Carbohydrates are molecules which consist of carbon atoms, oxygen atoms and hydrogen atoms. Sometimes nitrogen and sulphur atoms can be present too. The empirical formula of carbohydrates is (CH2O)n. All carbohydrates consist of an aldehyde or ketone molecule with at least one hydroxyl group attached. Monosaccharides are the simplest carbohydrates, they are aldehydes or ketones with at least two hydroxyls attached.
Carbohydrates have many isomers due to them containing numerous chiral centres. Stereoisomers are isomers with the same atoms but with a different arrangement of atoms in space. Carboydrates can form D or L isomers, called enantiomers.
Chiral centres occur when an alpha carbon has four different groups attached to it. The two compounds are called stereoisomers and are mirror images of each other for example Beta – D – glucose, and Beta – L – Glucose are stereoisomers and rotate the plane of polarised light in opposite directions. The D isomer rotates the plane of polarised light clockwise and so is dextrorotatory, and the L isomer rotates the plane of polarised light anticlockwise and so is laevorotatory.
- ↑ Berg J., Tymoczko J., Stryer L., Biochemistry 7th Edition 2012. New York. WH Freeman. page 330-335