Question

Gentiobiose, a rare disaccharide found in saffron and gentian, is a reducing sugar and forms only \(\mathrm{D}\) -glucose on hydrolysis with aqueous acid. Reaction of gentiobiose with iodomethane and \(\mathrm{Ag}_{2} \mathrm{O}\) yields an octamethyl derivative, which can be hydrolyzed with aqueous acid to give 1 equivalent of 2,3,4,6 -tetra- \(O\) -methyl-D-glucopyranose and 1 equivalent of 2,3,4 -tri- \(O\) -methyl-D-glucopyranose. If gentiobiose contains a \(\beta\) -glycoside link, what is its structure?

Short answer

Gentiobiose is \(\beta\)-D-glucopyranosyl-(1→6)-D-glucopyranose.

Step-by-step solution

Determine Sugar Linkage

Gentiobiose is a disaccharide composed of two glucose units. Given that it is a reducing sugar, one of the glucose units must have a free anomeric carbon, allowing the sugar to exist in equilibrium with its open chain form. This means that the glycosidic bond connects the anomeric carbon of one glucose to a hydroxyl group of the other glucose. The description specifies a \(\beta\)-glycoside linkage.

Consider Methylation Results

The formation of an octamethyl derivative indicates that all hydroxyl groups in the free sugars can be methylated. When hydrolyzed, this derivative provides 1 equivalent of tetra-O-methyl-D-glucopyranose (meaning all its hydroxyl groups except the anomeric one are methylated) and 1 equivalent of tri-O-methyl-D-glucopyranose (indicating that one hydroxyl group in this unit was involved in the glycosidic linkage).

Assign Methyl Groups Based on Hydrolyzed Products

The tetra-O-methyl-D-glucopyranose results from a glucose where the hydroxyl on the anomeric carbon was part of the glycosidic bond and not free, as it became methylated. The tri-O-methyl-D-glucopyranose likely had its anomeric carbon involved in the reducing end, not methylated, enabling the sugar to be reducing.

Structure Determination

The gentiobiose structure can now be deduced. It has a \(\beta\)-glycosidic linkage between the anomeric carbon of one glucose unit (that becomes tetra-O-methyl after methylation) to the fourth carbon (C4) hydroxyl of the other glucose unit (that becomes tri-O-methyl after methylation). Thus, the structure is \(\beta\)-D-glucopyranosyl-(1→6)-D-glucopyranose.

Key concepts

Gentiobiose

Gentiobiose is an intriguing disaccharide composed of two glucose units that form a rare sugar mainly found in saffron and gentian.
One notable characteristic of gentiobiose is that it is a reducing sugar, which means it has the ability to open its ring structure.
This is due to the presence of a free anomeric carbon in one of its glucose units, allowing for a dynamic equilibrium between its ring and open-chain forms.
Such properties are shared among sugars that can act as reducing agents, enabling them to undergo oxidation-reduction reactions.
Gentiobiose specifically features a beta-glycosidic linkage that connects its glucose units, which will be further discussed in later sections.

Reducing sugars

Reducing sugars are essential in carbohydrate chemistry because they can donate electrons during a glycosylation reaction.
This capability stems from the presence of a free aldehyde group or ketone group, found in the sugars' open form.
An open-chain glucose, for example, contains an aldehyde group on its first carbon, which is why many disaccharides, including gentiobiose, are reducing sugars.
In chemical reactions, reducing sugars can play a role in Maillard reactions, known for browning foods in cooking.
It's important to know whether a sugar is reducing, since the reactions it can undergo deeply influence its chemical properties and interactions.

Glycosidic linkage

The glycosidic linkage is a critical concept when studying complex carbohydrates such as disaccharides.
It describes the covalent bond formed between a sugar molecule and another molecule, which can be another sugar.
In the case of gentiobiose, this linkage involves a beta (\(\beta\)) glycosidic bond with a specific orientation that affects the sugar's structure and function.
Specifically, gentiobiose features a \D-glucopyranosyl-(1→6)-D-glucopyranose linkage. This means that the anomeric carbon of one glucose is connected to the fourth carbon on the second glucose, establishing its unique structure.
Such linkages not only affect the 3D-structure of the sugar, making them either digestible or indigestible depending on the organism, but also influence the sugar's solubility and reactivity.

Methylation

Methylation is a chemical reaction where a methyl group is added to a molecule, often changing its properties and function.
In carbohydrates, this process targets hydroxyl groups (\(\small{-OH}\)), resulting in the formation of methoxy groups (\(\small{-O-CH_3}\)).
Gentiobiose can be reacted with iodomethane in the presence of Ag\(\small{Ag_2O}\) to yield an octamethyl derivative.
This reaction effectively methylates all hydroxyl groups available except for one, preserving the reducing capabilities.
Methylation studies are important as they help determine the positions of glycosidic bonds and can also protect the structure of sugars during various chemical analyses.

Glucopyranose

Glucopyranose is a term that denotes a glucose molecule in its ring, or pyranose, form.
The majority of glucose in the natural world exists in this cyclic structure, which is a six-membered ring consisting of five carbon atoms and one oxygen atom.
Recognizing glucopyranose forms of glucose is vital in understanding the behavior and arrangement of sugars in biochemistry.
In gentiobiose, both glucose units exist as glucopyranose, which influences not only their chemical properties but also their biological interactions.
Moreover, the methylation products - tetra-\(O\)-methyl-D-glucopyranose and tri-\(O\)-methyl-D-glucopyranose - further emphasize the influence of ring structures on the sugar's reactivity and function.