Mutarotation refers to a change in optical rotation of chiral molecules due to the opening and closing of cyclic sugar rings as they switch between their alpha and beta anomers. Which of the following molecules is a β-carbohydrate?
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This question is testing your knowledge of α- and β-carbohydrates. The determination is based on the position of the OH group attached to the anomeric carbon and the CH2OH group attached to carbon 5. β-Carbohydrates have the OH and CH2OH group on the same side of the ring. Conversely, α- carbohydrates have the OH and CH2OH group on opposite sides of the ring.
Choice C (below) is the only molecule with the hydroxyl and hydroxymethyl group on the same side of the ring.
Thus, choice C, which is β-glucose, is the correct answer. The carbohydrates shown in A, B, and D are α-galactose, α-mannose, and α-glucose, respectively. As you can see from those answer choices, the OH attached to the anomeric carbon and the CH2OH are on opposite sides of the ring.
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