Tabla de Contenidos
Disaccharides are oligosaccharides formed by the union of two monosaccharide molecules or simple sugars. They are formed through a condensation reaction in which they bond through their hydroxyl groups, while losing a molecule of water.
The bond that is formed between the two sugar units is called a glycosidic bond and this can be of different types, which gives rise to a wide variety of different disaccharides, even when they are formed by the same two units.
The glycosidic bond
The glycosidic (also called glycosidic) bond always links the anomeric carbon of a carbohydrate to another molecule that may or may not be another carbohydrate. This means that it joins the hemiacetal or hemiketal group (depending on whether it is an aldose or a ketose) with the hydroxyl group of another molecule such as an alcohol or other carbohydrate.
To better understand the difference between the different examples of disaccharides that we present below, it is necessary to understand the different types of glycosidic bonds that can be formed.
Types of glycosidic bonds
Glycosidic bonds can be broadly classified as α or β bonds, depending on the position of the hydroxyl group on the anomeric carbon. In addition, the number of the two carbons that are part of the glycosidic bond is also usually indicated. Thus, a β bond (1 → 4) indicates that the hydroxyl of the anomeric carbon of the first sugar is in the beta position, and that the glycosidic bond is joining carbon number 1 of the sugar with carbon 4 of the second sugar molecule. sugar.
Examples of disaccharides
lactulose
This disaccharide is made up of galactose and fructose linked by a β (1 → 4) glycosidic bond. It is a carbohydrate that cannot be absorbed by the intestine, which is why it is frequently used as a laxative.
melibiosa
Reducing disaccharide with α (1 → 6) bond between a galactose unit and a glucose unit.
Lactose
It is the main sugar present in milk and other dairy products, hence its name. It forms between 2 and 8% by mass of milk and is obtained industrially by crystallization from a filtered whey solution. It is formed by a galactose unit linked to a fructose by means of a β (1 → 4) glycosidic bond.
Lactitol
This disaccharide is formed by a galactose unit and a sorbitol molecule through a β (1 → 3) glycosidic bond. This is a sugar alcohol that is used in low-calorie sweet preparations, since it is absorbed to a lesser extent than normal carbohydrates.
Trehalulose
Another isomer of sucrose but with the monosaccharides attached via an α(1→1) bond instead of an α(1→2) bond. It is found in the honey of bees and in the honeydew produced by some aphids.
Saccharose
Sucrose is common table sugar. It is sometimes called sucrose, in reference to its English name which is sucrose, but this name is incorrect. It is purified from sugar cane or sugar beet and is made up of a glucose unit and a fructose unit linked by an α (1 → 2) glycosidic bond.
Turanosa
This disaccharide is another isomer of sucrose, this time with an α (1 → 3) bond between the glucose unit and the fructose unit. Some species of bacteria are capable of using it as a source of energy and carbon.
leucrose
Leucrose is an isomer of sucrose. It also consists of a glucopyranose unit linked to a fructose unit, but instead of having an α (1 → 2) bond, it has an α (1 → 5) glycosidic bond.
Isomaltulose
This disaccharide is an isomer of maltulose. It consists of a glucose unit linked to a fructose by means of an α (1 → 6) bond. It has physical properties and flavor similar to sucrose (common sugar) but is half as sweet and much more difficult to digest, making it a commonly used sugar substitute in sugar-free confections.
Trehalose
Also known as tremalose or micosa, it is a natural disaccharide formed by two glucose molecules joined by an α (1 → 1) glycosidic bond. It has extremely high water retention properties which helps plants and animals survive long periods without water.
Maltose
Maltose is a sugar found in some cereals and candies. It is a product of starch digestion and can be purified from barley and other grains. The glycosidic bond in maltose is α (1 → 4) and joins two glucose molecules together.
Isomaltose
Isomaltose is an isomeric disaccharide of maltose. Like this, it is made up of two glucose units linked by α-glycosidic bonds, but in this case, the bond is α (1 → 6) instead of α (1 → 4). It is one of the products of the caramelization of glucose and is widely used in confectionery as a substitute for normal caramel.
soforosa
It is a rare disaccharide and has a β (1 → 2) glycosidic bond, which is very rare. It is made up of two glucose units.
cellobiose
This disaccharide contains two glucose molecules linked by a β (1 → 4) glycosidic bond. It is obtained by acid or enzymatic hydrolysis of the cellulose present in cotton or paper, for example.
β-maltose
Beta maltose is very similar to maltose. It is also made up of two glucose molecules joined by an α (1 → 4) bond. The difference is in the second glucose molecule, whose anomeric carbon has a β configuration instead of α as in normal mannose.
gentiobiosa
A disaccharide made up of two D-glucose units linked together by a β (1 → 6) glycosidic bond. This disaccharide is part of the carotenoid responsible for the color of saffron.
chitobiose
It is a disaccharide that is obtained by the breakdown of chitin, a polysaccharide commonly found in the cell wall of fungi and in the exoskeleton of arthropods.
2α-mannobiose
Disaccharide formed by the condensation of two mannose molecules joined by an α (1 → 2) glycosidic bond.
3α-mannobiose
This is an isomer of the above disaccharide whose only difference is that the glycosidic bond is α(1→3) instead of α(1→2).
rutinous
Disaccharide with α (1 → 6) bond between a rhamnose unit and a glucose unit. It is present in some flavonoid glycosides such as rutin, present in some citrus fruits.
xylobiose
This is a disaccharide made up of two identical units of xylose (hence the bi in xylobiose). Both monosaccharides are linked to each other through a β (1 → 4) glycosidic bond.
Practical summary of examples of disaccharides
The following table summarizes and organizes the most relevant structural characteristics of the disaccharides presented above. They are arranged according to the first and second units that make up each disaccharide, and then according to the type of glycosidic bond.
| disaccharide | First unit | Second unity | Glycosidic Bond Type |
| lactulose | Galactose | Fructose | β (1 → 4) |
| melibiosa | Galactose | Glucose | α (1 → 6) |
| Lactose | Galactose | Glucose | β (1 → 4) |
| Lactitol | Galactose | Sorbitol | β (1 → 3) |
| Trehalulose | Glucose | Fructose | α (1 → 1) |
| Saccharose | Glucose | Fructose | α (1 → 2) |
| Turanosa | Glucose | Fructose | α (1 → 3) |
| leucrose | Glucose | Fructose | α (1 → 5) |
| Isomaltulose | Glucose | Fructose | α (1 → 6) |
| Trehalose | Glucose | Glucose | α (1 → 1) |
| Maltose | Glucose | Glucose | α (1 → 4) |
| Isomaltose | Glucose | Glucose | α (1 → 6) |
| soforosa | Glucose | Glucose | β (1 → 2) |
| cellobiose | Glucose | Glucose | β (1 → 4) |
| β-maltose | Glucose | Glucose | β (1 → 4) |
| gentiobiosa | Glucose | Glucose | β (1 → 6) |
| chitobiose | Glucosamine | Glucosamine | β (1 → 4) |
| 2α-mannobiose | mannose | mannose | α (1 → 2) |
| 3α-mannobiose | mannose | mannose | α (1 → 3) |
| rutinous | rhamnosa | Glucose | α (1 → 6) |
| xylobiose | xylose | xylose | β (1 → 4) |
As can be seen in the table, from trehalose to gentioniose, they are all glucose dimers that differ only by the type of glycosidic bond, which shows that the same pair of sugars can form multiple different disaccharides.
If we consider the total number of known monosaccharides, it is easy to realize that there is a huge number of possible disaccharides, of which this is just a small sample made up of 21 common examples, all made up of hexoses.
However, these examples are more than enough to get a good idea of what characterizes a disaccharide and what aspects of its structure are important to define its properties.
