anomeric carbon

Meaning: The anomeric carbon is the former carbonyl carbon of a sugar. When an open-chain sugar cyclizes, a hydroxyl on the same chain attacks the carbonyl, forming a hemiacetal (aldose) or hemiketal (ketose). That carbon becomes a new stereocenter bonded to two oxygens. It is C1 in glucose and C2 in fructose. The two configurations are anomers. In a Haworth projection of a D-sugar, α puts the anomeric OH down (trans to the CH₂OH group) and β puts it up (cis to it). In solution the anomers interconvert through the open-chain form, called mutarotation.

On the MCAT: You may be asked to find the anomeric carbon in a ring, label a linkage α or β, or decide whether a sugar is reducing. A free anomeric OH lets the ring open to an aldehyde, so the sugar reacts with Benedict's or Tollens' reagent. A glycosidic bond at the anomeric carbon makes an acetal or ketal, which cannot open and mutarotate. Maltose and lactose keep one free anomeric carbon and are reducing; sucrose uses both and is nonreducing.

Common confusion: α-glucose and β-glucose are anomers, a special kind of epimer, and they are diastereomers, not enantiomers. They differ at one carbon and are not mirror images. Also, the anomeric carbon is not always C1; in fructose it is C2.