In 1891, Hermann Emil Fischer, a German chemist, Nobel Laureate in chemistry in 1902, developed a systematic method for the two-dimensional representation of molecules with chirality centers, the so-called Fischer projections or Fischer projection formulas.
Despite they are two-dimensional structures, Fischer projections preserve information about the stereochemistry of the molecules and, although not being a representation of how molecules might look in solution, are still widely used by biochemists to define the stereochemistry of amino acids, carbohydrates, nucleic acids, terpenes, steroids, and other molecules of biological interest.
By considering a molecule with a single chiral center, e.g. a carbon atom, for drawing the Fischer projections, the tetrahedral structure is rotated so that two groups point downward, whereas two groups point upwards. Then, you draw a cross, place the chiral center at the center of the cross, and arrange the molecule so that the groups pointing downward, that is, behind the plane of the paper, are attached to the ends of the vertical line, and the groups pointing upwards, that is, out front from the plane of the paper, are attached to the ends of the horizontal line.
For compounds with more than one chiral center, the same procedure is applied to each asimmetric center.
It is also possible to convert a Fischer projection into a three-dimensional representation, for example using the wedges and dashes of perspective formulas, where the two horizontal bonds are represented by solid wedges, whereas the vertical bonds are represented by dashed lines.
Since Fischer projections represent three-dimensional molecules on a two-dimensional sheet of paper, some rules must be respected to avoid changing the configuration.
- The projections must not be lifted out the plane of the paper, because this causes enantiomer is converted into the other enantiomer.
- If you rotate the projections in the plane of the paper, you obtain the same enantiomer if you rotate the structures by 180° in either direction, because the vertical groups must lie below the plane of the paper, whereas the horizontal groups above. Conversely, the rotation by 90° or 270° in either direction causes an enantiomer is converted into the other enantiomer.
- An odd number of exchanges of two groups leads to the other enantiomer.
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