Question

Propose structural formulas for the following polysaccharides. (a) Alginic acid, isolated from seaweed, is used as a thickening agent in ice creatn 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 \(a-1,4\)-glycosidic bonds. O=CC(O)C(O)C(O)C(O)C(=O)O O=CC(O)C(O)C(O)C(O)C(=O)O D-Mannuronic acid D. Galacturonic acid

Short answer

Answer: Alginic acid is a polymer of D-mannuronic acid in the pyranose form connected by β-1,4-glycosidic bonds, while pectic acid is a polymer of D-galacturonic acid in the pyranose form connected by α-1,4-glycosidic bonds.

Step-by-step solution

Identify the monomers and glycosidic bonds

First, we need to identify the monomers and glycosidic bonds involved in the formation of the polysaccharides in question. For alginic acid, we are given that it is a polymer of D-mannuronic acid in the pyranose form joined by β-1,4-glycosidic bonds. For pectic acid, it is a polymer of D-galacturonic acid in the pyranose form joined by α-1,4-glycosidic bonds.

Understand the pyranose form and glycosidic bonds

The pyranose form of a monosaccharide is a six-membered ring structure, with five carbons and one oxygen within the ring. The glycosidic bond is the linkage between two monomers in a polysaccharide, joining the anomeric carbon of one monomer to a hydroxyl group on another monomer. In this case, for alginic acid, the bond is β-1,4, meaning that it is a β-linkage between the first and fourth carbons in the pyranose ring. For pectic acid, the glycosidic bond is α-1,4, meaning it is an α-linkage between the first and fourth carbons.

Draw the D-mannuronic acid monomer structure

D-mannuronic acid has the following skeletal structure with an aldehyde group at carbon 1 and carboxylic acid groups at carbon 6: O=CC(O)C(O)C(O)C(O)C(=O)O From the given structure, the pyranose form of D-mannuronic acid can be drawn: a six-membered ring with the anomeric carbon (carbon 1) being a part of the ring along with the oxygen.

Draw the D-galacturonic acid monomer structure

D-galacturonic acid, similar to D-mannuronic acid, also has an aldehyde group at carbon 1 and a carboxylic acid group at carbon 6: O=CC(O)C(O)C(O)C(O)C(=O)O The pyranose structure of D-galacturonic acid can be drawn similar to D-mannuronic acid, with a six-membered ring containing one oxygen and carbons 1 through 6 in the ring.

Show the β-1,4 linkage in alginic acid

To draw the structural formula of alginic acid, we need to show how the β-1,4-glycosidic linkage forms between the monomers of D-mannuronic acid. This bond is formed between the hydroxyl group on the first carbon of one monomer and the oxygen on the fourth carbon of another monomer.

Show the α-1,4 linkage in pectic acid

Similarly, for the structural formula of pectic acid, we need to show the α-1,4-glycosidic bond connecting the hydroxyl group on carbon 1 of one D-galacturonic acid monomer with the oxygen on carbon 4 of another monomer. By following these steps, you can draw the structural formulas for alginic acid and pectic acid, showing the pyranose form of the monomers and the glycosidic linkages between them.

Key concepts

Structural Formulas

When discussing polysaccharides such as alginic acid and pectic acid, structural formulas are essential for visualizing the arrangement of atoms in the molecules. These structural depictions make clear the spatial arrangement and connectivity of the various constituent atoms.
For polysaccharides like alginic acid, which is composed of repeating units of D-mannuronic acid, its structural formula reveals these units in a sequence, interconnected by glycosidic bonds.
Similarly, pectic acid, made from repeating units of D-galacturonic acid, shows a comparable pattern, but the orientation and type of connection may differ.
Understanding these structural formulas helps us grasp how they form stable polymers used in various applications, such as food thickeners and jelly formation.
By examining the structural formula, one can identify important functional groups, such as hydroxyls and carboxyl groups, and analyze how these groups influence the polymer's properties.

Glycosidic Bonds

Glycosidic bonds are crucial for the formation of polysaccharides, as they connect individual sugar monomers into a long chain. These bonds are covalent and form between the anomeric carbon of one sugar and a hydroxyl group on another.
In the case of alginic acid, we see a specific type of glycosidic bond known as the β-1,4-linkage. This bond joins the first carbon (C1) on one D-mannuronic acid with the fourth carbon (C4) of another monomer, giving the polymer its unique structure.
For pectic acid, the glycosidic bonds are α-1,4-linkages. Here, the orientation of the bond is different from that of the β-linkage, impacting both the physical and chemical properties of the resulting polymer.
Understanding these linkages is fundamental because they determine not only the structural properties of the polysaccharide but also influence its solubility, flexibility, and biological functions.

D-mannuronic Acid

D-mannuronic acid is a key monomer in the structure of alginic acid. It is a type of uronic acid derived from mannose by oxidation of the C6 alcohol group to a carboxylic acid.
In its pyranose form, D-mannuronic acid features a six-membered ring encompassing one oxygen atom and carbons 1 through 6, with the anomeric carbon serving a pivotal role in the formation of glycosidic bonds.
One critical characteristic of D-mannuronic acid is its ability to exist in different isomeric forms, mainly due to variations in stereochemistry at its sugar hydroxyl groups.

• The aldehyde group at the first carbon can participate in glycosidic bond formation, allowing the creation of polymers like alginic acid.
• It naturally occurs in seaweeds and contributes significantly to their structural integrity and functionality.

D-mannuronic acid's role in alginates underscores its importance in food and pharmaceutical industries, as it imparts valuable rheological properties to products.

D-galacturonic Acid

D-galacturonic acid is the building block of pectic acid, playing a crucial role in the formation of fruit-based gels, such as pectins. This monosaccharide is derived from galactose by oxidizing its C6 alcohol group to a carboxylic acid.
In its pyranose structure, D-galacturonic acid forms a six-membered ring, similar to that of D-mannuronic acid, which is integral for its function as a monomer in polysaccharides.

• Its anomeric carbon is critical for forming α-1,4-glycosidic bonds, giving pectic acid its gel-forming properties.
• The structural flexibility and extensive hydrogen bonding capabilities of D-galacturonic acid help it trap water, which is essential in jelly preparation.

The presence and concentration of D-galacturonic acid significantly influence the setting time and the texture of pectins, thereby affecting their utility in culinary and industrial applications. Understanding its chemistry allows us to manipulate these systems for optimized food production.