A disaccharide or biose[1] is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides also dissolve in water, taste sweet and are called sugars.[2]
'Disaccharide' is one of the four chemical groupings of carbohydrates (monosaccharide, disaccharide, oligosaccharide, and polysaccharide).
There are two different types of disaccharides: reducing disaccharides, in which one monosaccharide, the reducing sugar, still has a free hemiacetal unit; and non-reducing disaccharides, in which the components bond through an acetal linkage between their anomeric centers and neither monosaccharide has a free hemiacetal unit. Cellobiose and maltose are examples of reducing disaccharides. Sucrose and trehalose are examples of non-reducing disaccharides. [3] [4]
Disaccharides are formed when two monosaccharides are joined together and a molecule of water is removed. For example; milk sugar (lactose) is made from glucose and galactose whereas the sugar from sugar cane and sugar beets (sucrose) is made from glucose and fructose.[citation needed]
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.[citation needed]
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.
| Disaccharide | Unit 1 | Unit 2 | Bond |
| Sucrose (table sugar, cane sugar, beet sugar, or saccharose) | glucose | fructose | α(1→2)β |
| Lactulose | galactose | fructose | β(1→4) |
| 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[5]:
| Disaccharide | Units | Bond |
| Kojibiose | two glucose monomers | α(1→2) [6] |
| Nigerose | two glucose monomers | α(1→3) |
| Isomaltose | two glucose monomers | α(1→6) |
| β,β-Trehalose | two glucose monomers | β(1→1)β |
| α,β-Trehalose | two glucose monomers | α(1→1)β[7] |
| Sophorose | two glucose monomers | β(1→2) |
| Laminaribiose | two glucose monomers | β(1→3) |
| Gentiobiose | two glucose monomers | β(1→6) |
| Turanose | a glucose monomer and a fructose monomer | α(1→3) |
| Maltulose | a glucose monomer and a fructose monomer | α(1→4) |
| Palatinose | a glucose monomer and a fructose monomer | α(1→6) |
| Gentiobiulose | a glucose monomer and a fructose monomer | β(1→6) |
| Mannobiose | two mannose monomers | either α(1→2), α(1→3), α(1→4), or α(1→6) |
| Melibiose | a galactose monomer and a glucose monomer | α(1→6) |
| Melibiulose | a galactose monomer and a fructose monomer | α(1→6) |
| Rutinose | a rhamnose monomer and a glucose monomer | α(1→6) |
| Rutinulose | a rhamnose monomer and a fructose monomer | β(1→6) |
| Xylobiose | two xylopyranose monomers | β(1→4) |
