Question

Polysaccharides known as dextrans have been used as substitutes for blood plasma in transfusions; they are made by the action of certain bacteria on (+)-sucrose. Interpret the following properties of a dextran: Complete hydrolysis by acid yields only \(\mathrm{D}-(+)\) -glucose. Partial hydrolysis yields only one disaccharide and only one trisaccharide, which contain only \(\alpha\) -glycoside linkages. Upon methylation and hydrolysis, there is obtained chiefly \(2,3,4\) -tri-O-methyl-D-glucose, together with smaller amounts of 2,4 -di-O-methyl-D-glucose and \(2,3,4,6\) -tetra- O-methyl-D- glocuse.

Short answer

In summary, dextran is a polysaccharide composed of glucose units connected through α-glycoside linkages. Its structure is primarily simple and regular, with the majority of glucose units linked through the 2,3,4 positions. Some branching occurs at the 2,4 and 2,3,4,6 positions, as evidenced by the smaller amounts of 2,4-di-O-methyl-D-glucose and 2,3,4,6-tetra-O-methyl-D-glucose obtained after methylation and hydrolysis.

Step-by-step solution

Understanding Complete Hydrolysis of Dextran

Complete hydrolysis of dextran yields only D-(+)-glucose which means that dextran is composed entirely of glucose units. Dextran is a polysaccharide formed by the action of bacteria on (+)-sucrose, and its complete hydrolysis confirms that it contains glucose as its primary component.

Analyzing Partial Hydrolysis of Dextran

Partial hydrolysis yields only one disaccharide and one trisaccharide, both containing only α-glycoside linkages. This suggests that dextran possesses a simple and regular structure with glucose units connected through α-glycoside linkages. The presence of only one type of disaccharide and trisaccharide further indicates that there is only one type of linkage between the glucose units in the dextran structure.

Interpreting Methylation and Hydrolysis Results

Methylation and hydrolysis of dextran result in the formation of primarily 2,3,4-tri-O-methyl-D-glucose, along with minor quantities of 2,4-di-O-methyl-D-glucose and 2,3,4,6-tetra-O-methyl-D-glucose. This finding gives insight into the positions of the linkages between glucose units in dextran. The prominence of 2,3,4-tri-O-methyl-D-glucose indicates that the majority of glucose units in dextran are connected through the 2,3,4 positions. The appearance of 2,4-di-O-methyl-D-glucose and 2,3,4,6-tetra-O-methyl-D-glucose in smaller amounts suggests the presence of branching in the dextran molecule, where glucose units are linked at either the 2 and 4 positions or the 2,3,4, and 6 positions. The presence of only α-glycoside linkages in these products confirms that dextran contains primarily α-linkages between glucose units. In conclusion, the given properties of dextran show that it is a polysaccharide composed of glucose units connected through α-glycoside linkages. The structure of dextran is simple and regular, with the majority of glucose units connected through the 2,3,4 positions. Some branching in the dextran molecule occurs at the 2,4 and 2,3,4,6 positions, as evidenced by the appearance of 2,4-di-O-methyl-D-glucose and 2,3,4,6-tetra-O-methyl-D-glucose in minor quantities after methylation and hydrolysis.

Key concepts

Polysaccharide Hydrolysis

Polysaccharide hydrolysis is a critical process in biochemistry where complex carbohydrates are broken down into simpler sugars, usually monosaccharides. This is achieved through the addition of water, with the help of enzymes or acids.

For dextran, a polysaccharide, complete hydrolysis results exclusively in the production of D-(+)-glucose units. This implies that dextran is a polymer entirely made up of glucose. When dextran undergoes complete hydrolysis in the presence of acid, all the complex molecule's glycosidic bonds are broken down. The fact that only D-(+)-glucose is formed confirms that no other monosaccharide is present in dextran's structure.

For a better understanding of this polysaccharide's hydrolysis, let's consider the experimental observations: Partial hydrolysis of dextran yields only specific oligosaccharides. This is indicative of a straightforward and regular branching pattern within dextran, suggesting that the same α-glycosidic linkages are cleaved during this controlled reaction.

α-Glycoside Linkages

The α-glycoside linkages refer to the type of covalent bond that joins individual sugar units in polysaccharides. In these linkages, the anomeric carbon of one sugar (carbon-1 if we designate the sugar as D-glucose) is attached to an oxygen which in turn is bonded to a carbon atom on the adjacent sugar molecule.

When analyzing the partial hydrolysis of dextran, we see that it results in the creation of a unique disaccharide and a unique trisaccharide, both containing only α-glycoside linkages. This specificity suggests a consistent pattern of α-glycosidic bonds in dextran. The 'α' designation implies that the OH group attached to the anomeric carbon is on the opposite side of the sugar ring as the CH2OH group at carbon-5.

Methylation of Carbohydrates

Methylation of carbohydrates is a technique used in structural analysis to protect the hydroxyl groups within a sugar molecule before further reactions, such as hydrolysis, or to deduce the pattern of linkage in the polysaccharide.

When dextran is methylated and subsequently hydrolyzed, the main product obtained is 2,3,4-tri-O-methyl-D-glucose. The presence of this heavily methylated glucose indicates that the original hydroxyl groups at the 2, 3, and 4 positions were free — not involved in α-glycoside linkages — and hence available for methylation. Methylation highlights branching in dextran's structure, as the minor products (2,4-di-O-methyl-D-glucose and 2,3,4,6-tetra-O-methyl-D-glucose) suggest that the molecule has certain points where the branching occurs through 2,4 or 2,3,4,6 linked glucose units.

D-(+)-Glucose

D-(+)-glucose is a naturally occurring monosaccharide and is one of two enantiomers of the glucose molecule. The 'D-' prefix indicates the configuration around the asymmetric carbon farthest from the aldehyde or keto group, and the '(+)' sign stands for the right-handed rotation of plane-polarized light by the sugar solution.

It is the building block of many polysaccharides, including dextran. In the context of dextran analysis, the exclusive yield of D-(+)-glucose upon complete hydrolysis reveals that the polysaccharide is composed purely of glucose in the 'D-' form. This monosaccharide's structural significance is profound, considering that it can form several different glycosidic linkages, contributing to the diverse structures observed in polysaccharides.