Chapter 25 Biomolecules: Carbohydrates

1. Chapter 25Biomolecules: Carbohydrates

2. The Importance of Carbohydrates • Carbohydrates are… • widely distributed in nature. • key intermediates in metabolism (sugar). • structural components of plants (cellulose). • key components of industrial products (wood, fibers). • key components of food sources (sugar, flour).

3. Chemical Formula • Carbohydrates are highly oxidized. • They have approximately as many oxygen atoms as carbon atoms. • Carbons of carbohydrates are usually bond to an alcohol and hydrogen atom; therefore, the empirical formula is roughly (C(H2O))n. D+ Glucose (C6H12O6)

4. Sources of Carbohydrates • Glucose is produced in plants from CO2 and H2O via photosynthesis. • Plants convert glucose into other small sugars and polymers (cellulose, starch). • Dietary carbohydrates provide the major source of energy required by organisms.

5. Classifications of Carbohydrates • Monosaccharide: simple sugars that can not be converted into smaller sugars by hydrolysis • Carbohydrate (Oligosaccharide, Polysaccharide): two or more simple sugars connected as acetals • Sucrose: disaccharide of two monosaccharides (glucose linked to fructose) • Cellulose: polysaccharide of several thousand glucose units connected by acetal linkages

6. Aldose and Ketose • The prefixes aldo- and keto- identify the nature of the carbonyl group. • Aldo: carbonyl is located at the end of the chain • Keto: carbonyl is located within the chain • The suffix -ose denotes a carbohydrate. • The number of carbons is indicated by the root.

7. Aldose and Ketose

8. Fischer Projections • Carbohydrates have multiple chiral centers. • A chiral center carbon is projected into the plane of the paper and other groups are drawn as horizontal and vertical lines. • The oxidized end of the molecule is always “up” on the paper.

9. Fischer Projections

10. Minimal Fischer Projections • In order to work with the structure of an aldose more easily, only the essential components are shown. • An alcohol is designated by a “-” and a carbonyl is designated by an “↑”. • The terminal OH in the CH2OH is not shown.

11. Stereochemical References • The reference compounds for stereochemistry are the two enantiomers of glyceraldehyde (C3H6O3). • The stereochemistry depends on the hydroxyl group attached to the chiral center farthest from the oxidized end of the sugar. • D: hydroxyl group is on the right • L: hydroxyl group is on the left

12. Stereochemical References

13. The “D” Sugar Family

14. D and L Sugars • The two enantiomers of glyceraldehyde were first identified by their opposite rotation of plane polarized light. • Naturally occurring glyceraldehyde rotates light in a clockwise rotation and is denoted as “+”. • The enantiomer rotates light counterclockwise and is denoted as “-”. • The direction of the rotation of light does not correlate to structural features.

15. Configurations of Aldoses • Because R and S designations are difficult to work with when multiple chiral centers are present, the D,L designations are used with aldoses.

16. Aldotetrose • Aldotetroses have two chiral centers; therefore, there are two pairs of enantiomers. • There and four sterioisomeric aldotertroses.

17. Aldopentose • Aldopentoses have three chiral centers, four enantiomers and eight stereoisomer. • Only D enantiomers are shown.

18. Aldohexose • Aldohexose has eight pairs of enantiomers: allose, altrose, glucose, mannose, gulose, idose, galactose, talose.

19. Hemiacetal Formation • Alcohols add reversibly to aldehydes and ketones to form hemiacetals.

20. Hemiacetals in Sugar • Intramolecular nuclephillic addition creates a cyclic hemiacetal in sugars. • Five- and six-membered rings are stable. • The formation of a cyclic hemiactal creates an additional chiral center creating two diasteromeric forms called anomer, which are designated α and β. • α: the OH at the anomer center is on the same side as the hydroxyl that determines D,L naming in the Fischer projection • β: the OH at the anomer center is on the opposite side of the hydroxyl that determines D,L naming in the Fischer projection

21. Fischer Projections of Anomers

22. Williamson Ether Synthesis • Treatment with a alkyl halide in the presence of a base • Silver oxide is used as a catalyst for base-sensitive compounds.

23. Glycosides • Carbohydrate acetals are named by sighting the alkyl group and replacing the -ose ending of the sugar with -oside. • Glycosides are stable in water; therefore, they require an acid catalyst for hydrolysis.

24. Glycoside Formation • Treatment of a monosaccharide hemiacetal with an alcohol and an acid catalyst yields an acetal in which the anomeric -OH has been replace with an -OR group.

25. Reduction of Monosaccharides • Treatment of an aldose or ketose with NaBH4 reduces it to a polyalcohol (alditol).

26. Oxidation of Monosaccharides • Br2 in water is an effective oxidizing reagent for converting an aldose to an aldonic acid (carboxylic acid).

27. Maltose and Cellobiose • Maltose: two D-glycopyranose units with a 1,4’-α-glycoside bond • Formed from the hydrolysis of starch • Cellobiose: two D-glycopyranose units with a 1,4’-β-glycoside bond • Formed from the hydrolysis of cellulose

28. Lactose • Lactose: 1,4-D-galactopyranosyl-D-glucopyranoside • Lactose is a disaccharide that occurs naturally in milk. • Lactose is cleaved during digestion to form glucose and galactose.

29. Sucrose • A disaccharide that hydrolyzes to glucose and fructose.

30. Cellulose • Cellulose: thousands of D-glucopyranosyl 1-4’-β-glucopyranosides • Cellulose molecules form a large aggregate structure held together by hydrogen bonds.

31. Starch • Starch: 1,4--glupyranosyl-glucopyranoside polymer • Starch is digested into glucose • Starch is made of two components • Amylose • insoluble in water – 20% of starch • Amylopectin • soluble in water – 80% of starch

32. Glycogen • Glycogen is a polysaccharide that serves the same energy storage function in animals that starch does in plants. • Glycogen is highly branched and contain up to 100,000 glucose units.