Question

Trehalose is found in young mushrooms and is the chief rarbohyclaste in the blood of certain insects. Trehalose is a disaccharide consisting of two D-monosacchatride umits, each joined to the other by an \(\alpha-1,1\)-glycosidic bond. Trehalose (a) Is trehalose a reducing sugar? (b) Does trehalose undergo mutarotation? (c) Name the two monosaccharide units of which trehalose is composed.

Short answer

Answer: Trehalose is not a reducing sugar, does not undergo mutarotation, and is composed of two D-glucose monosaccharide units.

Step-by-step solution

Answer for (a) - Is trehalose a reducing sugar?

A reducing sugar is a type of sugar that has the ability to reduce other compounds due to the presence of a free anomeric carbon with an open-chain form. In other words, it has an aldehyde or a hemiacetal group that can donate electrons. Since trehalose is connected by an \(\alpha-1,1\)-glycosidic bond, both anomeric carbons of its monosaccharide units are involved in the bond formation. This means that trehalose does not have a free anomeric carbon and there is no open-chain form possible for this sugar. Consequently, trehalose is not a reducing sugar.

Answer for (b) - Does trehalose undergo mutarotation?

Mutarotation is the process in which the optical rotation of an alpha or beta anomer of a reducing sugar changes over time, as a result of the interconversion between alpha and beta forms via the open-chain form. As we already established that trehalose does not have a free anomeric carbon and is not a reducing sugar, it cannot undergo mutarotation. Thus, trehalose does not experience mutarotation.

Answer for (c) - Name the two monosaccharide units of which trehalose is composed.

Trehalose is a disaccharide consisting of two D-monosaccharide units. In this case, both monosaccharide units that form trehalose are D-glucose. To summarize, trehalose is composed of two D-glucose monosaccharide units.

Key concepts

Reducing Sugars

Reducing sugars are fascinating chemical compounds that have the potential to donate electrons to other molecules, commonly referred to as reduction. These sugars must have a free anomeric carbon, which can produce an open-chain form of the molecule, typically involving an aldehyde or a hemiacetal group.

• Anomeric Carbon: This is the carbon derived from the carbonyl group (C=O) present in the sugar's cyclic form. In reducing sugars, this carbon must be free to create an open-chain form, necessary for reduction.
• Common Reducing Sugars: Glucose, lactose, and maltose are typical examples of reducing sugars due to their ability to revert between cyclic and open forms, presenting a free anomeric carbon.

Trehalose, an interesting disaccharide, does not exhibit the properties of reducing sugars. This is because its \(\alpha-1,1\)-glycosidic linkage involves both anomeric carbons of the D-glucose units, leaving them unavailable to shift to an open-chain form. Thus, since trehalose lacks a free anomeric carbon, it isn't considered a reducing sugar.

Mutarotation

Mutarotation is a unique property observed in certain carbohydrates, resulting in a change of the optical rotation of a solution as it reaches equilibrium between alpha (\(\alpha\)) and beta (\(\beta\)) anomers. This process requires a sugar to first open into its linear chain form. Here’s how it works:

• Open-Chain Form: The cyclic form of a sugar must be able to open into an open-chain form for mutarotation to occur.
• Interconversion: The sugar can flip between the \(\alpha\) and \(\beta\) forms through this open-chain state, leading to a change in optical rotation over time.

Since we established that trehalose is not a reducing sugar, meaning it does not have the free anomeric carbon for forming that open-chain configuration, it naturally cannot undergo mutarotation. Therefore, trehalose's optical consistency remains unchanged over time.

Monosaccharide Units

Monosaccharides are the building blocks of carbohydrates, the simplest form of sugar that cannot be hydrolyzed into simpler sugars. Trehalose's structure, a disaccharide, involves two such units joined together. Here's what is essential about these units:

• Main Function: Monosaccharides provide essential functions in metabolism, energy storage, and even as precursors in biosynthesis for nucleotides and amino acids.
• Examples: Glucose, fructose, and galactose are some of the most well-known monosaccharides due to their widespread occurrence in nature.

Trehalose specifically is composed of two D-glucose units. What makes these units intriguing is the type of bond they form – the \(\alpha-1,1\)-glycosidic bond. This bond is unique because it links the anomeric carbons of the two glucose molecules directly to each other, providing trehalose with distinct properties, such as its non-reducing nature. Understanding the composition of trehalose helps to appreciate how the bond type between monosaccharide units dictates the characteristics of the disaccharide.