Carbohydrates Biochemistry

Dr.

1. Carbohydrates z
Polyhydroxycompounds (poly-alcohols) that contain a
carbonyl (C=O) group
z
Elemental composition C
x
(H
2
O)
y
z
About 80% of human caloric intake
z
>90% dry matter of plants
z
Functional properties
–
Sweetness
–
Chemical reactivity
–
Polymer functionality

2. Types of Carbohydrates

3. Monosaccharides
z
Monosaccharidesare categorized by the number of
carbons (typically 3-8) and whether an aldehydeor ketone
z
Most abundant monosaccharidesare hexoses(6 carbons)
z
Most monosaccharidesare aldehydes, i.e. aldoses
C
C
O
H
HOH
C
CO
H
HOH
aldehyde
ketone

4. Fisher projections
OH HOH
H
OH HH HO
C C C C C CH
2
OH
H
O
OH HOH
H
O H HO
C C C C
C H OH
2CH OH
2
1
2
3
4
5
6
D-glucose
(an aldohexose)
D-fructose
(an ketohexose)
1
2
3
4
5
6

7. Cyclic Forms
z
Lowest energy state
H
C
C
C
O
C
C
OH
HO
OH
HO
H
H
H
CH
2
OH
H
1
2
3
4
5
6
β-D-glucopyranose (glucose)
—an aldose
a hexose
an aldohexose
—C
1
chairconformation
β-D-fructopyranose (fructose)
—a ketose
a hexulose
a ketohexose
—
1
C chairconformation
C
C
O
C
C
H
H
C
OH
H
OH
H
OH
OH
CH
2
OH
1
2
3
4
5
6
H

8. Ring Nomenclature z
pyranoseis a six-memberedring (a very stable
form due to optimal bond angles)
z
furanoseis a five-memberedring

9. Chirality
z
Geometric property of a rigid object (or spatial
arrangement of atoms) of being non-super-imposable
on its mirror image
OH HOH
H
OH HH HO
CCCCCC
its mirror image
is an "optical
isomer"
H
2
OH
H
O
4 chiralcenters
e.g. at C2 carbon: This
structure has a non-
superimposablemirror image
CHO
(CHOH)
3
CH
2
OH
OH H
C2

10. Isomers z
Isomersare molecules that have the same chemical
formula but different structures
z
Stereoisomerdiffers in the 3-D orientationof atoms
z
Diastereomersare isomers with > 1 chiralcenter.
–
Pairs of isomers that have opposite configurations at one or
more of the chiral centers but that are not mirror images of
each other.
z
Epimersare a special type of diastereomer.
–
Stereoisomers with more than one chiral center which differ in
chirality at only one chiral center.
–
A chemical reaction which causes a change in chirality at one
one of many chiral center is called an epimerisation.

11. Enantiomers z
Isomerism in which two isomers are mirror
images of each other. (D vsL)

13. Anomer
z
An anomeris one of a special pair of
diastereomericaldosesor ketoses
–
differ only in configuration about the carbonylcarbon
(C1 for aldoses and C2 for ketoses)

14. Carbonyl Group z
Carbonyl groups subject to nucleophilicattack,
since carbonyl carbon is electron deficient:
–
-OHgroups on the same molecule act as
nucleophile, add to carbonyl carbon to recreate ring
form

15. OH
H
O
O
H
O
H
O
H
OH
O
O
5
5
5
5
1
1
1
1
αanomerβanomer
Carbonyl carbon freely rotates
→Ocan attack either side

16. Specification of Conformation, chirality
and anomericform of sugars
z
Determination of chair conformation
–
Locate the anomeric carbon atom and determine if numbering
sequence is clockwise (n= +ve) or counterclockwise (n= -ve).
–
Observe if the puckered ring oxygen atom lies “above” (p= +ve)
the plane of the ring or below (p= -ve).
–
Multiply n*p. If the produc t is +ve then C1, -ve then 1C
z
Determination of chiralfamily
–
Locate the reference carbon atom contained within the ring and
determine whether the bulky substituent (OH or CH
2
OH) is
equatorial (r= +ve) or axial (r= -ve).
–
Multiply n*p*r. If product is +ve the chiral family is D, when it is
–ve the chiral family is L

17. z
Determination of Anomeric form:
–
Determine if the hydroxyl substituent on the anomeric carbon atom is
equatorial (a= +ve) or axial (a= -ve).
–
Multi[ly (n*p) by (n*p*r) by a. When the product is positive, the anomer
is β; when the product is negative the anomer is α
Specification of Conformation, chirality
and anomericform of sugars

18. Mutarotation
z
The α- and β- anomers of carbohydrates are typically stable solids.
z
In solution, a single molecule can interchange between
–
straight and ring form
–
different ring sizes
–
αand βanomeric isomers
z
Process is
–
dynamic equilibrium
–
due to reversibility of reaction
z
All isomers can potentially exist in solution
–
energy/stability of different forms vary

19. Mutarotation: interconversionof α-
and β-anomers
z
For example, in aqueous solution, glucose exists as a
mixture of 36% α-and 64% β-(>99% of the pyranose
forms exist in solution).

20. AnomerInterconverision
0
10
20
30
40
50
60
70
80
% of all isomers D-glucose D-fructose D-mannose D-galactose
α-pyranose β-pyranose α-furanose β-furanose
Generally only a few isomers predominate

21. TIME
+57.2
o
+112
o
+19
o
pure
α
-D-(+)-glucopyranose
1
pure
β
-D-(+)-glucopyranose
2
[
α
]
D
66%
β
34%
α
(min)

23. OH
HO
OH
O
O
HO
HO
OH
OH
O
HO
OH
OH
OH
H
C
OH OH
H
OH
H
CH
2
OH
OH H
O
OH
O
OH
OH
H
OH OH
H
OH
H
CH
2
OH
HO
OH
O
H
Mutarotationof ribose
hydrate(0.09%)
keto-form (0.04%)
α-pyranose (20.2%)
α-furanose (7.4%)
β-pyranose (59.1%)β-furanose (13.2%)

24. Stability of
Hemiacetals/Hemiketals z
As general rule the most stable ring
conformation is that in which all or most of
the bulky groups are equatorial to the axis of
the ring

25. Reactions
z
Isomerization
glucose fructose mannose
z
Oxidation
R-CHO R-COOH
R-CH
2
OH R-COOH
z
Reduction
sugar sugar alcohols
z
Acetalformation
sugar glycoside
z
Browning reactions
O
CCCCCC
HO
H
OH
H
2
OH
OHOH
HH
H
H
carbonyl group is key

26. Isomerization z
Isomerizationis possible because of the
“acidity”of the αhydrogen
O
C C C CC C
HO
H
OH
H
2
OH
OHOH
H H
H
H
αhydrogen
(on C next
to carbonyl)
O
C C C C CC
HO
OH
H
2
OH
OHOH
H H
H
H
O
C C C C C C
HO
OH
H
2
OH
OHOH
H H
H
H
base
keto form enol form

27. Isomerization
O
C C C C C C
HO
OH OH
H
2
OH
OH
H
H H
H
H
O
C C C C C C
HOHO
OH
H
2
OH
OH
H
H H
H
H
C C C C C C
HO
O
OH
H
2
OH
OH
H H
H
H
2
OH
D-glucose D-mannose D-fructose

28. Oxidation/Reduction
Oxidation
Increase oxygen or decrease hydrogen
Increase oxidation state
Remove electrons
Reduction
Decrease oxygen or increase hydrogen
Decrease oxidation state
Add electrons

29. Oxidation z
Carbonyl group can be oxidized to form
carboxylic acid
z
Forms “-onicacid”(e.g. gluconicacid)
z
Can not form hemiacetal
z
Very hydrophillic
–
Ca gluconate
z
Can react to form intramolecularesters:
–
lactones

30. Oxidation z
Also possible to oxidize alcohols to carboxylic
acids
–
“-uronicacids”
z
Galacturonic acids
z
Pectin
z
Reactivity
–
Aldehydesare more reactive than ketones
z
In presence of base ketones will isomerize
z
Allows ketones to oxidize

31. Reducing sugars z
Reducing sugarsare carbohydrates that can
reduce oxidizing agents
z
Sugars which form open chain structures
with free carbonyl group
z
Reduction of metal ions
–
Fehlingtest: CuSO
4
in alkaline solution

32. DNSA assay
• Colorimetric analysis: the sugars present reduce
3,5-dinitrosalicylic acid, DNSA, to 3-amino-5-
nitrosalicylic acid

33. Reduction z
Carbonyl group can be reduced to form alcohol
–
hydrogenation reaction
z
Forms sugar alcohol (“-itol”)
–
glucose glucitol(akasorbitol)
–
mannose mannitol
–
xylose xylitol
z
Sweet, same calories as sugar, non-cariogenic
z
Very hydrophillic
z
Good humectants

34. AcetalFormation z
In acid solution, sugars can react with alcohols
to form acetalsknown as glycosides
z
Reaction is a nucleophilicaddition of two
alcohols to aldehydes CH
2
OH
+ ROH
OH
+H
2
O
H
+
O
CH
2
OH
OR
O

35. 1. Protonation of OH
group
2. water removal to
form carbocation
3. alcohol addition and
release of proton
AcetalFormation

36. Sucrose Maltose
Trehalose Cellobiose

39. Stability of acetals z
Pyranose>>>> Furanose
z
β-glycosidic> α-glycosidic
z
1,6>1,4>1,3>1,2
z
Allow to predict stability of glycosidiclinkages
in terms of their resistance to hydrolysis
–
Gentiobiose

40. Acid catalyzed Rxns z
Acid hydrolysis of hemiactalsand hemiketals
(mutarotation)
z
Anhydrosugars
–
1C conformation
z
Reversion sugars
–
Formation of oligosaccharides under conditions of high sugar
concentration, dilute acid……. Maple syrup, fruit juice
concentrates
–
Detection of invert sugar in juices/honey
z
Enolizationand Dehydration
–
Formation of 3-deoxyosones and HMF/furfural

41. z
Hydrolysis of hemiactalsand hemiketals
(mutarotation)
–
Base catalyzed loss of H from anomeric–OH
z
Acetalsand Ketalsare stable
–
Sugar esters will be hydrolyzed in alkali
z
Enolization
–
Favored by alkali
z
Reduction of metal ions
–
Alkali prevents hydrolysis of non-reducing sugar
Base catalyzed Rxns