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2.4. The N-glucosidic and O-glucosidic bonds

N-glycosidic and O-glucosidic bonds

There are two types of bonds between a monosaccharide and other molecules:

  • The N-glucosidic bond, which is formed between an -OH and an amino compound (will give rise to amino sugars).
  • The O-glucosidic bond, which is made between two -OH of two monosaccharides, previously cyclized, eliminating a water molecule by condensation, being at least one of the OH groups that react from the anomeric carbon (C1).

The O-glucosidic bond is α-glucosidic if the first monosaccharide is α, and β-glucosidic if the first monosaccharide is β. For example, between C1 of an α-D-glucopyranose and C4 of another D-glucopyranose (α or β) an α-type bond is established (1→4).

Enlace glucosídico

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

As seen in the image, two α-D-glucose molecules are attached (they are α because the OH group of the anomeric carbon is in the trans position with respect to CH2OH).

In the O-glucosidic 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 loss of the water molecule, as occurs in the formation of the peptide bond.

If the OH in the reaction come from the two anomeric carbons, the disaccharide will be dicarbonyl and will not have reducing power. 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 a free OH group remains on the other anomeric carbon). This fact can be verified experimentally by reaction with Fehling's reagent or with Tollens's reagent.

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

To break the O-glucosidic bonds, hydrolysis will be necessary, adding a water molecule.

Glycosidic bonds are distinguished by the type of anomer of the monosaccharides involved and the carbons involved in the bond. Thus there are links α (1→6), β (1→4), etc.

Fundamental Ideas About Glycosidic Bonding

The O-glucosidic bond is the bond that occurs between two hydroxyl groups of two monosaccharides, releasing a water molecule. At the point where the union occurs, an oxygen atom remains.

  • If the bond occurs between the hydroxyl (OH) of the anomeric carbon of the first monosaccharide and another hydroxyl (OH) group of another non-anomeric carbon of the second monosaccharide, it is a  monocarbonyl bond. It has reducing power.
  • If the bond occurs between the hydroxyl (OH) of the anomeric carbon of the first monosaccharide and another hydroxyl (OH) group of the anomeric carbon of the second monosaccharide, it is a  dicarbonyl bond . It has no reducing power.

The N-glucosidic bond is the bond that occurs between the anomeric carbon of a monosaccharide and the carbon of an amino group of an amino acid, because at the point where the union occurs, a nitrogen atom remains.