1. Chapter 18: Carbohydrates
Types of Carbohydrates
18.4 and D and L Notations from Fischer Projections
Classification of Monosaccharides
Structures of Some Important Monosaccharides
18.10 and Cyclic Structures of Monosaccharides
Chemical Properties of Monosaccharides
Disaccharides
18.14 and Polysaccharides

2. Carbohydrates Most abundant organic compounds in nature
Produced by photosynthesis in plants
Composed of the elements C, H and O
Also called saccharides, which means “sugars”
A major source of energy from our diet
A source of C for synthesis of other biomolecules
Can be linked to cell membranes or proteins

3. Types of Carbohydrates
Monosaccharides are the simplest carbohydrates
Empirical formula = CH2O
Oligosaccharides contain a few monosaccharides (2-10)
Disaccharides consist of two monosaccharides
Polysaccharides contain many monosaccharides

4. Chiral Molecules Chiral molecules Achiral molecules
Have “handedness”
Nonsuperimposable on mirror images
Ex: hand, shoe
Achiral molecules
Do not have handedness
Ex: glass, spoon
Chiral molecules contain carbons with 4 different groups
Called chiral carbons or chiral center

5. Fischer Projections Used to represent carbohydrates
Places the most oxidized group at the top
All chiral carbons are represented as intersection of lines
C is not shown
Implied 3D arrangement of atoms
Horizontal lines for bonds that come forward
Vertical lines for bonds that go back

6. D and L Notations
By convention, the letter L is assigned to the structure with the —OH on the left
The letter D is assigned to the structure with
—OH on the right

7. D and L Monosaccharides
The —OH on the chiral atom farthest from the carbonyl group is used to assign the D or L configuration D L D

8. Learning Check Indicate whether each is the D or L isomer:
Ribose Threose Fructose

9. Classification of Monosaccharides
Monosaccharide = unbranched chain of 3-8 C atoms; one is carbonyl, others attached to -OH
Aldose
monosaccharide with an aldehyde group (1st carbon)
Ketose
monosaccharide with a ketone group (2nd carbon)
Aldose Aldose Ketose
CH2OH

10. Monosaccharides
Monosaccharides are also classified according to the number of carbon atoms
A triose has three carbons; a tetrose has four carbons; a pentose has five carbons; and a hexose has six carbons.
triose tetrose hexose
aldotriose aldotetrose ketohexose
CH2OH

11. Learning Check
Identify each as tetrose, pentose or hexose, and as aldose or ketose:
A B

12. D-Glucose Most common hexose Found in fruits, corn syrup, and honey
An aldohexose with the formula C6H12O6
Known as blood sugar in the body
Building block for many disaccharides and polysaccharides

13. D-Galactose Aldohexose Differ from D-glucose at C4
C1 is at the top
Not found in the free form in nature
Obtained from lactose, a disaccharide (milk products)
Important in cellular membranes in CNS (brain sugar)

14. D-Fructose Ketohexose C6H12O6
Differ from glucose at C1 and C2 (location of carbonyl)
The sweetest carbohydrate (2x sucrose)
Found in fruit juices and honey
Formed from hydrolysis of sucrose
Converts to glucose in the body

15. Learning Check
Draw the structure of D-fructose:

16. Hemiacetal Review What is a hemiacetal? How is a hemiacetal formed?
What if the alcohol and carbonyl are attached?

17. Hexose hemiacetals Favor formation of 5- or 6-membered rings
Hydroxyl group on C5 reacts with the aldehyde or ketone
The Haworth structure can be written from the Fischer projection
The cyclic structure of a D-isomer has the last CH2OH group located above the ring
The –OH group on the left (C3) is drawn up
The –OH groups on the right (C2, C4) are drawn down

18.  and  Anomers for D-Glucose
Anomers are isomers which differ in placement of hydroxyl on C1
The –OH is drawn down for the -anomer, and up for the -anomer
-D-Glucose -D-Glucose
Mashed potatoes or mashed paper?  

19. Mutarotation
In solution, -D-glucose is in equilibrium with β-D-glucose
Mutarotation involves the conversion of the cyclic anomers into the open chain
At any time, there is only a small amount of open chain
aldehyde

20. Cyclic Structure of Fructose
As a ketohexose, fructose forms a cyclic structure when the —OH on C5 reacts with the ketone on C2
Result is 5-atom ring
Anomeric carbon is C2
-D-Fructose
-D-Fructose

21. Learning Check
Write the cyclic form of -D-galactose:
-D-galactose

22. Review: Reactions of aldehydes
Oxidation to form carboxylic acids
Reduction to form alcohols
Formation of hemiacetal
Hemiacetal + alcohol → acetal
Now we’ll see all of these with monosaccharides…

23. 1. Oxidation of Monosaccharides
Monosaccharides are reducing sugars if their carbonyl groups oxidize to give carboxylic acids.
In the Benedict’s text, D-glucose is oxidized to D-gluconic acid. Glucose is a reducing sugar.

24. 2. Reduction of Monosaccharides
The reduction of the carbonyl group produces sugar alcohols, or alditols
D-Glucose is reduced to D-glucitol (also called sorbitol)

25. 3. Monosaccharides + alcohol
Formation of hemiacetal (cyclic structures)

26. 4. Monosaccharide hemiacetals + alcohol
When a cyclic monosaccharide reacts with an alcohol:
A glycoside is produced (acetal)
The bond is a glycosidic bond or glycosidic linkage
-D-Glucose Methanol Methyl--D-glucoside
+ H2O

27. Learning Check
Write the products of the oxidation and reduction of D-mannose.

28. Disaccharides A disaccharide consists of two monosaccharides
Disaccharide Monosaccharides
Maltose + H2O Glucose + Glucose
Lactose + H2O Glucose + Galactose
Sucrose + H2O Glucose + Fructose H+

29. Maltose
Malt sugar
A disaccharide in which two D-glucose molecules are joined by an -1,4- glycosidic bond
Obtained from starch
Used in cereals, candies, and brewing
A reducing sugar (has a hemiacetal)

30. Lactose
Milk sugar
Composed of galactose and glucose linked by a -1,4-glycosidic bond
Lactose intolerance
A reducing sugar

31. Sucrose
Table sugar
Is composed of glucose and fructose molecules joined by ,-1,2-glycosidic bond
Has no isomers because mutarotation is blocked
Not a reducing sugar (no hemiacetal)

32. Sweetness of Sweeteners
Sugars and artificial sweeteners differ in sweetness
Each sweetener is compared to sucrose (table sugar), which is assigned a value of 100

33. Learning Check
Identify the monosaccharides in lactose, maltose, and sucrose as glucose, fructose, and/or galactose:
A. Lactose
B. Maltose
C. Sucrose

34. Polysaccharides Polysaccharides are polymers of monosaccharides
“Complex” carbohydrates
Important polysaccharides of D-glucose are:
Starch (Amylose and Amylopectin)
Glycogen
Cellulose

35. Starch and Glycogen Storage polysaccharides Starch Glycogen
Form monosaccharides used for energy
Starch
Plants
Amylose is a continuous chain of glucose molecules linked by -1,4 glycosidic bonds.
Amylopectin is a branched chain of glucose molecules linked by -1,4- and -1,6-glycosidic bonds.
Glycogen
Humans, animals
Similar to amylopectin, but more highly branched.

36. Structures of Amylose and Amylopectin

37. Cellulose Structural polysaccharide
Plant cell walls (cellulose) and animal exoskeletons (chitin)
Cellulose is a polymer of glucose molecules linked by -1,4-glycosidic bonds
Enzymes in saliva can hydrolyze -1,4-glycosidic bonds in starch, but not -1,4-glycosidic bonds in cellulose

38. Learning Check Identify the types of glycosidic bonds in: 1) Amylose
2) Glycogen
3) Cellulose

39. End of Chapter 18!