Glycosidic bond

A glycosidic bond or glycosidic bond is defined as the chemical bond between the anomeric carbon of a carbohydrate ( glycon ) and the hetero or carbon atom of a rare aglycone or a second sugar molecule. Compounds containing a glycosidic linkage, are widely distributed in nature and are called glycosides.

When the aglycon is an alcohol or phenol, then comes the bridging oxygen atom from the aglycone. Glycoside bonds, also other hetero- atoms such as sulfur, selenium, nitrogen, and phosphorus, or rarely to carbon ( "C- glycosides " ) may have. The glycosidic linkage is cleaved by hydrolysis, wherein the reaction equilibrium is on the side of the cleavage products. The binding is kinetically very stable. It is made with low energy expenditure with elimination of water by a condensation reaction. In nature, referred to as glycosylation formation occurs enzymatically through an activated saccharide in the laboratory by special activation methods or by reaction of a sugar with a large excess of the alcohol under acid catalysis.

In a glycoside, the aldehyde of aldoses (e.g., glucose) or the keto group of ketoses (e.g., fructose) as the cyclic acetal is present. An acetal is the condensation product of an aldehyde or ketone and one or two alcohols ( " hemiacetal " or " acetal "). Acetals are stable to basic and neutral to weakly acidic aqueous solutions, however, hydrolyze in the presence of strong acids.

Nomenclature and examples

The α - and β -anomer of D-glucose. The position of the anomeric carbon atoms (red or green ), to determine relative CH2OH group at the C- 5, if it is (the same side of the ring ) to the α - compound ( different ring side ) or the β - compound.

Simple glycosides

The position of the hydroxy group of involved in binding the anomeric carbon atom determines the name of the compound and also the type of bond: Glucose comes in the cyclic pyranose ago as α -D -glucopyranose or β -D -glucopyranose. This nomenclature is included in the weave as a α - or β -glycosidic bond. In a simple glycoside, for example, from the monosaccharide glucose and an alcohol such as ethanol, is that as either ethyl - α -D-glucopyranoside or ethyl - β -D-glucopyranoside respectively. During a glycosidic bond in the normal case, a CO bond in glycosides with heteroatom (S, Se, N, P ), this is taken into account by your name in cursive names, eg Ethyl -S- α -D -glucopyranoside.

Definition α and β

The prefixes α and β anomeric configuration is defined. The decisive factor is the relative stereochemical configuration of the exocyclic oxygen for determining carbon atom of the sugar ( α = cis, β = trans). It is not always α and β axially not always equatorial to the ring. Examples: In α -D -glucopyranose the exocyclic oxygen is cis to the stereochemistry-determining C-5 is: the aglycone is axial ( β here is equatorial). The prime example of β -D -or L- arabinopyranose the exocyclic oxygen is cis to the stereochemistry-determining C-4 is. Here β thus axially. The distinction is best seen in the Fischer notation.

Di -, oligo- and polysaccharides

During the condensation of two carbohydrates linked by a glycosidic bond to the nomenclature of the disaccharide bond into account two factors:

  • On the one hand, depending on the starting compound, the α - or β -position of the parties to the bond first anomeric carbon atom.
  • The second part of the designation is from the position of the carbon atoms that are directly bonded to the oxygen bridge. When sucrose ( cane sugar ), the connection through the first carbon atom of the glucose with a hydroxy group in α -position relative to the second carbon atom of the β -D-fructose is established. The binding is therefore a α -1 → 2 glycosidic bond, and, therefore, is properly sucrose 1- α -D-glucopyranosyl -2- β -D - fructofuranoside, or 2- β -D - fructofuranosyl - 1- α -D-glucopyranoside.

It is also possible that two identical monosaccharides are joined to form a disaccharide such as cellobiose. In this case, the oxygen atom bridging the C1 atom of the first β -D-glucose molecule with the 4 carbon atom of a second, identical monomer. The compound is a β -1 → 4 glycosidic linkage, and therefore the 1- cellobiose β -D-glucopyranosyl -4- β -D-glucopyranoside. Since the first anomeric carbon atom of the second glucose molecule is free, it can further react with other monomers to form a polysaccharide such as cellulose. Analog can be linked to two molecules of glucose, α -1 → 4 -glycosidically to maltose (1- α -D-glucopyranosyl -4- α -D-glucopyranose ).

Another connection possibility occurs in gentiobiose: two glucose units are on the first carbon atom of the first monomer and the C 6 atom of the β -1 → 6 - bridged second, the connection means also 1- β -D-glucopyranosyl 6- β -D-glucose. In laminaribiose two glucose molecules β -1 → 3 are also linked.

In polysaccharides various glycosidic bonds occur:

  • Amylopectin, the major component of the starch is composed of α -1 → 4 -linked glucose monomers.
  • Amylose, which is also found in starch, contains only α -1 → 4 -bridged glucose which forms a helical structure.
  • Cellulose consists of glucose, the β -1 ,4- linked.
  • Glycogen consisting of glucose, wherein the main chain α -1 → 4 bonds has mainly, but also branches with α -1 → 6 - structure.

Complex glycosides and Heteroglycoside

Apart from simple alcohols and other saccharides, the sugar moiety may also be connected with a more complex alcohol (such as a steroid alcohol), a phenol or other hetero-atom such as nitrogen, sulfur or phosphorus. This case occurs, for example in the nucleosides and nucleotides, in which an N- glycosidic bond with a nitrogen bridge is present. The proportion of sugar here is mostly the ribose or deoxyribose. In nature, next come before very many O- and N- glycosides, rare S- glycosides, various functions such as flavorings ( amygdalin, glycyrrhizin ), defensive substances ( glucosinolates, barbaloin, Cycasin ), energy storage and transfer devices (ADP, ATP) meet dyes ( Azalein, betanin, Carmine, Malvin ) and storage compounds ( coniferin ). Also, some natural and artificial antibiotics such as azithromycin, bleomycin, Cethromycin, doramectin and streptomycin are glycosides.