Chemistry · MU Pleven

Disaccharides – types, properties, representatives.

Explains what disaccharides are and the three main types of glycosidic linkage found in nature, with the 1,4 linkage highlighted as the most common. Covers cellobiose, maltose, lactose and sucrose in turn, comparing their structures, how humans digest them, and the lactose intolerance that follows from lactase deficiency. Includes several structure pictures and notes that cellobiose and maltose differ only at the anomeric carbon.

Inside: 1 section · 8 labelled pictures · 20 flashcards · 4 min

Topic 42 of 47 in the MU Pleven syllabus for Chemistry (Medicine, Year 1). The free AceMed note covers it in 1 section, about 533 words (4 min of reading), with 20 flashcards.

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  1. Disaccharides – types, properties, representatives

Disaccharides – types, properties, representatives

Disaccharides are carbohydrates that yield two monosaccharide molecules on hydrolysis. Structurally, disaccharides are glycosides in which the alkoxy group attached to the anomeric carbon is derived from a second sugar molecule. In principle, the glycosidic bond might be between the anomeric carbon of one sugar and a hydroxyl group at any position of the other sugar. In naturally occurring disaccharides, however, there are three major types of glycosidic linkages:

  • 1,4’-link (the anomeric carbon C-1 is bonded to oxygen on C-4 of the second sugar, indicated with
  • 1,6’-link (the anomeric C-1 carbon is connected to C-6 of the second sugar)
  • 1,1’- or 1,2’-link (the anomeric carbon of one sugar unit is bonded through oxygen to the anomeric carbon of the second sugar unit)

Examples of the 1,4-linkage in cellobiose, maltose, and lactose. The most common glycosidic linkage in disaccharides is 1,4. Cellobiose is obtained by partial hydrolysis of cellulose.

Cellobiose contains two glucose monosaccharide units where the anomeric carbon of one glucose is connected through an equatorial C–O glycosidic bond to C-4 of the second glucose unit. The stereochemistry and points of connection are designated by (1,4). Such linkage is called (1,4)-glycosidic linkage. The letters designate stereochemistry at the anomeric position while the numerals specify the ring carbons that are connected. Humans

cannot assimilate cellobiose and, correspondingly – cellulose, because they lack digestive enzyme that can cleave the -glycosidic bond in cellobiose.

  • The 1,4’- -D-glucopyranosyl linkage in cellobiose is not attacked by any digestive enzyme
  • The 1,4’- -D-glucopyranosyl linkage in maltose is a substrate for digestive enzymes and cleaves to give glucose. Therefore maltose has nutritional value.

Cellobiose and maltose are closely related – they are diastereomers at C-1.

Maltose. is a disaccharide obtained by the hydrolysis of starch. Maltose possesses also two glucose monosaccharide units and 1,4-glycosidic bond but it is an (1,4)-linkage. The difference between cellobiose and maltose is in the configuration of the anomeric acetal carbon.

Lactose is a disaccharide in which C-1 of D-galactose and C-4 of D-glucose are linked by a (1,4)- glycosidic bond. Lactose is found in amounts 2-6% in milk and is known as “milk sugar”. The galactose ring as an acetal is linked by a -glycosidic bond to the position C-4 of the glucose ring. The latter has free anomeric OH group, therefore lactose is a reducing sugar.

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