Carbohydrates

Bettelheim, Brown, Campbell and Farrell

Chapter 20

Carbohydrates• Carbohydrate:Carbohydrate: polyhydroxyaldehyde or

polyhydroxyketone, or a substance that can be hydrolyzed to form these compounds

• Monosaccharide:Monosaccharide: a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate (simple sugar)

Monosaccharide:Monosaccharide: – 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

Monosaccharides• Monosaccharides are classified by their

number of carbon atoms

• “ose” ending for sugars

Hexose

Heptose

Octose

TrioseTetrose

Pentose

FormulaName

C3H6 O3C4H8 O4

C5H1 0O5

C6H1 2O6

C7H1 4O7C8H1 6O8

Monosaccharides• There are only two trioses

– Often simply call these trioses – Tells the number of carbons

Dihydroxyacetone (a ketotriose)

Glyceraldehyde (an aldotriose)

CHO

CHOH

CH2OH

CH2OH

C=O

CH2OH

Monosaccharides

• Glyceraldehyde, the simplest aldose, contains a stereocenter and exists as a pair of enantiomers

CHO

CH OH

CH2OH

CHO

C

CH2OH

HHO

Monosaccharides• Fischer projection:Fischer projection: a two dimensional

representation for showing the configuration of tetrahedral stereocenters– Horizontal lines represent bonds projecting

forward – Vertical lines represent bonds projecting to the

rear

CHO

CH OH

CH2OH

H OHCHO

CH2OH

convert to a Fischerprojection

D,L 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 (next to last) carbon is on the right

– L-monosaccharide:L-monosaccharide: the -OH on its penultimate (next to last) carbon is on the left

L-GlyceraldehydeD-Glyceraldehyde

CHOCHO

H OH

CH2OH CH2OH

HHO

[]25 = +13.5°D

[]25 = -13.5°D

D,L Monosaccharides– the most common D-tetroses and D-pentoses

– the three common D-hexoses

CH2OH

CHO

OHOHH

H

CH2OH

CHO

OHHHO

HH

CH2OH

CHO

OHOHH

OHH

CH2OH

CHO

OHHH

HOHH

D-Erythrose D-Threose D-Ribose 2-Deoxy-D-ribose

CHO

HOHH

HOOHH

CH2OHOHH

CHO

HOHH

HOHHO

CH2OHOHH

CH2OH

HHOC=O

OHH

CH2OHOHH

D-FructoseD-Glucose D-Galactose

Review: Addition of Alcohols to Carbonyls

• Addition of an alcohol to the carbonyl group of an aldehyde or ketone forms a hemiacetalhemiacetal (a half-acetal)– Functional group of a hemiacetal is a carbon

bonded to one -OH group and one -OR group– H of the alcohol adds to the carbonyl oxygen

and -OR adds to the carbonyl carbon

Addition of Alcohols to Carbonyl– Hemiacetals are generally unstable and are only

minor components of most equilibrium mixtures– Major exception: When both the carbonyl group

and the hydroxyl group are in the same molecule and can form a cyclic hemiacetal with a 5- or 6-member ring

– Cyclic hemiacetals predominate

H

O

O-HC

O O

H

H

O O-H

H

4-Hydroxypentanal A cyclic hemiacetal

123

45

1345

redraw to show the -OH and -CHO close

to each other2

Cyclic Structure• Aldehydes and ketones react with alcohols

to form hemiacetalshemiacetals– 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

Haworth Projections– D-Glucose forms these 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

Carbohydrate Rings• -OH group can react with C=O group to

give hemiacetal

• Fischer Fischer ring Haworth

Haworth Projections– Cyclic hemiacetal is represented as a planar

ring, lying roughly perpendicular to the plane of the paper

– Groups bonded to the carbons of the ring then lie either above or below the plane of the ring

– New carbon stereocenter created in forming the cyclic structure is called an anomeric carbonanomeric carbon

– Stereoisomers that differ in configuration only at the anomeric carbon are called anomersanomers

– Anomeric carbon of an aldose is C-1; that of the most common ketoses is C-2

Haworth Projections• In the terminology of carbohydrate

chemistry, form: -OH on the anomeric carbon is on the

same side of the ring as the terminal -CH2OH (up)

form: -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH2OH (down)

– Pyranose: Pyranose: 6-member ring sugar containing O – Furanose: Furanose: 5-member ring sugar containing O

PyranFuranOO

Haworth Projections– aldopentoses also form cyclic hemiacetals– the most prevalent forms of D-ribose and

other pentoses in the biological world are furanoses

OH ()

H

HOH OH

H HOHOCH2

H

OH ()

HOH H

H HOHOCH2

-D-Ribofuranose(-D-Ribose)

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

Haworth Projections– D-fructose also forms a five-membered cyclic

hemiacetal

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

Chair Conformations

• For pyranoses, the six-membered ring is more accurately represented as a chair chair conformationconformation

OCH2OH

HOHO

OHOH()

CHOH

HO

CH2OHOHHO

OHO

OH()HO

HO

CH2OHO

(-D-Glucose)

(-D-Glucose)

-D-Glucopyranose

-D-Glucopyranose

D-Glucose

anomericcarbon

Chair Conformations

– in both a Haworth projection and a chair conformation, the orientations of groups on carbons 1- 5 of -D-glucopyranose are up, down, up, down, and up

OCH2OH

HOHO

OHOH()H

H OH

HHO

HOH()

OH

H

CH2OHO

-D-Glucopyranose(chair conformation)

-D-Glucopyranose(Haworth projection)

123

4

5

6

1

23

4

5

6

Mutarotation• Mutarotation: Mutarotation: - and -anomers can have the

ring open and then form the other anomer.

• A change in specific optical rotation accompanies the equilibration of in aqueous solution– example: when either -D-glucose or -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

Fig 19.2, p.472

Mutarotation

[]D25 = + 18.7°

-D-Glucopyranose-D-Glucopyranose[]D

25 = +112°

OHOH

HOHO

CH2OHO HO OH

OC

CH2OH

HO

HOH

OCH2OH

HO

HOOH

HO

Open-chain form

Mutarotation

• Rings can open to form open chain compound. • New ring can be either - or -anomer.• Equilibrium among - and - forms (and open chain)

will be established. • Change in specific optical rotation accompanies this

equilibration – If either -D-glucose or -D-glucose is dissolved in water,

specific rotation changes to an equilibrium value of +52.7°– 64% beta and 36% alpha forms at equilibrium– Very little will exist in open chain form

Fig. 17.6 Another representation of ring

opening and closure

Fig 19.2, p.472

Mutarotation

• Shown in chair conformation

[]D25 = + 18.7°

-D-Glucopyranose-D-Glucopyranose[]D

25 = +112°

OHOH

HOHO

CH2OHO HO OH

OC

CH2OH

HO

HOH

OCH2OH

HO

HOOH

HO

Open-chain form

Physical Properties

• Monosaccharides – Colorless crystalline solids– Very soluble in water– Only slightly soluble in ethanol– Sweet taste

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 180

Formation of Glycosides• Reaction of a hemiacetal with alcohol

gives an 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

Hemiacetal

Acetals

Formation of Glycosides– Cyclic acetal derived from a monosaccharide is

called a glycosideglycoside– Bond from the anomeric carbon to the -OR group

is called a glycosidic bondglycosidic bond– Mutarotation is not possible in a glycoside

• Ring can’t open up• No equilibrium with the open-chain form

– Can be hydrolyzed in acidic solution to form alcohol and a monosaccharide

Oxidation and Reduction of simple sugars

• Aldehyde and ketone groups can be reduced to form alcohols– Requires reducing agents such as

NaBH4 or H2 with a transition metal catalyst

• Aldehyde group can be oxidized to produce a carboxylic acid

Reduction to Alditols

• The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of reducing agents, including NaBH4 and H2 in the presence of a transition metal catalyst– the reduction product is called an alditolalditol

OHOH

HOHO

CH2OHO

CHOOHHHHOOHH

CH2OHOHH

NaBH4

CH2OHOHHHHOOHH

CH2OHOHH

D-Glucitol(D-Sorbitol)

D-Glucose-D-Glucopyranose

Reduction to Alditols– sorbitol is found in the plant world in many berries

and in cherries, plums, pears, apples, seaweed, and algae

– it is about 60 percent as sweet as sucrose (table sugar) and is used in the manufacture of candies and as a sugar substitute for diabetics

– Other common alditols

CH2OH

CH2OH

OHHOHH

CH2OH

CH2OH

OHHHHOOHH

CH2OHHHOHHOOHH

CH2OHOHH

D-Mannitol XylitolErythritol

Oxidation to Aldonic Acids– the aldehyde group of an aldose is oxidized under

basic conditions to a carboxylate anion– the oxidation product is called an aldonic acidaldonic acid– any carbohydrate that reacts with an oxidizing

agent to form an aldonic acid is classified as a reducing sugarreducing sugar (it reduces the oxidizing agent)

OCH2OH

HOHO

OHOH

COHHHHOOHH

CH2OHOHH

O HC

OHHHHOOHH

CH2OHOHH

O O-

oxidizingagent

D-GluconateD-Glucose-D-Glucopyranose(-D-Glucose)

basicsolution

Common Disaccharides• Sucrose (table sugar)

– 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 (ring can’t open)

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

Fig. 17.10

Common Disaccharides• Lactose

– 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-galactose bonded by a -1,4-glycosidic bond to carbon 4 of D-glucose

– 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

Common Disaccharides

• Maltose– present in malt, the juice from sprouted barley

and other cereal grains – maltose consists of two glucose units joined

by an -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-glycosidicbond

1 4

Polysaccharides

• Polysaccharide:Polysaccharide: Polymer containing many monosaccharide units

• Starch:Starch: a polymer of D-glucose– Two forms: amylose and amylopectin– Amylose is composed of unbranched chains of up to

4000 D-glucose units joined by -1,4 bonds– Amylopectin contains chains up to 10,000 D-glucose

units also joined by -1,4-glycosidic bonds• Branched compound• New chains of 24 to 30 units attached by -1,6 linkages

Polysaccharides

• GlycogenGlycogen is the energy-reserve carbohydrate for animals– Glycogen is a branched polysaccharide of

approximately 106 glucose units joined by -1,4- and -1,6-glycosidic bonds

– Total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle

Amylose

Branched

Polysaccharides• CelluloseCellulose is a chain of D-glucose units joined

by -1,4-glycosidic bonds– Average chain has approximately 2200 glucose

units per molecule– cellulose chain is much stiffer than starch

• Chains line up side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds

– Arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength

– Cellulose is insoluble in water

Polysaccharides• Cellulose (cont’d)

– humans and other animals cannot digest cellulose– Can only digest starch and glycogen to get glucose– Many bacteria and microorganisms can digest

cellulose– 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

Cellulose

Blood Groups

• Several sugar groups attached to red blood cell (RBC) surface

• Difference in one sugar accounts for Types A, B, AB and O blood.

Sugars attached to RBC

(α 1,4) β (1,3)

X –-------- D-Galactose-----------N-Acetyl-D- RBC

| glucosamine

| (α 1,2)

|

L-fucose

Blood Groups -- Type A

• N-Acetyl-D-Galactosamine on RBC surface

Blood Groups -- Type B

• galactose on RBC surface

Blood Groups -- Type O

• Neither N-Acetyl-D-galactosamine nor Galactose attached to rest of sugar groups on RBC surface