Problem 20 - Biochemistry: Concepts and Connections

Question

Indicate whether each of the following disaccharides is a reducing (R) or nonreducing (NR) sugar by the criterion of reaction with Fehling's solution. (a) Glco(1 \(\rightarrow 2)\) Fruc \(\beta\) (b) \(\mathrm{Gal} \beta(1 \rightarrow 4) \mathrm{Glc}\) (c) Glca(1 \(\rightarrow\) 1)Glc \(\alpha\) (d) Glcax \((1 \rightarrow 4)\) Gle (e) \(\mathrm{Glc} \beta(1 \rightarrow 6) \mathrm{Glc}\)

Step-by-Step Solution

Answer

(a) NR, (b) R, (c) NR, (d) NR, (e) R

Step 1: Understanding Reducing and Nonreducing Sugars

Sugars that can open up to form an aldehyde group that can reduce Fehling's solution are known as reducing sugars. In disaccharides, the anomeric carbon of at least one sugar must be free for the sugar to be reducing. Non-reducing sugars have anomeric carbons that participate in the glycosidic linkage.

Step 2: Analyzing Disaccharide (a)

In (a) Glc(1 \(\rightarrow 2\))Fruc \(\beta\), glucose and fructose are linked through their anomeric carbons, making them non-reducing. The absence of a free anomeric carbon makes it impossible for this disaccharide to reduce Fehling's solution.

Step 3: Analyzing Disaccharide (b)

In (b) Gal \(\beta(1 \rightarrow 4)\) Glc, the anomeric carbon of glucose is free, meaning it can open into an aldehyde. Hence, this disaccharide is a reducing sugar as it can react with Fehling's solution.

Step 4: Analyzing Disaccharide (c)

In (c) Glca(1 \(\rightarrow\) 1)Glc \(\alpha\), both anomeric carbons are involved in the glycosidic bond, meaning neither can open to form an aldehyde group. Thus, this sugar is non-reducing.

Step 5: Analyzing Disaccharide (d)

In (d) Glcax \((1 \rightarrow 4)\) Gle, the disaccharide information is slightly ambiguous due to 'Gle'. Assuming 'Gle' corresponds to a glucose derivative with no functional free tip either way, this would be non-reducing.

Step 6: Analyzing Disaccharide (e)

In (e) \(\mathrm{Glc} \beta(1 \rightarrow 6) \mathrm{Glc}\), the linkage does not involve the anomeric carbon of one glucose molecule, leaving it free to form an aldehyde. This makes it a reducing sugar.

Key Concepts

Disaccharides

Disaccharides are a type of carbohydrate formed when two monosaccharides, or single sugar molecules, are linked together through a covalent bond. These bonds are known as glycosidic bonds.
These sugars play a significant role in our daily nutrition and biological processes. Familiar examples include sucrose, lactose, and maltose. The properties of disaccharides, such as whether they are reducing or non-reducing, hinge greatly on the structure and bonding of their monosaccharide components.
One important aspect of understanding disaccharides is whether their glycosidic bond involves the anomeric carbon of the participating monosaccharides.

Anomeric Carbon

The anomeric carbon is a specific carbon atom in sugars that determines whether a carbohydrate can act as a reducing sugar. In the context of monosaccharides, the anomeric carbon is the one that was a carbonyl carbon (either aldehyde or ketone) before ring closure, and it is key to the reactivity of the sugar chain.

• In a disaccharide, if the anomeric carbon of one sugar is involved in glycosidic bonding, it is not free to participate in reactions, including redox reactions that characterize reducing sugars.
• When one of the anomeric carbons is free, the sugar can potentially open to form an aldehyde group, enabling it to reduce certain chemical solutions.

Understanding the status of the anomeric carbon is crucial for determining the behavior of disaccharides in chemical reactions.

Fehling's Solution

Fehling's solution is a chemical reagent used to distinguish between reducing and non-reducing sugars. It is an alkaline solution of copper(II) sulfate, typically blue in color.
The solution reacts with free aldehyde groups in sugars, if present. During the reaction:

• Reducing sugars, which have free aldehyde (or ketone) groups, will react with Fehling's solution to form a brick-red precipitate of copper(I) oxide.
• Non-reducing sugars do not change the color of Fehling's solution, as they lack the necessary free aldehyde groups because their anomeric carbons are engaged in glycosidic bonds.

This chemical test provides a simple yet effective method to chemically analyze sugar composition in biochemistry.