1. Chapter 12
Carbohydrates

2. Carbohydrates
Carbohydrate: 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

3. Monosaccharides
Monosaccharides are classified by their number of carbon atoms

4. Monosaccharides There are only two trioses
often aldo- and keto- are omitted and these compounds are referred to simply as trioses
although this designation does not tell the nature of the carbonyl group, it at least tells the number of carbons

5. Monosaccharides
Glyceraldehyde, the simplest aldose, contains a stereocenter and exists as a pair of enantiomers

6. Monosaccharides
Fischer projection: a two dimensional representation for showing the configuration of tetrahedral stereocenters
horizontal lines represent bonds projecting forward
vertical lines represent bonds projecting backward

7. D,L Monosaccharides
In 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
L-monosaccharide: the -OH on its penultimate carbon is on the left

8. Table 19.1, p.468

9. Table 19.2, p.469

10. D,L Monosaccharides the most common D-tetroses and D-pentoses
the three common D-hexoses

11. Amino Sugars
Amino 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

12. Cyclic Structure
Aldehydes and ketones react with alcohols to form hemiacetals
cyclic hemiacetals form readily when the hydroxyl and carbonyl groups are part of the same molecule and their interaction can form a five- or six-membered ring

13. Haworth Projections
D-Glucose forms these cyclic hemiacetals

14. 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 an 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 ketoses is C-2

15. Haworth Projections In the terminology of carbohydrate chemistry,
b means that the -OH on the anomeric carbon is on the same side of the ring as the terminal -CH2OH
a 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

16. Haworth Projections aldopentoses also form cyclic hemiacetals
the most prevalent forms of D-ribose and other pentoses in the biological world are furanoses

17. Haworth Projections
D-fructose also forms a five-membered cyclic hemiacetal

18. Chair Conformations
For pyranoses, the six-membered ring is more accurately represented as a chair conformation

19. Chair Conformations
in both a Haworth projection and a chair conformation, the orientations of groups on carbons 1- 5 of b-D-glucopyranose are up, down, up, down, and up

20. Physical Properties
Monosaccharides are colorless crystalline solids, very soluble in water, but only slightly soluble in ethanol
sweetness relative to sucrose:

21. Formation of Glycosides
Treatment of a monosaccharide, all of which exist almost exclusively in a cyclic hemiacetal form, with an alcohol gives an acetal

22. 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
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

23. 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
the reduction product is called an alditol

24. Reduction to 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

25. 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
any carbohydrate that reacts with an oxidizing agent to form an aldonic acid is classified as a reducing sugar (it reduces the oxidizing agent)

26. Oxidation to Uronic Acids
Enzyme-catalyzed oxidation of the primary alcohol at C-6 of a hexose yields a uronic acid
enzyme-catalyzed oxidation of D-glucose, for example, yields D-glucuronic acid

27. D-Glucuronic Acid
D-glucuronic acid is widely distributed in the plant and animal world
in humans, it is an important component of the acidic polysaccharides of connective tissues
it is used by the body to detoxify foreign phenols and alcohols; in the liver, these compounds are converted to glycosides of glucuronic acid and excreted in the urine

28. Phosphate Esters
Mono- and diphosphoric esters are intermediates in the metabolism of monosaccharides
for example, the first step in glycolysis is conversion of D-glucose to a-D-glucose 6-phosphate
note that at the pH of cellular and intercellular fluids, both acidic protons of a phosphoric ester are ionized, giving it a charge of -2

29. Disaccharides Sucrose (table sugar)
sucrose is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets
sucrose is a nonreducing sugar

30. Disaccharides Lactose
lactose is the principal sugar present in milk; it makes up about 5 to 8 percent of human milk and 4 to 6 percent of cow's milk
it consists of D-galactopyranose bonded by a b-1,4-glycosidic bond to carbon 4 of D-glucopyranose
lactose is a reducing sugar

31. Disaccharides Maltose
present in malt, the juice from sprouted barley and other cereal grains
maltose consists of two units of D-glucopyranose joined by an a-1,4-glycosidic bond
maltose is a reducing sugar

32. Disaccharides Maltose
present in malt, the juice from sprouted barley and other cereal grains
maltose consists of two units of D-glucopyranose joined by an a-1,4-glycosidic bond
maltose is a reducing sugar

33. Polysaccharides
Polysaccharide: 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 a-1,4-glycosidic bonds
amylopectin contains chains up to 10,000 D-glucose units also joined by a-1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by a-1,6-glycosidic bonds

34. amylopectin
Fig 19.3, p.485

35. Polysaccharides
Glycogen is the energy-reserve carbohydrate for animals
glycogen is a branched polysaccharide of approximately 106 glucose units joined by a-1,4- and a-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

36. Polysaccharides
Cellulose is a linear polysaccharide of D-glucose units joined by b-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 the 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

37. Polysaccharides Cellulose (cont’d)
humans and other animals cannot use cellulose as food because our digestive systems do not contain b-glucosidases, enzymes that catalyze hydrolysis of b-glucosidic bonds
instead, we have only a-glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen
many bacteria and microorganisms have b-glucosidases and can digest cellulose
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

38. cellulose
Fig 19.4, p.486

39. Carbohydrates
End
Chapter 12