Question

Click the "draw structure" button to launch the drawing utility. Deduce the structure of the disaccharide isomaltose from the following data. [1] Hydrolysis yields D-glucose exclusively. [2[ Isomaltose is cleaved with a-glycosidase enzymes. [3] Isomaltose is a reducing sugar. [4] Methylation with excess CH31, Ag2O and then hydrolysis with H30+ forms two products: OH OCH3 OH CH₃OO CH₂Oo CH2O CH2O CHO CH20- (Both anomers are present.) CH, OH OH CHyo CH, OH Draw your structure as a three-dimensional representation using chair pyranose rings. HOT HO on OH HO- edit structure ...

Answer 1

Monosaccharides are the carbohydrates which cannot be hydrolyzed further to smaller compounds. These are also called as simple sugars. Glucose, fructose and mannose are the examples of simple sugars. Disaccharides are carbohydrates which are formed by the combination of two monosaccharide (sugar) units. The two sugar units are held together by glycoside linkage Cyclic structure of D-glucose: In D-glucose (aldohexose) the reaction of C5-OH group and an aldehyde forms a six membered pyranose ring. D-glucose forms two forms of ring structures a and B forms and they differ in the orientation of hydroxyl group C1 carbon atom (aldehydic carbon). If the hydroxy group is below the pyranose ring then it is a-form and if the hydroxy group is above the pyranose ring then it is B-form. These two forms are called as anomers. The two cyclic forms of D-glucose can be represented by Haworth projection, or chair conformation. The following are the cyclic structure of two forms of D-glucose. B-position нон,с он НоН-С ОН он он но но он Он Но Но. Он но он Он но Он он Но a-position a-D-gluopyranose PD-gluopyranose

Glycoside linkage: The combination of two monomeric sugar units takes place by the elimination of water molecule. During the elimination a new C-O-C bond is formed between two rings which is called glycoside linkage. The glycoside linkage indicates the point of linkage between two monomer units. For example, if the anomeric carbon of one monomer unit is bonded to the hydroxy group of C4 carbon of other monomer unit then the linkage is called as 1-4linkage The cleavage of glycosidic bond takes place on reaction with glycosidase enzyme. For example, the a-glycoside bond is cleaved by a-glycosidase enzyme Reducing and nonreducing sugars: The sugars which reduce Ag*ions are called as reducing sugars. For example, hemiacetals can reduce Ag*ions because in aqueous solutions they are in equilibrium with the open chain structure. Whereas, acetals cannot reduce Ag*ions because it cannot form open-chain structure in aqueous solutions thus they are called as nonreducing sugars.

It is asked to draw the structure of disaccharide isomaltose from the given data. (1) Hydrolysis of isomaltose yields only D-glucose, which indicates that the disaccharide is formed by the combination of two D-glucose monomer units. The two D-glucose units are joined together by a glycoside bond which may be aor B-glycosidic linkage. (2) It is given that a-glycosidase enzymes hydrolyze isomaltose, which indicates that a a- glycosidic linkage is present in between two D-glucose units. (3) It is given that isomaltose is a reducing sugar which indicates that it can reduce Ag*ions. It is in the form of hemiacetal and in aqueous solutions they are in equilibrium with the open chain structure. This indicates that one of the D-glucose units is in hemiacetal form.

(4) Isomaltose on methylation with excess of CH,I and Ag,O followed by acid hydrolysis yields only two products. The structures of the two products are shown below: ОСH ОН (both anomers are present) НСо НСо- Н,СО НСо- OCH3 ОСH In the given reaction, all the free hydroxyl group of disaccharide is methylated and hydrolysis of the methylated disaccharide cleaves the glycoside linkage to give two products. The above reaction gives information about point of linkage in two monomer units because after hydrolysis, the position of free hydroxyl groups indicates the glycoside bond. In the above structures, the first compound consists of hydroxyl group at C1 carbon (anomeric carbon) and the second compound consists of hydroxyl group at C1 and C6 carbons. As the given disaccharide is a reducing sugar, one of monomer unit should be in hemiacetal form. Thus the two D-glucose units are joined by the 1->6 a-glycosidic linkage which means the glycosidic bond is between anomeric carbon of one D-glucose unit and C6 carbon of another D- glucose unit.

Therefore, from the above all information the structure of isomaltose can be drawn as follows: ОН |НО НО. 1->6 a-glycoside linkage ОН НО Isomaltose НО ОН ОН The above structure satisfies all the given data