Carbohydrates - Introduction

A. Polyhydroxy Aldehydes or Ketones

B. Serve a variety of functions

1. Energy storage (Glucose, Glycogen, Starch) 2. Structural Support (Cellulose, Chitin) 3. Biochemical Control (DNA, Glycoproteins)

C. “Hydrates of Carbon” - General formula: Cn(H2O)n, n=1,2,3....

D. “Saccharides” - from Latin saccharum (sugar)

1. Monosaccharides - “simple” sugars 2. Disaccharides, trisaccharides, tetrasaccharides,... 3. Oligosaccharides 4. Polysaccharides

Carbohydrates - General Description

CARBON CHAIN C

OHO OH

HO OH CARBON CHAIN

OH OH

HO OH

CARBON CHAIN C

O

H

HO OH

HO OH

Carbohydrates - General NomenclatureA. “-OSE” ending - Glucose, Galactose, Sucrose, Cellulose

B. Classification by Number of Carbons

1. C3H6O3 - Tri + ose = Triose 2. C4H8O4 - Tetra + ose = Tetrose 3. C5H10O5 - Penta + ose = Pentose

C. Classification by Functional Groups

1. Aldoses contain the aldehyde functional group. 2. Ketoses contain the ketone functional group.

D. Combining “A”, “B”, and “C”

an aldopentose a ketohexose

HO

H2C

HCCH

HCC

O

HOH

OH

OH

H2CCH H

CC H

CCH2

O OH

OHOH

OH

OH

A. Simple Monosaccharide Structures

D-Glyceraldehyde Dihydroxyacetone (A simple aldose) (A simple ketose)

B. Note: Glyceraldehyde exists in two “enantiomeric” forms

Carbohydrates - Simple Structures

“D”

HOOH

A. Simple Monosaccharide Structures

D-Glyceraldehyde Dihydroxyacetone (A simple aldose) (A simple ketose)

B. Note: Glyceraldehyde exists in two “enantiomeric” forms

Carbohydrates - Simple Structures

“D” “L”

Stereochemistry - “Handedness” in Organic Compounds

Enantiomers - compounds that have the following characteristics:

1) Molecules of two compounds are mirror images of each other.

2) Molecules of two compounds are nonsuperimposable.

The characteristics of enantiomers are often the

result of a single “chiral” carbon atom.

Chirality and Optical Activity

Chiral compounds exhibit a property called optical activity and are said to be optically active.

Achiral molecules are optically inactive.

Optical activity is the ability of a compound to rotate the plane of plane-polarized light.

Chirality and Optical Activity

Chirality and Optical Activity

A chiral compound rotates the plane of plane-polarized light.

Chirality and Optical Activity

A sample tube containing an optically active

compound is placed between the observer and

plane-polarized light.

An analyzing filter allows the observer to quantitate

the degree to which the plane-polarized light is

rotated.

Optical Activity - The Polarimeter

Optical Activity of Enantiomers

Optical activity is the only property that distinguishes one enantiomer from the other. All other properties (MP, BP, solubility, etc.) are the same.

D-Lactic Acid L-Lactic Acid

m.p. 53 oC m.p. 53 oC

Very soluble in water Very soluble in water

[α] = - 2.6 o [α] = +2.6 o

Naming Chiral Compounds

1) The two enantiomers of a pair of enantiomers rotate the plane of plane-polarized light the same number of degrees but in opposite directions.

2) Rotation in the clockwise direction is called dextrorotatory (“+”) and in the anticlockwise direction, levorotary (“-”).

3)“D-” and “L-” are not directly related to (+) and (-), but are designations developed in carbohydrate chemistry to indicate a certain relationship between hydroxy groups.

4) When both the configuration and the optical rotation are known for a compound, both are indicated in the name:

D-(+)-glyceraldehyde D-(-)-fructose

5) “R-” and “S-” designate specific relationship between atoms attached to a “chiral” carbon atom and are based on priority rules discussed in more detail in you text.

Chiral Recognition

Most reactions in living cells are catalyzed by protein molecules called enzymes. (Enzymes are a subset of a group of biological macromolecules referred to as “receptors.”)

Enzymes are large molecules containing a surface site, called the active site, where the substrate or reactant binds. For most enzymes this site is chiral.

Chiral active sites are designed to interact strongly with one of the two possible substrate enantiomers, just as a left shoe interacts strongly with a left foot, and more or less excludes a right foot.

This phenomenon is called chiral recognition or chiral discrimination.

Other molecular properties are also used by enzymes to discriminate between substrate molecules: geometrical isomerism, size, shape, polarity, and charge.

Discrimination on these bases is called the complementarity principle.

Chiral Recognition

Chiral Recognition

Chiral Compounds - Biological Activity

Chiral Compounds - Biological Activity

Carbohydrates - Simple Structures

Important Aldopentoses

C

C

C

OHH

OHH

C

CH2OH

OHH

O HC

C

C

HH

OHH

C

CH2OH

OHH

O H

D-ribose 2-deoxy-D-ribose

Glucose and Galactose are epimers

Important Aldohexoses

D-glucose D-galactose

C

C

C

OHH

HHO

C

C

HHO

O H

CH2OH

OHH

C

C

C

OHH

HHO

C

C

OHH

O H

CH2OH

OHH

Carbohydrates - Simple Structures

An Important Ketohexose

D-fructose

CH2OH

C

C

O

HHO

C

C

OHH

CH2OH

OHH D-glucoseD-fructose

Comparison of the structures of D-fructose and D-glucose

CH2OH

C

C

O

HHO

C

C

OHH

CH2OH

OHH

C

C

C

OHH

HHO

C

C

OHH

O H

CH2OH

OHH

Carbohydrates - Simple StructuresHow to remember them

D-galactoseD-glucose

CHO

CH2OH

OHH

HHO

OHH

OHH

D-fructose

CH2OH

C

C

O

HHO

C

C

OHH

CH2OH

OHH

C

C

C

OHH

HHO

C

C

OHH

O H

CH2OH

OHH

D-glucose

C

C

C

OHH

HHO

C

C

OHH

O H

CH2OH

OHH

D-glucose

C

C

C

OHH

HHO

C

C

HHO

O H

CH2OH

OHH

C

Carbohydrates - Simple Structures

What is L-Glucose ?D-Glucose and L-Glucose

are enantiomers

Most naturally occurring monosaccharides are from

the D-series.

D-Glucose

C

C

C

OHH

HHO

C

C

OHH

O H

CH2OH

OHH

L-Glucose

C

C

C

HHO

OHH

C

C

HHO

O H

CH2OH

HHO

Emil Fischer (1852-1919)

1902 Nobel Prize

in Chemistry

Absolute Configurations

of the D-Aldoses

Absolute Configurations

of the D-Ketoses

Carbohydrates - Cyclic Structures

Previously Studied Reactions of Aldehydes and Ketones

Carbohydrates - Cyclic StructuresCyclic Forms of Glucose

1

5

1

6

1

5

cyclic hemiacetal formation

anomers

6

6

6

1

6

5

1

6

5

Carbohydrates - Cyclic Structures

In solution, glucose exists in a variety of forms.

Glucose is a reducing sugar

Carbohydrates - Reducing Sugars

Cu2+ Cu2O

The small amount of aldehyde present in a glucose solution is sufficient to reduce the copper ion from 2+ to 1+.

Carbohydrates - Cyclic Structures

Cyclic Forms of Other Carbohydrates

Carbohydrates - Cyclic Structures

OH

HO

H

HO

H

HOHH OH

OH

OHO

H

H

HO

H

HOHH OH

OH

β-D-Glucopyranose β-D-Galactopyranose

Carbohydrates - Reducing Sugars

Because they exist to a small extent in the aldehyde form in solution, most other simple sugars are reducing sugars.

Carbohydrates - Reducing Sugars

Because they exist to a small extent in the aldehyde form in solution, most other simple sugars are reducing sugars.

Carbohydrates - Glycoside Formation

glucopyranoside{methyl

β

D

Glycoside Nomenclature

Carbohydrates - Glycoside Formation

The linkage between the sugar and the alcohol is called a glycosidic linkage and must be designated as α or β. Glycosides of simple monosaccharides are not reducing sugars because the aldehyde functional group is no longer present. The alcohol which attaches through the glycosidic can be another carbohydrate.

Carbohydrates - Glycoside Formation

Carbohydrates - Sugar Phosphates

R O P

O

O

O

phosphate ester

Carbohydrates - Acidic Sugars

O

CH2OH

OHOH

OH

OH

O

COOH

OHOH

OH

OH

α-D-Glucose

α-D-Glucuronic Acid A building block for

hyaluronic acid

Carbohydrates - Amino Sugars

O

CH2OH

NH2

OH

OH

OH

O

CH2OH

HNOH

OH

OH

C OH3C

α-D-Glucosamine N-acetyl-α-D-Glucosamine Building block for chitin, the

exoskeleton of crustaceans

Carbohydrates - Sugar Sulphates

O

C

OOH

OH

OH

SO

O-O

O O-

O

CH2O

HNOH

OH

OH

SO

O

O-

SO

O-Oα-D-Glucuronic Acid 2-sulfate

N-sulfo-α-D-Glucosamine 6-sulfate

Building blocks for heparin

Monosaccharides - Physical Properties

1) Most monosaccharides are crystalline solids at room temperature and are very soluble in water where they can form highly viscous solutions.

2) Monosaccharides are slightly soluble in alcohols (methanol, ethanol) and are insoluble in less polar solvents (ethers, hydrocarbons).

3) Many monosaccharides taste sweet.

4) A solution of a reducing sugar may contain a mixture of α anomers, β anomers, and acyclic structures. The structures rapidly interconvert to form an equilibrium mixture. (This interconversion is called mutarotation.)

5) Usually, only a single form of a carbohydrate is drawn when drawing a carbohydrate in solution.

Mutarotation of Glucose

alpha anomer beta anomer“oxo” form

[α]D = +112º [α]D = +19º

Equilibrium Mixture[α]D = +53º36% 64%

Mutarotation of Glucose