Chapter 17:Carbohydrates

17.1 ─ Classes of Carbohydrates

17.2 and 17.3 ─ Stereochemistry and Fischer Projections

17.4 and 17.6 ─ Monosaccharides

17.5 ─ Cyclic Structures and Chemical Properties of Monosaccharides

17.7 ─ Disaccharides

17.8 ─ Polysaccharides

17.1 Carbohydrates Most abundant organic compounds in nature

Composed of the elements C (“carbo”), H and O (“hydrate”)

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

Carbohydrate Structure Polyhydroxy aldehydes or ketones

Examples:

C

CH OH

C

C

H OH

CH2OH

H OH

O H

CH2OH

C O

C

C

H OH

CH2OH

H OH

ribose ribulose

Types of Carbohydrates Monosaccharides are the simplest carbohydrates

Empirical formula = CH2O

Contain one aldehyde or ketone unit (saccharide)

Disaccharides consist of two monosaccharides Oligosaccharides contain a few monosaccharides (3-10) Polysaccharides contain many monosaccharides

17.2 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

or stereocenter

Chiral Molecules Compounds can have more

than one chiral carbon Example:

Glucose contains 6 carbons. How many of these carbons

are chiral?

C

CH OH

C

C

HO H

C

H OH

H O

CH2OH

OHH

1

2

3

4

5

6

Chiral Molecules

17.3 Fischer Projections Used to represent carbohydrates Places the most oxidized group (carbonyl)

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

C

CH OH

C

C

HO H

C

H OH

H O

CH2OH

OHH

1

2

3

4

5

6

CHO

H OH

HO H

H OH

CH2OH

OHH

1

2

3

4

5

6

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

group is used to assign a D or L configuration 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

D and L Monosaccharides

DD 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

17.4 Classification of Monosaccharides

Monosaccharide = unbranched chain of 3 or more carbon atoms one C is carbonyl, other C’s 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 triose, tetrose, pentose or hexose, and as aldose or ketose:

Learning Check

CH2OH

O

HHO

OHH

OHH

CH2OH

A B

CHO

HO H

HHO

HHO

CH2OH

17.6 Important Monosaccharides Ribose and deoxyribose Glucose Galactose Fructose

D-Ribose and D-Deoxyribose

Pentoses Used in the synthesis of DNA (deoxyribose) and

RNA (ribose)

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 C6H12O6

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 (73% sweeter than 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: