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.

C

C

C

C

C

O H

CH2OH

OH

H

OH

OH

H

HO

H

H

D-Glucose

CO H

CH2OH

L-Gluc ose

Mi rr or plane

C OHH

C HHO

C HHO

C HHO

Enantiomers

C

C

C

C

C

O H

CH2OH

OH

H

OH

OH

H

HO

H

H

D-Glucose

CO H

CH2OH

L-Mannose

C OHH

CH

C HHO

C 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:

CHO

dil. 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- -

O

CH3

HO

OH

OH

OH

L- Fucopyranose-

O

CH3

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