Carbohydrates

1 Carbohydrates

Carbohydrates are ▪ A major source of energy from our diet. ▪ Composed of the elements C, H, and O. ▪ Also called saccharides, which means “sugars.”

2 Carbohydrates

Carbohydrates ▪ Are produced by photosynthesis in plants. ▪ Such as glucose are synthesized

in plants from CO2, H2O, and energy from the sun. ▪ Are oxidized in living cells

(respiration) to produce CO2, H2O, and energy.

3 ▪ Carbohydrates – polyhydroxyaldehydes or polyhydroxy-ketones of formula (CH2O)n, or compounds that can be hydrolyzed to them. (sugars or saccharides) ▪ Monosaccharides – carbohydrates that cannot be hydrolyzed to simpler carbohydrates; eg. Glucose or fructose. ▪ Disaccharides – carbohydrates that can be hydrolyzed into two monosaccharide units; eg. Sucrose, which is hydrolyzed into glucose and fructose. ▪ Oligosaccharides – carbohydrates that can be hydrolyzed into a few monosaccharide units. ▪ Polysaccharides – carbohydrates that are are polymeric sugars; eg Starch or cellulose.

4 ▪ Aldose – polyhydroxyaldehyde, eg glucose ▪ Ketose – polyhydroxyketone, eg fructose ▪ Triose, tetrose, pentose, hexose, etc. – carbohydrates that contain three, four, five, six, etc. carbons per molecule (usually five or six); eg. Aldohexose, ketopentose, etc. ▪ Reducing sugar – a carbohydrate that is oxidized by Tollen’s, Fehling’s or Benedict’s solution. ▪ Tollen’s: Ag+ → Ag (silver mirror) ▪ Fehling’s or Benedict’s: Cu2+ (blue) → Cu1+ (red ppt) ▪ These are reactions of aldehydes and alpha-hydroxyketones. ▪ All monosaccharides (both aldoses and ketoses) and most* disaccharides are reducing sugars. ▪ *Sucrose (table sugar), a disaccharide, is not a reducing sugar.

5 Classification

◼ Monosaccharides (monoses or glycoses)

◼ Trioses, tetroses, pentoses, hexoses

◼ Oligosaccharides

◼ Di, tri, tetra, penta, up to 9 or 10

◼ Most important are the disaccharides

◼ Polysaccharides or glycans

◼ Homo and Heteropolysaccharides

◼ Complex carbohydrates

6 Types of Carbohydrates

Monosaccharides consist of ▪ 3 to 6 carbon atoms, typically. ▪ A carbonyl group (aldehyde or ketone). ▪ Several hydroxyl groups. ▪ also known as simple sugars ▪ classified by 1. the number of carbons and 2. whether aldoses or ketoses ▪ most (99%) are straight chain compounds ▪ D-glyceraldehyde is the simplest of the aldoses (aldotriose) ▪ all other sugars have the ending ose (glucose, galactose, ribose, lactose, etc…)

8 Properties

▪ Differences in structures of sugars are responsible for variations in properties ▪ Physical Crystalline form; ▪ solubility; rotatory power ▪ Chemical Reactions ▪ oxidations, reductions, condensations ▪ Physiological Nutritive value ▪ (human, bacterial); sweetness; absorption

9 Aldoses

Aldoses are monosaccharides O ▪ With an aldehyde group. ║ ▪ With many hydroxyl (─OH) C─H aldose groups. │ triose (3 C atoms) H─ C─OH tetrose (4 C atoms) │ pentose (5 C atoms) H─ C─OH hexose (6 C atoms) │ CH2OH

Erythose, an aldotetrose

10 Aldose sugars

H H H H H

C O C O C O C O C O

(H C OH)n H C OH H C OH H C OH H C OH CH OH H C OH H C OH H C OH 2 CH2OH

Aldose Aldotriose CH2OH H C OH H C OH n = 1 Aldotetrose CH2OH n = 2 H C OH Aldopentose CH OH n = 3 2 Aldohexose n = 4

11 Ketoses

Ketoses are monosaccharides CH2OH ▪ With a ketone group. │ ▪ With many hydroxyl (─OH) C=O ketose groups. │ OH ─ C─ H triose (3 C atoms) │ tetrose (4 C atoms) H─ C─OH pentose (5 C atoms) │ hexose (6 C atoms) H─C─OH │

CH2OH

Fructose, a ketohexose 12 Ketose sugars

CH2OH CH2OH CH2OH CH2OH C O CH2OH C O C O C O H C OH C O (H C OH)n H C OH CH2OH H C OH H C OH CH2OH CH2OH

CH2OH H OH Ketose Ketotriose Ketotetrose Ketopentose n = 0 n = 1 H C OH n = 2 CH2OH Ketohexose n = 3

13 Enantiomers and epimers

H H H H C O C O C O C O HO C H HO C H H C OH OH C H HO C H HO C H H C OH OH C H H C OH HO C H

CH2OH CH2OH H C OH H C OH these two aldotetroses are enantiomers. They are stereoisomers that are mirror CH2OH CH2OH images of each other these two aldohexoses are C-4 epimers. they differ only in the position of the hydroxyl group on one asymmetric carbon (carbon 4)

14 Enantiomers

▪ Pairs of stereoisomers ▪ Designated by D- or L- at the start of the name. ▪ They are mirror images ▪ that can’t be overlapped.

15 Enantiomers

16 17 Aldotetrose

18 Aldopentoses: C5, three chiral carbons, eight stereoisomers

19 Learning Check

Identify each as aldo- or keto- and as tetrose, pentose, or hexose: O CH2OH C H C O H C OH HO C H H C OH H C OH H C OH H C OH CH2OH

CH2OH A B

20 Solution

A. aldohexose

B. ketopentose

21 Carbohydrates

Structures of Monosaccharides

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 22 Structural representation of sugars ▪ Fisher projection: straight chain representation ▪ Haworth projection: simple ring in perspective ▪ Conformational representation: chair and boat configurations

23 Fischer Projections

A Fischer projection ▪ Is used to represent carbohydrates. ▪ Places the most oxidized group at the top. ▪ Shows chiral carbons as the intersection of vertical and horizontal lines.

In a Fischer projection, the −OH group on the ▪ Chiral carbon farthest from the carbonyl group determines an L or D isomer. ▪ Left is assigned the letter L for the L-isomer. ▪ Right is assigned the letter D for the D-isomer.

26 aldohexoses

CHO CHO CHO CHO CHO CHO H OH HO H HO H H OH H OH HO H H OH HO H H OH HO H HO H H OH H OH HO H H OH HO H H OH HO H H OH HO H H OH HO H H OH HO H

CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH

CHO CHO CHO CHO CHO CHO HO H H OH H OH HO H HO H H OH HO H H OH H OH HO H H OH HO H H OH HO H HO H H OH HO H H OH H OH HO H H OH HO H H OH HO H

CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH

CHO CHO CHO CHO H OH HO H HO H H OH HO H H OH HO H H OH HO H H OH HO H H OH H OH HO H H OH HO H

CH2OH CH2OH CH2OH CH2OH

27 Optical isomerism

▪ A property exhibited by any compound whose mirror images are non-superimposable ▪ Asymmetric compounds rotate plane polarized light

POLARIMETRY

Measurement of optical activity in chiral or asymmetric molecules using plane polarized light Molecules may be chiral because of certain atoms or because of chiral axes or chiral planes Measurement uses an instrument called a polarimeter Rotation is either (+) dextrorotatory or (-) levorotatory

28 POLARIMETER

29 Examples of D and L Isomers of Monosaccharides

O O C H C H O HO H H OH C H H OH HO H H OH H OH H OH H OH HO H H OH H OH CH OH CH2OH CH2OH 2 D-glucose D-ribose L-galactose

30 Learning Check

Identify each as the D or L isomer. A. B. C.

O CH2OH C H O O HO H C H HO H HO H H OH H OH HO H HO H H OH CH OH CH2OH CH2OH 2 __-ribose __- threose __- fructose

31 Solution

Identify each as the D or L isomer. A. B. C.

O CH2OH C H O O HO H C H HO H HO H H OH H OH HO H HO H H OH CH OH CH2OH CH2OH 2 L-ribose L-threose D-fructose

32 Epimers – stereoisomers that differ only in configuration about one chiral center.

CHO CHO H OH HO H HO H HO H H OH H OH H OH H OH

CH2OH CH2OH D-glucose D-mannose

epimers

33 D-Glucose

D-glucose is ▪ Found in fruits, corn syrup, and honey. ▪ An aldohexose with the formula C6H12O6. ▪ Known as blood sugar in the body. ▪ The monosaccharide in polymers of starch, cellulose, and Copyright © 2007 by Pearson Education, Inc glycogen. Publishing as Benjamin Cummings

34 Blood Glucose Level

In the body, ▪ Glucose has a normal blood level of 70-90 mg/dL. ▪ A glucose tolerance test measures blood glucose for Copyright © 2007 by Pearson Education, Inc Publishing as Benjamin several hours Cummings after ingesting glucose.

35 D-Fructose

D-fructose ▪ Is a ketohexose CH2OH

C6H12O6. C O ▪ Is the sweetest HO C H carbohydrate. H C OH ▪ Is found in fruit juices and honey. H C OH

36 D-Galactose

D-galactose is O

▪ An aldohexose C6H12O6. C H ▪ Not found free in nature. H C OH ▪ Obtained from lactose, a HO C H disaccharide. HO C H ▪ A similar structure to glucose except for the H C OH

–OH on C4. CH2OH D-Galactose

37 Learning Check

Draw the structure and Fischer projection of D-fructose.

38 Solution

CH OH CH2OH 2 │ C O C=O

HO C H HO H H C OH H OH H C OH H OH CH2OH CH2OH D-Fructose

Fischer projection 39 Carbohydrates

Cyclic Structures of Monosaccharides

CH2OH O

OH OH OH

Cyclic structures ▪ Are the prevalent form of monosaccharides with 5 or 6 carbon atoms.

O O

▪ Form when the hydroxyl group on C-5 reacts with the aldehyde group or ketone group.

41 Cyclic Haworth Structures

Stable cyclic hemiacetals form ▪ When the C=O group and the —OH are part of the same molecule. CH2OH ▪ For hexoses, the hydroxyl group O on C-5 reacts with the aldehyde group or ketone group. ▪ The cyclic structure of a D-isomer

has the last CH2OH group located above the ring.

42 Drawing the Cyclic Structure for Glucose

STEP 1 Number the carbon chain and turn clockwise to form a linear open chain. H O C 1 H C OH H 2 H H OH O HO C H 3 HOCH2 C C C C C 6 5 4 3 2 1 H 4C OH H

H 5C OH OH OH H OH

6CH2OH

43 Cyclic Structure for Glucose

STEP 2 Bend the chain to make a hexagon ▪ Bond the C5 –O– to C1. CH2OH 6 ▪ Place the C6 group above O the ring. 5 ▪ Write the –OH groups on 4 1 OH C2 and C4 below the ring. OH 3 2 OH ▪ Write the –OH group on C3 above the ring. OH ▪ Write a new –OH on C1.

44 Cyclic Structure for Glucose (cont)

STEP 3 The new –OH on C1 is drawn ▪ Down for the  anomer. ▪ Up for the  anomer.

CH OH 2 CH2OH  O O O H OH OH OH OH OH OH  OH

-D-glucose -D-glucose

45 Summary of the Formation of Cyclic Glucose

46 -D-Glucose and β-D-Glucose in Solution When placed in solution, ▪ Cyclic structures open and close. ▪ -D-glucose converts to β-D-glucose and back. ▪ There is only a small amount of open chain. CH OH CH OH CH OH 2 2 H 2 O O O O OH OH C OH OH H OH OH OH OH OH OH OH -D-glucose D-glucose (open) β-D-glucose (36%) (trace) (64%) 47 Cyclic Structure of Fructose

Fructose ▪ Is a ketohexose. ▪ Forms a cyclic structure. ▪ Reacts the —OH on C-5 with the C=O on C-2.

CH2OH CH OH CH OH C O 2 O CH2OH 2 O OH HO C H OH OH H C OH OH CH2OH H C OH OH OH CH2OH D-fructose α-D-fructose -D-fructose

48 ▪ In water each mutarotates to an equilibrium with [α] = +52.7 ▪ (63.6% β / 36.4% α)

49 Learning Check

Write the cyclic form of -D-galactose O C H H C OH HO C H HO C H H C OH

CH2OH

50 Solution

CH2OH O OH

OH OH

-D-galactose

51 Carbohydrates

Chemical Properties of Monosaccharides

52 Reducing Sugars

Reducing sugars ▪ Are monosaccharides with a carbonyl group that oxidizes to give a carboxylic acid. ▪ Undergo reaction with Benedict’s reagent (Cu2+) to give the corresponding carboxylic acid. ▪ Include the monosaccharides glucose, galactose, and fructose.

53 Oxidation reactions

▪ Aldoses may be oxidized to 3 types of acids ▪ Aldonic acids: aldehyde group is converted to a carboxyl group ▪ Uronic acids: aldehyde is left intact and primary alcohol at the other end is oxidized to COOH ▪ Saccharic acids (glycaric acids) – oxidation at both ends of monosaccharide)

54 Oxidation of D-Glucose

O O Glucose is oxidized to a carboxylic acid C H Benedicts C OH reagent H C OH H C OH HO C H [O] HO C H 2+ + Cu + Cu2O(s) H C OH H C OH H C OH H C OH Cu+ (reduced CH OH 2 CH2OH form) D-glucose D-gluconic acid Glucose is a reducing sugar 55 Br2 is a mild oxidant that gives good yields of aldonic acid products

56 Aldoses are oxidized in warm, dilute HNO3 to dicarboxylic acids called aldaric acids

57 Enzymatic oxidation at the –CH2OH end of aldoses yields uronic acids

58 Deoxy Sugars

▪ These are monosaccharides which lack one or more hydroxyl groups on the molecule ▪ one quite ubiquitous deoxy sugar is 2’-deoxy ribose which is the sugar found in DNA ▪ 6-deoxy-L-mannose (L-rhamnose) is used as a fermentative reagent in bacteriology

59 Amino Sugars

60 Reduction of Monosaccharides

The reduction of D-Glucitol monosaccharides ▪ Involves the carbonyl group. ▪ Produces sugar alcohols called alditols. ▪ Such as D-glucose gives D-glucitol also called sorbitol.

61 Learning Check

Write the products of the oxidation and reduction of D-mannose. O C H HO H HO H H OH H OH

CH2OH D-mannose

62 Solution

Write the products of the oxidation and reduction of D-mannose. O O C OH CH2OH C H HO H HO H HO H Reduction Oxidation HO H HO H HO H H OH H OH H OH H OH H OH H OH CH OH CH2OH CH2OH 2

D-mannitol D-mannose D-mannonic acid

63 Carbohydrates

Disaccharides

64 Important Disaccharides

A disaccharide ▪ Consists of two monosaccharides. Monosaccharides Disaccharide

Glucose + glucose maltose + H2O

Glucose + galactose lactose + H2O Glucose + fructose sucrose + H2O

65 Maltose

Maltose is ▪ A disaccharide also known as malt sugar. ▪ Composed of two D-glucose molecules. ▪ Obtained from the hydrolysis of starch. ▪ Linked by an -1,4-glycosidic bond formed from the  −OH on C1 of the first glucose and −OH on C4 of the second glucose. ▪ Used in cereals, candies, and brewing. ▪ Found in both the - and β - forms.

66 Formation of Maltose

Lactose ▪ Is a disaccharide of β- D-galactose and α- or β-D-glucose. ▪ Contains a β -1,4- glycosidic bond. ▪ Is found in milk and milk products. α-form

Sucrose or table sugar ▪ Is obtained from sugar cane and sugar beets. ▪ Consists of α-D-glucose and β-D-fructose.. ▪ Has an α,β-1,2-glycosidic bond.

α-D-glucose

Write the structures and names of the two monosaccharides that form when sucrose is hydrolyzed.

70 Solution

α-D-glucose

β-D-fructose

71 Sweeteners

Sugars and artificial TABLE 15.2 sweeteners ▪ Differ in sweetness. ▪ Are compared to sucrose (table sugar), which is assigned a value of 100.

60 000

72 Learning Check

Identify the monosaccharides in each of the following: A. lactose (1) α-D-glucose (2) β-D-fructose (3) β-D-galactose

B. maltose (1) α-D-glucose (2) β-D-fructose (3) β-D-galactose

C. sucrose (1) α-D-glucose (2) β-D-fructose (3) β-D-galactose

73 Solution

Identify the monosaccharides in each of the following: A. lactose (1) α-D-glucose (3) β-D-galactose

B. maltose (1) α-D-glucose

C. sucrose (1) α-D-glucose (2) β-D-fructose

74 Chapter 15 Carbohydrates

15.6 Polysaccharides

75 Polysaccharides

Polysaccharides CH2OH :Homopolysaccharides: O ▪ Are polymers of D-glucose. ▪ Include amylose and amylopectin, starches made of OH α-D-glucose. OH OH ▪ Include glycogen (animal starch OH in muscle), which is made of α- D-glucose. ▪ Include cellulose (plants and α-D-glucose wood), which is made of β-D-glucose.

76 Structures of Amylose and Amylopectin

Amylose is ▪ A polymer of α-D- glucose molecules. ▪ Linked by -1,4 glycosidic bonds. ▪ A continuous (unbranched) chain.

Amylopectin ▪ Is a polymer of α-D- glucose molecules. ▪ Is a branched-chain polysaccharide. ▪ Has α-1,4-glycosidic bonds between the glucose units. ▪ Has α-1,6 bonds to branches. ▪

Glycogen ▪ Is the polysaccharide that stores α-D-glucose in muscle. ▪ Is similar to amylopectin, but is more highly branched.

80 Cellulose

Cellulose ▪ Is a polysaccharide of glucose units in unbranched chains. ▪ Has β-1,4-glycosidic bonds. ▪ Cannot be digested by humans because humans cannot break down β-1,4- glycosidic bonds.

Identify the polysaccharides and types of glycosidic bonds in each of the following:

A. B. C.

82 Solution

A. Cellulose -1,4-glycosidic bonds

B. Amylose -1,4-glycosidic bonds Amylopectin -1,4-and -1,6-glycosidic bonds

C. Glycogen -1,4-and -1,6-glycosidic bonds (more branched than amylopectin)

83 Hetropolysaccharides: Mucopolysaccharides

▪ These materials provide a thin, viscous, jelly-like coating to cells. The most abundant form is hyaluronic acid.

84 Peptidoglycans

▪ Bacterial cell walls are composed primarily of an unbranched polymer of alternating units of N- acetylglucosamine and N-acetylmuramic acid. ▪ Peptide crosslinks between the polymer strands provide ▪ extra strength varies based on bacterium.