1. Bettelheim, Brown, Campbell and Farrell Chapter 20
Carbohydrates
Bettelheim, Brown, Campbell and Farrell
Chapter 20

2. Carbohydrates
Carbohydrate: polyhydroxyaldehyde or polyhydroxyketone, or a substance that can be hydrolyzed to form these compounds
Monosaccharide: a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate (simple sugar)

3. Monosaccharide:
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

4. Monosaccharides
Monosaccharides are classified by their number of carbon atoms
“ose” ending for sugars

5. Monosaccharides There are only two trioses
Often simply call these trioses
Tells the number of carbons

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

7. 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 to the rear

8. 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 (next to last) carbon is on the right
L-monosaccharide: the -OH on its penultimate (next to last) carbon is on the left

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

10. Review: Addition of Alcohols to Carbonyls
Addition of an alcohol to the carbonyl group of an aldehyde or ketone forms a hemiacetal (a half-acetal)
Functional group of a hemiacetal is a carbon bonded to one -OH group and one -OR group
H of the alcohol adds to the carbonyl oxygen and -OR adds to the carbonyl carbon

11. Addition of Alcohols to Carbonyl
Hemiacetals are generally unstable and are only minor components of most equilibrium mixtures
Major exception: When both the carbonyl group and the hydroxyl group are in the same molecule and can form a cyclic hemiacetal with a 5- or 6-member ring
Cyclic hemiacetals predominate

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. Carbohydrate Rings
-OH group can react with C=O group to give hemiacetal
Fischer Fischer ring Haworth

16. Haworth Projections
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
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
Anomeric carbon of an aldose is C-1; that of the most common ketoses is C-2

17. Haworth Projections In the terminology of carbohydrate chemistry,
b form: -OH on the anomeric carbon is on the same side of the ring as the terminal -CH2OH (up)
a form: -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH2OH (down)
Pyranose: 6-member ring sugar containing O
Furanose: 5-member ring sugar containing O

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

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

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

21. 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

22. Mutarotation
Mutarotation: a- and b-anomers can have the ring open and then form the other anomer.
A change in specific optical rotation accompanies the equilibration of in aqueous solution
example: when either a-D-glucose or b-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

23. Fig 19.2, p.472

24. Mutarotation

25. Mutarotation Rings can open to form open chain compound.
New ring can be either - or b-anomer.
Equilibrium among - and b- forms (and open chain) will be established.
Change in specific optical rotation accompanies this equilibration
If either a-D-glucose or b-D-glucose is dissolved in water, specific rotation changes to an equilibrium value of +52.7°
64% beta and 36% alpha forms at equilibrium
Very little will exist in open chain form

26. Another representation of ring opening and closure
Fig. 17.6
Another representation of ring opening and closure

27. Fig 19.2, p.472

28. Mutarotation
Shown in chair conformation

30. Physical Properties Monosaccharides Colorless crystalline solids
Very soluble in water
Only slightly soluble in ethanol
Sweet taste

31. Sweetness relative to Sucrose

32. Formation of Glycosides
Reaction of a hemiacetal with alcohol gives an acetal
Acetals
Hemiacetal

33. Formation of Glycosides
Cyclic acetal derived from a monosaccharide is called a glycoside
Bond from the anomeric carbon to the -OR group is called a glycosidic bond
Mutarotation is not possible in a glycoside
Ring can’t open up
No equilibrium with the open-chain form
Can be hydrolyzed in acidic solution to form alcohol and a monosaccharide

35. Oxidation and Reduction of simple sugars
Aldehyde and ketone groups can be reduced to form alcohols
Requires reducing agents such as NaBH4 or H2 with a transition metal catalyst
Aldehyde group can be oxidized to produce a carboxylic acid

36. 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

37. 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
Other common alditols

38. 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)

40. Common 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 (ring can’t open)

41. Fig

42. Common Disaccharides Lactose
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-galactose bonded by a b-1,4-glycosidic bond to carbon 4 of D-glucose
lactose is a reducing sugar

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

46. Polysaccharides
Polysaccharide: Polymer containing many monosaccharide units
Starch: a polymer of D-glucose
Two forms: amylose and amylopectin
Amylose is composed of unbranched chains of up to 4000 D-glucose units joined by a-1,4 bonds
Amylopectin contains chains up to 10,000 D-glucose units also joined by a-1,4-glycosidic bonds
Branched compound
New chains of 24 to 30 units attached by a-1,6 linkages

47. 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
Total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle

48. Amylose

49. Branched

50. Polysaccharides
Cellulose is a chain of D-glucose units joined by b-1,4-glycosidic bonds
Average chain has approximately 2200 glucose units per molecule
cellulose chain is much stiffer than starch
Chains line up side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds
Arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength
Cellulose is insoluble in water

51. Polysaccharides Cellulose (cont’d)
humans and other animals cannot digest cellulose
Can only digest starch and glycogen to get glucose
Many bacteria and microorganisms 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

52. Cellulose

53. Blood Groups
Several sugar groups attached to red blood cell (RBC) surface
Difference in one sugar accounts for Types A, B, AB and O blood.

54. Sugars attached to RBC
(α 1,4) β (1,3)
X – D-Galactose N-Acetyl-D- RBC
| glucosamine
| (α 1,2) |
L-fucose

55. Blood Groups -- Type A
N-Acetyl-D-Galactosamine on RBC surface

56. Blood Groups -- Type B
galactose on RBC surface

57. Blood Groups -- Type O
Neither N-Acetyl-D-galactosamine nor Galactose attached to rest of sugar groups on RBC surface