Anomer

In carbohydrate chemistry, anomers (from Greek ἄνω 'up, above' and μέρος 'part') are specific types of stereoisomers found in sugars.

Many common sugars, such as glucose, exist in both a linear (or open-chain) form and a cyclic (or ring) form. The ring is formed when one end of the sugar molecule connects to the other end. The carbon atom where this ring closure occurs is called the anomeric carbon. Depending on the direction from which the connection is made, this anomeric carbon can have its new group (–OH) pointing in one of two distinct orientations, typically visualized as "up" or "down" in a standard diagram. These two resulting molecules are the anomers and are labeled with the Greek letters alpha (α) or beta (β).

More formally, an anomer is an epimer at the hemiacetal/hemiketal carbon atom in a cyclic saccharide. The process of one anomer converting to the other is known as anomerization. Because they have different three-dimensional structures, anomers have distinct physical properties, such as melting point and specific rotation.