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Chapter 8: Problem 6

How many stereoisomers are possible for glucopyranose and for fructofuranose? How many are D sugars in each case, and how many are L sugars?

Short Answer

Expert verified
Glucopyranose has 32 stereoisomers, all of which are D-sugars. Fructofuranose has 8 stereoisomers, 4 of which are D-sugars and 4 of which are L-sugars.

Step by step solution

01

Determine number of stereoisomers

To determine the number of stereoisomers, identify the chiral carbons present (the carbons attached to four different groups). The number of possible stereoisomers can be calculated using the formula \(2^n\), where n is the number of chiral carbons. For glucose (glucopyranose) which has five chiral carbons, the number of stereoisomers is \(2^5\) = 32. For fructose (fructofuranose) which has three chiral carbons, the number of stereoisomers is \(2^3\) = 8.
02

Identify D and L Sugars

D-sugars are those with the hydroxyl group on the last chiral carbon on the right, and L-sugars are those with the hydroxyl group on the left. To identify these, we need to draw the Haworth projection. In glucopyranose, the hydroxyl group on the last chiral carbon (carbon 5) is on the right, making all the stereoisomers D-sugars. In fructofuranose, the hydroxyl group on the second last carbon (carbon 3) is on the right, and the last chiral carbon (carbon 4) can be oriented either to left or right, making half the stereoisomers D-sugars and half L-sugars.
03

Calculate number of D and L sugars

Applying the above, we find that all 32 stereoisomers of glucopyranose are D-sugars, and for fructofuranose, half of the 8 stereoisomers are D (4 sugars) and half are L (4 sugars)

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Most popular questions from this chapter

Draw Fischer projections for (a) L-mannose, (b) L-fucose (6-deoxy-L- galactose), (c) D-xylitol, and (d) D-iduronate.

Draw the structure of each of the following molecules and label each chiral carbon with an asterisk. (a) \(\alpha\)-D-Glucose 1-phosphate. (b) 2 -Deoxy- \(\beta\)-D-ribose 5 -phosphate. (c) D-Glyceraldehyde 3-phosphate. (d) L-Glucuronate.

In aqueous solution, virtually all D-glucose molecules \((>99 \%)\) are in the pyranose form. Other aldoses have a greater proportion of molecules in the open-chain form. D-Glucose may have evolved to be the predominant metabolic fuel because it is less likely than its isomers to react with and damage cellular proteins. Explain why D-glucose reacts less than other aldoses with the amino groups of proteins.

Why is the \(\beta\)-D-glucopyranose form of glucose more abundant than \(\alpha\)-D-glucopyranose in aqueous solution?

Honey is an emulsion of microcrystalline D-fructose and D-glucose. Although D-fructose in polysaccharides exists mainly in the furanose form, solution or crystalline Dfructose (as in honey) is a mixture of several forms with \(\beta\)-D-fructopyranose \((67 \%)\) and \(\beta\)-D-fructofuranose \((25 \%)\) predominating. Draw the Fischer projection for D-fructose and show how it can cyclize to form both of the cyclized forms above.
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