Question

Propose structural formulas for the following polysaccharides. (a) Alginic acid, isolated from seaweed, is used as a thickening agent in ice cream and other foods. Alginic acid is a polymer of D-mannuronic acid in the pyranose form joined by \(\beta-1,4\)-glycosidic bonds. (b) Pectic acid is the main component of pectin, which is responsible for the formation of jellies from fruits and berries. Pectic acid is a polymer of D-galacturonic acid in the pyranose form joined by \(\alpha-1,4\)-glycosidic bonds.

Short answer

Answer: The structural formulas for alginic acid and pectic acid are as follows: Alginic acid: ``` HO OH β1 HO OH | 4 | | HO-C-3-O- - -C-3 | | HO-C-2-O-C-4... | | HO-C-4 HO-C-4 | | HO-C-5 HO-C-5 | | HO-C-6 HO-C-6 || || O O ``` Pectic acid: ``` HO OH α1 HO OH | 4 | | HO-C-3-O- - -C-3 | | HO-C-1-O-C-5... | | HO-C-5 HO-C-5 | | HO-C-4 HO-C-4 | | HO-C-6 HO-C-6 || || O O ```

Step-by-step solution

Identify the monomers

The monomers for alginic acid and pectic acid are D-mannuronic acid and D-galacturonic acid, respectively. Both are in the pyranose form.

Draw the structures of the monomers

Draw the structures of D-mannuronic acid and D-galacturonic acid in their pyranose form. Remember that both monomers are hexoses. D-mannuronic acid: ``` HO OH | HO-C-3 | HO-C-2 | HO-C-4 | HO-C-5 | HO-C-6 || O ``` D-galacturonic acid: ``` HO OH | HO-C-3 | HO-C-1 | HO-C-5 | HO-C-4 | HO-C-6 || O ```

Determine the glycosidic bond types

Alginic acid has \(\beta-1,4\)-glycosidic bonds, while pectic acid has \(\alpha-1,4\)-glycosidic bonds. This information will be useful when joining the monomers together to form the polysaccharides.

Join the monomers using the appropriate glycosidic bonds

For both polysaccharides, draw the structures of the monomers with the glycosidic bonds between them. Alginic acid: ``` HO OH HO OH | | HO-C-3 HO-C-3 | β-1,4 | HO-C-2-O-C-4 | | HO-C-4 HO-C-4 | | HO-C-5 HO-C-5 | | HO-C-6 HO-C-6 || || O O ``` Pectic acid: ``` HO OH HO OH | | HO-C-3 HO-C-3 | α-1,4 | HO-C-1-O-C-5 | | HO-C-5 HO-C-5 | | HO-C-4 HO-C-4 | | HO-C-6 HO-C-6 || || O O ```

Combine the structures to form the polysaccharides

Finally, join the monomers together in the appropriate order to form the polysaccharides alginic acid and pectic acid. Alginic acid: ``` HO OH β1 HO OH | 4 | | HO-C-3-O- - -C-3 | | HO-C-2-O-C-4... | | HO-C-4 HO-C-4 | | HO-C-5 HO-C-5 | | HO-C-6 HO-C-6 || || O O ``` Pectic acid: ``` HO OH α1 HO OH | 4 | | HO-C-3-O- - -C-3 | | HO-C-1-O-C-5... | | HO-C-5 HO-C-5 | | HO-C-4 HO-C-4 | | HO-C-6 HO-C-6 || || O O ```