24.1 Introduction to Carbohydrates

Transcription

1 24.1 Introduction to Carbohydrates Carbohydrates (sugars) are abundant in nature: They are high energy biomolecules. They provide structural rigidity for organisms (plants, crustaceans, etc.). The polymer backbone on which DNA and RNA are assembled contains sugars. The term, carbohydrate, evolved to describe the formula for such molecules: C x (H 2 O) x. Carbohydrates are NOT true hydrates. WHY? 24-1

2 Carbohydrates (sugars) are polyhydroxy aldehydes or ketones. Consider glucose, which is made by plants: Describe the potential energy change that occurs during glucose photosynthesis. Is glucose a polyhydroxy aldehyde or ketone? 24-2

3 24.2 Classification of Monosaccharides Saccharides have multiple chiral centers, and they are often drawn as Fischer projections. Designate each chirality center in glucose as either R or S. 24-3

4 Saccharides have multiple chiral centers, and they are often drawn as Fischer projections. What does the suffix, ose mean? Define the following terms: Aldose and ketose Pentose and hexose 24-4

5 Glyceraldehyde is a monosaccharide with one chirality center. Natural glyceraldehyde is dextrorotatory (D): it rotates plane polarized light in the clockwise direction. 24-5

6 24.2 Classification of Monosaccharides Naturally occurring larger sugars can be broken down into glyceraldehyde by degradation. Such sugars are often called D-sugars. 24-6

7 24.2 Classification of Monosaccharides Recall that dextrorotatory versus levorotatory rotation cannot be predicted by the R or S configuration. Here, D no longer refers to dextrorotatory. Rather it refers to the R configuration at the chiral carbon farthest from the carbonyl. 24-7

8 24.3 Configuration of Aldoses There are four aldotetroses. Two are shown below. What are the other two structures? 24-8

9 24.3 Configuration of Aldoses Aldopentoses have three chirality centers. The number of isomers will be 2 3. Recall the 2 n rule from Section 5.5. The D-sugars are naturally occurring. 24-9

10 24.3 Configuration of Aldoses Ribose is a key building block of RNA. WHAT is RNA? More detail to come in Section Arabinose is found in plants. Xylose is found in wood

11 24.3 Configuration of Aldoses Based on the 2 n rule, how many aldohexoses are there? How many of the aldohexoses are D isomers. Glucose is the most common aldohexose. Mannose and galactose are also common

12 24.4 Configuration of Ketoses Relevant ketoses have between three and six carbons. For each naturally occurring D isomer, there is an L enantiomer

13 24.4 Configuration of Ketoses 24-13

14 24.5 Cyclic Structures of Monosaccharides Recall from Section 20.5 that carbonyls can be attacked by alcohols to form hemiacetals. The intramolecular reaction is generally favored for 5 and 6- membered rings. WHY? 24-14

15 24.5 Cyclic Structures of Monosaccharides For the following compound, draw the mechanism and resulting product that results from acid catalyzed ringclosing hemiacetal formation

16 24.5 Cyclic Structures of Monosaccharides Monosaccharides, like glucose, can also undergo ringclosing hemiacetal formation. The equilibrium greatly favors the closed form called pyranose

17 24-17

18 Distinguish between the α and β anomers

19 Anomeric effect Which would you predict to be more stable? Beta 67%, alpha 33%, open 0.01% 19

20 24.5 Cyclic Structures of Monosaccharides Ketoses form both furanose (5-membered) and pyranose (6-membered) rings: 24-24

21 24.5 Cyclic Structures of Monosaccharides 70% β 0.7% 23%-β 2% α 5%-α The equilibrium concentrations in water are above

22 24.5 Cyclic Structures of Monosaccharides The furanose form takes part in most biochemical reactions

23 24.6 Reactions of Monosaccharides Monosaccharides are generally soluble in water. WHY? To improve their solubility in organic solvents, the hydroxyl groups can be acetylated. WHY is pyridine added to the reaction? How might acetylation help in purification efforts

24 24.6 Reactions of Monosaccharides Monosaccharides can also be converted to ethers via the Williamson ether synthesis. Ether linkages are more robust than ester linkages. WHY? 24-28

25 24.6 Reactions of Monosaccharides When treated with an excess of an alcohol, the hemiacetal equilibrium can be shifted to give an acetal. When a sugar is used, alpha and beta glycosides are formed

26 24.6 Reactions of Monosaccharides The mechanism of glycoside formation is analogous to the acetal formation mechanism. Only the anomeric hydroxyl group is replaced

27 24.6 Reactions of Monosaccharides The mechanism of glycoside formation is analogous to the acetal formation mechanism. What factors would you consider when trying to predict whether the alpha or beta anomer will be the major product? Practice with CONCEPTUAL CHECKPOINTs and

28 24.6 Reactions of Monosaccharides Under strongly basic conditions, glucose and mannose interconvert. Mannose and glucose are epimers because they only differ in the configuration of one carbon center. Practice with CONCEPTUAL CHECKPOINT

29 24.6 Reactions of Monosaccharides Monosaccharides can be reduced to ALDITOLs shifting the equilibrium to the right. HOW? D-sorbitol or D-glucitol are sugar substitutes

30 Reducing sugars If the sugar has an OH attached to the anomeric carbon, then the sugar is a reducing sugar If it has OR, then it is not a reducing sugar 24-34

31 24.6 Reactions of Monosaccharides Practice with SKILLBUILDER

32 24.7 Disaccharides Disaccharides form when two sugars connect through a glycosidic linkage. The 1 4 glycosidic linkage is most common. The bottom ring is capable of mutarotation at its anomeric position. Because the anomeric position of the bottom ring is a HEMIACETAL rather than an acetal, it is in equilibrium with the open form. Thus, maltose is a reducing sugar

33 24.7 Disaccharides Cellobiose is similar to maltose. WHAT are the differences? Will cellobiose be a reducing sugar? 24-43

34 24.7 Disaccharides Lactose is another disaccharide. Some people have trouble digesting lactose

35 24.7 Disaccharides Sucrose (table sugar) is also a disaccharide. Honey bees can convert sucrose into a mixture of sucrose, fructose, and glucose. Fructose is very sweet. Sucrose is not a reducing sugar. WHY? 24-45

36 24.8 Polysaccharides Cellulose is a polysaccharide containing glucose units connected through glycosidic bonds. How is cellulose capable of giving plants like trees their rigidity and strength? 24-46

37 24.8 Polysaccharides Starch is a major components of grains and other foods, like potatoes. What is the difference between molecules of starch and molecules of cellulose? Starch is made of amylose and amylopectin

38 24.8 Polysaccharides Amylopectin has some 1 6- α-glycoside branches. We can eat corn and potatoes, but not grass or trees. WHY? 24-48

39 24.9 Amino Sugars Amino sugars like glucosamine are important biomolecules. Acetylated glucosamine can form an important polysaccharide called chitin

40 24.9 Amino Sugars The carbonyl groups in chitin allow for even stronger H- bonding between neighboring chains. Chitin is used in insect and arthropod exoskeletons. WHY? 24-50

41 24.10 N-Glycosides N-glycosides can be formed when sugars are treated with an amine and an acid catalyst. RNA and DNA incorporate important N-glycosides called nucleosides

42 24.10 N-Glycosides Ribose forms ribonucleosides in RNA. Deoxyribose forms deoxyribonucleosides in DNA

43 24.10 N-Glycosides There are four different heterocyclic amines that attach to deoxyribose molecules to form DNA nucleosides

44 24.10 N-Glycosides In DNA, the nucleosides are attached to phosphate groups forming nucleotides

45 24.10 N-Glycosides The phosphate groups of the nucleotides are connected together to make the DNA strand or POLYNUCLEOTIDE

46 24.10 N-Glycosides The nucleotides in DNA can attract one another through H-bonding of the DNA base pairs

47 24.10 N-Glycosides WHY does DNA form a double helix? 24-57

48 24.10 N-Glycosides RNA is structurally different from DNA : The sugar in RNA is ribose. WHAT is the sugar in DNA? RNA contains uracil instead of thymine. RNA translates the information stored in DNA into working molecules (proteins and enzymes)

49 RNA strands generally do not form double helices like DNA. RNA strands can fold into many different shapes, and some even act as catalysts called ribozymes. It is possible that RNA evolved self-replication as an early step in the evolution of life from small molecules N-Glycosides 24-59