Which of the following structures represents the Fischer projection of a β-D-glucose unit found in cellulose?

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11ecba2d_0e38_a26f_a3bf_79e484196506_MD0008_00 Fischer projections are two-dimensional representations of molecules that are often used to distinguish among carbohydrate anomers, epimers, and enantiomers. Carbohydrate enantiomers (L- and D-conformations) have the same chemical formula but differ at every stereocenter. In a Fischer projection, D-sugars always show the second-to-last carbon (carbon 5 in glucose) as having its oxygen atom to the right. Naturally occurring sugars generally adopt the D-conformation. Epimers are carbohydrates that differ at only one stereocenter. Fischer projections of D-glucose always show one hydroxyl group to the right of carbon 2, one to the left of carbon 3, and one to the right of carbon 4. Any carbohydrate that differs at exactly one of these positions is an epimer of D-glucose. Anomers are a special case of epimers that differ from each other at the anomeric carbon. They can exist only in cyclized sugars (furanose or pyranose) because in the linear form, the anomeric carbon is a carbonyl and cannot be a stereocenter. In Fischer projections, cyclic D-sugars with a hydroxyl group to the right of the anomeric carbon are in the α-conformation, whereas those with the hydroxyl group on the left are in the β-conformation. The opposite is true for L-sugars. Cellulose is composed of multiple β-D-glucose units in the pyranose form. Therefore, a Fischer projection of β-D-glucose must display a six-member ring with a hydroxyl group to the right of carbon 5 (D-sugar) and one to the left of the anomeric carbon (β-conformation). Of the choices available, only the Fischer projection in Choice B has the correct stereochemistry. (Choice A) This structure is α-D-mannose, a glucose epimer that differs at carbon 2. (Choice C) This is the structure of β-D-fructose. (Choice D) This is β-L-glucose, the enantiomer of β-D-glucose. Educational objective: Fischer projections help distinguish anomers, epimers, and enantiomers. Enantiomers are compounds with the same chemical formula that differ at every stereocenter, whereas epimers and anomers differ at only one stereocenter. Anomers can exist only in cyclized sugars and differ at the anomeric carbon.

Which of the Following Structures Represents the Fischer Projection of a β-D-Glucose