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Sucrose, a common disaccharide

A disaccharide 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.[1]

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

Contents

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]

Formation

It is 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 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)
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[3]:

Disaccharide Units Bond
Kojibiose two glucose monomers α(1→2) [4]
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[5]
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)

References

  1. ^ International Union of Pure and Applied Chemistry. "disaccharides". Compendium of Chemical Terminology Internet edition.
  2. ^ "Disaccharides and Oligiosaccharides". http://faculty.virginia.edu/mcgarveylab/Carbsyn/Carblist/html/disacch.html. Retrieved 2008-01-29.  
  3. ^ F.W.Parrish; W.B.Hahn,G.R.Mandels} (July 1968). [http://jb.asm.org/cgi/reprint/96/1/227.pdf "Crypticity of Myrothecium verrucaria Spores to Maltose and Induction of Transport by Maltulose, a Common Maltose Contaminant"]. J. Bacteriol. (American Society for Microbiology) 96 (1): 227-233. http://jb.asm.org/cgi/reprint/96/1/227.pdf. Retrieved 2008-11-21.  
  4. ^ Matsuda, K. (November 1957). "Kojibiose (2-O-alpha-D-Glucopyranosyl-D-Glucose): Isolation and Structure: Chemical Synthesis". Nature 180: 985. doi:10.1038/180985a0.  
  5. ^ T. Taga; Y. Miwa, Z. Min (1997). "α,β-Trehalose Monohydrate". Acta Cryst. C53: 234-236. doi:doi:10.1107/S0108270196012693. http://scripts.iucr.org/cgi-bin/paper?S0108270196012693.  

External links

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Simple English

[[File:|thumb|right|250px|Sucrose, a common disaccharide]] A disaccharide is a sugar (a carbohydrate) composed of two monosaccharides. 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.

Common disaccharides

Disaccharide Unit 1 Unit 2 Bond Notes
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) Produced during the malting of barley
Trehalose glucose glucose α(1→1)α Present in fungi and insects. Successfully produced at an industrial scale by enzymatic treatment of starch for use as a food ingredient.
Cellobiose glucose glucose β(1→4) -

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


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