anomer

When a sugar curls from its open chain into a ring, the carbon that was the carbonyl becomes a new chiral center — and a new chiral center can be made two ways. The two possibilities are the anomers: two ring forms of the same sugar that differ only at that single carbon, called alpha and beta. They are not separate sugars; they are two faces of the same molecule, and in water they slowly flip back and forth into each other.

Here is the picture. When the open-chain glucose closes, the chain's hydroxyl can attack the flat carbonyl carbon from the top face or the bottom face. The carbonyl carbon had no handedness; the ring's anomeric carbon does. If the new -OH ends up pointing down (on the opposite side from the CH2OH at the top, drawn below the ring), it is the alpha-anomer; if it points up (same side as the CH2OH), it is the beta-anomer. Only the configuration at this one carbon — the anomeric carbon — differs. Because they differ at only one of several stereocenters, anomers are a special case of diastereomers, given their own name. Left in water, pure alpha-glucose will partly open back to the chain and re-close as beta, until an equilibrium mix is reached — a process you can watch as a slow change in optical rotation called mutarotation.

Anomers are worth a name because that one carbon does a lot of work. Whether a glycosidic bond is alpha or beta decides whether a polymer of glucose is digestible starch or indigestible cellulose: humans have enzymes for alpha links but not beta ones, which is why we eat bread but not wood. The whole difference between food and firewood lives at the anomeric carbon.