Glycosidic bond

In the field of carbohydrates, the glycosidic bond is the one through which a carbohydrate is linked to another molecule, whether or not it is a carbohydrate.

The terms glycosidic and glycosidic can be considered synonymous, but some prefer to reserve glycosidic bonding for the combination in which all the monomers are exclusively glucose, and glycosidic bonding if the link is established with a monosaccharide other than glucose.

If two or more monosaccharides are joined (forming disaccharides or polysaccharides) using an oxygen atom as a bridge between both molecules,^Note its correct name is O-glycosidic bond. Similarly, there are also S-, N- and C-glycosidic bonds.

Note: The glycosydic link may seem similar to an ether, but strictly not such, it is an acetal or a cetal, resulting from the reaction between a hydroxyl group and hemiacetal (the cyclic aldose monosaccharide) or hemicetal (the cyclic-shaped cetosa)

Description

O-glucosydic link between two glucose monosaccharides.

In the scheme, two α-D-glucose molecules are joined together (they are α because the OH group of the anomeric carbon is in the trans position with respect to the CH_{2< /sub>OH}).

In the O-glycosidic bond, the OH (hydroxyl) group of the anomeric carbon of the first monosaccharide reacts with an OH attached to a carbon (anomeric or not) of the second monosaccharide. A disaccharide and a water molecule are formed. The process is really a condensation, it is called dehydration due to the characteristic of the loss of the water molecule, as it occurs in the formation of the peptide bond.

If the OH reaction comes from the two anomeric carbons, the disaccharide will be dicarbonyl, the two -OH groups are involved in the bond, and it will not have reducing power since it resides in the -OH group. However, if the OH of an anomeric carbon and another non-anomeric carbon participate in the bond, the disaccharide will be monocarbonyl and will have reducing power (since there remains a < b>OH free on the other anomeric carbon). This fact can be checked experimentally by reaction with Fehling's reagent or Tollens' reagent.

At the end of the process, both monosaccharides will be linked by oxygen (O).

Nomenclature

To name the disaccharides, the position of the bond with respect to the anomeric carbon is considered, composing the name as follows:

• Indicates the configuration of the anomial carbon that binds monomers
• Add the enantiomer (D or L) of each
• Indicates the structure of the ring of each monomer (furan or pyrane)
• Added the termination - Osil. to the first monosaccharide
• Between parentheses the carbon number of the first monomer joined by the link, an arrow and the carbon number of the second (X → X`)
• The second monosaccharide is written following the previous rules. If the link is dicarbon ends - Wearing.; if the link is monocarbonized ends - Yes..

So the example schema will be: α-D-glucopyranosyl (1→4) α-D-glucopyranose

Types of links

In nature there are 5 main types of glycosidic bonds between different types of monosaccharides:

• α (1→2) - as in the sucrose
• α (1→4) - as in the maltosa
• α (1→6) - as in isomaltosa
• β (1→4) - as in jealousy or lactose
• β (1→6) - as in gentiobiosa