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Chapter 20 CarbohydratesChapter 20

CarbohydratesCarbohydrate: A polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis.Monosaccharide: A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate.• Monosaccharides have the general formula CnH2nOn,

where n varies from 3 to 8.• Aldose: A monosaccharide containing an aldehyde

group.• Ketose: A monosaccharide containing a ketone group.

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Monosaccharides• The suffix -ose indicates that a molecule is a carbohydrate.• The prefixes tri-, tetra-, penta-, and so forth indicate the number of

carbon atoms in the chain.• Those containing an aldehyde group are classified as aldoses. • Those containing a ketone group are classified as ketoses.• There are only two trioses:

• Often aldo- and keto- are omitted and these compounds are referred to simply as trioses.

• Although “triose” does not tell the nature of the carbonyl group, it at least tells the number of carbons.

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MonosacharidesFigure 20-1 Glyceraldehyde, the simplest aldose, contains one stereocenter and exists as a pair of enantiomers.

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MonosaccharidesFischer projection: A two-dimensional representation for showing the configuration of tetrahedral stereocenters.• Horizontal lines represent bonds projecting forward

from the stereocenter. • Vertical lines represent bonds projecting to the rear.• Only the stereocenter is in the plane.

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MonosacharidesIn 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde.

• D-monosaccharide: the -OH on its penultimate carbon is on the right in a Fischer projection.

• L-monosaccharide: the -OH on its penultimate carbon is on the left in a Fischer projection.

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D,L-Monosaccharides• The most common D-tetroses and D-pentoses are:

• The three most common D-hexoses are:

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Amino SugarsAmino sugars contain an -NH2 group in place of an -OH group. • Only three amino sugars are common in nature:

D-glucosamine, D-mannosamine, and D-galactosamine. N-acetyl-D-glucosamine is an acetylated derivative of D-glucosamine.

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Cyclic Structure• Aldehydes and ketones react with alcohols to form

hemiacetals (Chapter 17).• Cyclic hemiacetals form readily when hydroxyl and

carbonyl groups are part of the same molecule and their interaction can form a five- or six-membered ring.

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Haworth Projections• Figure 20-2 D-Glucose forms these two cyclic

hemiacetals.

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Haworth Projections• A five- or six-membered cyclic hemiacetal is

represented as a planar ring, lying roughly perpendicular to the plane of the paper.

• Groups bonded to the carbons of the ring then lie either above or below the plane of the ring.

• The new carbon stereocenter created in forming the cyclic structure is called the anomeric carbon.

• Stereoisomers that differ in configuration only at the anomeric carbon are called anomers.

• The anomeric carbon of an aldose is C-1; that of the most common ketose is C-2.

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Haworth ProjectionsIn the terminology of carbohydrate chemistry, • β means that the -OH on the anomeric carbon is on the

same side of the ring as the terminal -CH2OH.• α means that the -OH on the anomeric carbon is on the

side of the ring opposite from the terminal -CH2OH. • A six-membered hemiacetal ring is called a pyranose,

and a five-membered hemiacetal ring is called a furanose because these ring sizes correspond to the heterocyclic compounds furan and pyran.

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Haworth Projections• Aldopentoses also form cyclic hemiacetals.• The most prevalent forms of D-ribose and other

pentoses in the biological world are furanoses.

• The prefix “deoxy” means “without oxygen.”

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Haworth ProjectionsD-Fructose (a 2-ketohexose) also forms a five-membered cyclic hemiacetal.

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Chair Conformations• For pyranoses, the six-membered ring is more accurately

represented as a strain-free chair conformation.

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Chair Conformations• In both Haworth projections and chair conformations, the

orientations of groups on carbons 1- 5 of β-D-glucopyranose are up, down, up, down, and up and all are equatorial.

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Mutarotation• Mutarotation: The change in specific rotation that

accompanies the equilibration of α- and β-anomers in aqueous solution.• Example: When either α-D-glucose or β-D-glucose is

dissolved in water, the specific rotation of the solution gradually changes to an equilibrium value of +52.7°, which corresponds to 64% beta and 36% alpha forms.

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Formation of Glycosides• Treatment of a monosaccharide, all of which exist almost

exclusively in cyclic hemiacetal forms, with an alcohol gives an acetal.

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Formation of Glycosides• A cyclic acetal derived from a monosaccharide is

called a glycoside.• The bond from the anomeric carbon to the -OR group is

called a glycosidic bond.• Mutarotation is not possible for a glycoside because an

acetal, unlike a hemiacetal, is not in equilibrium with the open-chain carbonyl-containing compound.

• Glycosides are stable in water and aqueous base, but like other acetals, are hydrolyzed in aqueous acid to an alcohol and a monosaccharide.

• Glycosides are named by listing the alkyl or aryl group bonded to oxygen followed by the name of the carbohydrate in which the ending -e is replaced by -ide.

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Reduction to Alditols• The carbonyl group of a monosaccharide can be reduced

to an hydroxyl group by a variety of reducing agents, including NaBH4 and H2 in the presence of a transition metal catalyst (H2/Pt).• The reduction product is called an alditol.• Alditols are named by changing the suffix -ose to -itol.

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Alditols• Sorbitol is found in the plant world in many berries and

in cherries, plums, pears, apples, seaweed, and algae.• It is about 60 percent as sweet as sucrose (table sugar)

and is used in the manufacture of candies and as a sugar substitute for diabetics.

• These three alditols are also common in the biological world. Note that only one of these is chiral.

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Oxidation to Aldonic Acids• The aldehyde group of an aldose is oxidized under

basic conditions to a carboxylate anion.• The oxidation product is called an aldonic acid.• A carbohydrate that reacts with an oxidizing agent to

form an aldonic acid is classified as a reducing sugar(it reduces the oxidizing agent).

• 2-Ketoses (e.g. D-fructose) are also reducing sugars.

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Sucrose• Table sugar, obtained from the juice of sugar cane and

sugar beet.

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Lactose• The principle sugar present in milk.

• About 5 - 8% in human milk, 4 - 5% in cow’s milk.

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Maltose• From malt, the juice of sprouted barley and other cereal

grains.

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PolysaccharidesPolysaccharide: A carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds.Starch: A polymer of D-glucose.• Starch can be separated into amylose and amylopectin.• Amylose is composed of unbranched chains of up to

4000 D-glucose units joined by α-1,4-glycosidic bonds.• Amylopectin contains chains up to 10,000 D-glucose

units also joined by α-1,4-glycosidic bonds. At branch points, new chains of 24 to 30 units are started by α-1,6-glycosidic bonds.

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Polysaccharides• Figure 20-3 Amylopectin is a branched polymer of D-

glucose units joined by α-1,4-glycosidic bonds. Branches consist of D-glucose units that start with an α-1,6-glycosidic bond.

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Polysaccharides• Glycogen is the energy-reserve carbohydrate for animals.

• Glycogen is a branched polysaccharide of approximately 106 glucose units joined by α-1,4- and α-1,6-glycosidic bonds.

• The total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle.

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PolysaccharidesCellulose is a linear polysaccharide of D-glucose units joined by β-1,4-glycosidic bonds.• It has an average molecular weight of 400,000 g/mol,

corresponding to approximately 2200 glucose units per molecule.

• Cellulose molecules act like stiff rods and align themselves side by side into well-organized water-insoluble fibers in which their -OH groups form numerous intermolecular hydrogen bonds.

• This arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength.

• It is also the reason why cellulose is insoluble in water.

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Polysaccharides• Figure 20-4 Cellulose is a linear polysaccharide of D-

glucose units joined by β-1,4-glycosidic bonds.

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PolysaccharidesCellulose (cont’d)• Humans and other animals can not digest cellulose

because their digestive systems do not contain β-glycosidases, enzymes that catalyze the hydrolysis of β-glycosidic bonds.

• Termites have such bacteria in their intestines and can use wood as their principal food.

• Ruminants (cud-chewing animals) and horses can also digest grasses and hay.

• Humans have only α-glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen.

• Many bacteria and microorganisms have β-glucosidases.

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Acidic PolysaccharidesAcidic polysaccharides: a group of polysaccharides that contain carboxyl groups and/or sulfuric ester groups, and play important roles in the structure and function of connective tissues.• There is no single general type of connective tissue.• Rather, there are a large number of highly specialized

forms, such as cartilage, bone, synovial fluid, skin, tendons, blood vessels, intervertebral disks, and cornea.

• Most connective tissues are made up of collagen, a structural protein, in combination with a variety of acidic polysaccharides.

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Acidic PolysaccharidesHeparin (cont’d)• Heparin is synthesized and stored in mast cells of

various tissues, particularly the liver, lungs, and gut.• The best known and understood of its biological

functions is its anticoagulant activity.• It binds strongly to antithrombin III, a plasma protein

involved in terminating the clotting process.

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Heparin• Figure 20-5 The repeating pentasaccharide unit of

heparin.

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Chapter 20 Carbohydrates

End Chapter 20

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