Disaccharide

A disaccharide is a sugar (a carbohydrate) composed of two monosaccharides.

'Disaccharide' is one of the four chemical groupings of carbohydrates (monosaccharide, disaccharide, oligosaccharide, and polysaccharide).

Classification

There are two basic types of disaccharides: reducing disaccharides, in which the monosaccharide components are bonded by hydroxyl groups; and non-reducing disaccharides, in which the components bond through their anometric centers.

Formation

It is formed when two sugars are joined together and a molecule of water is removed. For example; milk sugar (lactose) is made from glucose and galactose whereas cane sugar (sucrose) is made from glucose and fructose.

The two monosaccharides are bonded via a dehydration reaction (also called a condensation reaction or dehydration synthesis) that leads to the loss of a molecule of water and formation of a glycosidic bond.

Properties

The glycosidic bond can be formed between any hydroxyl group on the component monosaccharide. So, even if both component sugars are the same (e.g., glucose), different bond combinations (regiochemistry) and stereochemistry (alpha- or beta-) result in disaccharides that are diastereoisomers with different chemical and physical properties.

Depending on the monosaccharide constituents, disaccharides are sometimes crystalline, sometimes water-soluble, and sometimes sweet-tasting and sticky-feeling.

Common disaccharides

Disaccharide	Unit 1	Unit 2	Bond
Sucrose (table sugar, cane sugar, saccharose, or beet sugar)	glucose	fructose	α(1→2)
Lactose (milk sugar)	galactose	glucose	β(1→4)
Maltose	glucose	glucose	α(1→4)
Trehalose	glucose	glucose	α(1→1)α
Cellobiose	glucose	glucose	β(1→4)

Maltose and cellobiose are hydrolysis products of the polysaccharides, starch and cellulose, respectively.

Less common disaccharides include:

Disaccharide	Units	Bond
Gentiobiose	two glucose monomers	β(1→6)
Isomaltose	two glucose monomers	α(1→6)
Kojibiose	two glucose monomers	α(1→2)
Laminaribiose	two glucose monomers	β(1→3)
Mannobiose	two mannose monomers	either α(1→2), α(1→3), α(1→4), or α(1→6)
Melibiose	a glucose monomer and a galactose monomer	α(1→6)
Nigerose	two glucose monomers	α(1→3)
Rutinose	a rhamnose monomer and a glucose monomer	α(1→6)
Xylobiose	two xylopyranose monomers	β(1→4)

References

External links