Question

Trehalose is found in young mushrooms and is the chief carbohydrate in the blood of certain insects. Trehalose is a disaccharide consisting of two D-monosaccharide units, each joined to the other by an \(\alpha-1,1\)-glycosidic bond. (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, as it doesn't have any free aldehyde groups or hemiacetals due to both anomeric carbons being involved in a glycosidic linkage. It does not undergo mutarotation, as it lacks a free anomeric carbon. Trehalose is composed of two α-D-glucopyranose units joined by an α-1,1-glycosidic bond.

Step-by-step solution

(a) Is trehalose a reducing sugar?

A reducing sugar is a carbohydrate that is capable of getting oxidized by acting as a reducing agent due to the presence of a free aldehyde group or a hemiacetal. Since trehalose is a disaccharide containing two D-monosaccharide units joined by an \(\alpha-1,1\)-glycosidic bond, both of the anomeric carbons are involved in the glycosidic linkage. Therefore, there are no free aldehyde groups or hemiacetals present, and trehalose is not a reducing sugar.

(b) Does trehalose undergo mutarotation?

Mutarotation occurs when a carbohydrate with a free anomeric carbon is dissolved in water, causing the equilibrium between the two cyclic forms (any and b-form) of the carbohydrate. Since trehalose has both anomeric carbons linked by the glycosidic bond, it doesn't have a free anomeric carbon. Therefore, trehalose does not undergo mutarotation.

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

Trehalose is a disaccharide consisting of two D-monosaccharide units joined together by an \(\alpha-1,1\)-glycosidic bond. These two monosaccharide units are glucose molecules, specifically, they are two \(\alpha\)-D-glucopyranose units joined by \(\alpha-1,1\)-glycosidic bond.

Key concepts

Reducing Sugars

Reducing sugars are a type of carbohydrate that plays a crucial role in various biochemistry processes. Essentially, these sugars act as reducing agents due to their ability to donate electrons during chemical reactions.

A typical feature of reducing sugars is the presence of a free aldehyde group or a free ketone group, which can be found in their open-chain form. When present, these groups can be oxidized, and in the process, the sugar reduces another molecule. Common examples include glucose and fructose.

In the case of disaccharides like trehalose, whether it's a reducing sugar or not depends on the structure of the glycosidic bond. As explained in the textbook exercise, trehalose does not have a free aldehyde group as both anomeric carbons are involved in the α-1,1-glycosidic bond. Therefore, trehalose is not a reducing sugar and cannot take part in reactions typical for reducing sugars, such as reacting with Benedict's or Fehling's solution to yield a color change.

Mutarotation

Mutarotation refers to the change in the optical rotation that occurs when an alpha (α) form of a sugar spontaneously converts to its beta (β) form, and vice versa, in aqueous solution. This process is possible because of the equilibrium that can be established between the cyclic forms of monosaccharides and their corresponding open-chain forms.

However, mutarotation requires a free anomeric carbon - the carbon to which the carbonyl group is attached in the cyclic form of the sugar. In trehalose, each glucose unit is involved in a glycosidic bond at the anomeric carbon, preventing mutarotation from occurring. This static characteristic of trehalose makes it a non-mutarotating sugar, thus retaining a constant level of sweetness and structural stability over time.

α-1,1-Glycosidic Bond

Carbohydrates are known to connect via glycosidic bonds, which can be categorized based on their position and orientation. The α-1,1-glycosidic bond is a distinctive type found when two sugar molecules are linked between their anomeric carbons.

The prefix 'alpha' indicates that the glycosidic oxygen bridge points down in the same direction as the C6 carbon in a cyclic form of glucose. In trehalose, the presence of an α-1,1-glycosidic bond between two glucose units is what characterizes its non-reducing nature as this bond involves the anomeric carbons of both sugar units, rendering them unable to partake in oxidation-reduction reactions.

D-Monosaccharide Units

Monosaccharides are the simplest form of carbohydrates and can be thought of as the building blocks for more complex sugars. The 'D' indicates the configuration (right-handed) of the molecule, which is determined by the orientation of the hydroxyl group on the penultimate carbon atom.

D-monosaccharide units are found in nature and can exist in various structural forms like linear chains or rings. When two D-glucose molecules come together in trehalose, they do so by forming an α-1,1-glycosidic bond, resulting in a disaccharide that retains the stable D configuration but is non-reducing and does not undergo mutarotation. Therefore, understanding D-monosaccharide units is fundamental for grasping the biochemical properties and metabolism of various dietary sugars.