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Chapter 23: Q56P (page 1256)

Which of the sugars mentioned in Problem 23-53 and 23-54 are reducing sugars? Which ones undergo mutarotation?

Short Answer

Expert verified

Reducing sugars can undergo mutarotation and they are as:

2,3,4,6 - tetra - O - ethyl - D - mannopyranose

1,3,6 - tri - O - ethyl - D - fructofuranose

4 - O- (α - D - glucopyranosyl) - D - galactopyranose

6 - O - (β -D - galactopyranosyl) - D - glucopyranose

Step by step solution

01

Reducing and non-reducing sugars

Carbohydrates are also classified on the basis of reducing or non-reducing sugars. Reducing sugars are defined as those carbohydrates which have the ability to reduce Fehling’s solution and Tollen’s reagent while non-reducing sugars does not have the ability to reduce Fehling’s solution and Tollen’s reagent. All monosaccharides and disaccharides except the sucrose are classified as reducing sugars. Moreover, reducing sugars are hemiacetals while non-reducing sugars are acetals.

02

General rule for identifying reducing and non-reducing sugars

A sugar whose name ends with the suffix (-ose) is a reducing sugar while the sugar whose name ends with suffix (oside) is a non-reducing sugar.

03

Identification of the given sugars as reducing or non-reducing.

The name for the sugar ethyl - β - D - glucopyranoside ends with the suffix (-oside) . Hence, it is a non-reducing sugar (acetal).

The name for the sugar 2,3,4,6 - tetra - O - ethyl - D - mannopyranose ends with the suffix (-ose). Hence, it is a reducing sugar (hemiacetal).

The name for the sugar 1,3,6 - tri - O - ethyl - D - fructofuranose ends with the suffix (-ose) . Hence, it is a reducing sugar (hemiacetal).

The name for the sugar ethyl - 2,3,4,6 - tetra - O - methyl - β - D - galactopyranoside ends with the suffix (-oside) . Hence, it is a non-reducing sugar (acetal).

The name for the sugar 4 - O - (α - D - glucopyranosyl) - D - galactopyranose ends with the suffix . Hence, it is a reducing sugar (hemiacetal).

The name for the sugar α - D - fructopyranosyl -β- D - mannopyranoside ends with the suffix (-oside) . Hence, it is a non-reducing sugar (acetal).

The name for the sugar 6 - O -β -D - galactopyranosyl) - D - glucopyranose ( ends with the suffix (-ose). Hence, it is a reducing sugar (hemiacetal).

04

Mutarotation

A spontaneous change in the specific rotation of a solution of an optically active compound is known as mutarotation. This implies that the two samples are different but in solution they form an equilibrium mixture. Sugars which are reducing can undergo mutarotation while non-reducing sugars cannot undergo mutarotation.

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Most popular questions from this chapter

Cellulose is converted to cellulose acetateby treatment with acetic anhydride and pyridine. Cellulose acetate is soluble in common organic solvents, and it is easily dissolved and spun into fibers. Show the structure of cellulose acetate.

Question: Exposure to nitrous acid (see Section 19-16), sometimes found in cells, can convert cytosine to uracil.

  1. Propose a mechanism for this conversion.
  2. Explain how this conversion would be mutagenic upon replication.
  3. DNA generally includes thymine, rather than uracil(found in RNA). Based on this fact, explain why the nitrous acid-induced mutation of cytosine to uracil is more easily repaired in DNA than it is in RNA.

Draw the expected product of the reaction of the following sugars with excess methyl iodide and silver oxide.

  1. α-D-fructofuranose
  2. β-D-galactopyranose

Aldohexoses A and B both undergo Ruff degradation to give aldopentose C. On treatment with warm nitric acid, aldopentose C gives an optically active aldaric acid. B also reacts with warm nitric acid to give an optically active aldaric acid, but A reacts to give an optically inactive aldaric acid. Aldopentose C is degraded to aldotetrose D, which gives optically active tartaric acid when it is treated with nitric acid. Aldotetrose D is degraded to (+)-glyceraldehyde. Deduce the structures of sugars A,B,C and D, and use Figure 23-3 to determine the correct names of these sugars.

Erwin Chargaff’s discovery that DNA contains equimolar amounts of guanine and cytosine and also equimolar amounts of adenine and thymine has come to be known as Chargaff’s rule:

G = C and A = T

(a) Does Chargaff’s rule imply that equal amounts of guanine and adenine are present in DNA? That is, does G = A?

(b) Does Chargaff’s rule imply that the sum of the purine residues equals the sum of the pyrimidine residues? That is, does A + G = C + T?

(c) Does Chargaff’s rule apply only to double-stranded DNA, or would it also apply to each individual strand if the double helical strand were separated into its two complementary strands?
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