7P1-1

MonosaccharidesMonosaccharide stereoisomersCyclic structuresReactionsExamples and derivatives

Di and oligosaccharidesPolysaccharides

Homo and heteropolysaccharidesGlycoconjugates

Chapter 7: Outline

7P1-2

Originally, carbohydrates were those compounds having the formula Cn(H2O)n. Only monosaccharides or simple sugars fit the formula. As more complex carbohydrates were discovered, the term came to mean compounds associated with polyhydroxy aldehydes and ketones.

7P1-3

7.1 Monosaccharides

polyhydroxy

Aldehydesare aldoses

Ketonesareketoses

3=triose4=tetrose5=pentose6=hexose

Number of carbons

7P1-4

Monosaccharides: generic namesThe generic name for a simple sugar

begins with the carbonyl prefix aldo or keto and ends with the term for the number of carbons.

An aldose with three carbons is called an aldotriose. A ketose with three carbons is a ketotriose.

What is the name for a six carbon aldehyde sugar?What is the name for a five carbon ketone sugar?

aldohexose

ketopentose

7P1-5

DihydroxyacetoneDihydroxyacetone

Is a ketotriose

CH2OHCCH2OH

O

7P1-6

GlyceraldehydeGlyceraldehydeIs an aldotriose C

C

O H

CH2OHOHH

Glyceraldehyde exists in two stereoisomeric forms because the starred carbon is a stereocenter: it hasfour different groups attached.

7P1-7

The stereoisomers of glycer-aldehyde are designated D or L. The D isomer has the OH on the stereocenter to the right. The L isomer has the OH on the left .

CC

O H

CH2OHOHH

CC

O H

CH2OHHOH

the D isomer the L isomer

stereocenter

7P1-8

Glyceraldehyde: 3The stereoisomeric forms of glycer-

aldehyde are enantiomers: nonsuper-imposable mirror image molecules.

Perspective drawings of the two enantiomers of glyceraldehyde are on the next slide. A stereo view is on slide 11.

Remember, barred bonds ( ) recede behind the plane of the screen and wedge ( ) bonds project in front of the plane.

7P1-9

Perspective View

7P1-10

Carbonyl group

CH2OH group

OH groupH atom

View with blue lens on the left eye.

Stereoscopic view of glyceraldehyde

7P1-11

Fischer ProjectionsIn a Fischer projection, the

sugar molecule is oriented so that the most oxidized carbon is to the top. The stereocenter carbons are arranged so that the groups not part of the main chain project horizontally toward the viewer.

The molecule is in the all eclipsed form.

CO H

CH OHCCH2OH

H OH

7P1-12

Monosaccharides are drawn in Fischer projections with the most oxidized carbon closest to the top. The carbons are numbered from the top. If the the stereocenter with the highest number has the OH to the right, the sugar is D. If the OH is to the left, the sugar is L.

Most common sugars are in the D form.Note: Fisher projections represent an all

eclipsed conformation.

7P1-13

1

2

3

4

1

2

3

4

565

CC

O H

COHH

CH OHH

CH2OHOH

CH2OHCC

OOH

C OHHC

H

H OHCH2OH

D-ribosean aldopentose

D-fructosea ketohexose

7P1-14

CHOCC

OHOH

C HOHC

H

H OHCH2OH

H

D-glucosean aldohexose

D-galactosean aldohexose

CHOCC

OHOH

C OHHC

H

H OHCH2OH

H

These diastereomers are also epimers, they differ in configuration at only one stereo-center (colored dot).

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Cyclic forms for sugarsMost simple sugars of four or more

carbons exist in the cyclic (hemiacetal or hemiketal) form.

A hydroxy group in the sugar reacts with the carbonyl group.

The new OH bearing carbon is now a stereo center and is called an anomeric carbon.

If the OH on the ring is “up” the carbon is , if the OH is “down” it is .

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Cyclic forms for sugars-2 Fischer projections for D glucose

CCC

OHOH

C OHHC

H

HCH2OH

HOH

OH

CCC

OHOH

C OHHC

H

HCH2OH

HOHH C

CC

OHOH

C OHHC

H

HCH2OH

HOH H

D-glucose D-glucosecyclic form

O

D-glucosecyclic form

O

7P1-17

Cyclic forms for sugars-3 Haworth1. Draw a five- or six-membered ring

O O

pyranose form furanose form2. Anomeric C to right of O. Place OH up or down. Left on Fischer, up on ring.3. In D- sugars, the last C is always up.

7P1-18

Cyclic forms for sugars-4 HaworthCH2C O

OH

COH HC OHHCHCH2OH

OH

O

CH2OH

OHOH

OH

CH2OH

D-fructose-D-fructofuranose+ isomer

Anomeric C-OH

7P1-19

Cyclic forms for sugars-5 Glucose

Pyranose ring form

OCH2OH

HH

OHHOH

OHH

O

H

HOCH2OH

HH

OHHOH

OHH O

HH

OCH2OH

HH

OHHOH

OH

HH

O H

form(alpha)

form(beta)

arrows showelectron movement

7P1-20

Cyclic forms for sugars-6The alpha and beta forms of cyclic

sugars are said to be anomers. They differ in configuration about the hemiacetal or hemiketal carbon.

7P1-21

Cyclic forms for sugars-7Ribose also exists mainly in the cyclic

form.

OCH2OH

H

HH

OH

H

OH

O

HCH2OH

HH

OH

H

OH

O

H

O H

D-ribose D-ribofuranose(furanose ring form)

arrows showelectron movement

7P1-22

D-glucose: the chair conformer

OH

HH

H

HOH

H

O

OHOH

CH2OH

12

3

45

6

Four of the five bulky groups (OH and CH2OH on C 2,3,4,5) on the ring are in the more stable equitorial positions!

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Oxidation of Monosaccharides

OCH2OH

HH

OHH

OH

OH

H OO

CH2OH

HH

OHH

OH

OH

HOH

H

Aldoses react with Tollen’s reagent (Ag(NH3)2+) to give a

lactone (cyclic ester). The silver ion plates out as a mirror.

Ag(NH3)2+

+ Ag(mirror)

Cu2+

+ Cu2O (red-orange)Benedict’s reagent (a blue copper ion solution) also gives a lactone. The blue color fades as reaction occurs.

7P1-24

Aldehyde oxid’naldonic acid

Oxidation of Monosaccharides-2COOHCC

OHOH

C OHHC

H

H OHCH2OH

H

D-gluconic acid

Term CH2OH oxid’n

uronic acid

CHOCC

OHOH

C OHHC

H

H OHCOOH

H

D-glucuronic acid

7P1-25

Oxidation of Monosaccharides-3Aldehyde + term CH2OH oxid’n

aldaric acid COOHCC

OHOH

C OHHC

H

H OHCOOH

H

D-glucaric acid

7P1-26

Reduction of MonosaccharidesThe most important reduced sugar is deoxyribose. (In DNA)

When the carbonyl of a sugar is reduced to an alcohol, alditols are produced. The two shown above are used to sweeten nonsugar gum.

D-sorbitol D-xylitol

CH2OHCC

OHOH

C HOHC

H

H OHCH2OH

H

CH OHCH2OH

CH2OHCC

OHOH HHC

C

O H

CHH

CH OH

OHHCH2OH

D-deoxyribose

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Isomerization

Isomerization of mono-saccharides occurs through an enediol.

CCC

OHOH

C OHHC

H

H OHCH2OH

HH O C

CC

OHOH

C OHHC

H

H OHCH2OH

H OH

CH2CC

OOH

C OHHC

H

H OHCH2OH

OH

CCC

OHOH

C OHHC

H

H OHCH2OH

HH O

7P1-28

Esters of MonosaccharidesThe OH groups of sugars can react with

phosphoric acid to give phosphate esters.

OCH2OPO3

2-

HH

OHH

OH

OH

HH

OH

D-glucose-6-phosphate

7P1-29

GlycosidesThe anomeric OH can react with another

OH on an alcohol or sugar. Water is lost to form an acetal/ketal

OCH2OH

HH

OHH

OH

OH

HH

OH

CH3 O H

OCH2OH

HH

OHH

OH

OH

HH

O CH3

+ + H2O

Acetal link: R-O-C-O-R

Acetalcarbon

7P1-30

Important Monosaccharides

OCH2

HH

OHH

OH

OH

H OH

HOH

OCH2

H

H

OH

H

OH

OH

H OH

HOH

O

H

CH2

H

OH

OH

HCH2OH

OHOH

7P1-31

Amino Sugars

OCH2

HH

OHH

OH

NH3+

H OH

HOH

-D-glucosamine

OCH2

HH

OHH

OH

NH

H OH

HOH

C OCH3N-acetyl--D-glucosamine

7P1-32

Amino Sugars-2

NHCO

CH3O

R

OHOH

OH

COO-

CH OHCH OHCH2OH

N-acetylnuraminic acid sialic acid

R=