Question

Consider \(N\)-acetyl-D-glucosamine (Section 25.1D). (a) Draw a chair conformation for the \(\alpha\) - and \(\beta\)-pyranose forms of this monosaccharide. (b) Draw a chair conformation for the disaccharide formed by joining two units of the pyranose form of \(N\)-acetyl-D-glucosamine by a \(\beta-1,4\)-glycosidic bond. If you draw this correctly, you have the structural formula for the repeating dimer of chitin, the structural polysaccharide component of the shell of lobsters and other crustaceans.

Short answer

Question: Draw the chair conformations for (a) the α- and β-pyranose forms of N-acetyl-D-glucosamine and (b) the disaccharide formed by joining two units of the pyranose form of N-acetyl-D-glucosamine through a β-1,4-glycosidic bond.

Step-by-step solution

Part (a): Drawing chair conformations for α- and β-pyranose forms of N-acetyl-D-glucosamine

To draw the chair conformation for the α- and β-pyranose forms, we will start with the basic chair structure and then attach the necessary functional groups at the appropriate positions. 1. Draw a standard chair conformation with six-membered ring (consisting of five carbon atoms and one oxygen atom). Label these carbon atoms C1, C2, C3, C4, and C5, and the oxygen atom as O. 2. Add and label a sixth carbon atom (C6) connected to the C5 by a single bond. This will form an axial or equatorial bond, depending on the α- or β-form. 3. Attach an N-acetyl group (NHCOCH3) on C2 in an axial bond. For the α-pyranose form: 4. Add a hydroxyl group (OH) to C1 in an axial bond. For the β-pyranose form: 4. Add a hydroxyl group (OH) to C1 in an equatorial bond. 5. Add hydroxyl groups (OH) attached to C3 and C4 in equatorial bonds. 6. For both forms, add hydrogen atoms to the remaining attachment points on the carbon atoms to complete the chair conformations.

Part (b): Drawing chair conformation for the disaccharide formed by joining two units of N-acetyl-D-glucosamine by a β-1,4-glycosidic bond

To draw the disaccharide, we will start with two β-pyranose forms of N-acetyl-D-glucosamine (from part a) and connect them through a glycosidic bond. 1. Draw two β-pyranose forms of N-acetyl-D-glucosamine side by side. 2. Identify C1 of one unit and C4 of the other unit as the connecting points for the glycosidic bond. 3. Remove the hydrogen atom from C1's hydroxyl group (OH) and the hydrogen atom from C4's hydroxyl group (OH) on the other unit. 4. Form a single bond between the remaining oxygen atom on C1 of the first unit and the carbon atom (C4) of the second unit. 5. The resulting chair conformation represents the structural formula for the repeating dimer of chitin.