Disaccharides

DISACCHARIDES The disaccharides consists of two

monosaccharide units joined covalently by an O-glycosidic linkage.

SUCROSESaccharose or table sugarOn hydrolysis yields one molecule each of D-

glucose and D- fructose.

Sucrose contains no free anomeric carbon atom,therefore it is without reducing properties,NON REDUCING SUGAR.

INVERT SUGARSucrose is dextrorotatory,because glucose

and fructose(furanose form) are dextrorotatory.

When sucrose is hydrolysed it becomes levorotatory(its fructose is released in pyranose form that is levorotatory).

Levorotatory activity of fructose is more than glucose.

Because rotation is reversed the process is called INVERSION and the products are called INVERT SUGAR.

Inversion can be brought about by hydrolysis with dilute acids or with specific enzyme called invertase or sucrase(Mucosal lining the small intestine).

Honey is rich in invert sugar.Invert sugar is more sweeter than sucrose due

to the presence of free fructose.Jams and jellies also contain invert sugar.

Source of sucrose is sugar beets and sugar cane.Caramel is solid residue produced by heating sucrose.used pharmaceutically to make syrups.

MALTOSEIt is formed by O-glycosidic linkage between one

molecule of α-D- glucose with another molecule of α-D- glucose

Because of presence of free anomeric carbon it is reducing sugar.

Malt sugar,present in germinating seeds.Formed in vivo by the action of salivary

amylase on starch and in vitro by the partial hydrolysis of starch by acids.

Used as fementive reagent.

ISOMALTOSEIsomaltose is a disaccharide similar to

maltose, but with a α-(1-6)-linkage instead of the α-(1-4)-linkage.

It is a reducing sugar. The linkage between the sugars is 1,6.

LACTOSEContains O-glycosidic linkage between one

molecule of galactose with another molecule of glucose.

Milk sugar,cows milk contains 5% lactose,human milk contains 7%.

Contains free anomeric carbon so its reducing sugar.

Lactase in mucosal cells hydrolyzes lactose to its constituent monosaccharides.

Lactulosegalactose-b-(1,4)-fructosea semi-synthetic disaccharide (not naturally

occurring)not absorbed in the GI tractused as a laxative to treat constipation.

Cellobiose consists of 2 molecules of glucose linked by a beta-1,4 glycosidic bondIt is usually obtained by the partial hydrolysis of cellulose

Trehalose is a disaccharide that occurs naturally in insects, plants, fungi, and bacteria. The major dietary source is mushrooms. Trehalose is used in bakery goods, beverages, confectionery, fruit jam, breakfast cereals, rice, and noodles as a texturizer, stabilizer, humectant, or formulation aid with a low sweetening intensity

TrehaloseO

CH2OH

H

O O

H

H

H

HO

H

OH

OH

H

OH H

H

OH

OH

H

HOH2C

TREHALOSE

OLIGOSACCHARIDES