Question

Draw the structure of the disaccharide formed by two moles of \(\alpha\) -mannose (see Question 19).

Short answer

Question: Draw the structure of the disaccharide formed by two moles of α-mannose with an α(1→4) glycosidic linkage. Answer: To draw the structure of the disaccharide formed by two moles of α-mannose with an α(1→4) glycosidic linkage, first draw both α-mannose molecules with their hydroxyl groups in the correct stereochemical orientation. Then, remove the hydrogen atom from the hydroxyl group on C1 of the first mannose and the hydroxyl group on C4 of the second mannose. Finally, create a bond between the oxygen of the C1 hydroxyl group of the first mannose and C4 of the second mannose, maintaining the α configuration of the original hydroxyl group of C1 on the first mannose.

Step-by-step solution

Understand alpha-mannose structure

α-mannose is a monosaccharide with the molecular formula C6H12O6. It has an aldehyde functional group at the first carbon (hemiacetal form) and five hydroxyl (OH) groups on the other carbons. The α configuration indicates that the hydroxyl group on C1 is below the plane of the ring in the Haworth projection.

Draw the structure of α-mannose

Draw the six-carbon ring (pyranose) structure of α-mannose, with the stereochemical orientation of the hydroxyl groups. The structure should be represented as follows: - C1: down (alpha) configuration hydroxyl group and an aldehyde (hemiacetal form) - C2: up configuration hydroxyl group - C3: down configuration hydroxyl group - C4: up configuration hydroxyl group - C5: down configuration hydroxyl group - CH2OH group above the plane of the ring

Choose the glycosidic linkage

The disaccharide can be formed by different glycosidic linkages, for example, α(1→2), α(1→3), α(1→4), or α(1→6). The linkage is formed between the anomeric carbon (C1) of one α-mannose to any hydroxyl group on another α-mannose. In this exercise, we will consider a common linkage: α(1→4).

Draw the disaccharide structure

To form the disaccharide with an α(1→4) linkage: 1. Draw the α-mannose structure as described in Step 2. 2. Choose C1 of the first mannose and C4 of the second mannose for the linkage. 3. Remove the hydrogen atom from the hydroxyl group on C1 of the first mannose and the hydroxyl group on C4 of the second mannose. 4. Create a bond between the oxygen of C1 hydroxyl group of the first mannose and C4 of the second mannose, and keep the stereochemistry (α configuration) the same for the original hydroxyl group of C1 on the first mannose. Now, you have the structure of the disaccharide formed by two moles of α-mannose with an α(1→4) glycosidic linkage.

Key concepts

Understanding Alpha-Mannose

Alpha-mannose is an essential sugar molecule that plays a pivotal role in the realm of biochemistry, especially in the structure of disaccharides. Structurally, it is a type of monosaccharide, which means it's a single sugar unit generally characterized by the molecular formula C₆H₁₂O₆. In its alpha form, the distinguishing feature is the orientation of the hydroxyl (OH) group attached to the first carbon (C1) of the ring; it's directed down in relation to the plane of the cyclical structure in a Haworth projection. Additionally, alpha-mannose features an aldehyde group when in the linear form, which converts to a hemiacetal group in the cyclic form due to the intramolecular reaction of the aldehyde and the OH group on C5. This conversion contributes to the stability and ring structure of the sugar.

Glycosidic Linkage Explained

The glycosidic linkage is the 'bond' that holds together sugars to form more complex carbohydrates such as disaccharides, oligosaccharides, and polysaccharides. It's through such bonds that sugars can store energy or provide structural support in various organisms. Within the glycosidic bond, the anomeric carbon (carbon that was part of the carbonyl group in the open-chain form) of one sugar interacts with the hydroxyl (OH) group of another. This interaction results in the release of a water molecule, a process known as dehydration synthesis, and the formation of a covalent bond.

In the case of alpha-mannose, when it reacts with itself to form a disaccharide, the alpha designation of the linkage specifies that the OH group on C1 (which decides whether a sugar is alpha or beta) remains down in the Haworth projection post-bond formation, as it was before the linkage occurred. The specific carbon on which the bond forms (e.g., α(1→4), where 1 is C1 of the first alpha-mannose and 4 is C4 on the second sugar moiety) dictates the structural nature and functions of the resulting carbohydrate. The glycosidic linkage is not only essential for carbohydrate function but also instrumental in dictating the molecule's properties, such as solubility and digestibility.

The Haworth Projection

The Haworth projection is a widely-used method for visualizing the three-dimensional structure of cyclic sugars in a simple, two-dimensional form. It represents the cyclic structure of monosaccharides like alpha-mannose, wherein the ring is generally depicted from a side view, allowing chemists to easily discern the axial and equatorial placements of groups attached to the carbon atoms.

For alpha-mannose, the Haworth projection concretizes the position of particular groups around the hexagonal ring: each carbon, save for the sixth, is part of the ring structure itself, while the sixth (C6) extends above or below the plane depending upon its configuration. Conventionally in this projection, the groups projecting right are drawn downwards while those projecting left are drawn upwards. This projection is vital in understanding both the individual monosaccharide structure and the potential interactions it may have in forming glycosidic linkages. As such, the careful analysis of the Haworth projection is critical when anticipating how two alpha-mannose molecules might bond together to form an alpha-linkage disaccharide, the knowledge of which is foundational in broader topics such as enzyme specificity, carbohydrate metabolism, and even drug design.