Question

The trisaccharide raffinose occurs principally in cottonseed meal. Raffinose (a) Name the three monosaccharide units in raffinose. (b) Describe each glycosidic bond in this trisaccharicle. (c) Is raffinose a reducing sugar? (d) With how many moles of periodic acid will raffinose react?

Short answer

Question: Identify the monosaccharide units, describe the glycosidic bonds, determine if raffinose is a reducing sugar, and calculate the number of moles of periodic acid that raffinose will react with. Answer: Raffinose is a trisaccharide composed of D-galactose, D-glucose, and L-fructose units. The glycosidic bonds are an α-1,6-linkage between D-galactose and D-glucose and an α-1,2-linkage between D-glucose and L-fructose. Raffinose is not a reducing sugar, and it will react with three moles of periodic acid.

Step-by-step solution

Identify the monosaccharide units in raffinose

Raffinose is a trisaccharide made up of three monosaccharide units. The three monosaccharide units in raffinose are: D-galactose, D-glucose, and L-fructose.

Describe the glycosidic bonds in raffinose

In raffinose, two glycosidic bonds connect the three monosaccharide units. The first glycosidic bond is an α-1,6-linkage between the D-galactose and D-glucose units. The second glycosidic bond is an α-1,2-linkage between the D-glucose and L-fructose units.

Determine if raffinose is a reducing sugar

A reducing sugar is a carbohydrate that can act as a reducing agent because it has a free aldehyde or ketone group. In raffinose, both the D-glucose and D-galactose units have their anomeric carbons involved in glycosidic bonds, and the L-fructose unit has its ketone group at carbon 2. Since none of the monosaccharide units have a free aldehyde or ketone group, raffinose is not a reducing sugar.

Calculate the number of moles of periodic acid that raffinose will react with

Periodic acid reacts with vicinal diols (two hydroxyl groups on adjacent carbons) in a carbohydrate molecule, which results in the cleavage of the carbon-carbon bond. In raffinose, there are three such vicinal diols: one in each of the D-galactose, D-glucose, and L-fructose units. Therefore, raffinose will react with three moles of periodic acid. Summary: (a) The three monosaccharide units in raffinose are D-galactose, D-glucose, and L-fructose. (b) The glycosidic bonds in raffinose are an α-1,6-linkage between D-galactose and D-glucose and an α-1,2-linkage between D-glucose and L-fructose. (c) Raffinose is not a reducing sugar. (d) Raffinose will react with three moles of periodic acid.

Key concepts

Monosaccharide Units

Monosaccharides are the building blocks of carbohydrates, similar to how bricks form a house. In the case of raffinose, a specific trisaccharide, it consists of three monosaccharide units: D-galactose, D-glucose, and L-fructose. These are the tiny sugar units that hook up to make the larger, more complex sugar known as raffinose.
Monosaccharides can be classified based on the number of carbon atoms and the functional groups they contain. However, the essential takeaway is that each unit contributes distinct properties to the carbohydrate they form.
Knowing the specific monosaccharides in a compound can help predict its reactivity and interaction with other molecules. For example, D-galactose and D-glucose play vital roles in energy provision and storage in our bodies, while L-fructose is often associated with sweetness.

Glycosidic Bonds

When monosaccharide units link up, they do so through special connections called glycosidic bonds. These bonds are like the glue that holds the sugar units together, forming chains or branches of carbohydrates. In raffinose, there are two crucial glycosidic bonds. The first bond is an α-1,6 linkage that connects D-galactose to D-glucose. The second is an α-1,2 linkage that ties D-glucose to L-fructose.
The numbers in the bond names refer to the carbon atoms participating in the bond. For instance, the α-1,6 linkage means the first carbon of D-galactose is linked to the sixth carbon of D-glucose. Understanding how these links form helps in predicting the structure and properties of the carbohydrate.

• The type of glycosidic bond affects the chain length and branching.
• These bonds determine the digestibility and metabolism in organisms.
• Different enzymes are required to break different types of glycosidic linkages.

Reducing Sugars

Reducing sugars are carbohydrates capable of acting as reducing agents due to the presence of a free aldehyde or ketone group. This means they can donate electrons to other molecules, often changing color as a visible sign of their action. However, raffinose does not belong to this category because its sugar units lack free aldehyde or ketone groups.
In raffinose, both D-glucose and D-galactose have their reactive anomeric carbons engaged in glycosidic bonds. Likewise, L-fructose's ketone group is involved in bonding, preventing any reducing activity.
Identifying whether a sugar is reducing or not can be critical in food chemistry and nutrition, influencing both how sugars behave when heated or cooked and how they might react within our bodies.

Periodic Acid Reaction

Periodic acid is a reagent that reacts with carbohydrates containing vicinal diols, which are two hydroxyl groups on adjacent carbon atoms. This reaction is used to cleave carbon-carbon bonds between them, effectively breaking down the carbohydrate structure into smaller pieces.
In raffinose, there are three such vicinal diols: one in each monosaccharide unit—D-galactose, D-glucose, and L-fructose. Therefore, raffinose can react with three moles of periodic acid, one for each vicinal diol present.

• This reaction is helpful for studying carbohydrate structures.
• It assists in determining the number of hydroxyl groups in a sugar, clarifying its structure.
• The process helps in biochemical assays for determining sugar content and structure.

Understanding how periodic acid interacts with raffinose reveals insights into its structural features and chemical potential, towering over its breakdown mechanics.