Disaccharide

 ( Carbohydrate Chemistry ) 

A disaccharide (also called a double sugar) is a sugar formed when two are joined by glycosidic linkage. Like monosaccharides, disaccharides are white solids that are solubility in water. Common examples are sucrose, lactose, and maltose.

Related to disaccharides are other : monosaccharides, their precursors, and the larger and ). C The joining of monosaccharides into a double sugar happens by a condensation reaction, shown here in the case of two hexoses:

Breaking apart a double sugar into its two monosaccharides is accomplished by hydrolysis with the help of a type of enzyme called a disaccharidase. As building the larger sugar ejects a water molecule, breaking it down consumes a water molecule. These reactions are vital in metabolism. Each disaccharide is broken down with the help of a corresponding disaccharidase (sucrase, lactase, and maltase).

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Classification There are two functionally different classes of disaccharides:

• Reducing disaccharides, in which one monosaccharide, the reducing sugar of the pair, still has a free hemiacetal unit that can perform as a reducing aldehyde group; lactose, maltose and cellobiose are examples of reducing disaccharides, each with one hemiacetal unit, the other occupied by the glycosidic bond, which prevents it from acting as a reducing agent. They can easily be detected by the Woehlk test or Fearon's test on methylamine.
• Non-reducing disaccharides, in which the component monosaccharides bond through an acetal linkage between their centers. This results in neither monosaccharide being left with a hemiacetal unit that is free to act as a reducing agent. Sucrose and trehalose are examples of non-reducing disaccharides because their glycosidic bond is between their respective hemiacetal carbon atoms. The reduced chemical reactivity of the non-reducing sugars, in comparison to reducing sugars, may be an advantage where stability in storage is important.

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Formation The formation of a disaccharide molecule from two monosaccharide molecules proceeds by displacing a hydroxy group from one molecule and a Hydrogen atom (a proton) from the other, so that the new vacant bonds on the monosaccharides join the two together. Because of the removal of the water molecule from the product, the term of convenience for such a process is "dehydration reaction" (also "condensation reaction" or "dehydration synthesis"). For example, milk sugar (lactose) is a disaccharide made by condensation of one molecule of each of the monosaccharides glucose and galactose, whereas the disaccharide sucrose in sugar cane and sugar beet, is a condensation product of glucose and fructose. Maltose, another common disaccharide, is condensed from two glucose molecules.

Ellie Whitney (2025). 9780538734653, Wadsworth, Cengage Learning. . ISBN 9780538734653

The dehydration reaction that bonds monosaccharides into disaccharides (and also bonds monosaccharides into more complex ) forms what are called glycosidic bonds.

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Characteristic reactions Disaccharides can serve as by forming glycosidic bonds with other organic compounds, forming and .

Disaccarides characteristically undergo hydrolysis to give .

Some disaccharides can be hydrogenation to give useful sugar alcohol with retention of the acetal linkage. Commercial products include lactitol, isomalt, and maltitol. Isomalt production begins with a bacterial promoted conversion of sucrose to isomaltulose.

Sucrose undergoes acid catalyzed poly-dehydration to give hydroxymethylfurfural.ref>

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Assimilation Digestion of disaccharides involves breakdown into monosaccharides.

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Common disaccharides

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Sucrose (table sugar, cane sugar, beet sugar, or saccharose)	α(1→2)β
Lactose (milk sugar)	β(1→4)
Maltose (malt sugar)	α(1→4)
Trehalose	α(1→1)α
Cellobiose	β(1→4)
Chitobiose	β(1→4)

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

Less common disaccharides include:

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α(1→2)
α(1→3)
α(1→6)
β(1→1)β
α(1→1)β
β(1→2)
β(1→3)
β(1→6)
Trehalulose	One glucose and one fructose	α(1→1)
α(1→3)
α(1→4)
α(1→5)
α(1→6)
β(1→6)
Either α(1→2), α(1→3), α(1→4), or α(1→6)
α(1→6)
β(1→6)
α(1→6)
β(1→4)
α(1→6)
β(1→6)
β(1→4)

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External links

Categories: Carbohydrate Chemistry

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