Disaccharides

The objective of this article is to illustrate and visualize the structures and concepts of disaccharides^[1] that are difficult to visualize and illustrate by viewing two dimensional structures in textbooks.

Maltose and Cellobiose

(α-D-glucopyranosyl (1→4) D-glucopyranose) and (β-D-glucopyranosyl (1→4) D-glucopyranose) are both disaccharides made of D-glucopyranose. Comparing the two structures given below you can observe that both have a 1→4 glycosidic bond^[2]. C-1 (orange) of one glucose unit is bonded to the oxygen of C-4 of the second unit. The difference between the two is that maltose is α(1→4) and cellobiose is β(1→4). As you study the two structures notice that with cellobiose the second glucopyranose unit is rotated 180°, so that the oxygen bonds of both C-1 (β configuration) and C-4 of the second glucopyranose unit are projecting up so that oxygen has its normal angular geometry. In order to see the second glucopyranose in its normal position, rotate cellobiose 180° about the x axis so that C-6 is in the back of the ring and projecting upward. Both structures show the anomeric carbon (green) of the second glucose unit as the β anomer, but in an aqueous solution that designation would not be significant because there would be an equilibrium mixture of the α and β anomers and the open-chain structure. The open-chain structure in an aqueous solution provides an aldehyde group which can be oxidized, so maltose and cellobiose are reducing sugars^[3]. Also, notice the sharp bend in the maltose at the glycosidic bond. Most text books do not represent the structure of maltose in a way that shows this bend. View this or in spacefill. This kink produced by the α(1→4) glycosidic bond plays an important role in the structures of polysaccharides that have α(1→4) linkages.

The difference in the configuration of the glycosidic bonds in these two sugars is important in human digestion. The enzyme maltase is present in the GI tract of humans, catalyzes the hydrolysis of the α glycosidic bond in maltose, but is not able to cleave the β anomer, as a consequence humans are not capable of digesting cellobiose.

Lactose and Cellobiose

(β-D-galactopyranosyl (1→4) D-glucopyranose) and (β-D-glucopyranosyl (1→4) D-glucopyranose) are disaccharides whose monomeric units are connected by β(1→ 4) glycosidic bonds. The second glucose unit (contains green anomeric carbon) of lactose has been rotated 180° as was described for cellobiose above. As can be seen from their chemical names, the structural difference in the two disaccharides is that lactose contains a galactopyranose whereas both monomeric units in cellobiose are glucopyranose. Since the structures of glucose and galactose are very similar, the structures of these two disaccharides are different at only one stereogenic (chiral) carbon. Determine the one stereogenic center that is different in the two disaccharides. Do not be distracted by differences that are present in the location of hydroxyl groups that are on nonchiral carbons and by hydrogens that project differently from oxygens on which they are bonded. (Hint: Remember that glucose and galactose are epimers.) When lactose is in an aqueous solution, like cellobiose and maltose, it is a reducing sugars because the open-chain aldehyde is in equilbrium with the α and β anomers of the glucopyranose.

Even though both of these sugars have β(1→4) glycosidic bonds, lactose is the only one of the two that is catalytically hydrolyzed by lactase. Lactase distingishes between the two because of the difference in the one stereogenic center in the pyranosyl unit mentioned above.

Sucrose

The chemical name of is α-D-glucopyranosyl (1→2) β-D-fructofuranoside (). The chemical name indicates that the anomeric carbon (orange) of glucose has the α configuration and the anomeric carbon of fructose (green) has the β configuration. The α configuration of glucose can easily be seen in the structure of glucose, but the β of fructose is more difficult because the structure of fructose has been flipped 180° so the glycosidic bond can be formed with C-2. The anomeric carbon of fructose is on the left side of the fructose unit rather than the right side. , in order to see the β configuration of fructose in its normal position. The anomeric carbons of both monomeric units are involved in the glycosidic bond, C-1 (orange) of glucose and C-2 (green) of fructose. There are two important consequences of the (1→2) glycosidic bond. 1) Both monomeric units are glycosides, and therefore the chemical name has the ending of -oside instead of -ose like the above disaccharides. 2) There are no aldehyde or ketone groups present in an aqueous solution of the sugar, so sucrose is a non-reducing sugar.