anomer

In carbohydrate chemistry, an anomer is a special type of epimer. It is one of two stereoisomers of a cyclic saccharide that differs only in its configuration at the hemiacetal or hemiketal carbon, also called the anomeric carbon.[1] Put simply, the anomeric carbon is the carbonyl carbon, for example a ketone or aldehyde functional group, in a carbohydrate molecule. Anomerization is the process of conversion of one anomer to the other. Anomerization is the anomeric analogue of epimerization.

The anomeric centre of a sugar is a stereocentre created from the intramolecular formation of an acetal (or ketal) of a sugar hydroxyl group and an aldehyde (or ketone) group. The two stereoisomers formed from the two possible stereochemistries at the anomeric centre are called anomers. They are diastereoisomers of one another.

The configuration at the anomeric centre (that derived from the carbonyl carbon) is denoted alpha- (α-) or beta- (β-) by reference to the stereocentre that determines the absolute configuration. In a Fischer projection, if the substituent off the anomeric centre is on the same side as the oxygen of the configurational (D- or L-) carbon, then it is the α--anomer. If it is directed in the opposite direction it is the β-anomer.