General Index


Heteroside Glycosides

The internal hemiacetal of monosaccharides can react with -OH, -NH2 and -SH with loss of water forming a Glycosidic Bond. These compounds are called Glycosides. The compound that reacts with the monosaccharide is called Aglycone.

When the aglycone is another monosaccharide, the resulting gycoside is called Holoside. If it is a different compound, the glycoside is called Heteroside. Holosides will be studied later, being oligomers and polymers of monosaccharides that we know as Oligosaccharides and Polysaccharides. We usually reserve the name glycosides, somewhat incorrectly, for the Heterosides.

The concept of glycosidic bond can be illustrated by the reaction among D-Glucose and Methanol. If this reacts with the a form, we obtain



the aglycone being a methyl group. The glycoside is a a-glucoside.

If the reaction takes place with the b anomeric form, we obtain



Both compounds are different, and one cannot spontaneously change to the other. Formation of a glycosidic bond stabilizes the anomeric form of the monosaccharide.

Heteroside glycosides are widely distributed among the biomolecules. In general, Glycosilation is a strategy to increase the water solubility of the aglycone. On the other hand, many biomolecules have glycosidic structure, such as the Nucleosides, Cerebrosides, Gangliosides. An example can be seen in a Cerebroside, b-galactosyl ceramide:



Where R- is a fatty acid radical.

Many Natural products from Fungi and Plants appear as glycosides. Such is the case of Vanilloside, main component of vanilla flavor:



the glycosidic bond can also be formed between an anomeric carbon with an amino -NH2 group: the N-Glycosides. Important N-glycosides are the Nucleosides, resulting from the reaction of D-Ribose or D-Deoxyribose with a purine or pyrimidine base. As an example, we present Uridine, b-D-ribofuranosyl uracil:



Or Adenosine, b-D-ribofuranosyl adenine:



If glycosilation affects a thiol -SH group, we get the S-Glycosides. An example is



Natural product that appears in black pepper.




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