Disaccharides: Definition, Properties, Examples & Functions

Disaccharides can be defined as carbohydrates, which after acidic or enzymatic hydrolysis produces a pair of either similar or dissimilar monohydrate molecules. A disaccharide molecule is formed by two monosaccharide molecules joined together by a glycosidic linkage. Monosaccharides are the simplest fundamental units of carbohydrates that cannot be further hydrolyzed into simpler carbohydrates. Two such molecules of similar or dissimilar monosaccharides combine and by losing a water molecule from the reaction intermediate product, form a disaccharide. Let’s discuss disaccharides in detail along with some important questions.

Key Terms: Disaccharides, Carbohydrates, Hydroxyl Groups, Monosaccharides, Acidic or Enzymatic Hydrolysis.

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Properties of Disaccharides

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As the monosaccharide molecules contain a large number of hydroxyl (OH) groups, hence disaccharides are likely to form by glycosidic bond formation at any of the OH sites through elimination of water. It is to be noted that similar constituent monosaccharides can produce different varieties of disaccharides depending on the position of the glycosidic bond formation. For example, maltose, trehalose, cellobiose possess the same constituent monosaccharide, glucose. Difference in the position of glycosidic bonds results in contrasting physical and chemical properties of monosaccharides. Following are few common physical properties found in disaccharides:

Disaccharide molecules show high solubility in aqueous solutions due to the presence of a large number of hydroxyl groups which form hydrogen bonds with water molecules

• Disaccharides are polar compounds due to the presence of hydroxyl groups bearing partial negative charge and hydrogen atoms linked to the carbon chain, which bears partial positive charge 
• Disaccharides have a characteristic sweet taste, hence are used as sweetening agents
• Due to the large volume of the molecules, the diffusion gradient shown by disaccharides is almost nil and are hence impermeable across cell membranes.

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Disaccharides: Examples & Functions

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Given below is the list of the few disaccharides that are very common in day to day life:

• Sucrose

Sucrose is commonly known as the “table sugar'' that is widely used in cooking. The formula of sucrose is C₁₂H₂₂O₁₁ . It is a white crystalline solid that is highly soluble in water and the solubility increases with rising temperature. The melting point of sugar is 180°C and when warmed over the melting point, it forms a dark viscous substance, known as caramel. It is dextrorotatory in nature and exhibits a characteristic rotation of (+66.7°). Sucrose, when subjected to catalytic hydrolysis, gives one mole of D-Glucose and one mole of L-Fructose. The overall mixture shows a levorotatory turn because the laevorotation of fructose (-92.4) is greater than the dextrorotation of glucose (+52.5). The concoction structure of sucrose comprises both- the α type of glucose and β type of fructose. Sucrose is unable to exhibit mutarotation (α to β change) due to the unavailability of free hemiacetal hydroxide. The same reason accounts for the fact that sucrose cannot shape osazones.

Structure of Sucrose

• Lactose

Lactose is a disaccharide that is naturally abundant in milk and other dairy products, hence called “milk sugar”. It is one of the most commonly known sugars. Unlike other disaccharides, lactose does not taste sweet. β-D-glucose and β-D-galactose are the constituent monosaccharides of lactose. The constituent monosaccharides are bound together by a one-four glycosidic bond in a beta introduction i.e. the glycosidic bond lies between the first carbon atom of β-D-galactose and the fourth carbon atom of β-D-glucose. The presence of a free hemiacetal hydroxide in the structure of lactose is the reason behind its reactive nature. Lactose gives monocarboxylic acid when treated with bromine water.

Structure of Lactose

• Maltose

The constituent monosaccharides of maltose include two α-D-glucose molecules. The glycosidic bond between the identical monosaccharide molecules lies between the first carbon atom of one glucose molecule and the fourth carbon atom of another, which is termed as a one-four glycosidic bond. Catalytic hydrolysis of one mole of maltose when performed in an acidic environment produces two moles of D-glucose. Maltose exhibits mutarotation due to the presence of a free hemiacetal hydroxyl group i.e. there is existence of both α-Maltose and furthermore β-Maltose. For the same reason, Benedict’s and Tollens reagent can be used to identify maltose. Maltose is a reducing sugar.

Structure of Maltose

• Trehalose

Trehalose is a major component of the hemolymph of bugs, and serves as the storehouse of energy in the anatomy of those insects. Traces of trehalose are found in yeasts and different parasitic organisms. α-D-glucose forms the constituent monosaccharide of trehalose where the two anomeric carbon molecules of the two monosaccharides interface with one another. Hence the absence of any aldehyde gathering makes it a non-reducing sugar. The unavailability of free hemiacetal hydroxide accounts for the fact that trehalose cannot shape osazones. Catalytic hydrolysis of one mole of trehalose yields glucose. 

Structure of Trehalose

• Cellobiose

Cellobiose is a disaccharide that is formed when the polysaccharide cellulose undergoes enzymatic hydrolysis by cellulase enzyme. Cellulose is found in the cell wall of plants with a white crystalline form in solid state . There is a striking similarity between cellobiose and maltose except for the fact that the former has a β-1,4-glycosidic linkage whereas the latter has a α-1,4-glycosidic linkage. It exhibits a homogenous mixture of all three forms: α, β, and aldehyde. Catalytic hydrolysis of one mole of cellobiose yields glucose. It is dissoluble in water and is dextrorotatory. The presence of a free hemiacetal gathering accounts for the mutarotation shown by cellobiose. Its reducing nature and the property to structure an oxime or osazone also roots down to the same reason.

Structure of Cellobiose

Read more: Saturated and Unsaturated Fats

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Things to Remember

1. Disaccharides can be defined as carbohydrates, which after acidic or enzymatic hydrolysis produces a pair of either similar or dissimilar monohydrate molecules
2. Monosaccharides are the simplest fundamental units of carbohydrates that cannot be further hydrolyzed into simpler carbohydrates
3. disaccharides are likely to form by glycosidic bond formation at any of the OH sites of monosaccharides through elimination of water
4. Disaccharide molecules show high solubility in aqueous solutions
5. The formula of sucrose is C₁₂H₂₂O₁₁ 
6. β-D-glucose and β-D-galactose are the constituent monosaccharides of lactose

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