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2.5. Disaccharides

Disaccharides

Disaccharides are formed by the union of two monosaccharides through an O-glucosidic bond , which can be done in two ways:

  • By monocarbonyl bond between the anomeric carbon of the first monosaccharide and any non-anomeric carbon of the second. As an anomeric carbon remains with the free hemiacetal, it continues to have the reducing capacity. For example, the maltose, the cellobiose and lactoseThe name ending of the first monosaccharide is -osil and that of the second monosaccharide is -ose.

Reacción de síntesis de la maltosa

By Javier Velasco (Own work) [CC BY-SA 4.0], via Wikimedia Commons

  • By dicarbonyl bonding, when the two anomeric carbons of the two monosaccharides are involved, thereby losing the reducing capacity , such as sucroseThe name ending of the first monosaccharide is -osil and that of the second monosaccharide is -oside.

Disaccharides have the same properties as monosaccharides: they are soluble in water, crystallizable, colorless and sweet in taste. Its reducing capacity depends on whether it has a free anomeric group.

The main disaccharides of biological interest are:

Principales disacáridos

http://biogeo.iespedrojimenezmontoya.es/BIOLOGIAJM/BIOQUIMICA/TEMA3GLUCIDOS.htm, CC BY-SA 4.0, via Wikimedia Commons

Maltose

Disaccharide formed by two D-glucopyranose molecules linked by α (1 → 4) bond, between the oxygen of the first anomeric carbon (from -OH) of one glucose and the oxygen belonging to the fourth carbon of the other. The maltose is α-D-glucopyranosyl (1 → 4) -D-glucopyranose and having free the anomeric carbon of the second is a disaccharide reducer .

The maltose is also called malt sugar as it appears in the germinated barley grains.

Animation: Formation of a disaccharide (maltose).

Cellobiose

Disaccharide formed by two D-glucopyranose molecules linked by β bond (1 → 4). He is not free in nature. It is obtained by hydrolysis of cellulose.

Lactose

The lactose is a disaccharide formed by the union of a molecule D-galactopyranose and D-glucopyranose another joined by a β (1→4). It is found free in the milk of mammals.

Saccharose

Disaccharide formed by a molecule of α-D-glucopyranose and another of β-D-fructofuranose linked by means of an α bond (1 → 2). The bond is made between the -OH of the anomeric carbon of the first monosaccharide and the -OH of the anomeric carbon of the second. Due to this dicarbonyl bond , it is the only disaccharide of those mentioned that does not have reducing power on Fehling's liquor.

It is found in sugar cane and sugar beet, it is the sugar of habitual consumption.

Conservation in syrup

As you know, it is common to preserve fruit in syrup, based on highly concentrated sucrose solutions. But sucrose is also a good substrate for many microorganisms that can break down that food.

The preservation of food in syrup is based on the fact that the high concentration of sucrose creates a hypertonic environment (high osmotic pressure) that prevents the development of microorganisms that can cause food spoilage.

Isomalt

Disaccharide formed by two D-glucopyranose molecules joined by α bond (1→6). He is not free in nature. It is obtained by hydrolysis of amylopectin (a component of starch and glycogen ). It comes from the α branch points of these polysaccharides.

Isomaltosa By NEUROtiker (Own work) [Public domain], via Wikimedia Commons

Fundamental ideas about disaccharides

The disaccharides that you should know for the university entrance exam are: