Fischer projection

The Fischer projection is a method clearly represents the spatial structure of a chiral chemical compound two-dimensional. It was developed by Emil Fischer and uses the stereodescriptors D (Latin dexter 'right' ) and L (lat. laevus "left" ), which are represented as small caps, and separated by hyphens from the rest of the formula.

Regulate

In many chemical compounds the position of atoms in space is crucial for their properties, which makes a distinction between different stereoisomers necessary. Especially with carbon compounds, it is often difficult to make the spatial orientation of up to four binding partners in the tetrahedral angle of 109.5 ° clearly.

The Fischer projection solves this problem without perspective means by adherence to the following rules when drawing the molecule:

• A chain of carbon atoms is drawn from top to bottom, the most oxidized atom above. ( According to the rules of the nomenclature of hydrocarbons, the carbon atom number 1 above. This must be the most oxidized carbon atom not always, but it is in most cases). There are also agreed exceptions such as the galacturonic acid which is formally an oxidized form of the D-galactose. The highly oxidized carbon is in this case the C- atom to the No. 6, and it adheres to the numbering of D-galactose.

• Horizontal (horizontal ) lines show from the projection plane out towards the viewer. This can be also indicated by filled, toward the center, thinning wedges.

• Vertical (vertical ) lines run behind the projection plane, away from the viewer. This can be indicated by quergestrichelte, toward the center, thinning wedges.

In addition, the carbon atom is implicit, i.e., the "C" is not written in the particular chiral centers.

As an example, the Fischer projections of the two enantiomers of glyceraldehyde are given, and the corresponding wedge line formulas. With the stereodescriptors D and L then the configuration of the bottom stereocenter is specified, depending on whether one horizontal rest with the higher priority (in this case OH > H) points to the right (D) or left ( L). In the presence of multiple stereocenters can not be more Ds and Ls accumulate (see below)

It is important to note that the real spatial structure of the chiral molecules to the Fischer time was not known. We used the glyceraldehyde as a reference substance: the dextrorotatory enantiomer was arbitrarily assigned to the projection with the right-pointing OH radical and this, therefore, as the D-configuration (from the Latin dexter, " right" ), without that you could know if this is the corresponded to reality. Compounds other (sugar in particular) can be reduced to the dextrorotatory glyceraldehyde, they were under this assumption that is also D-configuration.

1951 was Johannes Martin Bijvoet demonstrated by the X-ray structure analysis of the glyceraldehyde the actual spatial structure. Here, the arbitrary assumption turned out to be correct: The D and (R ) - configurations of glyceraldehyde are identical. From D/L- or (R / S)- configuration can not automatically α to the angle of rotation or the direction of rotation [( ) or (- )] of the plane of polarization of linearly polarized light to be closed. Examples:

If there are several stereocenters, the configurations of these not ( such as in the Cahn -Ingold -Prelog convention ) can be specified after the other one. There is more than one chiral center, there are more than two stereoisomers, including diastereomers and enantiomers. When using the Fischer nomenclature, it is imperative to give diastereomers have different names:

Erythrose and threose are, for example, two different sugars, which differ only by the configurations of the two chiral centers. Specifically, they are diastereomers. Of the two, there is each a pair of enantiomers. The relative configuration of the upper stereocenter ie is determined by the name: The erythrose show the OH groups in the same, in the Threose in different directions. However, since, for example, C6 - aldoses ( including glucose) have four stereocenters, there are 24 = 16 stereoisomers. This corresponds to eight pairs of enantiomers, so you have to remember eight different names and eight different relative configurations for this group of sugar molecules.

Examples of the biochemistry of the D and L - compound, D- glucose (dextrose ), and the amino acid L- alanine.

Application

For a chiral

There are also a number of rules for the interpretation of a Fischer projection. This structures can be examined for isomerism, without having to imagine in three dimensions, which is often very difficult. For a projection with a central carbon and four substituents applies:

• Rotation of 180 ° (360 °, ...) provides the same molecule,
• Rotation of 90 ° (270 °, ... ) gives the corresponding enantiomer.

• A ( three, five, ...) exchange (s) of two substituents yields the enantiomer
• Two ( four, six, ...) permutations of two substituents that result in the same molecule.

It should firstly be noted that carbon that does not have four different substituents, but at least two of the same or a double bond, are not chiral and therefore are not required for the above rules. Secondly, it is noted - as it will appear trivial - that all of the substituent is to be compared with the detection of differences, i.e., on one side of the propyl and the other one depends ethyl chain, the substituents are different.

For more chiral

For multiple chiral centers, the same rules apply, namely for each center. To determine the isomerism of a molecule, there is the " Vertauschungsmethode " to. Are there differences between two structures only by the arrangement of the chiral centers, then:

• You are exactly identical if each center is identical, ie at each center two (or even zero) permutations are needed to convert a structure into another,
• They are then accurately enantiomers, if this is true for each center, ie at each center an exchange is necessary, in order to convert a structure into another,
• They are diastereomers if and only if the other two cases do not occur, that is, for example, if at one center and one at the other two permutations are needed.

The numbers go from bottom to top. The lowest carbon atom numbered as one is achiral and serves only to confusion. The carbons two and three are chiral. In the first case the center of two apparently identical with the center of the three permutations of two are needed, it is also identical: the molecules are identical. In the second case you need to center a two to number three two permutations: There are diastereomers. In the third case, a reflection is present ( at both centers a permutation ), there are enantiomers.

Meso- forms

In the Fischer projection can be easily seen a plane of symmetry and two isomers and meso forms. For a discussion we refer to the articles on meso forms.