Types of Carbohydrates Classification of Monosaccharides D and L Notations from Fischer Projections Structures of Some Important Monosaccharides

Carbohydrates

CarbohydratesCarbohydrates are: A major source of energy from our diet. Composed of the elements C, H and O. Also called saccharides, which means

“sugars”. Produced by photosynthesis in plants.

Photosynthesis in plants requires CO2, H2O, and energy from the sun and produces carbohydrates, such as glucose

[C6H12O6 = C(H2O)6].

In respiration, glucose is oxidized in living cells to produce CO2, H2O, and energy.

Photosynthesis and Respiration

Types of Carbohydrates

Monosaccharides are the simplest carbohydrates.

Disaccharides consist of two monosaccharides.

Polysaccharides contain many monosaccharides.

Most of the names of carbohydrates end in -ose

Glucose-What plants make Maltose- used in making beer (disaccharide) Fructose – found in fruit (monosaccharide) Sucrose- Table sugar (disaccharide) Lactose – In milk (disaccharide)

Classification of Monosaccharides

Aldoses are monosaccharides with an aldehyde group and many hydroxyl (-OH) groups.

Ketoses are monosaccharides with a ketone group and many hydroxyl (-OH) groups.

CHO

HO H

CH2OH

CHO

HO H

OHH

CH2OH

CH2OH

O

HHO

OHH

OH H

OH

AldoseAldose Ketose

Monosaccharides Monosaccharides are also classified according to the number of carbon atoms.

A triose has three carbons; a tetrose has four carbons; a pentose has five carbons; and a hexose has six carbons.

CHO

HO H

CH2OH

CHO

HO H

OHH

CH2OH

CH2OH

O

HHO

OHH

OH H

OH

Triose Tetrose Hexose

Th

e t

wo s

imp

lest

sug

ars 2

|1—C—3 | 4

2 |3—C—1 | 4

Note Numerous Chiral Carbons

Fischer Projection a two-dimensional representation of the three-

dimensional structure of organic molecules. usually used to represent the molecular structures of

monosaccharides and amino acids. devised by Hermann Emil Fischer in 1891.

Haworth Projection a common way of representing the

cyclic structure of monosaccharides with a simple three-dimensional perspective.

was named after the English chemist Sir Walter N. Haworth.

Fischer Projections Is used to represent carbohydrates. Places the most oxidized group at the top. Uses horizontal lines for bonds that come forward. Uses vertical lines for bonds that go back.

D and L Notations

By convention, the letter L is assigned to the structure with the —OH on the left.

The letter D is assigned to the structure with —OH on the right.

D and L Monosaccharides The —OH on the chiral atom farthest from the

carbonyl group is used to assign the D or L configuration.

The D form is usually the isomer found in nature.

L-Galactose

C

HO H

OHH

OHH

HHO

CH2OH

O

OHC

H OH

HHO

OHH

OHH

CH2OH

O

D-Glucose

HCHO

H OH

OHH

OHH

CH2OH

D-Ribose

D-Glucose Found in fruits, corn syrup, and honey.

An aldohexose with the formula C6H12O6

Known as blood sugar in the body.

Blood Glucose Level

In the body, glucose has a normal concentration of 80-110 mg/dL.

In a glucose tolerance test, blood glucose is measured for several hours after ingesting glucose.

D-Fructose

D-Fructose

CH2OH

C

C

O

C

H

OHH

HO

C

CH2OH

OHH

Is a ketohexose with formula of C6H12O6.

Is the sweetest carbohydrate. Is found in fruit, juices and

honey. Converts to glucose in the

body.

D-Galactose

Is not found free in nature.

Is obtained from lactose, a disaccharide.

Is important in cellular membranes.

CHO

C

C

OH

C

H

HHO

HO

C

CH2OH

OHH

H

D-Galactose

Cyclic Structures

Monosaccharides with 5-6 carbon atoms form cyclic structures

The hydroxyl group on C-5 reacts with the aldehyde group or ketone group

o O

Sugars Prefer To Be Cyclic

Haworth Structure for D-Isomers

The cyclic structure of a D-isomer has the final CH2OH group located above the ring.

oCH2OH

Numbered carbons

C

CC

C

CC

1'

2'3'

4'

5'

6'

O

Haworth Structure for D-Glucose

The new –OH on C1 has two possibilites: down for anomer, up for anomer

• Isomers that differ only in their configuration about the new asymmetric carbon are called anomers, the carbonyl carbon is called anomeric carbon.

• α-anomer • β-anomer

Cyclic Haworth Structures

Stable cyclic hemiacetals form when the C=O group and the

-OH are part of the same molecule.

For hexoses, the hydroxyl group on C-5 reacts with the aldehyde group or ketone group.

The cyclic structure of a D-isomer has the last CH2OH group located above the ring.

oCH2OH

Formation of Cyclic Glucose The Haworth structure can be written from the

Fischer Projection.

The –OH group on the left (C3) is drawn up.

The –OH groups on the right (C2, C4) are drawn down.

Haworth Structure for D-Glucose

oCH2OH

OHOH

OH

OH

oCH2OH

OHOH

OH

OH

-D-Glucose -D-Glucose

Glucose Anomers

OH

OH

OH

CH2OH

CHO

OHO

CH2OH

HH

OHH

OH

OH

HH

OH

D-(+)-glucose

UP

DOWN

on right = D

CONVERTING TO HAWORTH PROJECTIONS

-CH2OH up = D cis

= b

FISCHERPROJECTION

trans

= a

HAWORTHPROJECTIONS

O

CH2OH

HH

OHH

OH

OH

HOH

H

BOTH ANOMERS OFA D-SUGAR(D-glucose)

1

6

2

3

4

5

1

23

4

6

5

Mutarotation

• A small amount of open chain is in equilibrium with the cyclic forms.

• The most stable form of glucose is β-D-glucose.

-D-glucose D-glucose (open) β-D-glucose (36%) (trace) (64%)

-D-Glucose

and are “anomers” and differ in configuration at the new chiral center formed in the cycle

In nature, open chain forms are rare. All 5 forms coexist with the pyranose forms

occurring 99% of the time, furanose about 0.5% each and a trace of open chain