CHAPTER 16: Unit 4. Carbohydrates as Chiral Molecules
Carbohydrates as Chiral Molecules
The videos below illustrate the Chirality in carbohydrates.
Chiral carbon atoms in carbohydrates are atoms that are connected to 4 different groups/atoms.
An example of such chiral atom is given in the simplest carbohydrate namely the glyceraldehyde:
The carbon atom covered with the yellow circle is a chiral carbon atom since it is connected to 4 different atoms.
In the literature of chirality an asterisk * is posted at the carbon chiral atom (with 4 different atoms).
The L – glyceraldehyde and D – glyceraldehyde are mirror image to each other and there are non superimposable.
These characteristics or property of L and D glyceraldehyde is known as stereoisomerism. Both molecules are stereoisomers. The figure below illustrates this property:
L – Glyceraldehyde mirror D – Glyceraldehyde
Other examples are illustrated below:
Achiral objects or atoms are none chiral objects or atoms. Achiral objects or atoms are superimposable to each other. While chiral objects or atoms are non superimposable to each other. The non superimposable compounds are stereoisomers and called enantiomers.
The videos below illustrate the difference between the chiral and achiral objects or molecules:
https://www.youtube.com/watch?v=3JUZT0ajsCM
Epimers:
Epimers are optical isomers that differ in the configuration of a single carbon atom.
For example D – mannose and D – glucose differ in the hydroxyl group at Carbon atom number 2 at both structures.
Also D – glucose and D – galactose are epimers because they differ in the position of Carbon atom number 4.
Anomers:When a monosaccharide such as glucose is converted into its cyclic form, two isomers are reacted at Carbon atom number 1 (Hydroxyl group down is α anomer and Hydroxyl group up is β anomer). These two isomers are called anomers and Carbon atom number 1 is called anomeric carbon.
Anomers are special case of epimers where the OH group at Carbon atom number 1 can be up or down.
