D and L nomenclature

Although D-amino acids are occasionally found in nature, we usually assume the amino acids under discussion are the common L-amino acids. Remember once again that the d and l nomenclature, like the R and S designation, gives the configuration of the asymmetric carbon atom. It does not imply the sign of the optical rotation, (+) or (-), which must be determined experimentally. 

The absolute configuration at a chiral tetracoordinate center can be described using the D and l nomenclatures (but there are drawbacks, see [25]), or with the R and S nomenclature to be summarized below. 

An alternative definition specifies that the more highly oxidized terminal carbon atom be at the top of the Fischer projection. For carbohydrates and amino acids, there is ordinarily no conflict between these two definitions. For historical notes and an elaboration of the d and l nomenclature system, see Slocum, D. W. Sugarman, D. Tucker, S. P. /. Chem. Educ. 1971,48,597 reference 140a, pp. 88-92. 

The D and l nomenclature designations are like (R) and (S) designations in that they are not necessarily related to the optical rotations of the sugars to which they are applied. Thus, one may encounter other sugars that are d-(+) or d-( ) and ones that are L-(+) or l-(-). 

The same nomenclature has been adopted for amino-acids, the configurational family to which the a-carbon atom belongs being denoted by the prefixes d- and L-. 

The a-carbon of all amino acids, with the exception of glycine, has four different substituent groups and is therefore an asymmetric carbon atom. Such an atom can exist in two different spatial arrangements which are mirror images of each other. These structural forms of molecules are known as stereoisomers and the common notation of D and L forms is used, a nomenclature that refers to their absolute spatial configuration when compared with that of glyceraldehyde (Figure 10.4). 

The use of d- and L-prefixes is a nomenclature for orientation of atomic structure of sugar and amino acid molecules. It is a structural definition and is not related to the optical properties. 

Before 1968, the nomenclature for inositols assigned the symbols d- and l- to the highest-mm-bered chiral centre, C-6. This convention was based on the rules for naming carbohydrates. For substituted myo-inositols, in particular where C-1 and C-6 hydroxyl groups are trans, compounds identified in the literature before 1968 as d- are now assigned 1l-. 

Both D- and L-forms of the ketohexose related equally to altrose and allose are known. In conformity with the present system of nomenclature for ketone sugars, the name allulose is used in preference to the older terms pseudofriictose, psicose, 2-ketoribohexose, etc. 

Comment. This rule is simply an attempt to adapt the best organic nomenclature practices to coordination compounds. Difficulty arises because those who have worked with optically active inorganic compounds almost invariably have used d- and l- to designate the sign of rotation of the coordinated group and large capitals D and L for the sign of rotation of the coordination entity. 

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