Disaccharide
From Wikipedia, the free encyclopedia
A disaccharide is a sugar (a carbohydrate) composed of two monosaccharides.[1]
'Disaccharide' is one of the four chemical groupings of carbohydrates (monosaccharide, disaccharide, oligosaccharide, and polysaccharide).
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[edit] 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.[2]
[edit] 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.
[edit] 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.
[edit] 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) [3] |
| 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) |
[edit] References
- ^ International Union of Pure and Applied Chemistry. "disaccharides". Compendium of Chemical Terminology Internet edition.
- ^ "Disaccharides and Oligiosaccharides". Retrieved on 2008-01-29.
- ^ Matsuda, K. (November 1957). "Kojibiose (2-O-alpha-D-Glucopyranosyl-D-Glucose): Isolation and Structure: Chemical Synthesis". Nature 180: 985. doi:.
[edit] External links
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