Continued from 2-Carb-2
In this representation of a monosaccharide, the carbon chain is written vertically, with the lowest numbered carbon atom at the top. To define the stereochemistry, each carbon atom is considered in turn and placed in the plane of the paper. Neighbouring carbon atoms are below, and the H and OH groups above the plane of the paper (see below).
Conventional representation of a carbon atom (e.g. C-2 of D-glucose) in the Fischer projection. Representation (e) will be used in general in the present document.
The formula below is the Fischer projection for the acyclic form of D-glucose. The Fischer projections of the other aldoses (in the acyclic form) are given in Chart I (2-Carb-2.2).
Note. The Fischer projection is not intended to be a true representation of conformation.
2-Carb-4. Configurational symbols and prefixes
2-Carb-4.1. Use of D and L
The simplest aldose is glyceraldehyde (occasionally called glyceral ). It contains one centre of chirality (asymmetric carbon atom) and occurs therefore in two enantiomeric forms, called D-glyceraldehyde and L-glyceraldehyde; these are represented by the projection formulae given below. It is known that these projections correspond to the absolute configurations. The configurational symbols D and L should appear in print in small-capital roman letters (indicated in typescript by double underlining) and are linked by a hyphen to the name of the sugar.
2-Carb-4.2. The configurational atom
A monosaccharide is assigned to the D or the L series according to the configuration at the highest-numbered centre of chirality. This asymmetrically substituted carbon atom is called the 'configurational atom'. Thus if the hydroxy group (or the oxygen bridge of the ring form; see 2-Carb-6) projects to the right in the Fischer projection, the sugar belongs to the D series and receives the prefix D-.
2-Carb-4.3. Configurational prefixes in systematic names
In the systematic names of sugars or their derivatives, it is necessary to specify not only the configuration of the configurational atom but also the configurations of all CHOH groups. This is done by the appropriate configurational prefix. These prefixes are derived from the trivial names of the aldoses in Chart I (relevant portions of the structures are delineated in bold face). In monosaccharides with more than four asymmetrically substituted carbon atoms, where more than one configurational prefix is employed (see 2-Carb-8.3), each group of asymmetrically substituted atoms represented by a particular prefix has its own configurational symbol, specifying the configuration (D or L) of the highest numbered atom of the group.
The configurational prefixes are printed in lower-case italic (indicated in typescript by underlining), and are preceded by either D- or L-, as appropriate. For examples see 2-Carb-4.2 and 2-Carb-6.2
Note. In cyclic forms of sugars, the configuration at the anomeric chiral centre is defined in relation to the 'anomeric reference atom' (see 2-Carb-6.2).
2-Carb-4.4. Racemates and meso forms
Racemates may be indicated by the prefix DL-. Structures that have a plane of symmetry and are therefore optically inactive (e.g. erythritol, galactitol) are called meso forms and may be given the prefix 'meso-'.
2-Carb-4.5. Optical rotation
If the sign of the optical rotation under specified conditions is to be indicated, this is done by adding (+)- or (-)- before the configurational prefix. Racemic forms are indicated by (±)-.
D-Glucose or (+)-D-glucose
D-Fructose or (-)-D-fructose
DL-Glucose or (±)-glucose
21. IUPAC-IUB Joint Commission on Biochemical Nomenclature (JCBN), The nomenclature of lipids (Recommendations 1976), Eur. J. Biochem., 79, 11-21 (1977); Hoppe-Seyler's Z. Physiol. Chem., 358, 617-631 (1977); Lipids, 12, 455-468 (1977); Mol. Cell. Biochem., 17, 157-171 (1977); Chem. Phys. Lipids, 21, 159-173 (1978); J. Lipid Res., 19, 114-128 (1978); Biochem. J., 171, 21-35 (1978); ref. 2, pp. 180-191.
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