CHAPTER 23. Carbohydrates

Transcription

1 CAPTER 23 Carbohydrates 1

2 Introduction Carbohydrates are naturally occurring compounds of carbon, hydrogen, and oxygen. Carbohydrates have the empirical formula C 2. Carbohydrates have the general formula C n 2n n. eg. Glucose: C

3 Carbohydrates are defined as polyhydroxy aldehydes or ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones. Monosaccharides are carbohydrates that cannot be hydrolyzed to simpler carbohydrates = simple sugars. Disaccharides can be hydrolyzed to two monosaccharides. ligosaccharides yield 2 to 8 monosaccharides. 3Polysacccharides yield >8 monosaccharides

4 Some important monosacharides C C C 2 C C 2 C 2 C 2 D-glucose D- galactose D-fructose 4

5 Glucose Blood sugar. Mammals convert sucrose, lactose, maltose, and starch to glucose which is then used for energy glucose fats steroids (cholesterole) C 3 C protiens acetyl group in acetyl coenzyme A C energy 5

6 ther monosaccharides Fructose sweetest tasting sugar. It occurs in fruit and honey. Galactose found bonded to glucose in the disaccharide lactose. Ribose and deoxyribose form part of the polymeric backbone of nucleic acids. 6 C C 2 D-ribose C C C 2 C 2 2-deoxy-D-ribose deoxy = minus an oxygen

7 Classification of Monosaccharides The suffix ose is used to designate a carbohydrate. Carbohydrate names ending in ose (eg. Glucose, galactose). Mononsacchrides that contain aldehyde group Aldoses (aldehyde + ose) ( glucose and ribose). Monosacchrides with ketone group Ketoses (ketone + ose) ( frctose) 7

8 The number of carbon atoms are designated by tri, tetr-, etc ( for example a triose is three carbons) 8

9 Ketones are often given the ending ulose. example. Fructose, 9 C 2 C 2 Ketohexose or hexulose Ketopentose or pentulose C C 2 D-ribose aldopentose

10 Configuration of monosaccharides Mpnpsaccharides with the same number of carbons are structural isomers or diastereomers. Diastereomers are nonenentiomeric stereoisomers with two or more chiral centers but differ in the projection of at least one of them. If two diastereomers differ in the projection of only one chiral center epimers 10

11 Examples D-glucose, D- galactose, and D- fructose are all structural isomers ( diastereomers). D-glucose and D- galactose differ only in the projection at carbon 4 epimers C C C 2 C C 2 C 2 C 2 D-glucose D- galactose D-fructose 11

12 The D and L System If the on the last chiral carbon is projected to the right D If the on the last chiral carbon is projected to the left L 12

13 Examples C C 2 D-glyceraldehyde 13 C C 2 L-ripose C C 2 D- lyxose C C 2 L-aldohexose

14 14 The D Family of Aldoses

15 Cyclization of Monosaccharides 1 C only sugars seem to make stable hemiacetals 1 a hemiacetal C : : 4 4 C C 2 15 glucose glucopyranose

16 FURANSE AND PYRANSE RINGS : : a pyranose ring : two anomers are possible in each case a furanose ring 16 furan pyran for clarity no hydroxyl groups are shown on the chains or rings

17 ANMERS β-d-(+)-glucose : : β = up : : anomeric carbon (hemiacetal) for clarity hydroxyl groups on the chain are not shown 17 α-d-(+)-glucose α = down anomers differ in configuration at the anomeric carbon # 1

18 Glucose 2 C hemiacetals 2 C 34% α-d-(+)-glucose β-d-(+)-glucose [α] = [α] = % 1 2 C 2 C : : C 2 < 0.001% open chain Equilibrium mixture: [α] =

19 AWRT PRJECTINS It is convenient to view the cyclic sugars (glucopyranoses) as a aworth Projection, where the ring is flattened. AWRT PRJECTIN C 2 α-d-(+)-glucopyranose Standard Position upper-right back This orientation is always used for a aworth Projection 19

20 AWRT PRJECTINS ERE ARE SME CNVENTINS YU MUST LEARN 1) The ring is always oriented with the oxygen in the upper right-hand back corner. C 2 2) The -C 2 group is placed UP for a D-sugar and DWN for an L-sugar. D C 2 L 3) α-sugars have the anomeric hydroxyl group down. C 2 α 4) β-sugars have the anomeric hydroxyl group up 20 C 2 β

21 21 SME AWRT PRJECTINS -C 2 up = D -C 2 up = D C 2 C 2 cis = β trans = α BT F TESE ARE D-GLUCSE D-SUGARS β-d ANMERS α-d

22 CNVERTING FISCER PRJECTINS T AWRT PRJECTINS 22

23 CNVERTING T AWRT PRJECTINS 23 D-(+)-glucose U P C C 2 on right = D FISCER PRJECTIN D W N -C 2 up = D 4 6 C 2 5 C cis = β 1 trans = α BT ANMERS F A D-SUGAR (D-glucose) AWRT PRJECTINS

24 CNVERTING T ACTUAL CNFRMATINS -C 2 up = D C 2 C 2 cis = β trans = α β-d-(+)-glucopyranose C 2 C 2 AWRT CNFRMATIN 24 α-d-(+)-glucopyranose

25 AWRT PRJECTINS F L-SUGARS L-(+)-glucose U P on left = L C C 2 FISCER PRJECTIN D W N -C 2 down=l C 2 C 2 trans cis BT ANMERS F A L-SUGAR (L-glucose) AWRT PRJECTINS 25

26 CNVERTING FISCER T AWRT PRJECTINS Genral rules LEFT = UP RIGT = DWN β = up α = down These rules are the same for both D- and L- sugars The only difference when converting D- and L- sugars is : D-sugars -C 2 = UP L-sugars -C 2 = DWN 26

27 27 FRUCTFURANSES

28 FRUCTSE standard position C C 2 D-(-)-Fructose up = D.. : C cis = β 2 C β-d-(-)-fructofuranose anomeric carbon 28

29 Anomeric Effect β-anomer is the most stable anomer 29 C C 2 D-ribose β-d-ribopyranose (56%) C 2 β-d-ribofuranose (18%) + C 2 + α-d-ribopyranose (20%) α-d-ribofuranose (6%)

30 30 MUTARTATIN

31 Mutarotation of Glucose 2 C hemiacetals 2 C 36% α-d-(+)-glucose β-d-(+)-glucose [α] = [α] = % 2 C : : < 0.001% open chain Equilibrium mixture: [α] =

32 Glycoside Formation Glycosides are acetals at the anomeric carbon of carbohydrates R'' Remember 32 C C 2 RCR' a hemiacetal C 2 + R'' + RCR' β D-glucopyranose C 3 + An acetal C C 3 Methyl β Dglucopyranoside

33 Glycosides Glycoside has two or groups attached to the anomeric carbon two R R'' groups C RCR' 3 An acetal A glycoside two R groups Glycosides can be hydrolyzed in aqueous acid C 2 C Methyl β D-glucopyranoside + C 2 D-glucopyranose + C 3 aglycone

34 Glycoside formation ydrolysis of glycosides 34

35 xidation of Monosaccharides Aldoses and ketoses are easily oxidized when treated with Tollens reagent 35 C 2 a reducing sugar 2 C 2 C C 2 Ag(N 3 ) C Ag

36 Reducing Sugars Carbohydrates with hemiacetal linkages are reducing sugars because they react with Tollens reagent 36

37 Ketoses are also reducing sugars C 2 C C 2 Ag(N 3 ) C 2 - C C 2 + Ag D-fructose 37

38 Fructose is oxidized readily because it is in equlibrium with aldehydes through an endiol intermediate. A ketone An aldose 38

39 Glycosides are nonreducing sugars Glycosides are not in equilibrium with aldehydes nonreducing sugars C 3 + Ag(N 3 ) 2 no reaction - A glycoside 39

40 Aldonic Acids Bromine in water selectively oxidizes the aldehyde group of an aldose to the corresponding carboxylic acid 40

41 Example C C 2 Br 2, 2 p 5-6 C 2 C 2 D- glucose D-gluconic acid An aldonic acid 41

42 Aldaric Acids Dilute nitric acid oxidizes both the aldehyde and primary hydroxyl groups of an aldose to an aldaric acid 42

43 43 Example

44 Uronic Acids In biological systems, the terminal C 2 group can be oxidized without oxidation of the aldehyde group to uronic acid C 2 enzymes C 2 44 D-glucose D-glucoronic acid a uronic acid

45 Periodic Acid xidation: xidative Cleavage of Polyhydroxy Compounds Compounds with hydroxyl groups on adjacent carbons undergo cleavage of carbon-carbon bonds between the hydroxyl groups The products are aldehydes, ketones or carboxylic acids 45

46 The carbonyl group is oxidized to a carboxyl group, while the hydroxyl group is oxidized to an aldehyde or ketone. With three or more contiguous hydroxyl groups, the internal carbons become formic acid 46

47 Cleavage also takes place when a hydroxyl group is adjacent to an aldehyde or ketone group An aldehyde is oxidized to formic acid; a ketone is oxidized to carbon dioxide 47

48 48

49 49 No cleavage results if there are intervening carbons that do not bear hydroxyl or carbonyl groups

50 Reduction of Monosaccharides Aldoses and ketoses can be reduced to alditols 50

51 51 Example

52 Reaction at the ydroxyl Groups 52

53 Acetate Formation Carbohydrates react with acetic anhydride in the presence of weak base to convert all hydroxyl groups to acetate esters 53

54 Ether Formation Reaction of monosaccharides with excess dimethylsulfate and Na produces multi methyl ether. R - + C 3 SC 3 RC SC 3 C 2 (C 3 ) 2 S 2 - C 3 C 3 C 3 C 2 C 3 C 3 54

55 Cyclic Acetal Formation Carbohydrates form cyclic acetals with benzaldehyde selectively between 1,3-diol. C 6 5 C 2 C C 6 5 C 2 C + Cyclic acetal 55

56 Cyclic Ketal Formation Carbohydrates form cyclic ketals with acetone selectively between cis-vicinal hydroxyl groups. 56

57 Disaccharides A disaccharide is a carbohydrate compound of two units of monosaccharides joined together by glycoside link from carbon 1 of one unit to an of the other unit 57

58 C 2 b α-d-(+)-glucose enzyme mediated c C 2.. : a Maltose Contain two units of D- glucopyranose 1,4 -αglycosidic Linkage C 2 C 2 58 b c a Maltose umans can digest α-1,4

59 Cellobiose Two glucose units 59 C 2 b β-d-(+)-glucose 1,4 -β Glycosidic Linkage If continued, you get cellulose. umans can t digest β-1,4 C 2 b c C 2 c C 2.. : a a enzyme mediated Cellobiose

60 Sucrose (Table sugar) Sucrose is a disaccharide formed from D- glucose and D-fructose The glycosidic linkage is between C1 of glucose and C2 of fructose (both anomeric carbon atoms). Sucrose is a nonreducing sugar because of its acetal linkage 60

61 61 β-d-fructofuranosyl α-d-glucopyranoside 1,2`link

62 Polysaccharides A polysaccharide is a compound in which the molecule contain many units of monosaccharide joined together by glycoside link. Two important polysaccharides are starch and cellulose 62

63 Starch The two forms of starch are amylose and amylopectin Amylose consists typically of more than 1000 D- glucopyranoside units connected by α linkages between C1 of one unit and C4 of the next 1,4`α link 63

64 Amylopectin Amylopectin is similar to amylose but has branching points every glucose units Branches occur between C1 of one glucose unit and C6 of another 64