Transcript Carbohydrates

Carbohydrate
Basics
Carbohydrates (Overview):
- essential components of all living things.
- most abundant of the four major classes of biological molecules (other
classes are proteins, nucleic acids and lipids).
- range in complexity from simple 3-carbon sugars to enormous complexes
containing hundreds of polysaccharide chains linked to a protein core.
- serve as metabolic intermediates, energy stores, and fuels.
- components of DNA and RNA (ribose and deoxyribose).
- major structural components of plants and animals
- bound to proteins (glycoproteins) and lipids (glycolipids).
- are important for an enormous number of vital cellular functions, including
cell-cell recognition and signaling.
Aldoses & Ketoses
Chirality & Optical Activity
The term chiral is used to describe an object (molecule)
which is non-superimposable on its mirror image.
Optical activity is the ability of a
chiral molecule to rotate the plane of
plane-polarized light, measured using
a polarimeter. A simple polarimeter
consists of a light source, polarizing
lens, sample tube and analysing lens.
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Emil Fischer
Enantiomers and Diastereomers
Molecules with the same molecular formulas and order of
attachment of constituent atoms but which differ in the
arrangement of these atoms in space are called
stereoisomers.
Enantiomers are stereoisomers that are mirror images of each
other.
The mirror image of an enantiomer cannot be
superimposed on itself. In general, enantiomers have identical
chemical and physical properties and racemic mixtures of
enantiomers are difficult to separate into the individual
enantiomers.
Diastereomers are stereoisomers that are not mirror images
of each other. In general they have different chemical and
physical properties and can be separated from each other. If
they differ at one chiral center then they are epimers.
O
H
C
HO C H
H C OH
O
H
C
H C OH
HO C H
HO C H
HO C H
CH2OH
H
H
L-Gluc ose
C OH
C OH
CH2OH
Enantiomers
D-Glucose
Mi rr or p la n e
O
C
H
H C OH
H C OH
HO C H
HO C H
CH2OH
L-Man nose
O
H
C
H C OH
HO C H
H C OH
H C OH
Diastereomer s
CH2OH
D-Glucose
Racemic mixtures of enantiomers are often separated by derivatizing the mixture
with a chiral reagent thereby converting the components to a mixture of
diastereomers which can then be separated. The original compounds are then
regenerated.
Resolution of a Racemic Mixture
Jean Baptiste Biot's young protegé, Louis
Pasteur, was the first person to separate a
racemic mixture of optical isomers.
He initially used tweezers to physically
separate left- and right-handed crystals of
tartaric acid.
Pasteur subsequently devised the still
widely used procedure for resolving a
mixture of optical isomers based on
converting them to diastereomers with
different chemical and physical properties.
After separation of the diastereomers the
original optical isomers are then
regenerated..
Louis Pasteur
Oxidation:
CHO
~~~
Br2 H2O
COOH
an aldonic acid
~~~
Ketoses, such as fructose, are not oxidized.
However, in alkaline solution both aldoses and
ketoses are oxidized by either Ag+ (to give
metalic silver) or by Cu+2 to to give Cu+.
“reducing sugars”
Another oxidation:
CHO
COOH
dil. HNO3
CH2OH
COOH
An aldaric acid
Reduction:
CHO
NaBH4
~~~
~~~
CH2OH
CO
~~~
CH2OH
an alditol
CH2OH
NaBH4
OH
~~~
CH2OH
+
HO
~~~
Fig. 7-05, p.207
Fig. 7-05, p.207
Fehlings reaction for aldehyde
(can also detect ketones after tautamerization)
Fischer Projection Formulas
Fig. 7-06, p.208
Fig. 7-06, p.208
Fig. 7-06, p.208
Fig. 7-06, p.208
An omeric Co n figu ratio n s (Co n t'd)
Th e an o m er h av in g t h e sam e co n figu r at io n , in t h e Fisc h er
p r o ject io n , at t h e an o m er ic an d r efer en ce c ar b o n (h igh est n u m b er ed asy m m et r ic car b o n ) is d esign at ed 
In t h e case o f ald o h exo p y r an o ses, t h e C-6 su b st it u en t is o n t h e
sam e sid e o f t h e r in g as t h e an o m er ic h y d r o xy l gr o u p in t h e 
an o m er an d o n o p p o sit e sid e s in t h e  an o m er .
CH2OH
O
O
OH
CH3
OH
HO
- D-Glucopyranose
CH3
OH
HO
OH
O
OH
HO
OH
-L-Fucopyr anose
OH
OH
OH
 -L -Fucopyr anose
Possible conformations:
2 chairs
4 boats
6 skews
12 half chairs
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p.213
Common Alditols
Amino Sugars
• Sugars with an amino group at C-2 are amino sugars.
• They are found in many oligosaccharides and polysaccharides.
Figure 7.14
Muramic acid
Muramic acid is a component of the
polysaccharides of cell membranes
of higher organisms and also
bacterial cell walls.
Muramic acid is a glycosamine
linked to a 3-carbon acid at C-3.
(Murus is Latin for “wall”.)
Figure 7.15 Structure of muramic acid.
Sialic acids
The N-acetyl and N-glycolyl derivatives of neuraminic acid are
known as sialic acids.
Figure 7.15 Two depictions of a sialic acid.
Fig. 7-19, p.218