Question

Draw a Haworth projection for the disaccharide gentibiose, given the following information: (a) It is a dimer of glucose. (b) The glycosidic linkage is \(\beta(1 \rightarrow 6)\) (c) The anomeric carbon not involved in the glycosidic linkage is in the \(\alpha\) configuration.

Short answer

Gentibiose is composed of two glucose units connected by a \( \beta(1 \rightarrow 6)\) glycosidic bond. The glucose unit not involved in the linkage has C1 in the \alpha\-configuration.

Step-by-step solution

Identify the monosaccharides

Gentibiose is a dimer of glucose, which means it is composed of two glucose molecules. Each glucose molecule is a six-carbon sugar.

Determine the glycosidic linkage

The glycosidic linkage in gentibiose is \(\beta(1 \rightarrow 6)\), meaning the bond is between the \( \beta \)-OH group of the first glucose's anomeric carbon (C1) and the C6 hydroxyl group of the second glucose.

Draw the basic glucose structure

Draw the pyranose ring structure for glucose. The ring is formed between C1 and C5. Graphically, the oxygen in the ring is depicted at the top-right corner when the ring is drawn in its standard orientation.

Configure the first glucose molecule

In the first glucose unit, place the hydroxyl group at C1 in the \( \beta \)-position, which means it points upward. Also, place the hydroxyl groups at C2, C3, C4 and C6 following the standard Haworth projection rules for D-glucose.

Configure the second glucose molecule

Draw another glucose ring for the second molecule. The linkage \(\beta(1 \rightarrow 6)\) connects the first glucose at C1 (with an upward \( \beta \) OH group) to the C6 hydroxyl group of the second glucose. Also, at the anomeric carbon (C1) of the second glucose, place the hydroxyl group in the \( \alpha \)-position, pointing downwards.

Connect the two glucose units

Join the C1 of the first glucose unit to the C6 of the second glucose unit through the glycosidic bond. Ensure that the \( \beta \)-linkage is correctly represented (the bond follows an upward direction from the C1 hydroxyl).

Finalize the Haworth projection

Once both glucose units are connected, verify all glycosidic connections and hydroxyl orientations are as per the given information. The final diagram should accurately reflect gentibiose with a \( \beta(1 \rightarrow 6)\) linkage and \( \alpha\)-configuration at C1 of one glucose unit.

Key concepts

Glycosidic Linkage

A glycosidic linkage is a covalent bond that connects carbohydrate (sugar) molecules together. In the case of gentibiose, the type of glycosidic linkage is \(\beta(1 \rightarrow 6)\), which means this bond forms between the \(\beta\)-OH group at carbon 1 (C1) of the first glucose molecule and the OH group at carbon 6 (C6) of the second glucose molecule. This linkage is crucial for determining the structure and function of the resulting disaccharide.
In a \(\beta\) glycosidic bond, the bond between the two sugar units is oriented in such a way that the OH group on the anomeric carbon of the first glucose points upwards.  This linkage plays an important role in defining the disaccharide’s properties. Additionally, a glycosidic linkage can also affect the molecule’s digestibility and biochemical behavior.

Glucose Dimer

A glucose dimer is a molecule composed of two glucose units linked together. In gentibiose, these two glucose units are connected through a \(\beta(1 \rightarrow 6)\) glycosidic linkage. Glucose molecules are six-carbon sugars, which are often represented in a ring form called the pyranose form.
The formation of a glucose dimer through glycosidic linkages is significant in biochemistry because it often serves as an intermediate in the synthesis or breakdown of larger carbohydrates like starch or glycogen. By understanding how glucose dimers like gentibiose are formed, we can better understand carbohydrate metabolism and its role in biological systems.

Alpha and Beta Configurations

In carbohydrate chemistry, the terms \(\beta\) and \(\beta\) configurations refer to the orientation of the hydroxyl group (OH) attached to the anomeric carbon (C1). In Haworth projections, these configurations determine how the molecule is represented in its ring form. The anomeric carbon is the carbon derived from the carbonyl group during the formation of the ring.
If the OH group attached to the anomeric carbon points upwards, it is in the \(\beta\) configuration. If it points downwards, it is in the \(\beta\) configuration. This difference is key in determining the properties and digestion of carbohydrates.
For gentibiose:

• The first glucose unit has its OH group on C1 in the \(\beta\)-position, pointing upward.
• The second glucose unit has its OH group on C1 in the \(\beta\)-position, pointing downward.

Understanding these configurations is essential for drawing accurate Haworth projections and for predicting the behavior of these molecules in biological systems.