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 ActivityThe 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.
CCCCC
O H
CH2OH
OHHOHOH
HHO
HH
D-Glucose
CO H
CH2OH
L-Gluc ose
Mi rror plane
C OHHC HHO
C HHOC HHO
Enantiomers
CCCCC
O H
CH2OH
OHHOHOH
HHO
HH
D-Glucose
CO H
CH2OHL-Mannose
C OHHCH
C HHOC HHO
Diastereomers
OH
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
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”
CHO
~~~Br2 H2O COOH
~~~Ketoses, such as fructose, are not oxidized.
an aldonic acid
Oxidation:
CHOdil. HNO3
COOH
CH2OH COOH
An aldaric acid
Another oxidation:
CHO
~~~
NaBH4CH2OH
~~~
Reduction:
an alditol
CH2OH
~~~
CONaBH4
CH2OH
~~~
CH2OH
~~~
OH 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
Anomeric Configurations (Cont'd)
CH OH2
HOOH
OH
OH
D Glucopyranose- -
OCH3
HO
OH
OH
OH
L- Fucopyranose-
OCH3
HO
OH
OHOH
L - Fucopyranose-
O
The anomer having the same configuration, in the Fischerprojection, at the anomeric and reference carbon (highest num-bered asymmetric carbon) is designated
In the case of aldohexopyranoses, the C-6 substituent is on thesame side of the ring as the anomeric hydroxyl group in the anomer and on opposite sides in the anomer.
Possible conformations: 2 chairs 4 boats 6 skews 12 half chairs
p.213
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Common Alditols
Amino Sugars
Figure 7.14
• Sugars with an amino group at C-2 are amino sugars.• They are found in many oligosaccharides and polysaccharides.
Muramic acid
Figure 7.15 Structure of 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”.)
Sialic acidsThe 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