Configuration Fischer convention

The diirality of a molecule is described by specifying its configuration. The system that is used is the Cahn Ingold-Prel6g convention, which uses the descriptors R and S. The Fischer convention, employing the descriptors D and L, is historically important and is still used with certain types of molecules. 

In the Fischer convention, all common proteinogenic amino acids belong to the L series, but w ith the CIP convention the S configuration does not universally apply, (/ )-cysteine being an exception. 

The Fischer convention is widely used in sugar chemistry and for a-amino acids. For sugars, which contain a number of asymmetric centers, and for diastereomers with only two centers, the Fischer convention defines a series as D or L according to whether the configuration at the highest numbered asymmetric center is analogous to D- or L-glyceralde-... 

Figure 7 The configurations of D-(+) and L-(-)-gIyceraIdehyde, D-erythrose, and D threose, according to the Fischer Convention.

The Fischer convention is often inexact and difficult to use, especially when complicated chemical transformations are required to convert the molecule under investigation into a molecule of known configuration. In addition, the assigned configuration, D or L, is often confused with the... 

In the Fischer convention, the configuration of a molecule is described by the descriptors d and l, which are assigned by comparison with the reference molecule... 

With the exception of glycine, where R = H, a-amino acids have a stereogenic center at the a-carbon. All except glycine are therefore optically active. They have the L configuration relative to glyceraldehyde (Figure 17.1, page 495). Note that the Fischer convention, used with carbohydrates, is also applied to amino acids. 

In addition to the copolymer structure, proteins are also optically active. All amino acids except glycine possess at least one asymmetric carbon atom, as illustrated in equation (A2.1.1). The language commonly used to characterize the structure is different, however. The Fischer convention of designating the active group as D- or L- is used (Al). All the natural proteins contain the L configuration. Woe to the person who feeds an earth-bound creature the D configuration This difference, of course, has generated many a science fiction story. 

By means of the Fischer convention, the tetrahedral models for glucose, mannose, and arabinose are equivalent to the planar formulas given below. The formula for fructose is derived from the fact that fructose yields the same osazone as glucose when treated wdth phenylhydrazine (see above) it and glucose must have identical configurations for carbon atoms 3, 4, and 5. 

When the tail group attached to the ring contains an asymmetric carbon atom as in the heptopyranoses or the hexofuranoses and the corresponding glycosides, a convention is necessary to represent the configuration of this asymmetric atom. To do this, the Fischer convention may be applied for the tail group. Thus, glucofuranose would be ... 

In the Fischer convention, the configurations of other molecules are described by the descriptors D and L, which are assigned by comparison with the reference molecule glyceraldehyde. In employing the Fischer convention, it is convenient to use projection formulas. These are planar representations defined in such a way as to convey three-dimensional structural information. The molecule is oriented with the major carbon chain aligned vertically in such a manner that the most oxidized terminal carbon is at the top. The vertical bonds at each carbon are directed back, away from the viewer, and the horizontal bonds are directed toward the viewer. The D and L forms of glyceraldehyde are shown below with the equivalent Fischer projection formulas. 

Geometric isomers of alkenes are diastereomeric, since they are stereoisomers but not enantiomeric. The specification of the geometry of double bonds as cis and trans suffers from the same ambiguity as specifying configuration by the Fischer convention that is, it requires a subjective judgment about the similarity of groups. The sequence rule is the... 

A non-standard form of Fischer projection was used by Levene, in common with several other authors at about this time [6]. Bonds which should be interpreted as lying above the plane of the paper are shown as dotted lines, i.e. the configurations shown, if interpreted Uterally, are the converse of the Fischer convention prevailing at the time. 

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