Disaccharide

Disaccharides ( double sugars outdated, rarely Biosen ) are organic chemical compounds from the group of carbohydrates. Formally, disaccharides formed by elimination of water between two monosaccharides. The two monosaccharides ( simple sugar ) are linked covalently in a disaccharide glycosidic bond. Disaccharides belong to the group of oligosaccharides.

The most economically important disaccharide is the cane and beet sugar, sucrose. This is now industrially extracted from sugar cane and from sugar beet and is an important component of the human diet dar. Allen disaccharides together their sweet taste and especially sucrose is used today as a sweetener in the human diet.

Nomenclature

The numbering starts at the carbon of the carbohydrate with the highest end of the oxidized carbon atom bearing the open-chain form of the monosaccharide, or an aldehyde group adjacent to a ketone. The terms α and β define the stereochemistry at the anomeric center fixed.

A shorthand notation uses a three-letter abbreviation for the monosaccharides and the suffixes f and p for furanose and pyranose.

α -D-glucopyranose

Properties

Disaccharides are colorless solids that are sparingly soluble and insoluble in most organic solvents in water easily, in ethanol. We know both α - and β -linked disaccharides, as well as links to any position. The 1,4 - and 1,6- linked disaccharides occur in nature by far the most frequent. Disaccharides as well as the monosaccharides temperature sensitive and decompose when heated to temperatures above their melting point or already at the melting point and are therefore not be distilled in a vacuum. The cleaning therefore normally carried out via recrystallization or in small quantity by chromatography. In solution disaccharides are most stable at pH values ​​≥ 7, but hydrolyze already in the weakly acidic range. The acidic hydrolysis of disaccharides creates two monosaccharides, as arising from sucrose ( molecular formula C12H22O11 ), for example, the monosaccharides D- glucose ( C6H12O6 ) and D- fructose ( molecular formula C6H12O6 ). The most famous and also the most common disaccharide is sucrose, the cane and beet sugar.

Depending on how the two monosaccharides are linked, reducing and non-reducing disaccharides are distinguished. Wherein the at least one reducing disaccharides anomeric carbon atom is not involved in the glycosidic bond. At the non-reducing disaccharides, the two anomeric carbon atoms are directly connected. Reducing Disaccharides are indeed in the basic pH range stable against hydrolysis, however, very sensitive to oxidants.

In the crystalline state and in aqueous solution many disaccharides such as cellobiose, lactose and sucrose by intramolecular hydrogen bonds ( HBB ) are stabilized, whereby the most stable conformation of the molecule is determined. The HBB can also vary in non -aqueous solvents.

Occurrence

Disaccharides come in animal organisms are rare. The exceptions are the trehalose is present in the hemolymph of most insects and the lactose in the milk of mammals. Very common disaccharides are found in plants. Sucrose is found in many fruits and fruit and vegetable juices; from sugar cane or sugar beet, it is obtained as table sugar. Trehalose is also found in yeast and fungi, and algae. Maltose is not found in free form, but is formed during the enzymatic degradation of starch, such as in digestion. A well-known cleavage product is the invert which consists of equal parts of fructose and glucose, and the main ingredient of honey is and is formed from sucrose.

Common disaccharides

The two monosaccharide units of the disaccharide may be joined together and have, at the junction of different stereochemistry at various points. This allows a variety of disaccharides.

Enzymatic Degradation

In the food occurring disaccharides can not directly enter glycolysis, but must be hydrolyzed extracellularly to monosaccharides first. The corresponding degradation of enzymes from the group of glycosidases as " disaccharidases " means. They are localized in humans and many mammals in the microvilli of the small intestine. Examples of such hydrolases are the maltases ( Lysosomal α - glucosidase and maltase - glucoamylase ), lactase, β -galactosidase, invertase (also called " sucrose " ) and trehalase.