Chapter 18:Carbohydrates

18.3 - Types of Carbohydrates18.4 and 18.6 - D and L Notations from Fischer Projections

18.8 - Classification of Monosaccharides18.9 - Structures of Some Important Monosaccharides

18.10 and 18.11 - Cyclic Structures of Monosaccharides18.12 - Chemical Properties of Monosaccharides

18.13 - Disaccharides18.14 and 18.16 - Polysaccharides

Carbohydrates Most abundant organic compounds in nature Produced by photosynthesis in plants Composed of the elements C, H and O Also called saccharides, which means “sugars” A major source of energy from our diet A source of C for synthesis of other biomolecules Can be linked to cell membranes or proteins

Types of Carbohydrates Monosaccharides are the simplest carbohydrates

Empirical formula = CH2O Oligosaccharides contain a few monosaccharides (2-10) Disaccharides consist of two monosaccharides Polysaccharides contain many monosaccharides

Chiral Molecules Chiral molecules

Have “handedness” Nonsuperimposable on mirror images Ex: hand, shoe

Achiral molecules Do not have handedness Ex: glass, spoon

Chiral molecules contain carbons with 4 different groups Called chiral carbons or chiral center

CHO

C OH

CH3

Br

Fischer Projections Used to represent carbohydrates Places the most oxidized group at the top All chiral carbons are represented as intersection of lines

C is not shown Implied 3D arrangement of atoms

Horizontal lines for bonds that come forward 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

CHOH OH

OHHOHH

CH2OH

D-Ribose

D

C

H OH

HHO

OHH

OHH

CH2OH

O

D-Glucose

H

D

L-Galactose

C

HO H

OHH

OHH

HHO

CH2OH

O

OH

L

Learning Check Indicate whether each is the D or L isomer:

Ribose Threose Fructose

CHO

HO H

HHO

HHO

CH2OH

CHO

H OH

HHO

CH2OH

CH2OH

O

HHO

OHH

OH H

CH2OH

Classification of Monosaccharides

Monosaccharide = unbranched chain of 3-8 C atoms; one is carbonyl, others attached to -OH

Aldose monosaccharide with an aldehyde group (1st carbon)

Ketose monosaccharide with a ketone group (2nd carbon)

Aldose Aldose Ketose

CHO

HO H

CH2OH

CHO

HO H

OHH

CH2OH

CH2OH

O

HHO

OHH

OH H

OH

CH2OH

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.

triose tetrose hexose aldotriose aldotetrose ketohexose

CHO

HO H

CH2OH

CHO

HO H

OHH

CH2OH

CH2OH

O

HHO

OHH

OH H

OH

CH2OH

Identify each as tetrose, pentose or hexose, and as aldose or ketose:

CHO

HO H

HHO

HHO

CH2OH

Learning Check

CH2OH

O

HHO

OHH

OHH

CH2OH

A B

D-Glucose Most common hexose Found in fruits, corn

syrup, and honey An aldohexose with

the formula C6H12O6

Known as blood sugar in the body

Building block for many disaccharides and polysaccharides

D-Galactose Aldohexose Differ from D-glucose at C4

C1 is at the top Not found in the free form

in nature Obtained from lactose, a

disaccharide (milk products) Important in cellular

membranes in CNS (brain sugar)

CHO

C

C

OH

C

H

HHO

HO

C

CH2OH

OHH

H

D-Galactose

D-Fructose Ketohexose C6H12O6

Differ from glucose at C1 and C2 (location of carbonyl)

The sweetest carbohydrate (2x sucrose)

Found in fruit juices and honey Formed from hydrolysis of sucrose Converts to glucose in the body

D-Fructose

CH2OH

C

C

O

C

H

OHH

HO

C

CH2OH

OHH

Learning CheckDraw the structure of D-fructose:

Hemiacetal Review What is a hemiacetal? How is a hemiacetal formed?

What if the alcohol and carbonyl are attached?

C

O

HCH3 + CH3OH C

OH

HCH3

OCH3

H+

HOC

H

O

= C

OO

H

H

H+

C

OO

H

H

HOC

H

O

= C

OO

H

H

H+

C

OO

H

HCH2OH

HOCH2 HOCH2

Hexose hemiacetals Favor formation of 5- or 6-membered rings Hydroxyl group on C5 reacts with the aldehyde or ketone

The Haworth structure can be written from the Fischer projection The cyclic structure of a D-isomer has the last CH2OH group located

above the ring The –OH group on the left (C3) is drawn up The –OH groups on the right (C2, C4) are drawn down

Anomers are isomers which differ in placement of hydroxyl on C1

The –OH is drawn down for the -anomer, and up for the -anomer

-D-Glucose -D-Glucose Mashed potatoes or mashed paper?

OCH2OH

OHOH

OH

OH

OCH2OH

OHOH

OH

OH

and Anomers for D-Glucose

Mutarotation In solution, -D-glucose is in equilibrium with β-D-glucose Mutarotation involves the conversion of the cyclic anomers

into the open chain At any time, there is only a small amount of open chain

aldehyde

Cyclic Structure of Fructose As a ketohexose, fructose forms a cyclic

structure when the —OH on C5 reacts with the ketone on C2

Result is 5-atom ring Anomeric carbon is C2

CH2OH

C

C

C

C

CH2OH

O

HO H

OHH

OHH

D-Fructose

CH2OH

OH

OH

OH

CH2OHO

CH2OH

OH

OH

CH2OH

OHO

-D-Fructose -D-Fructose

Write the cyclic form of -D-galactose:Learning Check

C

C

C

C

C

CH2OH

OH

OHH

HO H

HHO

OHH

o

OH

OH

CH2OH

OH

OH

-D-galactose

Review:Reactions of aldehydes1. Oxidation to form carboxylic acids2. Reduction to form alcohols3. Formation of hemiacetal4. Hemiacetal + alcohol → acetal

Now we’ll see all of these with monosaccharides…

1. Oxidation of Monosaccharides Monosaccharides are reducing sugars if their carbonyl

groups oxidize to give carboxylic acids. In the Benedict’s text, D-glucose is oxidized to D-

gluconic acid. Glucose is a reducing sugar.

C

C

C

C

C

CH2OH

HO

OHH

HO H

OHH

OHH

C

C

C

C

C

CH2OH

OHO

OHH

HO H

OHH

OHH

+ Cu2+

D-Glucose D-Gluconic acid

+ Cu2O(s)

2. Reduction of Monosaccharides

The reduction of the carbonyl group produces sugar alcohols, or alditols

D-Glucose is reduced to D-glucitol (also called sorbitol)

3. Monosaccharides + alcohol Formation of hemiacetal (cyclic structures)

4. Monosaccharide hemiacetals + alcohol When a cyclic monosaccharide reacts with an

alcohol: A glycoside is produced (acetal) The bond is a glycosidic bond or glycosidic linkage

-D-Glucose Methanol Methyl--D-glucoside

glycosidic bond

O

OH

OH

CH2OH

O

OH

CH3

HOCH3+

O

OH

OH

CH2OH

OH

OH+ H2O

Learning CheckWrite the products of the oxidation and reduction of D-mannose.

C

HO H

HHO

OHH

OHH

CH2OH

O

H

D-Mannose

Disaccharides A disaccharide consists of two monosaccharides

Disaccharide Monosaccharides Maltose + H2O Glucose + Glucose Lactose + H2O Glucose + Galactose Sucrose + H2O Glucose + Fructose

H+

Maltose Malt sugar A disaccharide in

which two D-glucose molecules are joined by an -1,4-glycosidic bond

Obtained from starch Used in cereals,

candies, and brewing A reducing sugar (has

a hemiacetal)

Lactose Milk sugar Composed of

galactose and glucose linked by a -1,4-glycosidic bond

Lactose intolerance

A reducing sugar

Sucrose Table sugar Is composed of glucose

and fructose molecules joined by ,-1,2-glycosidic bond

Has no isomers because mutarotation is blocked

Not a reducing sugar (no hemiacetal)

Sweetness of Sweeteners Sugars and

artificial sweeteners differ in sweetness

Each sweetener is compared to sucrose (table sugar), which is assigned a value of 100

Learning CheckIdentify the monosaccharides in lactose, maltose, and sucrose as glucose, fructose, and/or galactose:

A. Lactose

B. Maltose

C. Sucrose

Polysaccharides Polysaccharides are

polymers of monosaccharides

“Complex” carbohydrates Important polysaccharides

of D-glucose are: Starch (Amylose and

Amylopectin) Glycogen Cellulose

Starch and Glycogen Storage polysaccharides

Form monosaccharides used for energy Starch

Plants Amylose is a continuous chain of glucose molecules

linked by -1,4 glycosidic bonds. Amylopectin is a branched chain of glucose molecules

linked by -1,4- and -1,6-glycosidic bonds. Glycogen

Humans, animals Similar to amylopectin, but more highly branched.

Structures of Amylose and Amylopectin

Cellulose Structural polysaccharide

Plant cell walls (cellulose) and animal exoskeletons (chitin) Cellulose is a polymer of glucose molecules linked by -1,4-

glycosidic bonds Enzymes in saliva can hydrolyze -1,4-glycosidic bonds in starch,

but not -1,4-glycosidic bonds in cellulose

Learning CheckIdentify the types of glycosidic bonds in:1) Amylose

2) Glycogen

3) Cellulose

End of Chapter 18!