1. Chapter 14 Biomolecules: Carbohydrates
Suggested Problems: 27,31-4,36-7,43,46-7

2. Importance of Carbohydrates
Distributed widely in nature
Key intermediates of metabolism (sugars)
Structural components of plants (cellulose)
Central to materials of industrial products: paper, lumber, fibers
Key component of food sources: sugars, flour, vegetable fiber
Contain OH groups on most carbons in linear chains or in rings

3. Chemical Formula and Name
Carbohydrates have roughly as many O’s as C’s (highly oxidized)
The empirical formulas are roughly (C(H2O))n
Appears to be “carbon hydrate” from formula

4. Sources
Glucose is produced in plants through photosynthesis from CO2 and H2O
Glucose is converted in plants to other small sugars and polymers (cellulose, starch)
Dietary carbohydrates provide the major source of energy required by organisms

5. 14.1 Classification of Carbohydrates
Simple sugars (monosaccharides) can't be converted into smaller sugars by hydrolysis.
Complex carbohydrates are made of two or more simple sugars connected as acetals (aldehyde and alcohol), oligosaccharides, and polysaccharides
Sucrose (table sugar): disaccharide from two monosaccharides (glucose linked to fructose),
Cellulose is a polysaccharide of several thousand glucose units connected by acetal linkages (aldehyde and alcohol)
Disaccharides: lactose galactose and glucose; sucrose fructose and glucose; maltose glucose and glucose
Enzyme linked hydrolysis converts polysaccharides into their constituent monosaccharides

6. Aldoses and Ketoses
aldo- and keto- prefixes identify the nature of the carbonyl group
-ose suffix designates a carbohydrate
Number of C’s in the monosaccharide indicated by root (tri-, tet-, pent-, hex-)

7. 14.2 Depicting Carbohydrate Stereochemistry: Fischer Projections
Carbohydrates have multiple chirality centers
A chirality center C is projected into the plane of the paper and other groups are on horizontal or vertical lines
Groups forward from paper are always in a horizontal line.
Groups back from the paper are always in a vertical line.
The oxidized end of the molecule is always higher on the page (“up”)
Fischer projections are a good way to represent molecules with numerous chirality centers.

8. Stereochemical Reference
Stereochemistry reference compounds are the two enantiomers of glyceraldehyde, C3H6O3
A compound is “D” if the hydroxyl group at the chirality center farthest from the oxidized end of the sugar is on the right or “L” if it is on the left.
D-glyceraldehyde is (R)-2,3-dihydroxypropanal
L-glyceraldehyde is (S)-2,3-dihydroxypropanal
The OH above the CH2OH group points to the right – D configuration

9. Stereochemical Reference
Carbohydrates with more than one stereocenters are shown in Fischer projection by stacking one center on top of the other
The molecules true three-dimensional conformation is one that is curled around itself like a bracelet
The bracelet sets the molecule up for form a hemiacetal. D-

10. Working With Fischer Projections
In determining R, S designations place the lowest priority group on the vertical axis (to the back)
With the lowest priority group to the back, prioritize the 3 highest groups – clockwise is R; counterclockwise is S
Switching any two groups changes R to S and vice versa
Switching any two groups again returns the original configuration.

11. Naturally Occurring D Sugars
Most have the —OH group at the bottom chirality center pointing to the right
Have an R configuration at the lowest chirality center
D sugars are typically what is found in nature. They have the R configuration at the lowest chiral center.
While glyceraldehyde rotates light in a positive direction, there is no correlation between between D and L and (+) and (-). The D configuration designation comes because Glyceraldehyde was the first sugar to have its rotation measured. The D refers to the configuration at the bottommost chiral center.

12. L Sugars
Most have the —OH group at the bottom chirality center pointing to the left
An L sugar is the mirror image (enantiomer) of the corresponding D sugar
Have an S configuration at the lowest chirality center
There are four chiral centers in this molecule and thus 16 possible stereoisomers of which only one occurs naturally.

13. 14.5 Cyclic Structures of Monosaccharides: Hemiacetal Formation
Alcohols add reversibly to aldehydes and ketones, forming hemiacetals

14. Internal Hemiacetals of Sugars
Intramolecular nucleophilic addition creates cyclic hemiacetals in sugars
Five- and six-membered cyclic hemiacetals are particularly stable
Five-membered rings are furanoses. Six-membered are pyanoses
Many carbohydrates exist in an equilibrium between open-chain and cyclic forms

15. Formation cyclic structures
Like cyclohexane rings, there are axial and equatorial positions
By convention the hemiacetal oxygen is place in the rear of the ring
An OH group on the right in a Fischer projection is on the bottom face of the pyranose ring when drawn with the acetal oxygen in the back right. For D sugars the CH2OH group is on the top of the ring.

16. Formation cyclic structures
Note the pyranose and furanose forms depending on which hydroxyl group forms the hemiacetal.

17. Monosaccharide Anomers: Mutarotation
Formation of the cyclic hemiacetal creates an additional chirality center giving two diasteromeric forms, designated  and b
These diastereomers are called anomers
In the alpha (a) anomer the C1 —OH group is trans to the —CH2OH substituent at C5
In the beta (b) anomer the C1 —OH group is cis to the —CH2OH substituent at C5
a anomer – OH down; b anomer – OH up

18. Monosaccharide Anomers: Mutarotation
Mutarotation refers to the interconversion of alpha and beta isomers.
Note the nomenclature indicating these forms for glucopyranose.
a anomer – OH down; b anomer – OH up

19. Monosaccharide Anomers: Mutarotation
Mutarotation is reversible
Slow at neutral pH
Catalyzed by acid and base
These forms can be isolated in pure form but in water they interconvert.

20. 14.7 Reactions of Monosaccharides
–OH groups can be converted into esters and ethers, which are often easier to work with than the free sugars and are soluble in organic solvents.
Esterification by treating with an acid chloride or acid anhydride in the presence of a base
All –OH groups react

21. Ethers
Treatment with an alkyl halide in the presence of base—the Williamson ether synthesis
Use silver oxide as a catalyst with base-sensitive compounds

22. Glycoside Formation
Treatment of a monosaccharide hemiacetal with an alcohol and an acid catalyst yields an acetal in which the anomeric –OH has been replaced by an –OR group
b-D-glucopyranose with methanol and acid gives a mixture of  and b methyl D-glucopyranosides
Structures like those on the lower right are called glycosides. Glycosides are stable in water; they are not in equilibrium with an open chain form and thus they do not mutarotate. They can be hydrolyzed back to the hemiacetal on treatment with acid, however.
Glycosides are abundant in nature.

23. Phosphorylation of Monosaccharides
In living organisms carbohydrates are often linked through their anomeric center to other biological molecules (called glycoconjugates)
Proceeds through a phosphorylation step to activate the anomeric —OH and make it a better leaving group
Blood types result from carbohydrates attached to cell surfaces. (polysaccharides)

24. The Eight Essential Monosaccharides (Continued)

25. 14.9 Disaccharides
A disaccharide combines a hydroxyl of one monosaccharide in an acetal linkage with another
A glycosidic bond between C1 of the first sugar ( or ) and the –OH at C4 of the second sugar is particularly common (a 1→4 link)

26. Sucrose
“Table Sugar” is pure sucrose, a disaccharide that hydrolyzes to glucose and fructose
Connected as an acetal from both anomeric carbons (aldehyde to ketone)
Draw maltose with an alpha (down) glycosidic linkage. Leave up on board for two slides.

27. 14.10 Polysaccharides
Complex carbohydrates in which very many simple sugars are linked
Cellulose and starch are the two most widely occurring polysaccharides

28. Cellulose
Consists of thousands of D-glucopyranosyl 1→4--glucopyranosides as in cellobiose
Cellulose molecules form a large aggregate structures held together by hydrogen bonds
Cellulose is the main component of wood and plant fiber
Cellulose is glucose in beta linkages

29. Starch and Glycogen
Starch is a 1→4--glupyranosyl-glucopyranoside polymer
It is digested into glucose
There are two components
amylose, insoluble in water – 20% of starch
1→4--glycoside polymer
amylopectin, soluble in water – 80% of starch

30. Amylopectin More complex in structure than amylose
Has 1→6--glycoside branches approximately every 25 glucose units in addition to 1→4--links
Amylopectin differs from amylose in that it has additional branches

31. Glycogen
A polysaccharide that serves the same energy storage function in animals that starch serves in plants
Highly branched and larger than amylopectin—up to 100,000 glucose units
Starch and glycogen are structurally similar but starch is found in plants and glycogen is found in animals.

32. 14.11 Cell-Surface Carbohydrates and Influenza Viruses
Polysaccharides are centrally involved in cell–cell recognition - how one type of cell distinguishes itself from another
Small polysaccharide chains, covalently bound by glycosidic links to hydroxyl groups on proteins (glycoproteins), act as biochemical markers on cell surfaces, determining such things as blood type

33. Let’s Work a Problem
The following figure is that of allose. Is this a furanose or pyranose ring form? Is it an a or b anomer? Is it an D or L sugar?

34. Answer
Examination of this structure lets us see that it has a 6-membered ring (pyranose), the C1 OH group is cis to the –CH2OH group (b anomer), and the –O at C5 is on the right side in the uncoiled form (D)