1

2

Carbohydrates: A First Class of Biochemicals

33

Carbohydrates are energy-yielding macronutrients in the

same class of nutrients as fats and proteins.

These polyhydroxy aldehydes or ketones include simple

carbohydrates like glyceraldehyde and dihydroxyacetone.

What are carbohydrates?

44

Carbohydrates are important in society because they provide;

(1) basic diets in the form of starch and sugar and…….

(2) clothing and shelter.

Many of the chemical properties of carbohydrates are

determined by the chemistry of the hydroxyl and carbonyl

functional groups.

555

Fischer projection formulas are drawn with thesecharacteristics: (1) The keto or aldehyde group is placed at the top of the projection.

(2) Each interior carbon atom is shown as an intersection point between two lines ( )

(3) The H atom and –OH group are written to left or right of the projection .

666

This is an example of a modified structural formula of glucose written as a Fischer projection formula.

D-glucose

(modif ied structural f ormula)

D-glucose(Fischer projection f ormula)

7

Classification of

Carbohydrates

88

The four major types of carbohydrates are……..

1. Monosaccharides

2. Disaccharides

3. Oligosaccharides

4. Polysaccharides

Types of Carbohydrates

9

Monosaccharides

• A monosaccharide is a carbohydrate that cannot be hydrolyzed to simpler carbohydrate units.

• The monosaccharide is the basic carbohydrate unit of cellular metabolism.

1010

Monosaccharides can be classified by the ….

(a) number of carbon atoms in the molecule

( e.g. a pentose versus a hexose)

(b) functional group ( aldoses versus ketoses)

(c) configuration ( D versus L isomers)

(d) optical activity [(+) versus (–) isomers]

(e) ring structure ( furanoses versus pyranoses)

(f) stereochemistry at an anomeric carbon ( versus isomers)

Types of Monosaccharides

1111

Types of Monosaccharides

Number of Carbons

The monosaccharides shown below are classified based on

the number of carbons in the molecule.

1212

Types of Monosaccharides

Functional Group

Mononosaccharides with a –CHO (aldehyde) group are known as aldoses while those with a (keto) group are known as ketoses.

C O

1313

Types of MonosaccharidesConfiguration

Monosaccharides with the –OH group on the right of thecarbon alpha to the terminal ROH carbon are D isomerswhile those with the –OH group on the left are L isomers.

1414

Types of MonosaccharidesOptical Activity

Monosaccharides that rotate plane-polarized light to the rightare known as (+) isomers while those that rotate it to the left are (–) isomers.

Note: The D and L designations do not indicate the direction of rotation, e.g., the D isomer of glucose could be either the (+) isomer or it can be the (–) isomer.

15

Learning Check

Identify each as the D or L isomer.

A. B. C.

__-Ribose __- Threose __- Fructose

CH2OH

HO H

HO H

HHO

O

C H

CH2OH

HO H

OHH

O

C H

CH2OH

H OH

H OH

HO H

O

CH2OH

16

Solution

Identify each as the D or L isomer.

A. B. C.

L-Ribose L-Threose D-Fructose

CH2OH

HO H

HO H

HHO

O

C H

CH2OH

HO H

OHH

O

C H

CH2OH

H OH

H OH

HO H

O

CH2OH

17

Learning Check

Write the projection formula for a D-aldopentose.

18

This is one example of a D-aldopentose. This molecule is a D-isomer because of the orientation of the hydroxyl group (see arrow). The molecule is an aldose because it is an aldehyde and a pentose because it contains five carbon atoms.

C

CH OH

C

C

HO H

CH2OH

H OH

OH

D-configuration

1919

Types of MonosaccharidesRing Structure

The cyclic form of monosaccharides that have five atomsin the ring are known as furanoses while those with six atomsare known as pyranoses based on the correspondingheterocyclic ring structures furan and pyran.

2020

Types of Monosaccharides

Anomeric Configuration

Monosaccharides that have an –OH below the ring at the anomeric carbon are known as (alpha) isomers while those with the –OH above the ring are (beta) isomers.

21

Disaccharides• A disaccharide yields two

monosaccharides – either alike or different – when hydrolyzed:

disaccharide + water 2 monosaccharidesH+ or

enzymes

22

Monosaccharides & Disaccharides

• Disaccharides are often used by plants or animals to transport monosaccharides from one cell to another.

• The monosaccharides and disaccharides generally have the ending –ose – for example, glucose, sucrose, and lactose.

• These are water-soluble carbohydrates, which have a characteristically sweet taste and are called sugars.

23

Oligosaccharides

• An oligosaccharide has two to six monosaccharide units linked together.

24

Polysaccharides• A polysaccharide is a macromolecular

substance that can be hydrolyzed to yield many monosaccharide units:

polysaccharide + water monosaccharidesH+ or

enzymes

• Polysaccharides are important structural supports, particularly in plants, and also serve as a storage depot for monosaccharides, which cells use for energy.

25

Importance of Carbohydrates

2626

Why are carbohydrates so important?

Carbohydrates are important because they are widely available and because they have exceptional utility.

The Utility of Carbohydrates

Energy-Yielding Nutrients(starch f rom plants )

Building Materials(cellulose is used in woodconstruction, paper, cottonbased clothing and cell wallsin higher plants)

Water-Soluble Molecules( mono- and disaccharides are used as sweetners)

27

MonosaccharidesMonosaccharides

2828

The most important monosaccharides are the pentoses and hexoses as shown in the diagram below .

Important Monosaccharides

Pentoses Hexoses

ribose deoxyribose glucose galactose fructose

( important source of celluar energy and in nutrition)

( important components of the nucleic acids DNA and RNA )

29

Monosaccharides

• The hexose monosaccharides are the most important carbohydrate sources of cellular energy.

• Three hexoses – glucose, galactose, and fructose – are of major significance in nutrition.– All three have the same formula, C6H12O6, and thus

deliver the same amount of cellular energy.– They differ in structure, but are biologically

interconvertible.

30

• “Tree” formulas• Shorthand formulas used in carbohydrate

configurations where the following symbols are used:

H C O=

= H C OH

= HO C H

= CH2OH

Example

is

H C O

H C OH

CH2OH

31

• Glucose is the most important of the monosaccharides.

• It is an aldohexose and is found in the free state in plant and animal tissue.

H

CHO

OH

HHO

OHH

OHH

CH2OH

or

3232

The concentration of glucose in blood is normally about

80-100 mg/100 mL

Glucose is known by either of the following names dextrose

(from dextrorotatory), grape sugar (found in grapes) , or blood

sugar (because it is transported in the blood).

Glucose

33

• Galactose is also an aldohexose and occurs, along with glucose, in lactose and in many oligo- and polysaccharides such as pectin and gums.

H

CHO

OH

HHO

HHO

OHH

CH2OH

or

34

• Fructose, also know as levulose, is a ketohexose that occurs in fruit juices, honey, and along with glucose, as a constituent of sucrose.

CH2OH

O

HHO

OHH

OHH

CH2OH

It is the sweetest common sugar being about two times sweeterthan glucose.

3535

Structures of the Pentoses (Ribose and Deoxyribose)

(both monosaccharides are aldopentoses)

36

Structures of Glucose and Other Aldoses

37

Epimers• Any two monosaccharides that differ

only in the configuration around a single carbon atom are called epimers.

• In the case of glyceraldehyde this atom is carbon 2.

3838

The D-family of aldoses. The red –OH group indicate the new chiral carbon added in case from top to bottom of the diagram.

39

Learning Check Draw the enantiomer of D-allose using the previous

slide.

40

The enantiomer of D-allose is L-allose. These molecules are mirror images.

CHO

OHH

OHH

OHH

OHH

CH2OH

D-allose

CHO

HO H

HO H

HO H

HO H

CH2OH

L-allose

41

Cyclic Structure of Glucose; Mutarotation

4242

Bonds that are missing atoms are understood to have H atoms.

Haworth Formulas

Haworth formulas are structural formulas that represent cyclicsugars. In the case of glucose the formula is drawn as a flat hexagon withH and –OH written above and below the plane of the ring.

Haworth formulas are sometimes shown in a abbreviated form as shown here.

OH’s removed

43

OH O

H

O O

H

44

OH O

H

O O

H

HHH H H H H H H HH HHH

HH

H

45

Most naturally occurring monosaccharides occur in the chair conformation shown.

-D-glucopyranose

4646

What is an anomer?

Anomeric carbon

This hydroxyl group on theanomeric carbon is above thering which means this is the anomer.

This is an anomeric carbon.

This hydroxyl group on theanomeric carbon is below thering which means this is the anomer.

The structure below is an alpha anomer.

An anomer is the or form of a monosaccharide as shown here.

4747

What is mutarotation? Mutarotation is the change in specific rotation of an anomer as it is converted into an equilibrium mixture of the and forms.

Fischer projection formulas showing mutarotation of D-glucose.

Emil Fischer

1852-1919

4848

Haworth formulas showing mutarotation of D-glucose.

Sir (Walter) Norman Haworth1883-1950

49

Comparison of Formulas

50

OH O

H

O O

H

H

51

Conversion of Fischer to Haworth Formulas

http://faculty.chemeketa.edu/lemme/CH%20123/Handouts/HaworthFormulas.pdf

52

Hemiacetals and AcetalsHemiacetals and Acetals

5353

Hemiacetals are structures that contain an alkoxy group and

a hydroxyl group on the same carbon atom.

Hemiacetals

This is the alkoxy part of the hemiacetal.

This is the hydroxyl group part of the hemiacetal.

5454

The cyclic structures of monosaccharides are intramolecular hemiacetals.Five and six -membered ring hemiacetals are stable but

theserings can open in aqueous solution to the straight-chain aldehyde.

5555

Acetals

Acetals are structures that contain two alkoxy groups on

the same carbon atom.

5656

Glycosides

Cyclic acetals are known as glycosides and glycosides are

derivatives of hemiacetals.

5757

Glycosides

Glycosides like the methyl isomers shown below are

less reactive than the corresponding monosaccharide.

The methyl isomers shown below will not undergo mutarotation.

58

Structures of Galactose, Fructose,

Ribose, andDeoxyribose

5959

Structure of Galactose

Galactose has the same structure as glucose except the configuration at carbon four is reversed as shown here.

6060

Structure of Galactose

Galactose is an aldohexose like glucose and like glucose it also exists in the alpha and beta cyclic pyranose forms.

6161

Structure of Fructose

Fructose is a ketohexose and like glucose it also exists in the open-chain and cyclic forms as shown here.

( open-chain form) ( cyclic form)

6262

Structure of Ribose and Deoxyribose

D-Ribose and its derivativeD-2-dexoyribose are pentosesfound in nuclei acids RNA and DNA.

Notice that the 2-deoxy in D-2-deoxyribose means an oxygen is omitted from theD-ribose molecule at carbon two.

63

DisaccharidesDisaccharides

6464

DisaccharidesDisaccharides are carbohydrates consisting of two monosaccharides.

The two monosaccharides are connected by a glycosidic linkage as shown here for the disaccharide lactose .

lactose

6565

DisaccharidesSucrose and lactose are important disaccharides foundin the free state in nature.

Sucrose is known as table sugar while lactose is known asmilk sugar. Both undergo hydrolysis in the presence of an acid or the enzymes sucrase or lactase respectively.

6666

Disaccharides

Maltose is not found in the free state but is the productwhen a polysaccharide is degraded during the sprouting of

grain. Maltose is known as grain sugar.

Maltose undergoes hydrolysis in the presence of acid ormaltase to produce two molecules of glucose.

67

Structures and Properties of Disaccharides

Structures and Properties of Disaccharides

6868

Formation of Maltose

6969

Structure of Lactose Shown here are Haworth structures using the stacked position convention and the bent structure convention.

stacked position

bent structure

7070

This is a Haworth projection formula of the disaccharide sucrose

71

This is an alternate Haworth projection formulas of the disaccharide sucrose

1

2

3 4

5

6

72

Sweeteners and Diet

Sweeteners and Diet

Invert sugar is sucrose that has been hydrolyzed to glucose and fructose.

7373

Importance of Sucrose as a Sweetener

Sucrose represents 40-60% of all sweeteners and is 20-30%

of the average caloric intake in the United States because of

its low price and sweet taste.

Sucrose is hydrolyzed to prevent crystallization in certain

food preparations and in these cases it is known as invert

sugar.

7474

Artificial Sweeteners

Artificial sweeteners have been developed with the intent to

balance the concerns for safety, relative sweetness, and

aftertaste.

Many artificial sweeteners have a higher relative sweetness

than the common sweeteners like sucrose, glucose or fructose

as shown in the table on the next slide.

75

Relative Sweetness of Sugars and Sugar Substitutesbased on fructose = 100

SugarsRelative

sweetnessSugar substitutes

Relative sweetness

Fructose 100Sucralose

(Splenda)3.5 × 104

Invert Sugar 75Saccharin(Sweet ‘N Low)

1.7 × 104

Sucrose 58Acesulfame potassium

(Sweet One)1.2 × 104

Glucose 43Asparatame

(Equal)1.0 × 104

Maltose 19Rebiana(Truvia, PureVia)

1.2 × 104

Galactose 19 

Neotame 

4.1 × 105

Lactose 9.2 

Stevia 

3.0 × 104

    

Xylitol 

58

76

NameCalories /

GramSweetness

IndexGlycemic

Index

Calories / Spoon-Equiv

Fructose 4 1.7 23 9

Sucrose 4 1 65 16

Glucose 4 0.75 100 21

Dextrose 4 0.75 100 21

Trehalose 4 0.45 70 36

Galactose 4 0.3 23 53

Maltose 4 0.3 105 53

Lactose 4 0.15 45 107

Sugars

77

NameCalories /

GramSweetness

IndexGlycemic

Index

Calories / Spoon-Equiv

Erythritol 0.2 0.65 1 1

Xylitol 2.4 1 12 10

Maltitol 2.4 0.9 35 11

Mannitol 1.6 0.5 2 13

Isomalt 2.1 0.5 2 17

Sorbitol 2.6 0.55 4 19

Lactitol 2 0.4 3 20

HSH 3 0.4 36 30

Sugar Alcohols

78

NameCalories /

GramSweetness

IndexGlycemic

Index

Calories / Spoon-Equiv

Honey 4 1.1 50 14

Maple Syrup 4 1 54 15

Coconut Palm Sugar

4 1 35 15

Sorghum Syrup

4 1 50 15

Natural Caloric Sweeteners

79

NameCalories /

GramSweetness

IndexGlycemic

IndexCalories /

Spoon-Equiv

Thaumatin 4 2,000 0 0

Monellin 4 1,500 0 0

Brazzein 4 1,000 0 0

Pentadin 4 500 0 0

LuoHanGuo 0 300 0 0

Stevia 0 300 0 0

Natural Zero Calorie Sweeteners

80

NameCalories /

GramSweetness

IndexGlycemic

IndexCalories /

Spoon-Equiv

Tagatose 4 0.92 0 7

Agave Syrup 4 1.5 15 10

HFCS-90 4 1.6 31 10

HFCS-55 4 1.2 58 13

HFCS-42 4 1.1 68 14

Golden Syrup

4 1.1 60 15

Barley Malt Syrup

4 0.5 42 32

Brown Rice Syrup

4 0.5 25 32

Modified Sugars

81

NameCalories /

GramSweetness

IndexGlycemic

IndexCalories /

Spoon-Equiv

Advantame 0 20,000 0 0

Neotame 0 8,000 0 0

Sucralose 0 600 0 0

Saccharin 0 300 0 0

AcesulfameK 0 200 0 0

Aspartame 4 180 0 0

Cyclamate 0 40 0 0

Artificial Sweeteners

82

Discovered in 1976

83

Discovered in 1879

84

Discovered in 1967

85

Discovered in 1965

86

Agave nectar (sometimes called agave syrup) is most often produced from the Blue Agaves that thrive in the volcanic soils of Southern Mexico. Agaves are large, spikey plants that resemble cactus or yuccas in both form and habitat, but they are actually succulents similar to the familiar Aloe Vera.

To make the agave nectar, sap is extracted from the pina, filtered, and heated at a low temperature, which breaks down the carbohydrates into sugars. Lighter and darker varieties of agave nectar are made from the same plants. Because of the low temperatures used in processing many varieties (under 118°F) raw foods enthusiasts generally regard agave nectar as a raw food.

The taste of agave nectar is comparable, though not identical, to honey. Many people who do not like the taste of honey find agave a more palatable choice. It also has none of the bitter aftertaste associated with artificial sweeteners.

http://www.allaboutagave.com/

87

Stevia (sweetleaf, sweet leaf or sugarleaf)

This sweetener is made from a crude preparation (powder or liquid) of dried stevia leaves. It may contain a mixture of many substances, only some of which are sweet.

88

Truvia™ natural sweetener is made from rebiana, the best tasting part of the stevia leaf, erythritol and natural flavors.

Rebiana is the common or usual name for a food-grade high-purity extract of the stevia leaf that is at least 97 percent rebaudioside-A, the best tasting sweet substance found in the stevia leaf.

Chemically, erythritol is simply a four-carbon sugar alcohol. Erythritol is made by fermenting glucose then separating andpurifying the resulting product.

89

Most fruits, berries and plants contain xylitol (also called wood sugar), the richest natural sources being plums, strawberries, raspberries, cauliflower and endives.

90

Xylitol is extremely toxic to dogs

The toxic dose of xylitol is 0.1 gm/kg body weight, while liver failure results from doses greater than 0.5 g/kg body weight. Translating these numbers into something usable in the every-day world is a little harder to do, since the amount of xylitol varies from one product to another. Two sticks of gum is enough to cause a serious drop in blood sugar for a small (under 20 lb) dog, while it might take 8 to 10 sticks to affect a large (over 60 lb) dog, but these amounts are only an estimate. As for baked goods containing xylitol, again, the amount in each cookie or muffin will vary. In one case, a Standard Poodle died after eating 5 or 6 cookies sweetened with xylitol.

9191

The history of sodium cyclamate illustrates the difficultyin balancing consumer safety with the needs of the consumer market.

This sweetener was banned in 1970 because of research that indicated risks of cancer from consuming the sweetener.

Discovered in 1937

92

Redox Reactions of Monosaccharides

Redox Reactions of Monosaccharides

9393

Oxidation of AldohexosesThe aldehyde group in monosaccharides can be oxidized

to

monocarboxylic acids with a mild oxidizing agent.For example glucose is oxidized to gluconic acid in thepresence of bromine water.

9494

Oxidation of AldohexosesDicarboxylic acids are formed when aldohexoses are

treated

with stronger oxidizing agents.For example glucose is oxidized to glucaric acid in the presence of nitric acid.

9595

Reduction of Aldohexoses

Hexahydric alcohols ( six –OH groups) are formed when

aldohexoses are treated with reducing agents.

For example glucose is reduced to glucitol (sorbitol) in the presence of H2/Pt.

Sorbitol is in many moisturizers

96

Learning Check

Write the products of the mild oxidation and reduction of D-mannose.

H

O

CH2OH

H OH

H OH

HO H

HHO

C

D-Mannose

97

Solution

Write the products of the mild oxidation and reduction of

D-mannose.

D-Mannitol D-Mannose D-Mannonic acid

9898

Redox Tests for Carbohydrates

A reducing sugar is a compound that will reduce Ag + → Ag or Cu2+ → Cu+.

A reducing sugar will have one of the following groups;

(a) An aldehyde group ( e.g. glyceraldehyde)(b) A hydroxyketone ( e.g. fructose) (c) A cyclic hemiacetal group ( e.g. glucose or maltose)

9999

Redox Tests for Carbohydrates

The Benedict, Barfoed, and Fehling tests are based on the formation of a brick red copper(I) oxide precipitate as a positive result while the Tollens test is based on theformation of a silver mirror.

100

The Barfoed test (a solution of cupric acetate and acetic acid) is more sensitive in that it can distinguish a reducing monosaccharide from a reducing disaccharide.

Monosaccharides form a precipitate within 3 minutes and Disaccharides take a bit longer.

– +

101

Redox Test for Carbohydrates

Benedict test for reducing sugars. The tube on the right contains Benedict reagent. The tube on the left shows the brick-red precipitate of Cu2O when glucose is added.

102

103103

Reduction of Hemiacetals

Sugars with the hemiacetal structure can be reduced under alkaline conditions because the ring opens as shown belowforming an aldehyde group.

Therefore glucose, lactose, and maltose have the hemiacetal structure and are reducing sugars but the disaccharide sucroseis not a reducing sugar because it does not have the hemiacetal structure.

104

Osazone FormationOsazone Formation

105

Phenylhydrazine (C6H5NHNH2) reacts with carbons #1 and #2 of reducing sugars to form derivatives called osazones. The formation of these distinctive crystalline derivatives is useful for comparing the structures of sugars. Glucose and fructose react as shown on next slide:

106

C

C

C

C

C

CH2OH

H

H OH

HO H

H

H OH

OH

O

C6H5NHNH2

C

C

C

C

C

CH2OH

H

H

HO

H

H

OH

H

OH

OH

NNHC6H5

C6H5NHNH2

C

C

C

C

C

CH2OH

H

HO

H

H

H

OH

OH

NNHC6H5

O

D-glucose

107

C

C

C

C

C

CH2OH

H

HO

H

H OH

OH

H

NNHC6H5

NNHC6H5

osazone

C6H5NHNH2

108

CH2OH

C

C

C

C

CH2OH

HO

H

H OH

OH

H

O

C6H5NHNH2

CH2OH

C

C

C

C

CH2OH

HO H

H OH

H OH

NNHC6H5

C6H5NHNH2

C

C

C

C

C

CH2OH

HO

H

H OH

OH

H

HO

NNHC6H5

D-fructose

109

C

C

C

C

C

CH2OH

H

HO

H

H OH

OH

H

NNHC6H5

NNHC6H5

osazone

C6H5NHNH2

110

Identical osazones are obtained from D-glucose and D-fructose. This demonstrates that carbons #3 through #6 of D-glucose and D-fructose molecules are identical. The same osazone is also obtained from D-mannose. This indicates that carbons #3 through #6 of the D-mannose molecule are the same as those of D-glucose and D-fructose molecules. In fact, D-mannose differs from D-glucose only in the configuration of the –H and –OH groups on carbon #2.

111

112

113

114

115

Polysaccharides Derived from

Glucose

Polysaccharides Derived from

Glucose

116116

Major Polysaccharides Derived from Glucose

Cellulose (used to construct cell walls in plants)

Glycogen (energy-storage in animals i.e. liver and muscle tissues)

Starch (energy-storage in plants)

Glucose Based Polysaccharides

There are three types of naturally occurring polysaccharides; cellulose, glycogen, and starch as shown below.

117117

Starch, glycogen, and cellulose all yield D-glucose

when hydrolyzed as shown here.

118118

StarchStarch is a polysaccharide composed of amylose and

amylopectin.

Amylose is a large molecule consisting of unbranched

chains composed of about 25-1300 -D-glucose units

joined by -1,4-glycosidic linkages.

Amylopectin is a large molecule with branched chains

composed of -1,4-glycosidic linkages in the main chain

and -1,6-glycosidic linkages at branch points as seen

in the next slide.

119119

Molecular Structure of Amylose

Amylose Glucose units

in Amylose

No Branching

120120

Molecular Structure of amylopectin.

Amylopectin

Glucose unitsin Amylopectin

Branching

121121

Hydrolysis of Starch

An important reaction during digestion is the hydrolysis

of starchy foods as shown below.

Starch is not soluble in cold water and will form a colloidal dispersion

in hot water.

Starch solutions form a blue-black

color in the presence of free iodine.

122122

Glycogen

Glycogen is a carbohydrate polymer that is stored in the liver

and muscle tissues in animals.

Glycogen has a structure similar to amylopectin except it

is more highly branched with the -1,6-glycosidic linkages

occurring more frequently along the polymer chain.

123123

Cellulose

Cellulose, like starch and glycogen, is a glucose based

polymer.

Cellulose is the most abundant organic substance found in nature and it is the chief structural component of plants and wood.

124124

Cellulose

However the glucose units in cellulose are join by -1,4-glycosidic linkages instead of -1,4-glycosidiclinkages.

This change in stereochemistry at the anomeric carbon

allows extensive hydrogen bonding in cellulose as shown

in the next slide.

125125

Two representations of cellulose. In the three-dimensional model note the hydrogen bonding that links the extended cellulose polymers to form cellulose fibers.

Haworth formula “Bent”

Three-dimensional model of cellulose

126

Cellulose: stacked structure

CH2OH

OH

OH

O

O

O

OH

OH

O

O

CH2OH

OH

OH

O

O

CH2OH

OH

OH

O

O

CH2OH

127