Disaccharides and Polysaccharides

Chapter 35

Carbohydrates II. Disaccharides and Polysaccharides

Disaccharides

• Disaccharides are carbohydrates that are made up of two monosaccharide units. On hydrolysis a molecule of disaccharide yields two molecules of monosaccharide.

• We shall study four disaccharides: f)-maltose (malt sugar), (+)-cellobiose, (+)-lactose (milk sugar), and (+)-sucrose (cane or beet sugar). As with the monosaccharides, we shall focus our attention on the structure of these molecules: on which monosaccharides make up the disaccharide, and how they are attached to each other. In doing this, we shall also learn something about the properties of these disaccharides.

(+)-Maltose

• (-f)-Maltose can be obtained, among other products, by partial hydrolysis of starch in aqueous acid, (-f)-Maltose is also formed in one stage of the fermentation of starch to ethyl alcohol; here hydrolysis is catalyzed by the enzyme diastase, which is present in malt (sprouted barley).

• Let us look at some of the facts from which the structure of (-f)-maltose has been deduced.

• (-f)-Maltose has the molecular formula C12 H22On. It reduces Tollens' and Fehling's reagents and hence is a reducing sugar. It reacts with phenyl hydrazine to yield an oxazine, Ci2H2oO9(^NNHC6H5)2. It is oxidized by bromine water to a monocarboxylic acid, (CnH2iO10) COOH, malt bionic acid. (+)-Maltose exists in alpha ([a] = + 168) and6eta([a] = + 11 2) forms which undergo mutarotation in solution (equilibrium [a] = +136).

• All these facts indicate the same thing: (-f)-maltose contains a carbonyl group that exists in the reactive hemiacetal form as in the monosaccharides we have studied. It contains only one such "free" carbonyl group, however, since.

• (a) the oxazine contains only two phenyl hydrazine residues, and (b) oxidation by bromine water yields only we Wo carboxylic acid. ^

• These facts indicate that (+)-maltose has structure I, which is given the name 4-O-(-i>glucopyranosyl)-D-glucopyranose. It is the OH group on C-4 that serves as the alcohol in the glucoside formation; both halves of the molecule contain the six-membered, pyranose ring.

• Let us see how we arrive at structure I from the experimental facts. First of all, the initial oxidation labels (with a COOH group) the D-glucose unit that contains the "free" aldehyde group. Next, methylation labels (as OCH3) every free OH group. Finally, upon hydrolysis, the absence of a methoxy group shows which OH groups were not free.

 

(+)-Cellobiose

• When cellulose (cotton fibers) is treated for several days with sulfuric acid and acetic anhydride, a combination of acetylation and hydrolysis takes place; there is obtained the Octa acetate of (+)-cellobiose. Alkaline hydrolysis of the Octa acetate yields (-l-)-cellobiose.

• Like (+)-maltose, (H-)-cellobiose has the molecular formula C12H22On, is a reducing sugar, forms an ozone, exists in alpha and beta forms that undergo mutarotation, and can be hydrolyzed to two molecules of r+)-glucose. The sequence of oxidation, methylation, and hydrolysis (as described for (+)-maltose) shows that (+)-cellobiose contains two pyranose rings and a glucoside linkage to an OH group on C-4. 

• (+)-Cellobiose differs from (+)-maltose in one respect: it is hydrolyzed by the enzyme emulsion (from bitter almonds), not by maltase. Since emulsion is known to hydrolyze only 0-giucoside linkages, we can conclude that the structure of (+)- cellobiose differs from that of (-f)-maltose in only one respect: the D-glucose unit are joined by a beta linkage rather than by an alpha linkage, (-f)-Cellobiose is therefore 4-O-(jB-D-glucopyranosyls)-D-glucopyranose.

(+)-Lactose

• (+)-Lactose makes up about 5% of human milk and of cow's milk. It is obtained commercially as a by-product of cheese manufacture, being found in the whey, the aqueous solution that remains after the milk proteins have been coagulated. Milk sours when lactose is converted into lactic acid (sour, like all acids) by bacterial action (e.g., by Lactobacillus bulgaricus).

• (-f)-Lactose has the molecular formula C12 H2 2On, is a reducing sugar, forms an oxazine, and exists in alpha and beta forms which undergo mutarotation. Acidic hydrolysis or treatment with emulsion (which splits /3-linkages only) converts (-f)-lactose into equal amounts of o- (+)-glucose and D+)-galactose. (4-) -Lactose is evidently a j8-glycoside formed by the union of a molecule of D- (-f)-glucose and a molecule of o-(-f)-galactose.

• The question next arises: which is the reducing monosaccharide unit and which the non-reducing unit? Is (-f)-lactose a glucoside or a galactose? Hydrolysis of lactarane yields D-(+)-galactose and D-fluconazole; hydrolysis of autobiopic acid (monocarboxylic acid) yields D-gluconic acid and D-(+)-galactose (see Fig. 35.2). Clearly, it is the D-(+)-glucose unit that contains the "free" aldehyde group and undergoes Osa zone formation or oxidation to the acid, (-f )-Lactose is thus a substituted D-glucose in which a D-galactosyl unit is attached to one of the oxygens; it is a galactose, not a glucoside.

• The sequence of oxidation, methylation, and hydrolysis gives results analogous to those obtained with (+)-maltose and (4- >cellobiose: the glycoside linkage involves an OH group on C-4, and both units exist in the six-membered, pyranose form. (+)-Lactose is therefore 4-O(j8-D-galactopyranosyl)-D-glucopyranose. 

(+)-Sucrose

• (+)-Sucrose is our common table sugar, obtained from sugar cane and sugar beets. Of organic chemicals, it is the one produced in the largest amount in pure form.

• (-f)-Sucrose has the molecular formula^! 2 H2 2Qii. It does not reduce Tollens' or Fehling's reagent. It is a non-reducing sugar, and in this respect, it differs from the other disaccharides we have stuffier Moreover, (+)-sucrose does not form an ozone, does not exist in anomeric forms, and does not show mutarotation in solution. All these facts indicate that (-f)-sucrose does not contain a "free" aldehyde or ketone.

• When (-t-)-sucrose is hydrolyzed by dilute aqueous acid, or by the action of the enzyme invertase (from yeast), it yields equal amounts of o- (+)-glucose and D- ()-fructose. This hydrolysis is accompanied by a change in the sign of rotation from positive to negative; it is therefore often called the inversion of (-f )-sucrose, and the levorotatory mixture of D- (+)-glucose and D- (-)-fructose obtained has been called invert sugar. (Honey is mostly invert sugar; the bees supply the invertase.) While (-h)-sucrose has a specific rotation of -f 66.5 and D-(+)-glucose has a specific rotation of 4-52.7, D-(-)-fructose has a large negative specific rotation of -92.4, giving a net negative value for the specific rotation of the mixture. (Because of their opposite rotations and their importance as components of (-l-)-sucrose, D- (+)-glucose and D- (-)-fructose are commonly called dextrose and levuloses.)

• The weight of evidence, including the results of x-ray studies and finally the synthesis of (-f)-sucrose (1953), leads to the conclusion that (+)-sucrose is a beta D-fructose and an alpha D-glucoside. (The synthesis of sucrose, by R. U. Lemieux of the Prairie Regional Laboratory, Saskatoon, Saskatchewan, has been described as "the Mount Everest of organic chemistry.") 

Polysaccharides

• Polysaccharides are compounds made up of many hundreds or even thousands monosaccharide units per molecule. As in disaccharides, these units are held together by glycoside linkages, which can be broken by hydrolysis.

• Polysaccharides are naturally occurring polymers, which can be considered as derived from aldoses or ketoses by condensation polymerization. A polysaccharide derived from hexoses, for example, has the general formula (C6H10O5) n. This formula, of course, tells us very little about the structure of the polysaccharide. We need to know what the monosaccharide units are and how many there are in each molecule; how they are joined to each other; and whether the huge molecules thus formed are straight-chained or branched, looped or.

• By far the most important polysaccharides are cellulose and starch. Both are produced in plants from carbon dioxide and water by the process of photosynthesis, and both, as it happens, are made up of D-(+)-glucose units. Cellulose is the chief structural material of plants, giving the plants rigidity and form. It is probably the most widespread organic material known. Starch makes up the reserve food supply of plants and occurs chiefly in seeds. It is more water-soluble than cellulose, more easily hydrolyzed, and hence more readily digested.

• Both cellulose and starch are, of course, enormously important to us. Generally speaking, we use them in very much the same way as the plant does. We use cellulose for its structural properties: as wood for houses, as cotton or rayon for clothing, as paper for communication and packaging. We use starch as a food: potatoes, corn, wheat, rice, cassava, etc.

Starch

• Starch occurs as granules whose size and shape are characteristic of the plant from which the starch is obtained. When intact, starch granules are insoluble in cold water; if the outer membrane has been broken by grinding, the granules swell in cold water and form a gel. When the intact granule is treated with warm water, a soluble portion of the starch diffuses through the granule wall; in hot water the granules swell to such an extent that they burst. 

• In general, starch contains about 20% of a water-soluble fraction called amylose, and 80% of a water-insoluble fraction called amylopectin. These two fractions appear to correspond to different carbohydrates of high molecular weight and formula (C6Hj O5) n, upon treatment with acid or under the inf fence of enzymes, the components of starch are hydrolyzed progressively to debt in (a mixture of low molecular weight polysaccharides), (+)-maltose, and finally D- (+)- glucose. (A mixture of all these is found in corn sirup, for example.) Both amylose and amylopectin are made up of D-(+)-glucoses units but differ in molecular size and shape.

Structure of amylose. End group analysis

• (-h)-Maltose is the only disaccharide that is obtained by hydrolysis of amylose, and D-(4* )-glucose is the only monosaccharide. To account for this, it has been proposed that amylose is made up of chains of many D-(+)-glucose units, each unit joined by an alpha glycoside linkage to C-4 of the next one.

• We could conceive of a structure for amylose in which a- and /^-linkages regularly alternate. However, a compound of such a structure would be expected to yield (+)- cellobiose as well as (+)-maltose unless hydrolysis of the /^-linkages occurred much faster than hydrolysis of the a-linkages. Since hydrolysis of the ^-linkage in (+)-cellobioses is actually slower than hydrolysis of the a-linkage in (+)-maltose, such a structure seems unlikely. 

• How many of these a-o-(+)-glucose units are there per molecule of amylose, and what are the shapes of these large molecules? These are difficult questions and attempts to find the answers have made use of chemical and enzymatic methods, and of physical methods like x-ray analysis, electron microscopy, osmotic pressure and viscosity measurements, and behavior in an ultracentrifuge.

• Valuable information about molecular size and shape has been obtained by the combination of methylation and hydrolysis that was so effective in studying the structures of disaccharides. D-(+)-Glucose, a monosaccharide, contains five free OH groups and forms a pentamethyl derivative, methyl tetra-O-methyl-D-glucopyranoside. When two o-(+)-glucose units are joined together, as in (-f)-maltose, each unit contains four free OH groups; an octamethyl derivative is formed. If each D-(+)-glucose unit in amylose is joined to two others, it contains only three free OH groups; methylation of amylose should therefore yield a compound containing only three Ocha groups per glucose unit. What are the facts?

 Structure of amylopectin

• Amylopectin is hydrolyzed to the single disaccharide (-h)-maltose; the sequence of methylation and hydrolysis yields chiefly 2,3,6-tri-O-methyl-D-glucose. Like amylose, amylopectin is made up of chains of D-glucose units, each unit joined by an alpha glycoside linkage to C-4 of the next one. However, its structure is more complex than that of amylose.

• Molecular weights determined by physical methods show that there are up to a million D-glucose units per molecule. Yet hydrolysis of methylated amylopectin gives as high as 5% of 2,3,4,6-tetra-O-methyl-D-gIucose, indicating only 20 units per chain. How can these facts be reconciled by the same structure?

• The answer is found in the following fact : along with the trimethyl and tetramethyl compounds, hydrolysis yields 2,3-di-O-methyl-D-glucose and in an amount nearly equal to that of the tetramethyl derivative. 

• Amylopectin has a highly branched structure consisting of several hundred short chains of about 20-25 D-glucose units each. One end of each of these chains is joined through C-l to a C-6 on the next chain.

Structure of cellulose

• Cellulose is the chief component of wood and plant fibers; cotton, for instance, is nearly pure cellulose. It is insoluble in water and tasteless; it is a non-reducing carbohydrate. These properties, in part at least, are due to its extremely high molecular.

• Cellulose has the formula (C6Hi O5)n . Complete hydrolysis by acid yields r>(+ )-glucose as the only monosaccharide. Hydrolysis of completely methylated cellulose gives a high yield of 2,3,6-tri-O-methyl-D-glucose. Like starch, therefore, cellulose is made up of chains of D-glucose units, each unit joined by a glycoside linkage 10 C-4 of the next.

• Cellulose differs from starch, however, in the configuration of the glycoside 

• linkage. Upon treatment with acetic anhydride and sulfuric acid, cellulose yields octa-O-acetyl cellobiose; there is evidence that all glycoside linkages in cellulose, like the one in ( + )-cellobiose, are beta linkages.

Reactions of cellulose

• We have seen that the glycoside linkages of cellulose are broken by the action of acid, each cellulose molecule yielding many molecules of r>-( + )-glucose. Now let us look briefly at reactions of cellulose in which the chain remains essentially intact. Each glucose unit in cellulose contains three free OH groups; these are the positions at which reaction occurs.

• These reactions of cellulose, carried out to modify the properties of a cheap, available, ready-made polymer, are of tremendous industrial importance.

Cellulose nitrate

• Like any alcohol, cellulose forms esters. Treatment with a mixture of nitric and sulfuric acids converts cellulose into cellulose nitrate. The properties and uses of the product depend upon the extent of nitration.

• Guncotton, which is used in making smokeless powder, is very nearly completely nitrated cellulose, and is often called cellulose trinitrate (three nitrate groups per glucose unit)

• Pyroxylin is less highly nitrated material containing between two and three nitrate groups per glucose unit. It is used in the manufacture of plastics like celluloid and collodion, in photographic film, and in lacquers. It has the disadvantage of being flammable, and forms highly toxic nitrogen oxides upon burning.

Cellulose acetate

• In the presence of acetic anhydride, acetic acid, and a little sulfuric acid, cellulose is converted into the triacetate. Partial hydrolysis removes some of the acetate groups, degrades the chains to smaller fragments (of 200-300 units each), and yields the vastly important commercial cellulose acetate (roughly a ^//acetate).

• Cellulose acetate is less flammable than cellulose nitrate and has replaced the nitrate in many of its applications, in safety-type photographic film, for example. When a solution of cellulose acetate in acetone is forced through the fine holes of a spinnerets, the solvent evaporates and leaves solid filaments. Threads from these filaments make up the material known as acetate.

Rayon. Cellophane

• When an alcohol is treated with carbon disulfide and aqueous sodium hydroxide, there is obtained a compound called a xanthate. 

• Cellulose undergoes an analogous reaction to form cellulose xanthate, which dissolves in the alkali to form a viscous colloidal dispersion called viscose.

• When viscose is forced through a spinneret into an acid bath, cellulose is regenerated in the form of fine filaments which yield threads of the material known as rayon. There are other processes for making rayon, but the viscose process is still the principal one used in the United States.

• If viscose is forced through a narrow slit, cellulose is regenerated as thin sheets which, when softened by glycerol, are used for protective films (Cellophane). Although rayon and Cellophane are often spoken of as " regenerated cellulose," they are made up of much shorter chains than the original cellulose because of degradation by the alkali treatment.

Cellulose ethers

• Industrially, cellulose is alkylated by the action of alkyl chlorides (cheaper than sulfates) in the presence of alkali. Considerable degradation of the long chains is unavoidable in these reactions. Methyl, ethyl, and benzyl ethers of cellulose are important in the production of textiles, films, and various plastic objects.