Selective oxidation to a ketone is then possible. Finally, direct di-O-isopropylidene derivatization of glucose by reaction with excess acetone results in a change to a furanose structure in which the C-3 hydroxyl is again unprotected. However, the same reaction with D-galactose, shown in the blue-shaded box, produces a pyranose product in which the C-6 hydroxyl is unprotected. Both derivatives do not react with Tollens' reagent. This difference in behavior is attributed to the cis-orientation of the C-3 and C-4 hydroxyl groups in galactose, which permits formation of a less strained five-membered cyclic acetal, compared with the trans-C-3 and C-4 hydroxyl groups in glucose.
Derivatizations of this kind permit selective reactions to be conducted at different locations in these highly functionalized molecules.
The ring size of these cyclic monosaccharides was determined by oxidation and chain cleavage of their tetra methyl ether derivatives. To see how this was done for glucose Click Here. Glycosides Acetal derivatives formed when a monosaccharide reacts with an alcohol in the presence of an acid catalyst are called glycosides.
This reaction is illustrated for glucose and methanol in the diagram below. In naming of glycosides, the "ose" suffix of the sugar name is replaced by "oside", and the alcohol group name is placed first. As is generally true for most acetals , glycoside formation involves the loss of an equivalent of water.
The diether product is stable to base and alkaline oxidants such as Tollen's reagent. Since acid-catalyzed aldolization is reversible, glycosides may be hydrolyzed back to their alcohol and sugar components by aqueous acid. From the structures in the previous diagram, we see that pyranose rings prefer chair conformations in which the largest number of substituents are equatorial.
In the case of glucose, the substituents on the beta-anomer are all equatorial, whereas the C-1 substituent in the alpha-anomer changes to axial. Glycosides abound in biological systems. By attaching a sugar moiety to a lipid or benzenoid structure, the solubility and other properties of the compound may be changed substantially. Because of the important modifying influence of such derivatization, numerous enzyme systems, known as glycosidases, have evolved for the attachment and removal of sugars from alcohols, phenols and amines.
Chemists refer to the sugar component of natural glycosides as the glycon and the alcohol component as the aglycon. Two examples of naturally occurring glycosides and one example of an amino derivative will be displayed above by clicking on the diagram. Salicin, one of the oldest herbal remedies known, was the model for the synthetic analgesic aspirin.
A large class of hydroxylated, aromatic oxonium cations called anthocyanins provide the red, purple and blue colors of many flowers, fruits and some vegetables. Peonin is one example of this class of natural pigments, which exhibit a pronounced pH color dependence.
The oxonium moiety is only stable in acidic environments, and the color changes or disappears when base is added. The complex changes that occur when wine is fermented and stored are in part associated with glycosides of anthocyanins. Finally, amino derivatives of ribose, such as cytidine play important roles in biological phosphorylating agents, coenzymes and information transport and storage materials.
For a discussion of the anomeric effect Click Here. For examples of structurally and functionally modified sugars Click Here. Disaccharides When the alcohol component of a glycoside is provided by a hydroxyl function on another monosaccharide, the compound is called a disaccharide. Four examples of disaccharides composed of two glucose units are shown in the following diagram. The individual glucopyranose rings are labeled A and B, and the glycoside bonding is circled in light blue.
Notice that the glycoside bond may be alpha, as in maltose and trehalose, or beta as in cellobiose and gentiobiose. Acid-catalyzed hydrolysis of these disaccharides yields glucose as the only product.
Enzyme-catalyzed hydrolysis is selective for a specific glycoside bond, so an alpha-glycosidase cleaves maltose and trehalose to glucose, but does not cleave cellobiose or gentiobiose. A beta-glycosidase has the opposite activity.
The bonding between the glucopyranose rings in cellobiose and maltose is from the anomeric carbon in ring A to the C-4 hydroxyl group on ring B. This leaves the anomeric carbon in ring B free, so cellobiose and maltose both may assume alpha and beta anomers at that site the beta form is shown in the diagram. Gentiobiose has a beta-glycoside link, originating at C-1 in ring A and terminating at C-6 in ring B. Its alpha-anomer is drawn in the diagram.
Because cellobiose, maltose and gentiobiose are hemiacetals they are all reducing sugars oxidized by Tollen's reagent. Trehalose, a disaccharide found in certain mushrooms, is a bis-acetal, and is therefore a non-reducing sugar. A systematic nomenclature for disaccharides exists, but as the following examples illustrate, these are often lengthy.
Maltose, sometimes called malt sugar, comes from the hydrolysis of starch. It is about one third as sweet as cane sugar sucrose , is easily digested by humans, and is fermented by yeast. Cellobiose is obtained by the hydrolysis of cellulose. It has virtually no taste, is indigestible by humans, and is not fermented by yeast.
Some bacteria have beta-glucosidase enzymes that hydrolyze the glycosidic bonds in cellobiose and cellulose. The presence of such bacteria in the digestive tracts of cows and termites permits these animals to use cellulose as a food. Finally, it may be noted that trehalose has a distinctly sweet taste, but gentiobiose is bitter. Disaccharides made up of other sugars are known, but glucose is often one of the components.
Two important examples of such mixed disaccharides will be displayed above by clicking on the diagram. Lactose, also known as milk sugar, is a galactose-glucose compound joined as a beta-glycoside. It is a reducing sugar because of the hemiacetal function remaining in the glucose moiety. Many adults, particularly those from regions where milk is not a dietary staple, have a metabolic intolerance for lactose.
Infants have a digestive enzyme which cleaves the beta-glycoside bond in lactose, but production of this enzyme stops with weaning. Cheese is less subject to the lactose intolerance problem, since most of the lactose is removed with the whey.
Sucrose, or cane sugar, is our most commonly used sweetening agent. It is a non-reducing disaccharide composed of glucose and fructose joined at the anomeric carbon of each by glycoside bonds one alpha and one beta. For examples of some larger saccharide oligomers Click Here. Polysaccharides As the name implies, polysaccharides are large high-molecular weight molecules constructed by joining monosaccharide units together by glycosidic bonds.
They are sometimes called glycans. The most important compounds in this class, cellulose, starch and glycogen are all polymers of glucose. This is easily demonstrated by acid-catalyzed hydrolysis to the monosaccharide.
Since partial hydrolysis of cellulose gives varying amounts of cellobiose, we conclude the glucose units in this macromolecule are joined by beta-glycoside bonds between C-1 and C-4 sites of adjacent sugars.
Partial hydrolysis of starch and glycogen produces the disaccharide maltose together with low molecular weight dextrans, polysaccharides in which glucose molecules are joined by alpha-glycoside links between C-1 and C-6, as well as the alpha C-1 to C-4 links found in maltose. Polysaccharides built from other monosaccharides e. Over half of the total organic carbon in the earth's biosphere is in cellulose. As a polymer of glucose, cellulose has the formula C6H10O5 n where n ranges from to 5,, depending on the source of the polymer.
The glucose units in cellulose are linked in a linear fashion, as shown in the drawing below. The beta-glycoside bonds permit these chains to stretch out, and this conformation is stabilized by intramolecular hydrogen bonds. A parallel orientation of adjacent chains is also favored by intermolecular hydrogen bonds. Although an individual hydrogen bond is relatively weak, many such bonds acting together can impart great stability to certain conformations of large molecules.
Most animals cannot digest cellulose as a food, and in the diets of humans this part of our vegetable intake functions as roughage and is eliminated largely unchanged.
Some animals the cow and termites, for example harbor intestinal microorganisms that breakdown cellulose into monosaccharide nutrients by the use of beta-glycosidase enzymes. Cellulose is commonly accompanied by a lower molecular weight, branched, amorphous polymer called hemicellulose.
In contrast to cellulose, hemicellulose is structurally weak and is easily hydrolyzed by dilute acid or base. Also, many enzymes catalyze its hydrolysis. Hemicelluloses are composed of many D-pentose sugars, with xylose being the major component. Mannose and mannuronic acid are often present, as well as galactose and galacturonic acid. Starch is a polymer of glucose, found in roots, rhizomes, seeds, stems, tubers and corms of plants, as microscopic granules having characteristic shapes and sizes.
Most animals, including humans, depend on these plant starches for nourishment. The structure of starch is more complex than that of cellulose. The intact granules are insoluble in cold water, but grinding or swelling them in warm water causes them to burst. The released starch consists of two fractions. Molecules of amylose are linear chains of several thousand glucose units joined by alpha C-1 to C-4 glycoside bonds.
Amylose solutions are actually dispersions of hydrated helical micelles. The majority of the starch is a much higher molecular weight substance, consisting of nearly a million glucose units, and called amylopectin. Molecules of amylopectin are branched networks built from C-1 to C-4 and C-1 to C-6 glycoside links, and are essentially water insoluble. Representative structural formulas for amylose and amylopectin will be shown above by clicking on the diagram.
To see an expanded structure for amylopectin click again on the diagram. The branching in this diagram is exaggerated, since on average, branches only occur every twenty five glucose units. Hydrolysis of starch, usually by enzymatic reactions, produces a syrupy liquid consisting largely of glucose.
When cornstarch is the feedstock, this product is known as corn syrup. It is widely used to soften texture, add volume, prohibit crystallization and enhance the flavor of foods. Glycogen is the glucose storage polymer used by animals.
It has a structure similar to amylopectin, but is even more highly branched about every tenth glucose unit. The degree of branching in these polysaccharides may be measured by enzymatic or chemical analysis. For examples of chemical analysis of branching Click Here.
Synthetic Modification of Cellulose Cotton, probably the most useful natural fiber, is nearly pure cellulose. The manufacture of textiles from cotton involves physical manipulation of the raw material by carding, combing and spinning selected fibers. For fabrics the best cotton has long fibers, and short fibers or cotton dust are removed.
Solutions for nmr study can be prepared in deuteriochloroform. An analysis of the nmr spectra reveals how in cyclohexane type systems the difference in the chemical shift of a n axial versus a n eauatorial proton and the dependence of the coupling c o n s t k t on the dihedral angle between vicinal protons can be used t o distinguish between the diastereomeric glucose pentaacetates.
The facts that each of the two isomers is essentially conformationally uniform and also that each is easily available make this an especially attractive system to illustrate these principles. However, the chemical shift for the methine proton a t C-1 is farther downfield than any of the others because the anomeric carbon atom is bonded to two oxygen atoms.
Lemieux showed that this farthest downfield absorption is due to the anomeric proton and used the chemical shift and coupline constant data for this absorption to unequivocally codfirm the structures for a- and 8-D-glucose pentaacetate 4, 5. There is some virtual coupling present especially in the 6-anomer but this does not change the size of the coupling constants measured appreciably and so does not affect the structure assignment. A comparison of our data to those of Lemieux appears in the table and reveals close agreement.
The absorption for an equatorial proton is generally found 0. An alternate exnlanation for this chemical shift difference is that paramagnetic ring current effects exist in the cvclohexane rine svstem. The eauatorial proton falls in the ;egion in whicb the magnetic-lines of force set up by the ring current more distinctly align with the applied magnetic field and so appears deshielded in a manner analogous to aromatic protons.
A large class of hydroxylated, aromatic oxonium cations called anthocyanins provide the red, purple and blue colors of many flowers, fruits and some vegetables. Chemists refer to the sugar component of natural glycosides as the glycon and the alcohol component as the aglycon.
Over half of the total organic carbon in the earth's biosphere is in cellulose. Because cellobiose, maltose and gentiobiose are hemiacetals they are all reducing sugars oxidized by Tollen's reagent. Note that despite the very low concentration of the open chain aldehyde in this mixture, typical chemical reactions of aldehydes take place rapidly.
Some chain shortening occurs unavoidably in the preparations. Solutions for nmr study can be prepared in deuteriochloroform. Derivatizations of this kind permit selective reactions to be conducted at different locations in these highly functionalized molecules. When cornstarch is the feedstock, this product is known as corn syrup.
These Haworth formulas are convenient for displaying stereochemical relationships, but do not represent the true shape of the molecules. By clicking on the diagram a second time this relationship will be displayed above. The released starch consists of two fractions. A methyl glycoside derivative of this compound see below leaves the C-2 and C-3 hydroxyl groups exposed to reactions such as the periodic acid cleavage, shown as the last step.
This is shown in the following diagram by a red dot where the symmetry axis passes through the projection formula. These less desirable cellulose sources are widely used for making paper.
Five and six-membered rings are favored over other ring sizes because of their low angle and eclipsing strain. The released starch consists of two fractions. Cellulose Nitrate, first prepared over years ago by treating cellulose with nitric acid, is the earliest synthetic polymer to see general use. Consequently, fresh solutions of either alpha or beta-glucose crystals in water should establish an equilibrium mixture of both anomers, plus the open chain chain form. Two different crystalline forms of glucose were reported in
Crude cellulose is also available from wood pulp by dissolving the lignan matrix surrounding it. A parallel orientation of adjacent chains is also favored by intermolecular hydrogen bonds. An analysis of the nmr spectra reveals how in cyclohexane type systems the difference in the chemical shift of a n axial versus a n eauatorial proton and the dependence of the coupling c o n s t k t on the dihedral angle between vicinal protons can be used t o distinguish between the diastereomeric glucose pentaacetates.
Trehalose, a disaccharide found in certain mushrooms, is a bis-acetal, and is therefore a non-reducing sugar. By clicking on the diagram a second time this relationship will be displayed above. Cyclic Forms of Monosaccharides As noted above, the preferred structural form of many monosaccharides may be that of a cyclic hemiacetal. The search for scientific truth often proceeds in stages, and the structural elucidation of glucose serves as a good example. As a polymer of glucose, cellulose has the formula C6H10O5 n where n ranges from to 5,, depending on the source of the polymer.
Because of the important modifying influence of such derivatization, numerous enzyme systems, known as glycosidases, have evolved for the attachment and removal of sugars from alcohols, phenols and amines. Cellulose is commonly accompanied by a lower molecular weight, branched, amorphous polymer called hemicellulose. The upper bond to this carbon is defined as beta, the lower bond then is alpha.
The majority of the starch is a much higher molecular weight substance, consisting of nearly a million glucose units, and called amylopectin. Oxidation of erythrose gave an achiral optically inactive aldaric acid. The anomeric carbons are colored red.