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Chapter 7: Outline. Monosaccharides Monosaccharide stereoisomers Cyclic structures Reactions Examples and derivatives Di and oligosaccharides Polysaccharides Homo and heteropolysaccharides Glycoconjugates.

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Chapter 7: Outline

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Chapter 7 outline l.jpg

Chapter 7: Outline

Monosaccharides

Monosaccharide stereoisomers

Cyclic structures

Reactions

Examples and derivatives

Di and oligosaccharides

Polysaccharides

Homo and heteropolysaccharides

Glycoconjugates


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  • 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.


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7.1 Monosaccharides

polyhydroxy

Number of carbons

Aldehydes

are aldoses

Ketones

are

ketoses

3=triose

4=tetrose

5=pentose

6=hexose


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Monosaccharides: generic names

  • The 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?

aldohexose

What is the name for a five carbon

ketone sugar?

ketopentose


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Dihydroxyacetone

  • Dihydroxyacetone

Is a ketotriose


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Glyceraldehyde

Glyceraldehyde

Is an aldotriose

Glyceraldehyde exists in two

stereoisomeric forms because the

starred carbon is a stereocenter: it has

four different groups attached.


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stereocenter

  • 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 .


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Glyceraldehyde: 3

  • The 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.


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Perspective View


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View with blue lens on the left eye.

Carbonyl group

OH group

H atom

CH2OH group

Stereoscopic view of glyceraldehyde


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Fischer Projections

  • In 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.


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  • 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.


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1

2

1

2

3

3

4

4

5

5

6


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These diastereomers are also epimers, they

differ in configuration at only one stereo-

center (colored dot).


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Cyclic forms for sugars

  • Most 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 b, if the OH is “down” it is a.


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Cyclic forms for sugars-2

  • Fischer projections for a D glucose


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Cyclic forms for sugars-3 Haworth

  • 1. Draw a five- or six-membered ring

2. 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.


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Cyclic forms for sugars-4 Haworth

Anomeric C

b-OH


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Cyclic forms for sugars-5 Glucose

Pyranose

ring form


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Cyclic forms for sugars-6

  • The alpha and beta forms of cyclic sugars are said to be anomers. They differ in configuration about the hemiacetal or hemiketal carbon.


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Cyclic forms for sugars-7

  • Ribose also exists mainly in the cyclic form.


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a D-glucose: the chair conformer

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

+ Ag(mirror)

Ag(NH3)2+

Cu2+

+ Cu2O (red-orange)

Benedict’s reagent (a blue copper ion solution) also gives a

lactone. The blue color fades as reaction occurs.

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

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


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Oxidation of Monosaccharides-2

Term CH2OH oxid’n

uronic acid

  • Aldehyde oxid’n

  • aldonic acid


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Oxidation of Monosaccharides-3

  • Aldehyde + term CH2OH oxid’n 

  • aldaric acid


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Reduction of Monosaccharides

The 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.


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Isomerization

  • Isomerization of mono-

  • saccharides occurs

  • through an enediol.


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Esters of Monosaccharides

  • The OH groups of sugars can react with phosphoric acid to give phosphate esters.


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Glycosides

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

Acetal

carbon

  • The anomeric OH can react with another OH on an alcohol or sugar. Water is lost to form an acetal/ketal


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Important Monosaccharides


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Amino Sugars


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Amino Sugars-2


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