Chapter 18: Carbohydrates

1. Chapter 18:Carbohydrates 18.3 - Types of Carbohydrates 18.4 and 18.6 - D and L Notations from Fischer Projections 18.8 - Classification of Monosaccharides 18.9 - Structures of Some Important Monosaccharides 18.10 and 18.11 - Cyclic Structures of Monosaccharides 18.12 - Chemical Properties of Monosaccharides 18.13 - Disaccharides 18.14 and 18.16 - 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 • 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 D L D and L Monosaccharides • The —OH on the chiral atom farthest from the carbonyl group is used to assign the D or L configuration

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

9. CH2OH 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

10. CH2OH 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

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. aldehyde 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

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 DisaccharideMonosaccharides • 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 • 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!