Question

Draw Haworth projection formulas for dimers of glucose with the following types of glycosidic linkages: (a) \(A \beta(1 \rightarrow 4)\) linkage (both molecules of glucose in the \(\beta\) form) (b) \(\operatorname{An} \alpha, \alpha(1 \rightarrow 1)\) linkage (c) \(\mathrm{A} \beta(1 \rightarrow 6) \text { linkage (both molecules of glucose in the } \beta \text { form })\)

Short answer

Draw Haworth projections for glucose. For β(1 → 4), link the anomeric carbon of one with C-4 of another. For α, α(1 → 1), link both anomeric carbons. For β(1 → 6), link the anomeric carbon of one with C-6 of another.

Step-by-step solution

Understand Haworth Projection

Haworth projections represent the three-dimensional structure of cyclic sugars in a two-dimensional manner. For glucose, the ring is usually a six-membered (pyranose) form.

Draw β-D-Glucose in Haworth Projection

Draw each glucose molecule with its hydroxyl groups positioned correctly. In the β form, the OH group on carbon-1 (anomeric carbon) is positioned up relative to the ring.

Form β(1 → 4) Glycosidic Linkage

Connect the anomeric OH of one β-D-glucose to the hydroxyl group on carbon-4 of another β-D-glucose. The bond forms by the loss of a water molecule.

Draw α-D-Glucose in Haworth Projection

For the α, α(1 → 1) linkage, represent both glucose molecules in their α forms with the OH group on carbon-1 positioned down relative to the ring.

Form α, α(1 → 1) Glycosidic Linkage

Link the anomeric OH groups (carbon-1) of two α-D-glucose molecules. The bond forms with the elimination of a water molecule.

Form β(1 → 6) Glycosidic Linkage

For the β(1 → 6) linkage, use the β form for both glucose molecules. Connect the anomeric OH of one β-D-glucose to the hydroxyl group on carbon-6 of another β-D-glucose.

Key concepts

Haworth Projection

Haworth projections are a way to depict the three-dimensional structure of cyclic sugars in two dimensions. They are particularly useful in carbohydrate chemistry, where understanding the spatial arrangement of atoms within the molecule is crucial.

For glucose, which commonly exists in a six-membered ring form known as pyranose, Haworth projections help visualize how the hydroxyl groups and other substituents are arranged around the ring. In these projections, the ring is typically drawn with a slight perspective view to indicate depth.

The orientation of the substituents like the hydroxyl (-OH) groups on the ring carbons (especially at carbon-1, known as the anomeric carbon) determines whether the molecule is in its alpha (α) or beta (β) form. Correct depiction of these orientations is crucial for illustrating glycosidic linkages and other reactions.

Glycosidic Linkage

Glycosidic linkages are the bonds formed between sugar molecules when they join together. These linkages are vital in forming complex carbohydrates like starch and cellulose.

Glycosidic bonds occur specifically between the anomeric carbon of one sugar molecule and a hydroxyl group of another. The configuration of the anomeric carbon (whether in the α or β form) and the specific carbons involved in the linkage determine the type of glycosidic linkage.

For example, a β(1 → 4) linkage involves the β form of the anomeric carbon on one sugar and the hydroxyl group on the fourth carbon of another sugar. This specificity helps create the diverse structures and functions of carbohydrates found in nature.

β-D-Glucose

β-D-Glucose is one of the two anomeric forms of D-Glucose, the most common sugar in nature. In the β form, the hydroxyl group on the anomeric carbon (carbon-1) is oriented upwards relative to the plane of the ring in the Haworth projection.

This orientation affects how β-D-glucose interacts and forms bonds with other molecules. For instance, in β(1 → 4) glycosidic linkages, the upward-facing hydroxyl group at the anomeric carbon of one β-D-glucose reacts with the hydroxyl group on the fourth carbon of another glucose molecule.

β-D-glucose is a key building block for many polysaccharides, including cellulose, which is crucial for the structural integrity of plants.

α-D-Glucose

α-D-Glucose, like β-D-Glucose, is a form of D-Glucose but has a different configuration at the anomeric carbon. In the α form, the hydroxyl group on carbon-1 is oriented downwards relative to the plane of the ring in the Haworth projection.

This downward orientation makes α-D-glucose behave differently in chemical reactions compared to its β counterpart. For example, when forming an α, α(1 → 1) glycosidic linkage, two α-D-glucose molecules interact through the hydroxyl groups on their respective anomeric carbons, creating a unique bond.

α-D-glucose is essential in the formation of many important carbohydrates, including the energy storage polysaccharide glycogen in animals.

Carbohydrate Chemistry

Carbohydrate chemistry is the study of sugars and their derivatives, encompassing their structures, properties, and roles in biological systems.

Carbohydrates are classified based on their complexity into monosaccharides (simple sugars like glucose), disaccharides (such as sucrose, made of two monosaccharides), oligosaccharides, and polysaccharides (complex carbohydrates like starch and cellulose).

The study involves understanding the formation of glycosidic linkages, which are crucial for building disaccharides and polysaccharides. Specific linkages and the arrangement of monosaccharides give rise to a wide variety of carbohydrate structures with different physical and biological properties.

Carbohydrate chemistry is fundamental to biochemistry, as carbohydrates play critical roles in energy storage, structural integrity of cells, and cell signaling.