A limitation of the d/l system may be seen from the example of (+)-camphor (10), which was assigned configuration D. Comparison with the formula 3 for D-glyceraldehyde reveals that the inter-relationship is, at best, tenuous. Some suppliers still offer D-camphor for sale. 

This system labels each chiral center in a molecule (and also has an extension to chiral molecules not involving chiral centers). It thus has greater generality than the D/L system, and can label, for example, an (R,R) isomer versus an (R,S) — diastereomers. 

For this reason, the D/L system remains in common use in certain areas, such as amino acid and carbohydrate chemistry. It is convenient to have all of the common amino acids of higher organisms labeled the same way. In D/L, they are all L. In R/S, they are not, conversely, all S — most are, but cysteine, for example, is R, again because of sulfur s higher atomic number. 

The D, L system, was arbitrarily based on the configuration of the enantiomeric glyceraldehyde molecules the (-l-)-isomer was taken to have the structure implied by formula 1 and this arrangement of atoms was called the d configuration. Conversely, formula 2 was designated as representing the l configuration ... 

Lactic acid provides a simple example of how the d, l system could be applied to other compounds. The relative configuration of lactic acid is determined by... 

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