Question

How many disaccharides of D-gluco-pyranose are possible?

Answer 1

Monosaccharides are the basic unit of carbohydrates such as disaccharides, oligosaccharides and polysaccharides. The gluco-pyranose is the glucose molecule formed by intramolecular attraction between the hydroxyl groups of carbon and oxygen. The open structure binds to form a cyclic ring. The mutarotation causes the interconversion of cyclic hemi-acetals with the open chain structure of disaccharide.

Complete answer:
Disaccharides are sugars composed of two monosaccharides units that are joined together by the glycosidic linkage. This type of linkage is formed between the carbon-oxygen-carbon linkages. The glycosidic bond can be formed between the hydroxyl groups of two glucose molecules.
There are various types of disaccharides depending on the monosaccharide constituents and the specific type of glycosidic linkage connecting them. Glucose is the building block of disaccharides such as lactose and sucrose, oligosaccharide such as raffinose and of polysaccharide such as starch and amylopectin, glycogen or cellulose.
D-glucopyranose exists in both alpha and beta forms. In the structure of D-glucopyranose the possible linkages can be found in many types, such as linkage between alpha-alpha glucose, alpha-beta glucose, beta-alpha glucose and beta-beta glucose molecules. Mutarotation occurs between glucose molecules to reverse the ring structure and form new bonds. Glucose has six carbon molecules so there are possibilities that one carbon molecule can be linked to other five molecules and form different compounds. The molecule formed by beta 1 and alpha 1 is the same as beta-beta (1-1) glucose.

So, there are 19 possible disaccharides of D-glucopyranose.

Note:
Mutarotation is the type of isomeric rotation which reverses the ring forms resulting in the open chain form and followed by forming the ring again. The different hydroxyl group is used for closing the ring. This causes switching between alpha and beta forms. The open chain form of compounds cannot be detected easily. The ring forms of disaccharides are more stable than the open ones.