Chapter Eighteen Carbohydrates Ch 18 | # 2 of 52 Carbohydrates cont’d.

Chapter EighteenChapter EighteenCarbohydrates

Ch 18 | # 2 of 52

Carbohydrates cont’d

Ch 18 | # 3 of 52

Carbohydrates

• Major source of ______ from our diet

• Composed of the elements _, _ and _

• Produced by photosynthesis in plants

• Also called saccharides (sugars)

Ch 18 | # 4 of 52

Carbohydrates, Chemically

• Polyhydroxy aldehydes or polyhydroxy ketones

CH

CH

CH

CH

CH

CH

CH3

O

OH

OH

OH

OH

OH

CH2

C

CH

CH

CH

CH3

OH

OH

OH

OH

O

Ch 18 | # 5 of 52

Monosacchrides

• Three Carbons = ________

• Four Carbons = ________

• Five Carbons = ________

• Six Carbons = ________

Ch 18 | # 6 of 52

Monosacchrides

• Aldoses are monosacchrides with an _________________ group and many hydroxyl (-OH) groups.

• Ketoses are monosacchrides with a __________________ group and many hydroxyl (-OH) groups.

Ch 18 | # 7 of 52

Glucose

C

C

CH2OH

H OHC

OHH C

HHO

H OH

C OH

D-Glucose

Ch 18 | # 8 of 52

Galactose

D-galactose

Ch 18 | # 9 of 52

Fructose

C

C

CH2OH

H OHC

OHH C

HHO

O

CH2OH

D-Fructose

Ch 18 | # 10 of 52

Types of Carbohydrates

• MonosacchridesA carbohydrate that contains a single

polyhydroxy aldehyde or polyhydroxy ketoneunit.

• DisaccharidesContain 2 monosacchride units

• Polysacchrides Contain many monosacchride units

Ch 18 | # 11 of 52

Different Forms of Monosaccharides

• Most monosaccharides exist in two different forms– “left-handed” and – “right-handed”

• The two forms are related to each other the same way that mirror images are related to each other.

Ch 18 | # 12 of 52

← Fig. 18.3 The mirror image of the right hand is the left hand. Conversely, the mirror image of the left hand is the right hand.


Ch 18 | # 13 of 52

→ Fig. 18.4 A person’s left and right hands are not superimposable upon each other.


Ch 18 | # 14 of 52

Mirror Images

• Mirror images– The reflection of an object in a mirror

• Superimposable mirror images– Images that coincide at all points when the images are laid upon

each other

• Nonsuperimposable mirror images– Images where not all points coincide when the images are laid

upon each other

• The easiest way to determine if two mirror images are superimposable or not, is to make models

Ch 18 | # 15 of 52

Chiral Objects

• Chiral molecules have mirror images that are not superimposable

• Chiral molecules contain at least one chiral center• A chiral carbon atom (“chiral center”) has four

different groups attached to it (typically marked with *)

Ch 18 | # 16 of 52

Fig. 18.5 Examples of simple molecules that are chiral.


Ch 18 | # 17 of 52

Stereoisomerism

• Structural isomers– Isomers in which atoms are connected to each other in

different ways

• Stereoisomers– Isomers whose atoms are connected in the same way

but which differ in the orientation of these atoms in space

• Stereoisomers always have a chiral center and structural rigidity• Cis-trans isomers are one form of stereoisomerism

Ch 18 | # 18 of 52

Stereoisomers

• 2 different kinds of stereoisomers– Enantiomers

• Stereoisomers whose molecules are nonsuperimposable mirror images of each other (the left- and right-hand versions of a single molecule)

– Diastereomers• Stereoisomers whose molecules are not mirror images of each

other

Ch 18 | # 19 of 52

→ Fig. 18.6 Enantiomers are stereoisomers whose molecules are nonsuperimposable mirror images of each other. a) Enantiomersb) Diastereomers –

molecules are not mirror images


Ch 18 | # 20 of 52

← Fig. 18.7 The “thought process” used in classifying molecules as enantiomers or diastereomers.


Ch 18 | # 21 of 52

Ch 18 | # 22 of 52

→ Fig. 18.8 Emil Fischer was one of the early greats in organic chemistry. He is credited with the development and use of the Fischer Projection Formulas. We have already used this notation in the tetrahedral drawing in a two dimensional form. See page 519 in your text.


Edgar Fahs Smith Collection, University of Pennsylvania Library

Ch 18 | # 23 of 52

Fischer Projections

• A two-dimensional structural notation for showing the spatial arrangement of groups about chiral centers in molecules– Vertical lines = bonds to groups directed into the page (away

from you)– Horizontal lines = bonds to groups directed out of the page

(towards you)

CH

C

CH3

O

OHH

CH

C

CH3

O

OH H

Ch 18 | # 24 of 52

Properties of Chiral Centers

• Most important property is a solution of a stereoisomer is its ability to rotate plane polaraized light.

• Light passing through polarized filters has waves only in one direction, i.e., the light is polarized.

Ch 18 | # 25 of 52

← Fig. 18.9 Vibrational characteristics of ordinary and polarized light.


Ch 18 | # 26 of 52

Fig. 18.10 Schematic depiction of how a polarimeter works.


Ch 18 | # 27 of 52

• Rotation of light:– Levorotatory: Rotation of light to the left– Dextrorotatory: Rotation of light to the right

• Not to be confused with D and L forms or the “handedness” of molecules. They are different.

Ch 18 | # 28 of 52

D notation

C

C

CH2OH

OHH

OHH

CO

H

Right = D

Ch 18 | # 29 of 52

Chiral Centers – Why Do We Care?

• Monosaccharides often contain more than one chiral center– This means that there are at least two different forms of

each monosaccharide, a left-handed form and a right-handed form

– Each form elicits a different chemical response

• Biologically active monosaccharides are the right-handed versions of molecules

Ch 18 | # 30 of 52

← Fig. 18.11 The distinctly different natural flavors of spearmint and caraway are caused by enantiomeric molecules.


Ch 18 | # 31 of 52

Intermolecular Interactions

• Interactions where there is binding or contact at three points.

• Look at enantiomeric pair and see how each would bind differently.

Ch 18 | # 32 of 52

→ Fig. 18.12 Epinephrine binds to the receptor at three points.


Ch 18 | # 33 of 52

Ch 18 | # 34 of 52

Ch 18 | # 35 of 52

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 (to form a hemiacetal or a hemiketal)

Ch 18 | # 36 of 52

Haworth Structure for D-Isomers

• Two-dimensional structural notations that specifies the 3-dimensional structure of a cyclic form of a monosaccharide

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

Ch 18 | # 37 of 52

→ Fig. 18.16 The cyclic hemiacetal forms of D-glucose result from the intramolecular reaction between the carbonyl group and the hydroxyl group on carbon 5.

Ch 18 | # 38 of 52

Haworth Structure for D-Glucose

-D-Glucose -D-Glucose

Ch 18 | # 39 of 52

← Fig. 18.17 Walter Norman Haworth was a British carbohydrate chemist.


© Hulton-Deutsch Collection / CORBIS

Ch 18 | # 40 of 52

→ Fig. 18.19 The three forms of maltose present in aqueous solution.

Ch 18 | # 41 of 52

Disaccharides

• A disaccharide consists of two monosaccharides

• Glucose + Glucose Maltose + water• Glucose + Galactose Lactose + water• Glucose + Fructose Sucrose + water

Ch 18 | # 42 of 52

Ch 18 | # 43 of 52

← Fig. 18.22 The polymer chain of a polysaccaride may be unbranched or branched.

Ch 18 | # 44 of 52

Amylose, Amylopectin, and Glycogen

• Amylose is a continuous chain of glucose molecules linked by -1,4 glycosidic bonds

• Amylopectin is a branched chain of glucose molecules linked by a -1,4- and -1,6-glycosidic bonds.

• Glycogen is similar to amylopectin, but more highly branched

Ch 18 | # 45 of 52

Fig. 18.24 Two perspectives on the structure of the polysaccaride amylopectin.

Ch 18 | # 46 of 52

Cellulose

• 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

Ch 18 | # 47 of 52


Ch 18 | # 48 of 52

Fig. 18.28 The structures of cellulose (a) chitin (b).


Ch 18 | # 49 of 52

Phosphate Ester Formation

• When a cyclic monosaccharide reacts with phosphoric acid, a phosphate ester is produced

• Phosphodiesters link the monomers in DNA

Ch 18 | # 50 of 52

Amino Sugar Formation

• When a cyclic monosaccharide reacts with an amine, an amine sugar is produced

Ch 18 | # 51 of 52

Glycolipids and Glycoproteins

• A glycolipid is a lipid molecule that has one or more carbohydrate (or carbohydrate derivative) units covalently bonded to it.

• A glycoprotein is a protein molecule that has one or more carbohydrate (or carbohydrate derivative) units covalently bonded to it.

Ch 18 | # 52 of 52

Chapter Eighteen Carbohydrates Ch 18 | # 2 of 52 Carbohydrates cont’d.

Documents

Transcript of Chapter Eighteen Carbohydrates Ch 18 | # 2 of 52 Carbohydrates cont’d.