习题第一章糖.doc

第一章 糖
1. When D-glyceraldehyde is reduced to glycerol, it is no longer designated D or L. Why?
2. A solution of one enantiomer(对映体) of a given monosaccharide rotates plane-polarized light to the left(conterclockwise) and is called the levorotatory isomer, designated (-); the other enantiomer rotates plane-polarized light to the same extent but to the right(clockwise) and is called the dextrorotatory isomer, designated (+). An equimolar mixture of the (+) and (-) forms does not rotate plane-polarized light.
The optical activity of a stereoisomer is expressed quantitatively by its optical rotation, the number of degrees by which plane-polarized light is rotated on passage through a given path length of a solution of the compound at a given concentration. The specific rotation [α]tλ of an optically active compound is defined thus:

The temperature(t) and the wavelength of the light(λ) employed(usually, as here, the D line of sodium, 589nm) must be specified.
A freshly prepared solution of α-D-glucose shows a specific rotation of +112°. Over time, the rotation of the solution gradually decreases and reaches an equilibrium value corresponding to =+52.5°. In contrast, a freshly prepared solution of β-Dglucose has a specific rotation of +19°. The rotation of this solution increases over time to the same equilibrium value as that shown by the αanomer.
(a) draw the Haworth perspective formulas of theαandβforms of D-glucose. What feature distinguishes the two forms?
(b) Why does the specific rotation of a freshly prepared solution of theαform gradually decrease with time? Why do solutions of theαandβforms reach the same specific rotation at equilibrium?
(c) Calculate the percentage of each of the two forms of D-glucose present at equilibrium.
3. Which bond(s) inα-D-glucose must be broken to change its configuration to β-D-glucose? Which bond(s) to convert D-glucose to D-mannose? Which bond(s) to convert one “chair” form of D-glucose to the other?
4. Describe the common structural features and the differences for each pair: (a) D-glucose and D-fructose; (b) maltose and sucrose; (c) cellulose and glycogen.
5. Draw the structural formula forα-D-glucosyl-(1→6)-D-mannosamine and indicate the part of this structure that makes the compound a reducing sugar.
6. The hydrolysis of sucrose(specific rotation +66.5°) yields an equimolar mixture of D-glucose(specific rotation +52.5°) and D-fructose(specific rotation -92°).
(a) suggest a convenient way to determine the rate of hydrolysis of sucrose by an enzyme preparation extracted from the lining of the small intestine.
(b) Explain why, in the food industry, an equimolar mixture of D-glucose and D-fructose formed by hydrolysis of sucrose is called invert sugar.
(c) The enzyme invertase (now commonly called sucrase) is allowed to act on a 10% solution of sucrose until hydrolysis is complete. What will be the observed optical rotation of the solution in a 10 cm cell? (Ignore a possible small contribution from the enzyme.)
7. Lactose exists in two anomeric forms, but no anomeric forms of sucrose have been reported. Why?
8. Draw the structure of N-acetyle-β-D-glucosamine. Is it a reducing sugar? What about D-gluconate? Is the disaccharide GlcN(α1→1α)Glc a reducing sugar?