Question

Pectin, which occurs in plant cell walls, exists in nature as a polymer of p-galacturonic acid methylated at carbon 6 of the monomer. Draw a Haworth projection for a repeating disaccharide unit of pectin with one methylated and one unmethylated monomer unit in \(\alpha(1 \rightarrow 4)\) linkage.

Short answer

Draw two galacturonic acid units, one with COOCH3 at C6 and one with COOH at C6, linked via an \(\alpha(1 \rightarrow 4)\) bond.

Step-by-step solution

Identify the Monomer Units

The monomer unit for pectin is p-galacturonic acid. It has a carboxylic acid group at the 6th carbon, and it can be methylated to form its methyl ester.

Understand the Linkage

The monomers are linked through an \(\backslash\)alpha(1 \rightarrow 4) linkage. This means that the \(\backslash\)alpha-anomeric carbon (Carbon 1) of one monomer is connected to the 4th carbon of another monomer.

Draw the Unmethylated Monomer

Draw the Haworth projection of p-galacturonic acid. The carboxylic acid group (COOH) is attached to the 6th carbon, and hydroxyl groups (OH) are located at the positions 2, 3, and 4.

Draw the Methylated Monomer

On another p-galacturonic acid molecule, replace the carboxylic acid group with its methyl ester (COOCH3) at Carbon 6. This is the methylated monomer.

Connect the Monomers

In an \(\backslash\)alpha(1 \rightarrow 4) linkage, the anomeric carbon (Carbon 1) of the methylated monomer forms a glycosidic bond with the hydroxyl group on the 4th carbon of the unmethylated monomer.

Final Structure

Combine the drawings from Steps 3 and 4 according to the connection in Step 5. The first unit has the methyl ester at carbon 6 and the second unit has the free carboxylic group at carbon 6.

Key concepts

Haworth Projection

The Haworth projection is a common way to depict the cyclic structure of sugars. This method simplifies the three-dimensional arrangement of atoms in a sugar molecule.
For pectin, which is a polymer of p-galacturonic acid, you would draw a repeating disaccharide unit. The Haworth projection focuses on the shape and the ring structure of the sugar.
Each ring in a Haworth projection of p-galacturonic acid usually contains 5 or 6 members, including oxygen.
You can start by identifying the anomeric carbon (usually Carbon 1) and draw the rest of the carbon atoms in a ring. The OH (hydroxyl) groups and other substituents are written above or below the plane of the ring.
In the case of pectin, you need to show the \(\alpha(1 \rightarrow 4)\) linkage. This shows how the anomeric carbon of one sugar is connected to the fourth carbon of another sugar in the chain.

P-Galacturonic Acid

p-Galacturonic acid is the primary monomer unit of pectin. It is a type of sugar acid derived from galactose.
It has a carboxylic acid group at the 6th carbon, which can be represented as COOH. Hydroxyl groups (OH) are attached to the 2nd, 3rd, and 4th carbons of the sugar chain.
In pectin, some of these carboxylic acid groups are methylated to form methyl esters, noted as COOCH3. This esterification typically occurs at Carbon 6.
When drawing the Haworth projection, you will start with p-galacturonic acid in a five or six-membered ring structure. The carboxylic acid group or its methyl ester will be shown at the 6th position. By showing both methylated and unmethylated forms, you can illustrate the diversity in pectin's structure.

Glycosidic Bond

A glycosidic bond is crucial for connecting sugar monomers in polysaccharides like pectin. This bond forms between the anomeric carbon (Carbon 1) of one sugar and another carbon atom, typically the 4th carbon of the next sugar, in what is called an \(\alpha(1 \rightarrow 4)\) linkage.
To create this bond in the context of pectin, you need to know where the connecting points are:

• The anomeric carbon (C1) will have a linkage with the hydroxyl group (OH) on the fourth carbon (C4) of the neighboring sugar.
• These connections are often illustrated in the Haworth projection by a straight or curved line indicating the bond.
• For pectin, ensure you display this \(\alpha(1 \rightarrow 4)\) linkage as it is essential for understanding pectin's structure and how its monomer units bond together.

The glycosidic bonds give pectin its structural firmness and functionality in plant cell walls.