Disaccharide


A disaccharide is a sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are white solids that are soluble in water. Common examples are sucrose, lactose, and maltose.
Related to disaccharides are other carbohydrates: monosaccharides, their precursors, and the larger oligosaccharides and polysaccharides). C
The joining of monosaccharides into a double sugar happens by a condensation reaction, shown here in the case of two hexoses:
Breaking apart a double sugar into its two monosaccharides is accomplished by hydrolysis with the help of a type of enzyme called a disaccharidase. As building the larger sugar ejects a water molecule, breaking it down consumes a water molecule. These reactions are vital in metabolism. Each disaccharide is broken down with the help of a corresponding disaccharidase.

Classification

There are two functionally different classes of disaccharides:
  • Reducing disaccharides, in which one monosaccharide, the reducing sugar of the pair, still has a free hemiacetal unit that can perform as a reducing aldehyde group; lactose, maltose and cellobiose are examples of reducing disaccharides, each with one hemiacetal unit, the other occupied by the glycosidic bond, which prevents it from acting as a reducing agent. They can easily be detected by the Woehlk test or Fearon's test on methylamine.
  • Non-reducing disaccharides, in which the component monosaccharides bond through an acetal linkage between their anomeric centers. This results in neither monosaccharide being left with a hemiacetal unit that is free to act as a reducing agent. Sucrose and trehalose are examples of non-reducing disaccharides because their glycosidic bond is between their respective hemiacetal carbon atoms. The reduced chemical reactivity of the non-reducing sugars, in comparison to reducing sugars, may be an advantage where stability in storage is important.

    Formation

The formation of a disaccharide molecule from two monosaccharide molecules proceeds by displacing a hydroxy group from one molecule and a hydrogen nucleus from the other, so that the new vacant bonds on the monosaccharides join the two monomers together. Because of the removal of the water molecule from the product, the term of convenience for such a process is "dehydration reaction". For example, milk sugar is a disaccharide made by condensation of one molecule of each of the monosaccharides glucose and galactose, whereas the disaccharide sucrose in sugar cane and sugar beet, is a condensation product of glucose and fructose. Maltose, another common disaccharide, is condensed from two glucose molecules.
The dehydration reaction that bonds monosaccharides into disaccharides forms what are called glycosidic bonds.

Characteristic reactions

Disaccharides can serve as functional groups by forming glycosidic bonds with other organic compounds, forming glycosides and glycoconjugates.
Disaccarides characteristically undergo hydrolysis to give monosaccharides.
Some disaccharides can be hydrogenated to give useful disaccharide alcohols with retention of the acetal linkage. Commercial products include lactitol, isomalt, and maltitol. Isomalt production begins with a bacterial promoted conversion of sucrose to isomaltulose.
Sucrose undergoes acid catalyzed poly-dehydration to give hydroxymethylfurfural.

Assimilation

Digestion of disaccharides involves breakdown into monosaccharides.

Common disaccharides

Maltose, cellobiose, and chitobiose are hydrolysis products of the polysaccharides starch, cellulose, and chitin, respectively.
Less common disaccharides include: