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).

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

Less common disaccharides include: Gentiobiose, that consists of two glucose monomers with an β(1→6) linkage; Isomaltose, that consists of two glucose monomers with an α(1→6) linkage; Kojibiose, that consists of two glucose monomers with an α(1→2) linkage ; Laminaribiose, that consists of two glucose monomers with a β(1→3) linkage; Mannobiose, that consists of two mannose monomers with either an α(1→2), α(1→3), α(1→4), or an α(1→6) linkage; Melibiose, that consists of a glucose monomer and a galactose monomer with an α(1→6) linkage; Nigerose, that consists of two glucose monomers with an α(1→3) linkage; Rutinose, that consists of a rhamnose monomer and a glucose monomer with an α(1→6) linkage; and Xylobiose, that consists of two xylopyranose monomers with a β(1→4) linkage.