Carbohydrates - Faculty Personal Homepage- KFUPMfaculty.kfupm.edu.sa/CHEM/ajhamdan/CHEM202/CHAPTER...
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CHAPTER 23
Carbohydrates
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Introduction
• Carbohydrates are naturally occurring compounds of carbon, hydrogen , and oxygen.
• Carbohydrates have the empirical formula CH2O.
• Carbohydrates have the general formula CnH2nOn. eg. Glucose: C6H12O6
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• Carbohydrates are defined as polyhydroxyaldehydes or ketones or substances that hydrolyze to yield polyhydroxy aldehydesand ketones.
• Monosaccharides are carbohydrates that cannot be hydrolyzed to simpler carbohydrates = simple sugars.
• Disaccharides can be hydrolyzed to two monosaccharides.
• Oligosaccharides yield 2 to 8 monosaccharides.
• Polysacccharides yield >8 monosaccharides
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Some important monosacharides
CH2OH
H OH
OHH
HHO
OH
CHO
CH2OH
H OH
HHO
HHO
H OH
CHOH C
CH2OH
H OH
OHH
HHO
O
CH2OH
D-glucose D- galactose D-fructose
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Glucose• Blood sugar. • Mammals
convert sucrose, lactose, maltose, and starch to glucose which is then used for energy
glucose CH3C
O
acetyl groupin acetyl coenzyme A
fatssteroids(cholesterole)
protiens
CO2 + H2O + energy
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Other monosaccharides• Fructose sweetest
tasting sugar. It occurs in fruit and honey.
• Galactose found bonded to glucose in the disaccharide lactose.
• Ribose and deoxyribose form part of the polymeric backbone of nucleic acids.
CH2OH
OHH
OHH
H OH
CHOCH2
CH2OH
OHH
OHH
CHO
D-ribose 2-deoxy-D-riboseC5H10O5 deoxy = minus
an oxygen
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Classification of Monosaccharides
• The suffix –ose is used to designate a carbohydrate. Carbohydrate names ending in –ose (eg. Glucose, galactose).
• Mononsacchrides that contain aldehydegroup Aldoses (aldehyde + ose) ( glucose and ribose).
• Monosacchrides with ketone group Ketoses (ketone + ose) ( frctose)
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• The number of carbon atoms are designated by tri, tetr-, etc ( for example a triose is three carbons)
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• Ketones are often given the ending –ulose. example. Fructose,
CH2OH
H OH
OHH
HHO
O
CH2OH
CH2OH
OHH
OHH
H OH
CHO
D-riboseKetohexoseor hexulose Ketopentose
or pentulosealdopentose
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Configuration of monosaccharides
• Mpnpsaccharides with the same number of carbons are structural isomers or diastereomers.
• Diastereomers are nonenentiomericstereoisomers with two or more chiralcenters but differ in the projection of at least one of them.
• If two diastereomers differ in the projection of only one chiral center epimers
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Examples• D-glucose, D-
galactose, and D-fructose are all structural isomers ( diastereomers).
• D-glucose and D-galactose differ only in the projection at carbon 4 epimers
CH2OH
H OH
OHH
HHO
OH
CHO
CH2OH
H OH
HHO
HHO
H OH
CHOH C
CH2OH
H OH
OHH
HHO
O
CH2OH
D-glucose D- galactose D-fructose
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The D and L System
• If the OH on the last chiral carbon is projected to the right D
• If the OH on the last chiral carbon is projected to the left L
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Examples
CH2OH
OHH
HHO
HO H
CHO
CH2OH
H OH
CHO
CH2OH
HO H
OHH
HHO
H OH
CHO
D-glyceraldehyde
D- lyxose
CH2OH
HHO
HHO
HO H
CHO
L-aldohexose
L-ripose
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The D Family of Aldoses
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CyclizationCyclization of of MonosaccharidesMonosaccharides
O
C
CH2 OH
OH
OHH
HOH
OHH
H
H
C
CH2 OH
H
OHH
HOH
OHH
H
O
OH
..
..
: :
1 1
2 2
3 3
4 4
5 5
6 6
a hemiacetalonly sugars seem to makestable hemiacetals
glucose glucopyranose
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FURANOSE AND PYRANOSE RINGSFURANOSE AND PYRANOSE RINGS
: :
OH
H
O
H
OHO
H
H
OO
O
HOH
:..
a pyranosering
a furanosering
6
5
two anomersare possiblein each case
furan pyran
O O
for clarity nohydroxyl groups are shown on the chains or rings
123
4 5
123
4
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ANOMERS ANOMERS
: :
OH
HO
H
OHO
H
H
OO
α-D-(+)-Glucose
β-D-(+)-Glucose
: :
O
H
HO
for clarityhydroxyl groups on the chain are not shown
anomericcarbon
(hemiacetal)
anomers differ in configurationat the anomeric carbon # 1
β = OH up
α = OH down
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: :
OH
HOHO
OHOH
CH2
OH
H
OHOHO
OHOH
CH2
OH
H
H
OOHO
OHOH
CH2
OH
α-D-(+)-Glucose β-D-(+)-Glucose[α] = + 112.2° [α] = + 18.7°
[α] = + 52.7°
Equilibrium mixture:
GlucoseGlucose
hemiacetals
66%34%
open chain
O
C
CH2 OH
OH
OHH
HOH
OHH
H
H
..
..
1
2
3
4
5
6 < 0.001%
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HAWORTH PROJECTIONSHAWORTH PROJECTIONS
It is convenient to view the cyclic sugars (glucopyranoses)as a “Haworth Projection”, where the ring is flattened.
upper-rightback
This orientation isalways used for aHaworth Projection
OCH2OH
HH
OHH
OH
OH
HOH
H O
Standard PositionHAWORTHPROJECTION
α-D-(+)-glucopyranose
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HAWORTH PROJECTIONSHAWORTH PROJECTIONS
HERE ARE SOME CONVENTIONS YOU MUST LEARN
2) The -CH2OH group is placed UP for a D-sugar andDOWN for an L-sugar.
1) The ring is always oriented with the oxygen in the upper right-hand back corner.
O
OCH2OH
OCH2OH
3) α-Sugars have the anomeric hydroxyl group down.
4) β-Sugars have
the anomeric hydroxyl group up
D
L
OCH2OH
OH
OCH2OH
OH
α
β
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SOME HAWORTH PROJECTIONSSOME HAWORTH PROJECTIONS
OCH2OH
HH
OHH
OH
OH
HH
OH
OCH2OH
HH
OHH
OH
OH
HOH
H
cis
= β
trans
= α
-CH2OH up = D
-CH2OH up = D
β-D
DD--SUGARSSUGARS
α-D
ANOMERS
BOTH OF THESE ARE D-GLUCOSE
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CONVERTING CONVERTING FISCHER PROJECTIONSFISCHER PROJECTIONS
TO HAWORTH PROJECTIONSTO HAWORTH PROJECTIONS
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CONVERTING TO HAWORTH PROJECTIONS
OHOHOH
CH2OH
CHOOH
OCH2OH
HH
OHH
OH
OH
HH
OH
D-(+)-glucose
UP
DOWN
on right= D
-CH2OH up = D cis
= β
FISCHERFISCHERPROJECTIONPROJECTION
trans
= α
HAWORTHHAWORTHPROJECTIONSPROJECTIONS
OCH2OH
HH
OHH
OH
OH
HOH
H
BOTH ANOMERS OFA D-SUGAR(D-glucose)
1
6
2
3
4
5
1
23
4
6
5
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CONVERTING TO ACTUAL CONFORMATIONS
OCH2OH
HH
OHH
OH
OH
HOH
H
OH
HH
H
HOH
H
O
OHOH
CH2
OHtrans
= α
α-D-(+)-glucopyranose
OCH2OH
HH
OHH
OH
OH
HH
OH
H
HH
H
OHOH
H
O
OHOH
CH2
OH
cis
= β-CH2OH up = D
β-D-(+)-glucopyranose
HAWORTH
O
O
CONFORMATION
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HAWORTH PROJECTIONS OF L-SUGARS
L-(+)-glucose
trans
cis
OH
CH2OHOH
HOH
H
H
OHH
OH
OH
CH2OHOH
HOH
H
H
OHOH
H
HAWORTHHAWORTHPROJECTIONSPROJECTIONS
BOTH ANOMERS OFA L-SUGAR(L-glucose)
-CH2OH down=L
DOWN
OHOHOH
CH2OH
CHOOH
UP
on left= L
FISCHERFISCHERPROJECTIONPROJECTION
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CONVERTING FISCHER TO HAWORTH PROJECTIONS
Genral rules
LEFT = UPRIGHT = DOWN
β = upα = down
These rules are the same for bothD- and L-sugars
The only difference whenconverting D- and L- sugars is :
D-sugars -CH2OH = UP
L-sugars -CH2OH = DOWN
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FRUCTOFURANOSES
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Ostandard positionFRUCTOSE
cis = βup = DCH2OH
OOH
OHOH
CH2OH
1
2
3
4
5
6
..
..
..:
O
HOH
H
H
OHCH2OH
OHCH2OH anomericcarbon
1
2
34
5
6
β-D-(-)-FructofuranoseD-(-)-Fructose
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Anomeric Effect
CH2OH
OHH
OHH
H OH
CHO O
OH
OHOH
O
O OHOHOH
O
H H+
CH2
OH OH
OHO CH2
OH OHOHO
+
HO HO
β-anomer is the most stable anomer
β-D-ribopyranose(56%) α-D-ribopyranose
(20%)
D-ribose
α-D-ribofuranose(6%)β-D-ribofuranose
(18%)
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MUTAROTATION
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: :
OH
HOHO
OHOH
CH2
OH
H
OHOHO
OHOH
CH2
OH
H
H
OOHO
OHOH
CH2
OH
α-D-(+)-Glucose β-D-(+)-Glucose[α] = + 112.2° [α] = + 18.7°
[α] = + 52.7°
Equilibrium mixture:
MutarotationMutarotation of Glucoseof Glucose
hemiacetals
64%36%
open chain< 0.001%
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Glycoside Formation• Glycosides are acetals at the anomeric carbon
of carbohydrates
RCHOR'
OH+ R''OH
H+RCHOR'
OR''
+ H2O
CH2OH
H OH
OHH
HHO
H OH
CHO
OH
OH
OH
OH
CH2OHO CH3OH
H+OH
OH
OH
CH2OHO OCH3
Remember
a hemiacetal An acetal
β –D-glucopyranoseMethyl β –D-glucopyranoside
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Glycosides
RCHOR'
OR'' O
OCH3
Glycoside has two or groups attached to the anomeric carbon
An acetalA glycoside
two OR groups
two OR groups
Glycosides can be hydrolyzed in aqueous acid
OH
OCH3
OH
OH
CH2OHO
H+
+ H2O
OH
OH
OH
CH2OHO
OH + CH3OH
D-glucopyranoseaglycone
Methyl β –D-glucopyranoside
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Glycoside formation
Hydrolysis of glycosides
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Oxidation of Monosaccharides
• Aldoses and ketoses are easily oxidized when treated with Tollens’ reagent
CH
OH
OH
OH
CH2OHOH O
Ag(NH3)2+
OH-
CO2-
OH
OH
OH
CH2OHOH
+ Ag
OH
OH
OH
CH2OHO
OHH2O
a reducing sugar
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Reducing Sugars• Carbohydrates with hemiacetal linkages
are reducing sugars because they react with Tollens’ reagent
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Ketoses are also reducing sugars
CHOH
CH2OH
H OH
OHH
HHO
CO2-
+ Ag
C
CH2OH
H OH
OHH
HHO
O
CH2OH
Ag(NH3)2+
OH-
D-fructose
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Fructose is oxidized readily because it is in equlibrium with aldehydes through an endiol intermediate.
An aldoseA ketone
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Glycosides are nonreducingsugars
• Glycosides are not in equilibrium with aldehydesnonreducing sugars
Ag(NH3)2+
OH-
O OCH3
no reaction
A glycoside
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Aldonic Acids
• Bromine in water selectively oxidizes the aldehyde group of an aldose to the corresponding carboxylic acid
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Example
CH2OH
H OH
OHH
HHO
OH
CO2H
H
CH2OH
H OH
OHH
HHO
OH
CHO
H
Br2 , H2OpH 5-6
D-gluconic acidD- glucoseAn aldonic acid
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Aldaric AcidsDilute nitric acid oxidizes both the aldehydeand primary hydroxyl groups of an aldose to an aldaric acid
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Example
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Uronic AcidsIn biological systems, the terminal CH2OH group
can be oxidized without oxidation of the aldehyde group to uronic acid
OH
OH
OH
CH2OHO
OH Oenzymes
OH
OH
OH
CO2HO
OH
D-glucose D-glucoronic acida uronic acid
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Periodic Acid Oxidation:Oxidative Cleavage of PolyhydroxyCompounds
Compounds with hydroxyl groups on adjacent carbons undergo cleavage of carbon-carbon bonds between the hydroxyl groupsThe products are aldehydes, ketones or carboxylic acids
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• The carbonyl group is oxidized to a carboxyl group, while the hydroxylgroup is oxidized to an aldehyde or ketone.
• With three or more contiguous hydroxyl groups, the internal carbons become formic acid
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Cleavage also takes place when a hydroxyl group is adjacent to an aldehyde or ketone group–An aldehyde is oxidized to formic acid; a ketone is oxidized to carbon dioxide
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No cleavage results if there are intervening carbons that do not bear hydroxyl or carbonyl groups
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Reduction of Monosaccharides
• Aldoses and ketoses can be reduced to alditols
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Example
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Reaction at the Hydroxyl Groups
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Acetate Formation
Carbohydrates react with acetic anhydride in the presence of weak base to convert all hydroxyl groups to acetate esters
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Ether FormationReaction of monosaccharides with excess dimethylsulfate and NaOH produces multi methyl ether.
RO-+ CH3 OSOCH3
O
O
ROCH3 + -OSOCH3
O
O
O
HOHO
OH OH
CH2
OH
(CH3O)2SO2
OH-
O
CH3OCH3O
O OCH3
CH2
OCH3
CH3
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Cyclic Acetal FormationCarbohydrates form cyclic acetals with benzaldehyde selectively between 1,3-diol.
HOHO
OH
H2COH
OH HOO
OH
H2C
OH
OC6H5CH
C6H5CHOH+
Cyclic acetal
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Cyclic Ketal FormationCarbohydrates form cyclic ketals with acetone selectively between cis-vicinal hydroxyl groups.
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Disaccharides
A disaccharide is a carbohydrate compound of two units of monosaccharides joined together by glycoside link from carbon 1 of one unit to an OH of the other unit
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O
CH2OH
H
OH
H OH
HH
OHOH
O
CH2OH
H
OH
H OH
HH
OHOH
O
CH2OH
H
OH
H OH
H
OH
H O
CH2OH
H
OH
H OH
HH
OHO
:..
bc
a
α-D-(+)-Glucose
bc a
Maltose
1,41,4’’--ααGlycosidic LinkageGlycosidic Linkage
MaltoseMaltose
enzymemediated
Humans can
digest α-1,4
Contain two units of D-glucopyranose
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O
CH2OH
H
OH
H OH
HOH
HOH
O
CH2OH
H
OH
H OH
HOH
HOH
O
CH2OH
H
OH
H OH
H
HOH
O
CH2OH
H
OH
H OH
HOH
HO
:..
bc
a
β-D-(+)-Glucose
b
ca
Cellobiose
1,41,4’’--β β GlycosidicGlycosidic LinkageLinkage
CellobioseCellobiose
enzymemediated
If continued, youget cellulose.
Humans can’tdigest β-1,4
Two glucose units
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Sucrose (Table sugar)
Sucrose is a disaccharide formed from D-glucose and D-fructose–The glycosidic linkage is between C1 of glucose and C2 of fructose (both anomericcarbon atoms).–Sucrose is a nonreducing sugar because of its acetal linkage
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β-D-fructofuranosyl α-D-glucopyranoside1,2`link
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Polysaccharides
• A polysaccharide is a compound in which the molecule contain many units of monosaccharide joined together by glycoside link.
• Two important polysaccharides are starch and cellulose
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Starch•The two forms of starch are amylose and amylopectin•Amylose consists typically of more than 1000 D-glucopyranoside units connected by α linkages between C1 of one unit and C4 of the next
1,4`α link
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Amylopectin•Amylopectin is similar to amylose but has branching points every 20-25 glucose units–Branches occur between C1 of one glucose unit and C6 of another