Alpha and beta carbohydrates are determined by the position of the hydroxyl group attached to the anomeric carbon. Which of the following molecules is a β-carbohydrate?
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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