Isomers, types and examples of isomers, and the structural arrangement of monosaccharides

1. Chemistry of carbohydrates
Part 2
Namrata Chhabra
MD, MHPE
Principal-in charge,
Professor & Head, Department of Biochemistry
S.S.R Medical College,Mauritius

2. Learning objectives
1. What are isomers
2. What is the basis of isomerism in monosaccharides
3. Types and examples of biologically important isomers
4. Structural representation of monosaccharides
5. Reflection
6. Questions

3. Let us first recall the classification of monosaccharides
The monosaccharides can be divided into groups based on the
number of carbon atoms in the molecules, thus:
● trioses have 3-C atoms,
● tetroses have 4-C atoms,
● pentoses have 5-C atoms, and
● hexoses have 6-C atoms.

4. MONOSACCHARIDES - Classification
2- According to nature of reactive group - depending on the
presence of Aldehyde or keto group:
Aldose sugars e.g. glyceraldehyde
Ketose sugars e.g. dihydroxyacetone

5. Classification of monosaccharides
Number of carbon atoms Aldose Ketose
3 (Trioses) Glyceraldehyde Dihydroxyacetone
4 (Tetroses) Erythrose Erythrulose
5 (Pentoses) Ribose, Arabinose,
Xylose
Ribulose, Xylulose
6 (Hexoses) Glucose, Galactose,
Mannose
Fructose
7(Heptoses) - Sedoheptulose

6. Isomers
● Within each of these groups there are different compounds, each
with the same molecular formula.
● As an example glucose, galactose and fructose are hexoses
(C6H12O6) but they have different chemical and physical
properties.
● These types of compounds are called isomers.

7. Isomers
● Any of two or more
compounds with the
same molecular formula
but with different
structure and properties
are called isomers

8. Structural representation of carbohydrates
1) Fischer projection: straight chain representation
2) Haworth projection: simple ring in perspective
Conformational representation:
3) Chair and boat conformations

9. Fischer projection and numbering of carbon atoms
● Carbon atoms are numbered
beginning from the reactive end
of the molecule, the CHO
(aldehyde)
● Each carbon atom is then
numbered in order through the
end of the chain.

10. Haworth projections

11. Chair and Boat Conformations

12. Fischer projection and numbering of carbon atoms
● Carbon atoms are numbered
beginning from the reactive end
of the molecule, the CHO
(aldehyde)
● Each carbon atom is then
numbered in order through the
end of the chain.

13. Asymmetric carbon atom
● The sugar molecules having
asymmetric carbon atoms exhibit
isomerism.
● Asymmetric carbon atom- It is the
carbon atom that is attached to
four different groups

14. Asymmetric carbon atom
All monosaccharides except- Dihydroxyacetone, have asymmetric
carbon atoms

15. Asymmetric carbon atom
In glucose, the position of the OH
group on each of the asymmetric carbon
atoms, numbers two, three, four, and
five could be flipped, producing a
distinct stereoisomer each time, for a
total of 16 or 24 stereoisomers.
However, not all of these actually exist
in nature. For fructose, there are only
three asymmetric carbons, so only 8 or
23 stereoisomers can be produced.

16. Asymmetric carbon atom and isomerism
Based on the presence of asymmetric carbon atoms the following types of
isomerism of monosaccharides are observed in the human system:
1. D&L isomerism
2. Optical Isomerism
3. Epimers
4. Aldose-ketose isomerism
5. Anomers

17. Isomers of Glucose
1. D & L isomers
The orientation of the —H and —OH
groups around the carbon atom
adjacent to the terminal primary
alcohol carbon (carbon 5 in glucose)
determines whether the sugar belongs
to the D or L series.

18. D & L isomers
● The designation of a sugar isomer as the D form
● or of its mirror image as the L form is determined by
● comparison of its spatial relationship to the parent compound of
the carbohydrates,
● the three-carbon sugar glycerose (glyceraldehyde), also called
reference sugar.

19. D-Glyceraldehyde and L-glyceraldehyde are
enantiomers, or mirror images of each other.
These differences do not affect the physical
properties but can affect the biochemical
properties due to changing the shape of the
molecule.
D & L isomers
The L is taken from the Latin word for left, Laever, and the D is taken from the Latin word
for right, Dexter.

20. Biological significance of D & L Isomerism
• Most of the monosaccharides
occurring in mammals are D
sugars, and the enzymes
responsible for their metabolism
are specific for this configuration.
• D-Glucose, D-mannose, and D
-galactose are abundant
six-carbon aldoses.

21. Biological significance of D & L Isomerism
Some sugars naturally occur in
the L form e.g. L-Arabinose and
L-Fucose are found in
glycoproteins
L- Xylulose is produced during
the metabolism of Glucose in
Uronic acid pathway, which is
subsequently converted to its D
form.

22. D and L Isomers of Fructose

23. 2) Optical Isomerism
● The presence of asymmetric carbon atoms also confers optical
activity on the compound.
● When a beam of plane-polarized light is passed through a solution
of an optical isomer, it rotates either to the right, dextrorotatory
(+), or to the left, levorotatory (–).

24. Polarimetry
● Measurement of optical activity
in chiral or asymmetric
molecules using plane polarized
light is called Polarimetry.
● The measurement of optical
activity is done by an
instrument called Polarimeter.

25. 2) Optical Isomerism
● The direction of rotation of polarized light is independent of the
stereochemistry of the sugar, so it may be designated D (–), D (+),
L (–), or L (+).
● For example, the naturally occurring form of fructose is the D (–)
isomer.
● In solution, glucose is dextrorotatory, and glucose solutions are
sometimes known as dextrose.

26. 3) Epimers
● The compounds with the same molecular formula, but differing
in spatial configuration around a single carbon atom are called
epimers.
● In hexoses, Isomers differing as a result of variations in
configuration of the —OH and —H on carbon atoms 2, 3, and 4
are known as epimers.

27. 3) Epimers
Biologically, the most important epimers of glucose are mannose and galactose, formed
by epimerization at carbons 2.

28. 3) Epimers
Mannose and Galactose are not epimers of each other as they differ in
configuration around 2 carbon atoms

29. 4) Aldose-ketose isomerism
● Compounds with the same molecular formula
but differing in nature of functional group
(aldehyde or keto) are aldose ketose isomers.
● Examples- Fructose and Glucose are aldose
ketose isomers.
● Fructose has the same molecular formula as
glucose but it differs in its structural formula,
since there is a potential keto group in position
2, the anomeric carbon of fructose , whereas
there is a potential aldehyde group in position 1,
the anomeric carbon of glucose.

30. 4) Aldose-ketose isomerism
● Glyceraldehyde and Dihydroxyacetone,
● Ribose and Ribulose are other examples of aldose -ketose isomers.

31. 5) Anomers
● In biological system the monosaccharides tend to exist in a ring
form.
● The ring structure of an aldose is a hemiacetal, since it is formed
by combination of an aldehyde (C1) and an alcohol group (Mostly
C5).
●

32. 5) Anomers
The ring can open and
reclose allowing the rotation
to occur around the carbon
bearing the reactive carbonyl
group yielding two possible
configurations- α and β of the
hemiacetal and hemiketal.

33. 5) Anomers
● The carbon about which this
rotation occurs is called Anomeric
carbon and the two stereoisomers
are called Anomers.
● In alpha anomer the orientation
of the OH group is towards the
right side
● whereas in the beta anomer, it is
towards the left side.

34. 5) Anomers
Similarly, the ring
structure of a ketose is a
hemiketal.

35. Haworth projections

36. Rules for drawing Haworth projections
1) Draw either a six or 5-membered ring including oxygen as one
atom.
2) Most aldohexoses are six-membered
aldotetroses, aldopentoses, ketohexoses are 5-membered

37. Rules for drawing Haworth projections
Number the ring clockwise starting next to the oxygen
if the substituent is to the right in the Fischer projection, it will be
drawn down in the Haworth projection (Down-Right Rule)

38. Rules for drawing Haworth projections
In Haworth configuration all
groups to the right of carbon
backbone in Fischer projection are
oriented down while all groups to
the left of carbon backbone are
oriented up, except those around
C5,the reverse orientation occurs.

39. Rules for drawing Haworth projections
When drawn in the Haworth projection, the α configuration places
the hydroxyl downward. While the β is the reverse.

40. Pyranose and Furanose forms of Glucose

41. Pyranose and Furanose forms of Ribose