The Disaccharides

For an elementary study of the sugars, it is not suggested that the student should remember the structure of the disaccharides their chief and clturactet-istic reactions will, ho. e er, be readily understood if the following facts arc borne in mind. 

Hydrolysis by acids. Place 15 ml. of starch solution in a boiling-tube, add I ml. of cone. HCl, mix well and place in a boiling water-bath for 20 minutes. Cool and add 2 drops of iodine solution to i ml. of the solution no blue coloration is produced. On the remainder, perform tests for glucose in particular show that glucosazone can be formed. Neutralise the excess of acid before carrying out these tests. (Note that a more concentrated acid is required to hydrolyse starch than to hydrolyse the disaccharides, such as sucrose.)... 

FIGURE 25 11 Diagram of a cell surface glycoprotein showing the disaccharide unit that IS recognized by an invading influenza virus... 

Utilize the suciose-containing trisaccharide raffiaose [512-69-6] as a substrate for levan synthesis, releasing the disaccharide melihiose as a side-product. 

The disaccharide stmcture of (12) (trade name SPLENDA) is emphasized by the manufacturer as responsible for a taste quaUty and time—intensity profile closer to that of sucrose than any other high potency sweetener. The sweetness potency at the 10% sucrose solution sweetness equivalence is between 450 and 500X, or about two and one-half times that of aspartame. When compared to a 2% sugar solution, the potency of sucralose can be as high as 750X. A moderate degree of synergy between sucralose and other nonnutritive (91) or nutritive (92) sweeteners has been reported. 

In nature, fmctose (levulose, fmit sugar) is the main sugar in many fmits and vegetables. Honey contains ca 50 wt % fmctose on a dry basis. Sucrose is composed of one unit each of fmctose and dextrose combined to form the disaccharide. Fmctose exists in polymeric form as inulin in plants such as Jemsalem artichokes, chicory, dahlias, and dandeHons, and is Hberated by treatment with acid or enzyme. 

FIGURE 25.6 Molecular models of the disaccharides maltose and cellobiose. Two D-glucopyranose units are connected by a glycoside linkage between C-1 and C-4. The glycosidic bond has the a orientation in maltose and is p in cellobiose. Maltose and cellobiose are diastereomers. 

A disaccharide is added to a pyridine SO3 complex solution, which is prepared by reacting 5 to 6 times the molar amount of liquid SO3 as much as that of disaccharide with 5 to 10 times the amount of pyridine as that of the disaccharide at 0°C to 5°C, for sulfation at 50°C to 70°C for 3 to 7 hours. After the completion of sulfation, the greater part of pyridine Is removed by decantation. The obtained solution exhibits an acidity that is so strong that it is improper to apply the reaction with aluminum ion and, therefore, sodium hydroxide is added for neutralization. After the remaining pyridine is removed by concentration, 100 unit volumes of water per unit volume of the residue is added thereto. To the solution is then added aluminum ion solution mainly containing aluminum dihydroxychloride, the pH of which is 1.0 to 1.2, in such an amount that the aluminum ion Is present in an amount of 4 to 6 molar parts of the amount of disaccharide to provide a pH of 4 to 4.5. The mixture is reacted under stirring at room temperature and the formed disaccharide poly sulfate-aluminum compound is allowed to precipitate. After filtration, the residue is washed with water and dried. 

Galactose, a constituent of the disaccharide lactose found in dairy products, is metabolized by a patiiwav that includes the isomerization of UDP-galactose to UDP-glucose. where UDP = uridylyl diphosphate. The enzyme responsible for the transformation uses NAD+ as cofactor. Propose a mechanism. 

The glycosidic bond to an anomeric carbon can be either a or (3. Maltose, the disaccharide obtained by enzyme-catalyzed hydrolysis of starch, consists of two cv-D-glucopyranose units joined by a 1->4-o-glycoside bond. Cellobiose, the disaccharide obtained by partial hydrolysis of cellulose, consists of two /3-o-glucopyranose units joined by a 1—>4-/3-glycoside bond. 

Draw the structure of the disaccharide formed by two moles of a-mannose (see Question 19). 

In the crucial step of the synthesis (Scheme 16), the disaccharide glycosyl donor 17 was used once again, in condensation with the acceptor 19 having two free OH groups, to produce the octasaccharide 20. This product was then converted in 7 steps via the 1-trichloroacetamido compound, analogous to the synthesis described in Scheme 15, into the final compound 21. 

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