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The D and L Notations
The notations D and L are used to describe the configurations of carbohydrates and amino acids. Glyceraldehyde has been chosen as arbitrary standard for the D and L notation in sugar chemistry. Because, this has an asymmetric carbon and can exist as a pair of enantiomers. In a Fischer projection, the carbonyl group is always placed on the top position for monosaccharide. From its structure, if the โ OH group attached to the bottom-most asymmetric center (the carbon that is second from the bottom) is on the right, then, the compound is a D- sugar. If the โ OH group is on the left, then, the compound is a L-sugar. Almost all sugars found in nature are D-sugar.
Like R and S , D and L indicate the configuration of an asymmetric carbon, but they do not indicate whether the compound rotates polarized light to the right or to the left. For example, D-glyceraldehyde is dextrorotatory, whereas D-lactic acid is levorotatory. In other words, optical rotation, like melting or boiling points, is a physical property of a compound, whereas โ R , S , D, and Lโ are conventions humans use to indicate the configuration of a molecule. D/L Configuration of the Aldoses and Ketose Aldohexose (Glucose), Ketohexose (Fructose) :
Mechanism of Epimerisation
โ In the presence of base, D-glucose may be converted into D-mannose via the removal of hydrogen at C- 2 carbon followed by protonation of the enolate
Anomers:
โ Anomer is the stereoisomer, having different configuration at anomeric carbon. โ It is a type of geometric isomer found at certain atoms in carbohydrate molecules. โ The anomeric carbon is the carbon derived from the carbonyl carbon (the ketone or aldehyde functional group) of the open-chain form of the carbohydrate molecule. โ Anomerization is the process of conversion of one anomer to the other. โ The different anomers have different physical properties, melting points and specific rotation.
