In chemistry, a glycosidic bond is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate.
A glycosidic bond is formed between the hemiacetal group of a saccharide (or a molecule derived from a saccharide) and the hydroxyl group of some organic compound such as an alcohol. If the group attached to the carbohydrate residue is not another saccharide it is referred to as an aglycone. If it is another saccharide, the resulting units can be termed as being at the reducing end or the terminal end of the structure. This is a relative nomenclature where the reducing end of the di- or polysaccharide is towards the last anomeric carbon of the structure, and the terminal end is in the opposite direction.
In the literature, the bond between an amino group or other nitrogen-containing group and the sugar is often referred to as a glycosidic bond (although IUPAC seems to suggest that the term is a misnomer). For example, the sugar-base bond in a nucleoside may be referred to as a glycosidic bond. A substance containing a glycosidic bond is a glycoside.
The hemiacetal group of carbohydrates (which contains the anomeric carbon) is reactive, and glycosidic bonds form readily in the presence of acid. This is a condensation reaction as one molecule of water is released. This condensation is known as the Fischer glycosidation. Glycosidic bonds are fairly stable; however they can be broken chemically by strong aqueous acids. A glycosidic bond, once formed, is an example of an acetal functional group.
Reducing sugars, those with a free hydroxyl at the anomeric position, in aqueous solution exist in both a minor linear form and a major cyclic form. These forms readily interconvert. Only the cyclic, acetal forms have an anomeric carbon(Carbonyl group) and can form a glycosidic bond; once a glycosidic bond to an aglycone or another carbohydrate residue is formed, the saccharide unit can no longer equilibrate to the linear form without breaking the glycosidic bond.
A glycosidic bond can join two monosaccharide molecules to form a disaccharide, as, for instance, in the linkage of glucose and fructose to create sucrose. More complicated polysaccharides such as starch (an important nutrient), glycogen, cellulose (cell walls of plants) or chitin (found in fungi) consist of numerous monosaccharide units joined by glycosidic bonds.
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