Chapter 20: Carbohydrates

Carbohydrate:Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis.

Chapter Overview:Chapter Overview:•Monosaccharides

• Fischer projections• Haworth projections• Hemiacetals and Acetals• Oxidation and reduction

•Disaccharides•Polysaccharides

Monosaccharide:Monosaccharide: A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate.• Monosaccharides have the general formula CCnnHH2n2nOOnn, where nn varies from 3 to 8.• Aldose:Aldose: A monosaccharide containing an aldehyde group.• Ketose:Ketose: A monosaccharide containing a ketone group.• The prefixes tri-tri-, tetratetra, pentapenta, and so forth indicate the number of carbon atoms in the chain.

Dihydroxyacetone (a ketotriose)

Glyceraldehyde (an aldotriose)

CHO

CHOH

CH2OH

CH2OH

C=O

CH2OH

Chapter 20: Monosaccharides

Fructose( ____________ )

Chapter 20: Fischer projections

Fischer projection:Fischer projection: A two-dimensional representation for showing the configuration of tetrahedral stereocenters.• Horizontal lines represent bonds projecting forward from the

stereocenter. • Vertical lines represent bonds projecting to the rear.

CHO

CH OH

CH2OH

H OHCHO

CH2OH

convert to a Fischerprojection

Chapter 20: Monosaccharides

In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde.

• D-monosaccharide:D-monosaccharide: the -OH on its penultimate carbon is on the right in a Fischer projection.

• L-monosaccharide:L-monosaccharide: the -OH on its penultimate carbon is on the left in a Fischer projection.

L-GlyceraldehydeD-Glyceraldehyde

CHOCHO

H OH

CH2OH CH2OH

HHO

[]25 = +13.5°D

[]25 = -13.5°D

Chapter 20: Monosaccharides

The most common monosaccharides:The most common monosaccharides:

D-Ribose D- Glucose D- Galactose D- Fructose

Chapter 20: Cyclic Structure

• Aldehydes and ketones react with alcohols to form hemiacetalshemiacetals (Chapter 17).• 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.

O-HH

O

CO O

H

H

O O-H

H4-Hydroxypentanal

A cyclic hemiacetal

14

14

redraw to show -OH and -CHO

close to each other

Chapter 20: Haworth Projections

• D-Glucose forms these two cyclic hemiacetals.

CHO

OH

H

OH

H

HO

H

H OH

CH2OH

HH OH

HHO

HOH

OH

H

CH2OHO

C

H OH

HHO

HOH

H

CH2OHOH

O

H

OHH OH

HHO

HH

OH

H

CH2OHO

D-Glucose

-D-Glucopyranose (-D-Glucose)

()

()

-D-Glucopyranose (-D-Glucose)

+

anomericcarbon

5

5

1

1

redraw to show the -OH on carbon-5 close to thealdehyde on carbon-1

anomericcarbon

Chapter 20: Anomers

• The anomeric carbon of an aldose is C-1; • The anomeric carbon of most ketoses is C-2. • ββ means that the -OH on the anomeric carbon is on the same

side of the ring as the terminal -CH2OH.

• αα means that the -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH2OH.

• A six-membered hemiacetal ring is called a pyranosepyranose, and a five-membered hemiacetal ring is called a furanosefuranose because these ring sizes correspond to the heterocyclic compounds furan and pyran.

PyranFuranOO

Chapter 20: Monosaccharides

OH ()

H

HOH OH

H HOHOCH2

H

OH ()

HOH H

H HOHOCH2

-D-Ribofuranose(-D-Ribose)

-2-Deoxy-D-ribofuranose(-2-Deoxy-D-ribose)

The prefix “deoxydeoxy” means “without oxygen.”

Chapter 20: Monosaccharides

HO

HOCH2 OH

HHO

CH2OH

OHH

H

C=O

CH2OH

HOH

CH2OH

OHH

HO HOH

HOHOCH2

HO HCH2OH

OH

D-Fructose

1

2

5

5

5

1

2

2

()

-D-Fructofuranose(-D-Fructose)

-D-Fructofuranose(-D-Fructose)

()

1

Fructose Fructose is a ketose and it also forms cyclic hemiacetals

Chapter 20: Monosaccharides

• Mutarotation: Mutarotation: The change in specific rotation that accompanies the equilibration of a- and b-anomers 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.

[]25 = + 18.7°-D-Glucopyranose-D-Glucopyranose

[]25 = +112°

OHOH

HOHO

CH2OHO HO OH

OC

CH2OH

HO

HOH

OCH2OH

HO

HOOH

HO

Open-chain form

D D

Chapter 20: Monosaccharides

Hemiacetal + Alcohol = Acetal

HH OH

HHO

HOH

OH

H

CH2OHO

CH3OHH+

-H2O

OCH2OH

H

OH

OCH3H

HOH

OHH

H

OCH2OH

H

OH

HH

HOH

OHH

OCH3

(-D-Glucose)-D-Glucopyranose

Methyl -D-glucopyranoside(Methyl -D-glucoside)

anomeric carbon

+

+

Methyl -D-glucopyranoside(Methyl -D-glucoside)

glycosidicbond

• The acetal obtained from a monosaccharide is called glycoside• Mutarotation is not possible in glycosides

Chapter 20: Disaccharides

Disaccharide:Disaccharide: a carbohydrate containing two monosaccharide units joined by a glycosidic bond.Sucrose (table sugar) = Glucose + Fructose• 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.

O

HOOH

OH

CH2OH

O

OH

HOO

CH2OH

HOCH2

OHO

HO

O

OH

CH2OH

OH

HOO

CH2OH

HOCH2

1

1

2

1

2

1

a unit of -D-glucopyranose

a unit of -D-fructofuranose

-1,2-glycosidic bond

Chapter 20: Monosaccharides

Lactose = Galactose + Glucose• Lactose is the 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-galactopyranose bonded by a β-1,4-glycosidic

bond to carbon 4 of D-glucopyranose.• Lactose is a reducing sugar.

O

OH

HOOH

O

CH2OH

O

HOOH

OH

CH2OHOOH O

OH

OH

CH2OH

O OH

OH

OH

CH2OH

1

1

4

4

-1,4-glycosidic bond

Chapter 20: Monosaccharides

Maltose = Glucose + Glucose• Present in malt, the juice from sprouted barley and other cereal

grains. • Maltose consists of two units of D-glucopyranose joined by an a-

1,4-glycosidic bond.• Maltose is a reducing sugar.

OHO

HOOH

OOHO OH

OH

CH2OH

CH2OHO

OH

O

OHHO

O OH

HO

OH

CH2OH

HOCH2 1

4

-1,4-glycosidicbond1 4

Chapter 20: Physical Properties

Monosaccharides are colorless crystalline solids, very soluble in water, but only slightly soluble in ethanol.

Sweetness relative to sucrose:

Carbohydrate

fructose

glucose

galactose

sucrose (table sugar)

lactose (milk sugar)

honey

SweetnessRelative to Sucrose

1.741.000.970.74

0.320.16

Artificial Sweetener

SweetnessRelative to Sucrose

maltose 0.33

saccharin 450acesulfame-K 200aspartame 180sucralose 600

Chapter 20: Polysaccharides

Polysaccharide:Polysaccharide: A carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds.

Starch:Starch: A polymer of D-glucose.• Starch can be separated into amylose and amylopectin.• Amylose is composed of unbranched chains of up to 4000 D-

glucose units joined by α-1,4-glycosidic bonds.• Amylopectin contains chains up to 10,000 D-glucose units also

joined by α-1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by α-1,6-glycosidic bonds.

Chapter 20: Polysaccharides

Amylopectin, a branched polymer of approximately 10,000 units of D-glucose joined by -1,4-glycosidic bonds.

Chapter 20: Polysaccharides

GlycogenGlycogen is the energy-reserve carbohydrate for animals.• Glycogen is a branched polysaccharide of approximately 106

glucose units joined by α-1,4- and α-1,6-glycosidic bonds.• The total amount of glycogen in the body of a well-nourished

adult human is about 350 g, divided almost equally between liver and muscle.

Chapter 20: Polysaccharides

CelluloseCellulose is a linear polysaccharide of D-glucose units joined by β-1,4-glycosidic bonds.• It has an average molecular weight of 400,000

g/mol, corresponding to approximately 2200 glucose units per molecule.

• Cellulose molecules act like stiff rods and align themselves side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds.

• This arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength.

• It is also the reason why cellulose is insoluble in water.

Chapter 20: Polysaccharides

Cellulose is a linear polymer containing as many as 3000 units of D-glucose joined by β-1,4-glycosidic bonds.

• Humans and other animals can not digest cellulose because their digestive systems do not contain β-glycosidases, enzymes that catalyze the hydrolysis of β-glycosidic bonds.

• 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. • Instead, we have only α-glucosidases; hence, the polysaccharides we use as

sources of glucose are starch and glycogen.• Many bacteria and microorganisms have β-glucosidases.