Carbohydrates

CARBOHYDRATES

polyhydroxy aldehydes and ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones.

Most abundant class of biological molecules on Earth

Originally produced through CO2 fixation during photosynthesis

Empirical formula = (CH2O)n

ROLES OF CARBOHYDRATES Energy storage (glycogen,starch) Structural components (cellulose,chitin) Cellular recognition Carbohydrate derivatives include DNA, RNA,

co-factors, glycoproteins, glycolipids

CARBOHYDRATES CLASSIFICATION

Monosaccharides (simple sugars) cannot be broken down into simpler sugars under mild conditions

Oligosaccharides = "a few" - usually 2 to 10 monosaccharides. It is further divided into disaccharides or trisaccharides.

Polysaccharides are polymers of the simple sugars with high molecular weight. It is of two types, Homopolysaccharides and heteropolysaccharides

CLASSIFICATION OF MONOSACCHARIDES

Monosaccharides are classified according to:(1) The number of carbon atoms present in the

molecules.(2) whether they contain an aldehyde or keto group.

Monosaccharides

Aldoses (-HC=O)

Ketoses(-C=O)

Trioses (C3H6O3) Glyceraldehyde Dihydroxyacetone

Tetroses (C4H8O4)

Erythrose Erythrulose

Pentoses (C5H10O5)

Ribose Ribulose

Hexoses (C6H12O6)

Glucose Fructose

MONOSACCHARIDES

C

C*

O

C*

C*

CH2OH

H OH

HO H

H OH

H

CH2OH

C

C*

C*

CH2OH

O

HO H

H OH

D-ribose D-ribulose

MONOSACCHARIDES ARE CHIRAL Aldoses with 3C or more

and ketoses with 4C or more are chiral

The number of chiral carbons present in a ketose is always one less than the number found in the same length aldose

Number of possible steroisomers = 2n (n = the number of chiral carbons)

C

C*

O

C*

C*

C*

CH2OH

H OH

HO H

H OH

H OH

H

CH2OH

C

C*

C*

C*

CH2OH

O

HO H

H OH

H OH

D-glucose D-fructose

STEREOCHEMISTRY

•Enantiomers = mirror images

•Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers

•Epimers = Two sugars that differ in configuration at only one chiral center

C

C*

O

C*

C*

C*

CH2OH

H OH

HO H

H OH

H OH

H

C

C*

O

C*

C*

C*

CH2OH

HO H

H OH

HO H

HO H

H

C

C*

O

C*

C*

C*

CH2OH

H OH

HO H

H OH

H OH

H

C

C*

O

C*

C*

C*

CH2OH

HO H

HO H

H OH

H OH

H

C

C*

O

C*

C*

C*

CH2OH

HO H

HO H

H OH

H OH

H

C

C*

O

C*

C*

C*

CH2OH

H OH

HO H

HO H

H OH

H

D-glucoseL-glucose

Enantiomers

D-glucose D-mannose

Epimers

D-mannose D-galactose

Diastereomers

D VS L DESIGNATION

D & L designations are based on the configuration about the single asymmetric C in glyceraldehyde.

The lower representations are Fischer Projections.

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

L-glyceraldehydeD-glyceraldehyde

L-glyceraldehydeD-glyceraldehyde

SUGAR NOMENCLATURE

O H O H

C C

H – C – OH HO – C – H

HO – C – H H – C – OH

H – C – OH HO – C – H

H – C – OH HO – C – H

CH2OH CH2OH

D-glucose L-glucose

For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group.

Most naturally occurring sugars are D isomers.

CYCLIZATION OF ALDOSE AND KETOSES INTRODUCES ADDITIONAL CHIRAL CENTER Aldose sugars (glucose) can cyclize to form a

cyclic hemiacetal An aldehyde can react with an alcohol to form a hemiacetal.

Ketose sugars (fructose) can cyclize to form a cyclic hemiketal

A ketone can react with an alcohol to form a hemiketal

HC

R1

O

OH R2

HO

C*

OR2

R1H

H

ALCOHOL

ALDEHYDE

HEMIACETAL

NEW CHIRAL CARBON

RC

R1

O

OH R2

HO

C*

OR2

R1R

H

ALCOHOL

KETONE

HEMIKETAL

NEW CHIRAL CARBON

Pentoses and hexoses can cyclize as the ketone or aldehyde reacts with a distal OH.Glucose forms an intra-molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6-member pyranose ring, named after pyran.

These representations of the cyclic sugars are called Haworth projections.

H O

OH

H

OHH

OH

CH2OH

H

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose -D-glucose

23

4

5

6

1 1

6

5

4

3 2

H

CHO

C OH

C HHO

C OHH

C OHH

CH2OH

1

5

2

3

4

6

D-glucose (linear form)

HAWORTH PROJECTIONS

Anomeric carbon(most oxidized)

-OH up = beta-OH down = alpha

1

23

4

5

6

For all non-anomeric carbons, -OH groups point down in Haworth projections if pointing right in Fischer projections

C1

C2 OHH

HO

C3

C4

C5

CH2OH

HHO

OHH

OHH

Fructose forms either a 6-member pyranose ring, by reaction of

the C2 keto group with the OH on C6, or a 5-member furanose ring, by reaction of

the C2 keto group with the OH on C5.

CH2OH

C O

C HHO

C OHH

C OHH

CH2OH

HOH2C

OH

CH2OH

HOH H

H HO

O

1

6

5

4

3

2

6

5

4 3

2

1

D-fructose (linear) -D-fructofuranose

MONOSACCHARIDES CAN CYCLIZE TO FORM PYRANOSE / FURANOSE FORMS

= 64% = 36%

= 21.5% = 58.5%

= 13.5% = 6.5%

REDUCING SUGARS When in the

uncyclized form, monosaccharides act as reducing agents.

Free carbonyl group from aldoses or ketoses can reduce Cu2+ and Ag+ ions to insoluble products

DERIVATIVES OF MONOSACCHARIDES

SUGAR ALCOHOLS - LACKS AN ALDEHYDE OR KETONE; E.G…

COOH

C

C

C

C

H OH

HO H

H OH

D-gluconic acid D-glucuronic acid

CH2OH

OHH

CHO

C

C

C

C

H OH

HO H

H OH

COOH

OHH

sugar acid - the aldehyde at C1, or OH at C6, is oxidized to a carboxylic acid; e.g., gluconic acid, glucuronic acid.

SUGAR DERIVATIVES

amino sugar - an amino group substitutes for a hydroxyl. An example is glucosamine.

The amino group may be acetylated, as in N-acetylglucosamine.

H O

OH

H

OH

H

NH2H

OH

CH2OH

H

-D-glucosamine

H O

OH

H

OH

H

NH

OH

CH2OH

H

-D-N-acetylglucosamine

C CH3

O

H

AMINO SUGARS

MONOSACCHARIDE STRUCTURES YOU NEED TO KNOW

Glucose Fructose Ribulose Glyceraldehyde Dihydroxyacetone

GLYCOSIDIC BONDS

The anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond:

R-OH + HO-R' R-O-R' + H2O

GLYCOSIDIC LINKAGE

O

CH2OH

OH

OH

OHOH

O

CH2OH

OH

OH

OH

OH

H2OH2O

O

CH2OH

OH

OH

OH

O

O

CH2OH

OH

OH

OH

alcohol

hemiacetal

glycosidic linkage

Hydrolysis

Condensation

acetal

DIASACCHARIDES

H O

O H

H

O HH

O H

CH 2O H

H

O H

O H

H

O HH

O H

CH 2O H

H

O

HH

1

23

5

4

6

1

23

4

5

6

m altose

-D-glucosyl-(1->4)--D-glucopyranose)

DIASACCHARIDES

-D-glucosyl-(1->4)--D-glucopyranose)

H O

O H

H

O HH

O H

CH 2O H

H

O O H

H

H

O HH

O H

CH 2O H

H

H

H

O1

23

4

5

6

1

23

4

5

6

cellobiose

DISACCHARIDES

O

CH2OH

OH

OH

H

O

O

CH2OH

OH

OH

OH

H

OH

H

H

H

H

H

H

lactose

sucrose

-D-galactosyl-(1->4)--D-glucopyranose)

-D-glucosyl-(1->2)--D-fructofuranose)

O

CH2OH

OH

OH

OH

H

H

H

H

OCH2OH

H

H

OH

OH

H

O

CH2OH