anomer

Meaning: Anomers are two cyclic forms of a sugar that differ in configuration at the anomeric carbon, the former carbonyl carbon (C1 in glucose, C2 in fructose). Ring closure occurs when a hydroxyl in the same molecule attacks the carbonyl, forming a hemiacetal (aldose) or hemiketal (ketose) and a new stereocenter. In a Haworth projection of a D-sugar, α has the anomeric OH down, trans to the CH₂OH group; β has it up, cis to that group.

On the MCAT: Expect to label a drawn ring α or β, or to name the relationship between two sugars. Mutarotation appears as polarimetry: a freshly dissolved pure anomer's optical rotation drifts to a constant intermediate value as α and β interconvert through the open-chain form. Linkages matter too: starch and glycogen contain α-1,4 bonds, cellulose contains β-1,4 bonds, and human amylase cleaves the α links.

Common confusion: Anomers are a special kind of epimer, but epimers that differ at a different carbon are not anomers. Glucose and galactose are C4 epimers. Anomers are diastereomers, not enantiomers. The α/β label refers to the anomeric carbon, while D/L refers to the highest-numbered stereocenter, so α-D-glucose and α-L-glucose are enantiomers.