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
Current terminology: natural materials that contain many hydroxyls and other oxygen-containing groups

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.
Carbohydrates are made of two or more simple sugars (complex carbohydrates) 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)

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 and common sets of atoms
A chirality center C is projected into the plane of the paper and other groups are 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”)

8. Stereochemical Reference
The 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 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. 14.3 D, L Sugars
Glyceraldehyde exists as two enantiomers, first identified by their opposite rotation of plane polarized light
Naturally occurring glyceraldehyde rotates plane-polarized light in a clockwise direction, denoted (+) or “D” (dextrorotary) and is designated “(+)-glyceraldehyde”
The enantiomer gives the opposite rotation and has a (–) or “L” (levorotatory) prefix
The direction of rotation of light does not generally correlate to any structural feature

12. 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

13. 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

14. 14.4 Configurations of Aldoses
Stereoisomeric aldoses are distinguished by trivial names, rather than by systematic designations
Enantiomers have the same names but different D,L prefixes
R,S designations are difficult to work with when there are multiple similar chirality centers
Systematic methods for drawing and recalling structures are based on the use of Fischer projections

15. Configurations of the Aldoses (Continued)
Aldotetroses have two chirality centers
There are 4 stereoisomeric aldotetroses, two pairs of enantiomers: erythrose and threose
D-erythrose is a diastereomer of D-threose and L-threose
Aldopentoses have three chirality centers and 23 = 8 stereoisomers, four pairs of enantiomers: ribose, arabinose, xylose, and lyxose
Aldohexoses have four chirality centers and 24 = 16 stereoisomers, eight pairs of enantiomers: allose, altrose, glucose, mannose, gulose, iodose, galactose, and talose

16. Configurations of the Aldoses (Continued)

17. 14.5 Cyclic Structures of Monosaccharides: Hemiacetal Formation
Alcohols add reversibly to aldehydes and ketones, forming hemiacetals

18. 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

19. Formation cyclic structures
Like cyclohexane rings, there are axial and equatorial positions
By convention the hemiacetal oxygen is placed in the rear of the ring

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

21. 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

22. Monosaccharide Anomers: Mutarotation
a anomer – OH down; b anomer – OH up

23. Monosaccharide Anomers: Mutarotation
Mutarotation is reversible
Slow at neutral pH
Catalyzed by acid and base

24. 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

25. 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

26. 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

27. 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

28. The Eight Essential Monosaccharides (Continued)

29. 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)

30. 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)

31. 14.10 Polysaccharides
Complex carbohydrates in which very many simple sugars are linked
Cellulose and starch are the two most widely occurring polysaccharides

32. 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

33. Starch and Glycogen
Starch is a 1→4--glucopyranosyl-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

34. Amylopectin More complex in structure than amylose
Has 1→6--glycoside branches approximately every 25 glucose units in addition to 1→4--links

35. 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

36. 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

37. 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?

38. 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)