CHAPTER 27 CARBOHYDRATES SOLUTIONS TO REVIEW QUESTIONS

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1 27 09/17/ :12:35 Page 397 APTER 27 ARBYDRATES SLUTINS T REVIEW QUESTINS 1. In general, the carbohydrate carbon oxidation state determines the carbon s metabolic energy content. The more oxidized a carbon is, the less energy it can provide in biological systems. 2. The notations D and L in the name of a carbohydrate specify the configuration on the last chiral carbon atom (from -1) in the Fischer projection formula. If the is written to the right of that carbon the compound is a D-carbohydrate. If the is written to the left it is an L-carbohydrate. For example, D-glyceraldehyde and L-glyceraldehyde differ only at the chiral ; D-glyceraldehyde has the on the right while L-glyceraldehyde has the on the left. 3. The notations (þ) and ( ) in the name of a carbohydrate specify whether the compound rotates the plane of polarized light to the right (þ) or to the left ( ). 4. Galactosemia is the inability of infants to metabolize galactose. The galactose concentration increases markedly in the blood and also appears in the urine. Galactosemia causes vomiting, diarrhea, enlargement of the liver, and often mental retardation. If not recognized a few days after birth it can lead to death. 5. There are four pairs of epimers among the D-aldohexoxes in Figure They are: allose and altrose; glucose and mannose; gulose and idose; and galactose and talose. Glucose and mannose are epimers at carbon A carbohydrate forms a five-member or six-member heterocyclic ring (one oxygen atom, the rest carbon atoms). If it forms a five-member ring, it is termed a furanose, after the compound furan. If it forms a six-membered ring it is termed a pyranose, after the compound pyran. Furan Pyran a-d-glucopyranose and b-d-glucopyranose differ in the configuration at the number 1 carbon in the cyclic structure. In the open-chain structure, carbon 1 is the aldehyde group, and is not chiral. In the cyclic structure that carbon contains a hemiacetal structure, which is chiral. When the ring forms, carbon 1 can have two configurations leading to the two structures called a and b. 2 2 In the aworth structure for glucose the on carbon 1 is written down for the a structure and up for the b structure

2 27 09/17/ :12:35 Page hapter The cyclic forms of monosaccharides are hemiacetals, because the number one carbon has an ether and an alcohol group; whereas a glycoside is an acetal which contains two ether linkages. 9. Mutarotation is the phenomenon by which the a or b form of a sugar, when in solution, will undergo change to reach an equilibrium mixture, not necessarily 50%-50%, of the two forms. To achieve this equilibrium, the chain must open up and then reclose. n closing, it has the possibility of closing in either the a or b form as the equilibrium mixture is achieved. 10. Major sources: (a) sucrose: sugar beets and sugar cane lactose: milk (c) maltose: sprouting grain and partially hydrolyzed starch 11. The following parts are related to the eight D-aldohexoses shown in the text (Figure 27.1): (a) If each of the aldohexoses is oxidized by nitric acid to dicarboxylic acids, allose and galactose would become meso forms. ƒƒ 2 allose N 99: 3 ƒƒ ƒƒ 2 galactose N 99: 3 ƒƒ

3 27 09/17/ :12:35 Page hapter 27 - Names and structures of enantiomers of D-altrose and D-idose: ƒƒ ƒƒ ƒƒ ƒƒ D-altrose L-altrose D-idose L-idose 12. Invert sugar is sweeter than sucrose because it is a mixture of fructose and glucose. Glucose is somewhat less sweet than sucrose, but fructose is much sweeter, so the mixture is sweeter. 13. Sucralose differs from sucrose in two important ways: (a) sucralose has three chlorines in place of three hydroxyl groups; sucralose has a different configuration at one chiral carbon compared with sucrose. These two differences make sucralose a non-nutritive sweetener. 14. Sugar alcohols are poorly absorbed by the small intestine and, thus, can contribute only few dietary calories. 15. Based on Figure 27.3, the alpha glucose anomer gives a specific optical rotation of þ Amylose is a linear polymer that forms a helix in solution. In contrast, amylopectin is a branching polymer that forms a tree-like shape in solution. 17. Glycoaminoglycans protect our joints from osteoarthritis by acting as a shock absorber and by keeping the bones from rubbing against each other. 18. Glucose provides metabolic energy via two important metabolic processes. They are glycolysis and the citric acid cycle. 19. Blood types A and B differ in one place. In the fourth pyranose ring at -2, blood type B has an group, while blood type A has an amide group. 20. Branch chains of glucose in amylopectin are linked to the main chain by a single a-(1,6) linkage. All the other glucose units are linked by a-(1,4) linkages. learly, upon hydrolysis, many more maltose molecules than isomaltose molecules will be formed. (See Figure 27.7.) 21. Yes. L-glucose is a reducing sugar. The linear structure has an aldehyde group which is reducible by Benedict solution; or an hemiacetal structure which opens to form an aldehyde structure

4 27 09/17/ :12:35 Page hapter 27 - SLUTINS T EXERISES 1. There are no chiral carbons in dihydroxyacetone D-glyceraldehyde L-glyceraldehyde D-aldopentose. The chiral carbon furthest from the aldehyde group has the on the right in the Fischer projection formula. 4. L-aldopentose. The chiral carbon furthest from the aldehyde group has the on the left in the Fischer projection formula. 5. A common carbohydrate with the formula, 4 8 4, can be either 2 2 aldotetrose 2 ketotetrose but not a ketopentose. The correct answer is the aldotetrose since it has two chiral carbons

5 27 09/17/ :12:35 Page hapter A common carbohydrate with the formula, , can be either 2 2 aldohexose 2 ketohexose but not an aldopentose. The correct answer is the ketohexose since it has 3 chiral carbons. 7. An enantiomer is a mirror-image stereoisomer. In the Fischer formula, the positions of the atoms around chiral carbons must be carefully shown. For achiral carbons, the positions of the atoms are not critical An enantiomer is a mirror-image stereoisomer. In the Fischer formula, the positions of the atoms around chiral carbons must be carefully shown. For achiral carbons, the positions of the atoms are not critical

6 27 09/17/ :12:36 Page hapter (a) An epimer is a stereoisomer that differs at one chiral carbon. For example, the following epimer differs from D-galactose at the carbon adjacent to the carbonyl carbon. 2 An enantiomer is a mirror-image stereoisomer. 2 (c) A diastereomer is any stereoisomer that is not a mirror image. For example, the following sugar differs from D-galactose at all chiral carbons except the bottom-most and it is not a mirror image of D-galactose

7 27 09/17/ :12:36 Page hapter (a) An epimer is a stereoisomer that differs at one chiral carbon. For example, the following epimer differs from D-fructose at the carbon just below the carbonyl carbon. 2 2 An enantiomer is a mirror-image stereoisomer. 2 2 (c) A diastereomer is any stereoisomer that is not a mirror image. For example, the following sugar differs from D-fructose at all chiral carbons except the bottom-most and it is not a mirror image of D-fructose

8 27 09/17/ :12:36 Page hapter Since D-glucose and D-mannose are epimers, they differ at only one chiral center. Their structures are almost identical. 12. Since D-galactose and D-mannose are diastereomers, they are non-mirror image stereoisomers. They may differ at one or more chiral carbons but they cannot be enantiomers. 13. (a) 2 (c) (a) 2 2 (c) The hemiacetal carbon is the only carbon in this structure that is directly bonded to two oxygens. 2 *

9 27 09/17/ :12:36 Page hapter The hemiketal carbon is the only carbon in this structure that is directly bonded to two oxygens. 2 * The monosaccharide composition of: (a) sucrose: one glucose and one fructose unit glycogen: many glucose units (c) amylose: many glucose units (d) maltose: two glucose units 18. The monosaccharide composition of: (a) lactose: one glucose and one galactose unit amylopectin: many glucose units (c) cellulose: many glucose units (d) sucrose: one glucose and one fructose unit 19. Both cellobiose and isomaltose are disaccharides composed of two glucose units. owever, in cellobiose monosaccharides are linked by a b-1,4-acetal bond while for isomaltose the linkage is a-1, cellobiose 2 2 isomaltose

10 27 09/17/ :12:36 Page hapter Both maltose and isomaltose are disaccharides composed of two glucose units. The glucose units in maltose are linked by an a-1,4-glycosidic bond while the glucose units in isomaltose are linked by an a-1,6-glycosidic bond. 2 maltose isomaltose 21. Lactose will show mutarotation; sucrose will not. The hemiacetal structure in lactose will open allowing mutarotation. Since sucrose has an acetal structure and no hemiacetal, it will not undergo mutarotation. 22. Both maltose and isomaltose will show mutarotation. Both disaccharides contain a hemiacetal structure which will open allowing mutarotation sucrose 2 isomaltose

11 27 09/17/ :12:36 Page hapter maltose 2 2 lactose 25. isomaltose, a-d-glucopyranosyl-(1,6)-a-d-glucopyranose; sucrose, a-d-glucopyranosyl-(1,2)-(b-dfructofuranose. 26. maltose, a-d-glucopyranosyl-(1,4)-a-d-glucopyranose; lactose, b-d-galactopyranosyl-(1,4)-a-dglucopyranose. 27. (a)

12 27 09/17/ :12:36 Page hapter (a) Br Br Br Br

13 27 09/17/ :12:36 Page hapter /Pt reduces the aldehyde group on D-ribose to a primary alcohol /Pt reduces the aldehyde group on D-mannose to a primary alcohol Warm, dilute nitric acid oxidizes both primary alcohols and aldehyde groups to carboxylic acids

14 27 09/17/ :12:36 Page hapter Warm, dilute nitric acid oxidizes both primary alcohols and aldehyde groups to carboxylic acids. 35. (a) Amylopectin and glycogen share the same structure except that glycogen has more 1,6-links and, thus, more branches. D-glucose and D-galactose share the same structure except for the configuration at the -4 chiral carbon. (c) Lactose and maltose are both disaccharides with a 1,4 linkage. Each has one a-d-glucopyranose unit. Lactose has a b-d-galactopyranose for the other unit while maltose has an a-dglucopyranose. 36. (a) D-ribose and D-ribulose share the same structure except that D-ribose is an aldose while D-ribulose is a ketose (and, thus, has one less chiral carbon). Maltose and isomaltose share the same structure except that maltose contains a (1,4) acetal bond while isomaltose contains a (1,6) acetal bond. (c) ellulose and amylose share the same structure except that cellulose has b-anomer glucose units while amylose has a-anomer glucose units maltose cellobiose

15 27 09/17/ :12:36 Page hapter Glucose is called blood sugar because it is the most abundant carbohydrate in the blood and is carried by the bloodstream to all parts of the body. 42. Two advantages of aspartame over sucrose: (a) aspartame sweetens without added calories; aspartame does not cause dental caries. Two advantages of neotame over aspartame: (a) neotame does not release phenylalanine, an amino acid that is dangerous to people with phenylketonuria; neotame is intensely sweet and can be used in very small amounts. 43. Glucose molecules change from one anomer to the other in solution. This process is called mutarotation. A solution with a specific optical rotation of þ18:7 contains primarily the b-anomer of glucose and, as the optical rotation becomes more positive, more of the a-anomer is forming

16 27 09/17/ :12:36 Page hapter When D-galactose reacts to form a cyanohydrin, the carbonyl group reacts with N. This reaction is called addition because cyanide adds to the carbonyl carbon while adds to the carbonyl oxygen. The double bond is lost in this process. Two isomers form in this reaction. N N (a) D-2-deoxyribose has two chiral carbons (marked with asterisks in the following structure). * * 2 2 When D-2-deoxyribose forms the ringed structure, a-d-2-deoxyribofuranose, a new chiral carbon forms at the hemiacetal position. a-d-2-deoxyribofuranose has three chiral carbons as marked by asterisks in the following structure: 2 * * *

17 27 09/17/ :12:36 Page hapter (a) The sugar acid could be most easily derived from b-d-mannopyranose. n 47. ompound A must be a reducing disaccharide because it produces a reddish color in the Benedict test. Sucrose is nonreducing with the Benedict test so compound A must be maltose. 48. A nonreducing disaccharide composed of two molecules of a-d-galactopyranose can have no hemiacetal structures. Thus, the hemiacetal structure of one a-d-galactopyranose must be used to form the glycosidic link to the hemiacetal structure of the other a-d-galactopyranose unit forming an acetal structure ellulose, amylose, and amylopectin are polymers of glucose. ellulose exists in the form of fibers, is not digestible by humans, and therefore remains in the digestive tract as fibers. Amylose and amylopectin are digested to glucose which is dissolved into the bloodstream. 50. (a) D-galactose and D-glucose differ only at carbon 4. Thus, D-galactose must be changed at carbon 4 to be converted to D-glucose. D-galactose is an epimer of D-glucose. 51. No, the classmate should not be believed. Although D-glucose and D-mannose are related as epimers, it is pairs of enantiomers which have equal and opposite optical rotation. 52. b-d-glucopyranose differs at two chiral carbons from the structure shown (a-d-galactopyranose). To convert a-d-galactopyranose to b-d-glucopyranose, the a-anomer must be changed to the b-anomer and the hydroxyl at -4 must be changed from up to down. The structure for b-d-glucopyranose follows:

18 27 09/17/ :12:36 Page hapter (a) acid (c) 2 2 Fructose is sweeter than sucrose. When sucrose is broken down to its two component monosaccharides, glucose and fructose, the candy becomes sweeter