D,L system

We mentioned in Section 7 6 that the d l system of stereochemical notation while outdated for most purposes is still widely used for carbohydrates and amino acids Likewise Fischer projections find their major application m these same two families of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 discoveries of optical activity and enantiomeric structures (see the box, page 97) made it important to develop suitable nomenclature for chiral molecules. Two systems are in common use today the so-called d,l system and the (R,S) system. 

The newer (R,S) system of nomenclature is superior to the older d,l system in one important way. The configuration of molecules with more than one... 

Because of the somewhat ambiguous definitions of D and L, another system of nomendature was devised for asymmetric compounds, and it has largely replaced the old D, L-system since the 1960s. However, the D, L-nomendature is still used for amino adds and sugars. 

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. 

Often confused with die D/L system of nomenclature to which it has absolutely no relationship. The d stand for dex-tro or dextrorotatory1 and simply signifies that plain polarised light is rotated to the right (clockwise). The 1 stands for laevo or lacvorotatory1 and signifies that the molecule rotates plane polarised to the left (anticlockwise). The system is outdated and largely replaced with die ( + )/(-) nomenclature. 

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

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. 

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. 

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