Epimers can be called isomers

List of experiments on carbohydrates

To understand why there are so many carbohydrates and why they react or taste so differently, why they have so different functions as biomolecules, one has to look at their structure. They all seem to have the same composition: They each have a carbonyl function and hydroxyl groups.
There are many isomers. There are 12 isomers of the hexoses alone, 6 of the pentoses, 3 of the tetroses and two trioses - that is, 23.
If you put them all together, we have 45 isomers. (A triose, dihydroxyacetone, is not a chial compound.)
We didn't even take into account sugars such as deoxyribose or amino sugar. Or sugars like heptoses, which play an important role in photosynthesis or gluconeogenesis.


What is a Chirality Center?
The word chiral comes from the Greek and means something like "handiness". If you look at your hands, you will see that they look like a picture and a mirror image. They are congruent, that is, they match, but they are not the same. Your hands are chiral.
The same can happen with molecules.

These two molecules are chiral because they too behave like an image and a mirror image. This property is due to the spatial structure of the tetrahedron. The atom in the middle is that Chirality center.

Chirality can only ever occur if four different substituents are bound to an atom and this results in the possibility of a spatially different arrangement. This is why this carbon atom is called asymmetric atom. Compounds that differ only in the arrangement of the substituents around one or more centers of chirality are also referred to as Stereoisomers.

The phenomenon of chirality also occurs with carbohydrates. Every atom of the carbon chain, with the exception of the first and last carbon atoms, can have a center of chirality. Other well-known examples from biochemistry are the amino acids.


What are enantiomers?
If two compounds differ from one another only in the arrangement of the substituents at the center of chirality, they will Enantiomers called. An example of this is the simplest carbohydrate Glyceraldehyde.
An aid for determining the arrangement of the substituents around a center of chirality is the Fischer projection. The three-dimensional molecule is shown in a plane, i.e. planar.

Since glyceraldehyde has only one asymmetric carbon atom, there are only two possible variants of the substituent arrangement and thus two possible enantiomers. They are identified by the symbols D (lat. dexter, right) and L (lat. laevus, left). In the case of glyceraldehyde, it denotes the absolute position of the hydroxyl group.

How do you actually know in which of the two forms the OH group is on the right or left? We have an extra website for this.

Now there are also molecules with several centers of chirality. An example is glucose. Only when their molecules behave like image and mirror image in all centers of chirality are they enantiomers. The enantiomerism thus affects the entire molecule. So there is D- and L-glucose.


What are diastereomers?
In the case of compounds that have more than one center of chirality, it is possible that they do not differ in all centers, i.e. H. arrangements can arise that do not behave like an image and a mirror image. To clarify, let's consider the Amino acid threonine. There are four positional isomers of this, as it has two centers of chirality.

Stereoisomers of threonine

The naturally occurring L-amino acid has the second structure from the right. The symbol L refers to the amino group.

Two of these behave like an image and a mirror image. However, these two groups cannot be mapped to one another. They are called Diastereomers.

These are stereoisomers that do not behave like an image and a mirror image. They also have different structures, which in turn affects their chemical and physical properties.

Well-known examples of diastereomers from sugar chemistry are a-D-glucose and-D-glucose, which only differ in the position of the OH group on C atom 1. They are called Anomers. They are chemically very different. The plant builds starch from a-D-glucose, and from Cell-D-glucose it makes cellulose.


What are epimers?
Under Epimers one understands two diastereomers, which differ only in the position of the substituents at the chiral center on the carbon atom 2 (Greek epi, on, over, regarding the tip). The best-known examples of this are the carbohydrates D-glucose and D-mannose, which only differ at the carbon atom 2. Fructose is not one of them because it has no center of chirality in C.2.
Epimers are characterized by the fact that they form identical osazones. This is where its name comes from.

Sometimes the pair D-galactose and D-glucose is also referred to as epimeric. They only differ at the carbon atom 4, so they are diastereomeric. However, they make different Osazones.


What is a meso shape?
There is another form of stereoisomers and that is the meso compounds. Let us first consider a well-known, historically significant example: tartaric acid.

Structural isomers of tartaric acid

Meso compounds contain two or more centers of chirality. However, their molecules are congruent with their mirror image. So they are identical. The reason: you own one intramolecular mirror planewhich makes it possible to map one half onto the other and thus mirror one center of chirality onto the other. So two meso compounds are not enantiomers, even though they look like that at first glance.

There is an example of this from everyday life: The simple cup is one meso-Shape. So there are no cups for left-handers, as is sometimes jokingly said. Scissors, on the other hand, are available for left- and right-handed people!

An example from sugar chemistry is the oxidation product of galactose, mucic acid.


More about stereoisomerism and the exact nomenclature can be found here:
Snail kings and the hands of molecules

For specialists: Description of chiral molecules according to Cahn-Ingold-Prelog


Further texts on the subject of `` carbohydrates ''