Disaccharides

Mechanism of formation and general characteristics

Monosaccharide molecules, which include the well-known glucose and fructose, are capable of actively entering into chemical interaction with each other. This process proceeds via a condensation mechanism. Two molecules combine into a larger structure, and the byproduct of this reaction is the release of a water molecule. In a similar manner, not only disaccharides are formed, but also, with the further addition of carbohydrate residues, trisaccharides and complex polysaccharides.

The most important criterion determining the chemical behavior of the resulting disaccharide is which specific functional groups of the monomers participate in the bonding. If the hemiacetal hydroxyl groups of both starting monosaccharides are involved in the reaction, the resulting disaccharide completely loses the ability to transition into an open-chain form. As a result, it lacks aldehyde reducing properties, and such a substance is called non-reducing.

Sucrose as an example of a non-reducing disaccharide

A classic representative of non-reducing disaccharides is sucrose. Its molecule consists of an $\alpha$-glucose residue and a $\beta$-fructose residue. An important structural feature of sucrose is that, in order to correctly depict the bond formation mechanism in the chemical formula, the fructose molecule must be mentally rotated in a plane by 180 degrees.

The bond between monosaccharides in sucrose is formed due to the hemiacetal hydroxyl groups of both initial molecules (both glucose and fructose). A direct consequence of this is that sucrose is a glycoside on both sides simultaneously. Due to the complete absence of a free hemiacetal hydroxyl group, the molecule is unable to open to form an aldehyde group, which is why sucrose is strictly classified as a non-reducing disaccharide.

The physical properties of this substance are also quite notable:

• It has an extremely high solubility in water: about 203 grams of the substance per 100 grams of water at standard room temperature (20 °C).
• Upon heating, the solubility of sucrose increases up to ~97%.
• If the highly concentrated solution is cooled, classic sugar crystallization does not occur. Instead, a transparent amorphous mass is formed — caramel (commercial variants of which often contain additional impurities and other disaccharides).

Lactose and its chemical features

Unlike sucrose, lactose belongs to the class of reducing disaccharides. Its molecule is constructed from a $\beta$-galactose residue and a glucose residue.

During the condensation reaction, the functional groups behave differently:

1. In the galactose molecule, the hemiacetal hydroxyl group participates in (is replaced during) bond formation.
2. In the glucose molecule, only the hydrogen in the hydroxyl group located at the fourth carbon atom (designated as $^{4}C$) is substituted.

Due to this asymmetric type of bonding, an intact hemiacetal hydroxyl group is preserved in the glucose residue. The presence of this free reactive group allows lactose to exhibit reducing properties characteristic of aldehydes.

Hydrolysis products of polysaccharides: maltose and cellobiose

During the enzymatic or chemical breakdown of complex polymeric carbohydrates, specific disaccharides are formed that also possess reducing properties. The most important of these include maltose and cellobiose.

• Maltose is a natural product of the enzymatic hydrolysis of starch. By its chemical structure, it is an $\alpha$-D-glucose glycoside linked to another glucose molecule via the hydroxyl group at the $^{4}C$ atom.
• Cellobiose is formed as a result of the hydrolysis of the structural polysaccharide cellulose. It is a $\beta$-D-glucose glycoside similarly linked to a second glucose molecule via the hydroxyl group at the $^{4}C$ atom.

Note: Both of these natural disaccharides show a structural analogy to lactose — they retain a free hemiacetal hydroxyl at the second glucose residue. It is this detail that allows maltose and cellobiose to be unquestionably classified as reducing disaccharides.

Class and exam questions

What else to study on this topic

• Spatial structure of the fructose molecule and the necessity of its 180° rotation in formulas
• Chemical mechanisms of monosaccharide condensation reactions and the elimination of water molecules
• Enzymatic hydrolysis of starch to maltose
• Mechanism of cellulose hydrolysis with the formation of cellobiose
• Open-chain form of sugars: chemical conditions for the transition in reducing disaccharides

Other topics in "Chapter 18. Major Classes of Bioorganic Compounds"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.