1. Chemistry of Carbohydrates
R.C. Gupta
Professor and Head
Dept. of Biochemistry
National Institute of Medical Sciences
Jaipur, India

2. Carbohydrates Synthesized in plants by photosynthesis
Used as source of energy by animals
Largest source of energy in our daily diet
Some perform other functions also

3. Constituents of nucleic acids Ribose and deoxyribose
Constituents of nervous tissue
Glycolipids
Form some hormones and blood group substances
Glycoproteins
Mucin
Constituent of mucus
Mucopoly-saccharides
Structural constituents of tissues 3 3 3 3

4. Definition
Carbohydrates are polyhydroxyaldehydes or polyhydroxyketones or compounds that give polyhydroxyaldehydes or poly-hydroxyketones on hydrolysis
Carbohydrates are aldehyde or ketone derivatives of polyhydric alcohols OR

5. Carbohydrates can be classified into:
Classification
Carbohydrates can be classified into:
Monosaccharides
Disaccharides
Polysaccharides

6. Monosaccharides are the smallest carbo-hydrates
They can’t be hydrolysed into smaller carbohydrates
Made up of carbon, hydrogen and oxygen
Have general formula CnH2nOn

7. Disaccharides are made up of two monosaccharides
The constituent monosaccharides may be identical or different
The common disaccharides are sucrose, lactose and maltose
Monosaccharides and disaccharides are called sugars because of their sweet taste

8. Polysaccharides are made up of a large number of monosaccharide molecules
Those having 3-6 monosaccharide units are called oligosaccharides
Those having more than 6 monosaccharide units are called polysaccharides

9. Accordingly, they can be divided into aldoses and ketoses
Monosaccharides
Monosaccharides may be aldehyde or ketone derivatives of polyhydric alcohols
Accordingly, they can be divided into aldoses and ketoses

10. Monosaccharides having an aldehyde group
Aldoses
Monosaccharides having an aldehyde group
Ketoses
Monosaccharides having a keto group

11. Trioses - Three carbon atoms
Aldoses and ketoses may be sub-divided on the basis of number of carbon atoms:
Trioses - Three carbon atoms
Tetroses - Four carbon atoms
Pentoses - Five carbon atoms
Hexoses - Six carbon atoms
Heptoses - Seven carbon atoms

12. Some common monosaccharides
No. of
carbon
atoms
Trioses Glyceraldehyde Dihydroxyacetone
Tetroses Erythrose Erythrulose
Pentoses Ribose Ribulose
Hexoses Glucose Fructose
Aldose Ketose

13. Trioses The smallest monosaccharides
Include glyceraldehyde and dihydroxyacetone
Aldehyde and ketone derivatives of trihydric alcohol, glycerol
Formed during metabolism of hexoses

14. | | | C1 CH2OH CHO CH2OH C2 CHOH CHOH C = O | | | C3 CH2OH CH2OH CH2OH
  | |   |
C2 CHOH CHOH C = O
  | | |
C3 CH2OH CH2OH CH2OH
Glycerol Glyceraldehyde Dihydroxyacetone

15. Glyceraldehyde and dihydroxyacetone share the same molecular formula (C3H6O3)
They differ in their structural formulae
They are isomers of each other
This is a simple aldose-ketose isomerism

16. Glyceraldehyde shows another type of isomerism
It contains an asymmetric carbon atom
All the four groups attached to C2 are different from each other
This produces two stereo-isomers of glyceraldehyde

17. D-Glyceraldehyde L-Glyceraldehyde
The ‒OH group is on right hand side of carbon 2
D-Glyceraldehyde
The ‒OH group is on left hand side of carbon 2
L-Glyceraldehyde

18. CHO CHO
| |
H—C—OH HO—C—H
       CH2OH CH2OH
D-Glyceraldehyde L-Glyceraldehyde

19. Asymmetric carbon atom also confers optical activity
On passing polarised light, its plane is rotated to the left or the right
One stereoisomers causes laevorotation (rotation to left) and the other causes dextrorotation (rotation to right)

20. Stereoisomerism and optical activity are present in higher monosaccharides also
The higher monosaccharides possess more than one asymmetric carbon atoms
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They have several stereoisomers and optical isomers

21. The D/L assignment depends upon the orientation of –OH group relative to the asymmetric carbon atom most remote from the aldehyde or the ketone group
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This will be carbon atom 3 in tetroses, carbon atom 4 in pentoses and carbon atom 5 in hexoses

22. If the –OH group is on the right of these carbon atoms, the isomer will be D
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If the –OH group is on the left, the isomer will be L
Most of the carbohydrates important in human biochemistry are D-isomers

23. A method to show the configuration of mono-saccharides on paper was devised by Emil Fischer
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His formulas are known as Fischer projection formulas
The monosaccharides are shown as linear molecules in these formulas

24. The orientation of the chain is such that carbon 1 (C1) is at the top and ‒CH2OH at the bottom
All the bonds are shown by horizontal or vertical lines
The chain of carbon atoms is shown to be oriented vertically
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25. The hydrogen atoms and hydroxyl groups are on left or right of the carbon atoms
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In aldoses, the carbon of the aldehyde group is C1
In ketoses, the carbon of the keto group is C2

26. Tetroses
The only tetrose of some importance in human beings is D-erythrose
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This is formed as an intermediate (as erythrose-4- phosphate) during the metabolism of glucose via the hexose monophosphate shunt pathway
The corresponding ketotetrose is D-erythrulose

28. Pentoses
D-Ribose and 2-deoxy-D-ribose are the most important pentoses which are the constituents of nucleic acids and nucleotides
E M B - R C G
D-Ribose and its corresponding ketopentose, D-ribulose are formed as intermediates in the hexose monophosphate shunt

30. Another pentose formed in HMP shunt pathway is D-xylulose
E M B - R C G
Its corresponding aldopentose is D-xylose
D-Xylose is used as a diagnostic agent to study intestinal absorption

31. E M B - R C G An L-pentose occurring in human beings is L-xylulose
L-Xylulose is formed as an intermediate in the uronic acid pathway of carbohydrate metabolism
It is excreted in urine in detectable amounts in a hereditary disease, essential pentosuria

33. Hexoses
The important aldohexoses in human beings are D-glucose, D-galactose and D-mannose
E M B - R C G
The important ketohexose is D-fructose which is the ketoisomer of D-glucose

34. D-Glucose is the most important carbohydrate in human beings
CHO |
H — C — OH
HO — C — H
D-Glucose
CH2OH

35. The carbohydrates are transported in blood in the form of D-glucose
This is the form in which carbohydrates are used by the tissues to obtain energy
Most other carbohydrates are converted into D-glucose in the body
The important polysaccharides, starch, dextrin and glycogen are made up of D-glucose
E M B - R C G

36. D-Galactose is present in glycolipids which are an important constituent of nervous tissue
It is also present in milk in the form of the disaccharide, lactose
Amino derivatives of D-galactose and D-mannose are present in mucopolysaccharides and glyco-proteins
E M B - R C G

38. Formed in some path- ways of carbohydrate metabolism
Also present in seminal fluid; provides nourish- ment to sperms

39. Heptoses
The only heptose important in human beings is D-sedoheptulose which is a ketoheptose
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It is formed as an intermediate in HMP shunt pathway of carbohydrate metabolism

41. Anomerism
Aldose-ketose isomerism, stereoisomerism and optical isomerism have been seen earlier
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These can be explained easily by Fischer projection formulas
A problem arose with the discovery of two different methyl glucosides derived from glucose.

42. Glucose reacts with methanol in the presence of a mineral acid to form two distinct methyl glucosides
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One is known as methyl-a-D-glucoside and the other is known as methyl-b-D-glucoside
Both have cyclic structures

43. C |
H — C — OH
HO — C — H
H — C
CH2OH
Methyl-a-D-glucoside Methyl-b-D-glucoside O
H O‒CH3
H3C‒O H

44. It was later found that higher monosaccharides also exist in solution in a cyclic hemi-acetal form
Ring is formed by a reaction between carbonyl group and the ‒OH group attached to C4 or C5
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If cyclisation involves C4, it results in the formation of a five-membered ring similar to furan
A monosaccharide having this type of ring structure is designated as a furanose

45. If cyclisation involves C5, it results in the formation of a six-membered ring
This ring (cyclic 1,5-oxide) is similar in structure to pyran
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Therefore, a monosaccharide having this type of ring structure is called a pyranose

46. Furan
Pyran

47. For showing the ring form of monosaccharides on paper, Haworth introduced a projection formula
In this representation, the plane of the ring is perpendicular to the plane of the paper
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The substituent groups project upwards or downwards from the ring
The ring oxygen is away from the viewer

48. This is C1 in case of aldoses and C2 in case of ketoses
Cyclisation creates an additional asymmetric carbon atom in the molecule.
This carbon is known as anomeric carbon atom
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49. The anomeric carbon produces an additional type of isomerism called anomerism
The additional isomers are called a-anomer and b- anomer
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In a-anomer, ‒OH group attached to anomeric carbon projects below the plane of the ring
In b-anomer, it projects above the plane of the ring

50. The a and b anomers of glucose in pyran ring form
CH2OH H OH O 1 2 4 5 6 3
Pyran a-D-Glucopyranose b-D-Glucopyranose ↑ ↓
The a and b anomers of glucose in pyran ring form

51. Ketohexoses exist in the form of a five membered ring resembling furan
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The monosaccharides in furan ring form also exhibit anomerism

52. The additional centre of asymmetry in keto- hexoses is at carbon atom 2
The ‒OH group attached to C2 projects below the plane of the ring in the a-anomer
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It projects above the plane of the ring in the b-anomer

53. 6 1 5 2 ↓ 4 3 6 ↑ 5 2
CH2OH 4 3 1

54. Aldopentoses, e.g. ribose, also exist in the form of five membered furan ring form
Sometimes even aldohexoses exist in furan ring form
E M B - R C G

55. 5 H a-D-Ribofuranose OH HOH2C O 4 1 6 3 2 CH2OH H OH a-D-Glucofuranose| H OH
a-D-Glucofuranose
H‒C‒OH O 5 4 1 5 3 2 OH H
b-D-Ribofuranose
HOH2C O 4 1 3 2

56. Mutarotation
Carbohydrates possessing an asymmetric carbon atom are optically active
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The specific rotation caused by each carbo- hydrate is quite characteristic
Before the ring structures of carbohydrates were established, it had been shown that glucose existed in two optically distinct forms

57. When glucose, crystallized from alcohol-water, is dissolved in water, its specific rotation is +112°
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When glucose, crystallized from a concentrated aqueous solution at 110°C, is dissolved in water, it has a specific rotation of +19°
When either form is allowed to stand, the specific rotation gradually changes to +52.5°, and then becomes constant

58. This change in specific rotation is known as mutarotation
E M B - R C G
On discovery of ring structures of carbohydrates, it was found that the glucose crystallized from alcohol-water is a-D-glucose
Glucose crystallized from a concentrated aqueous solution at 110°C, is b-D-glucose

59. E M B - R C G a-D-Glucose has a specific rotation of +112°
b-D-Glucose has a specific rotation of +19°
E M B - R C G
On standing, a-D-glucose changes into b-D-glucose and vice versa
The inter-conversion continues until an equilibrium mixture is formed

60. The equilibrium mixture contains 36% a-D- glucose and 64% b-D-glucose
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This equilibrium mixture has a specific rotation of °

61. Interesting, right?
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