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CHAPTER 27
CARBOHYDRATES
SOLUTIONS TO REVIEW QUESTIONS
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 C-1) in the Fischer projection formula. If the
is written
is written to
to the right of that carbon the compound is a D-carbohydrate. If the
the left it is an L-carbohydrate. for example, D-glyceraldehyde and L-glyceraldehyde
differ only at the chiral C ¬ OH; D-glyceraldehyde has the
on the right while
L-glyceraldehyde has the
on the left.
3.
The notations
and
in the name of a carbohydrate specify whether the compound
rotates the plane of polarized light to the right
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 27.1. They are: allose
and altrose; glucose and mannose; gulose and idose; and galactose and talose.
6.
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.
Pyran
Furan
O
7.
C4H4O
C5H6O
O
-D-glucopyranose and -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 and b.
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7.
(cont.)
CH2OH
O
CH2OH
O
␣
OH
␤
OH
OH
HO
OH
HO
OH
OH
on carbon 1 is written down for the a
In the Haworth structure for glucose the
structure and up for the structure.
8.
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 or 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. On
closing, it has the possibility of closing in either the or form as the equilibrium
mixture is achieved.
10.
Major sources:
(a) sucrose: sugar beets and sugar cane
(b) 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.
COOH
CHO
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
CH 2OH
allose
HNO
3
99:
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
COOH
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CHO
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
HO ¬¬¬¬¬ H
ƒ
ƒ
HO ¬¬¬¬¬ H
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
COOH
HNO
3
99:
CH 2OH
galactose
(b)
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
HO ¬¬¬¬¬ H
ƒ
ƒ
HO ¬¬¬¬¬ H
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
COOH
Names and structures of enantiomers of D-altrose and D-idose:
CHO
ƒ
ƒ
HO ¬¬¬¬¬ H
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
CH 2OH
D-altrose
CHO
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
HO ¬¬¬¬¬ H
ƒ
ƒ
HO ¬¬¬¬¬ H
ƒ
ƒ
HO ¬¬¬¬¬ H
ƒ
ƒ
CH 2OH
L-altrose
CHO
ƒ
ƒ
HO ¬¬¬¬¬ H
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
HO ¬¬¬¬¬ H
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
CH 2OH
D-idose
CHO
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
HO ¬¬¬¬¬ H
ƒ
ƒ
H ¬¬¬¬¬ OH
ƒ
ƒ
HO ¬¬¬¬¬ H
ƒ
ƒ
CH 2OH
L-idose
12.
Invert sugar is sweeter than sucrose because it is a 50–50 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 two chlorines in place of two hydroxyl groups;
(b) sucralose has a different configuration at one chiral carbon compared with sucrose.
These two differences make sucralose a non-nutritive sweetener.
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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 +112º.
16.
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 C-2, blood type B
has an OH group, while blood type A has an amide group.
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SOLUTIONS TO EXERCISES
1.
There are no chiral carbons in dihydroxyacetone.
CH2 OH
C
O
CH2 OH
2.
D-glyceraldehyde
H
L-glyceraldehyde
O
O
CH
CH
C
OH
HO
C
H
CH2 OH
CH2 OH
3.
D-aldopentose.
The chiral carbon furthest from the aldehyde group has the –OH on the
right in the Fischer projection formula.
4.
L-aldopentose.
5.
A common carbohydrate with the formula, C4H8O4, can be either
The chiral carbon furthest from the aldehyde group has the –OH on the
left in the Fischer projection formula.
O
CH
CH2OH
CHOH
C
CHOH
CHOH
CH2OH
CH2OH
aldotetrose
ketotetrose
O
but not a ketopentose. The correct answer is the aldotetrose since it has two chiral
carbons.
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6.
A common carbohydrate with the formula, C6H12O6, can be either
O
CH
CH2OH
CHOH
C
CHOH
CHOH
CHOH
CHOH
CHOH
CHOH
CH2 OH
CH2 OH
aldohexose
O
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.
O
CH
HO
C
H
H
C
OH
H
C
OH
HO
C
H
CH2 OH
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8.
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.
CH2 OH
C
O
H
C
OH
HO
C
H
H
C
OH
CH2 OH
9.
(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.
O
CH
HO
C
H
HO
C
H
HO
C
H
H
C
OH
CH2 OH
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(b)
An enantiomer is a mirror image stereoisomer.
O
CH
HO
C
H
H
C
OH
H
C
OH
HO
C
H
CH2 OH
(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 bottommost and it is not a mirror image of D-galactose.
O
CH
HO
C
H
H
C
OH
H
C
OH
H
C
OH
CH2 OH
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10.
(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.
CH2 OH
C
O
H
C
OH
H
C
OH
H
C
OH
CH2 OH
(b)
An enantiomer is a mirror image stereoisomer.
CH2 OH
C
O
H
C
OH
HO
C
H
HO
C
H
CH2 OH
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(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.
CH2 OH
C
O
H
C
OH
HO
C
H
H
C
OH
CH2 OH
11.
(a)
(c)
CH 2OH
O
CH 2OH
O
OH HO
OH
HO
OH
HO
OH
OH
(b)
HOCH2
O
OH
OH
OH
12.
CH2OH
(a)
CH2 OH
O
(b)
O
OH
OH
HO
OH
OH
HO
OH
OH
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(c)
HOCH2 O
OH
OH
13.
OH
The hemiacetal carbon is the only carbon in this structure that is directly bonded to two
oxygens.
HOCH2 O
OH
*
OH
14.
The hemiketal carbon is the only carbon in this structure that is directly bonded to two
oxygens.
HOCH 2 O
OH
HO
*
CH 2 OH
OH
15.
The monosaccharide composition of:
(a) sucrose: one glucose and one fructose unit
(b) glycogen: many glucose units
(c) amylose: many glucose units
(d) maltose: two glucose units
16.
The monosaccharide composition of:
(a) lactose: one glucose and one galactose unit
(b) amylopectin: many glucose units
(c) cellulose: many glucose units
(d) sucrose: one glucose and one fructose unit
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17.
Both cellobiose and isomaltose are disaccharides composed of two glucose units.
However, in cellobiose monosaccharides are linked by a -1,4-acetal bond while for
isomaltose the linkage is -1,6.
CH2OH
O
CH2OH
O
CH2OH
O
OH
OH
OH
HO
O
OH
OH
OH
O
HO
OH
cellobiose
CH2
O
OH
OH
HO
OH
isomaltose
18.
Both maltose and isomaltose are disaccharides composed of two glucose units. The
glucose units in maltose are linked by an -1,4-glycosidic bond while the glucose units in
isomaltose are linked by an -1,6-glycosidic bond.
CH2OH
O
OH
CH2OH
O
OH
CH2OH
O
OH
O
HO
OH
OH
OH
HO
OH
maltose
O
CH2
O
OH
HO
OH
OH
isomaltose
19.
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.
20.
Both maltose and isomaltose will show mutarotation. Both disaccharides contain a
hemiacetal structure which will open allowing mutarotation.
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21.
CH2OH
O
CH2OH
O
OH
OH
HO
isomaltose
HO
sucrose
OH
HOCH2 O
OH
HO
CH2OH
CH2OH
OH
O
O
OH
OH
OH
OH
CH2OH
CH2OH
lactose
O
O
HO
OH
CH2OH
O
HO
O
OH
OH
maltose
CH
OH 2
O
HO
22.
O
O
OH
OH
OH
OH
OH
23.
isomaltose, α-D-glucopyranosyl-(1,6)-α-D-glucopyranose; sucrose, α-D-glucopyranosyl(1,2)-β-D-fructofuranose.
24.
maltose, α-D-glucopyranosyl-(1,4)-α-D-glucopyranose; lactose, β-D-galactopyranosyl(1,4)-α-D-glucopyranose.
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25.
(a)
CH2OH
O
CH2OH
O
OH
OH HO
O
HO
OH
OH
(b)
CH2OH
CH2OH
O
HO
O
O
OH
OH
OH
OH
OH
26.
(a)
CH2OH
CH2OH
O
O
O
OH
OH
OH
HO
OH
OH
(b)
CH2OH
O
HO
OH
O
OH
CH2
O
OH HO
HO
OH
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27.
H2/Pt reduces the aldehyde group on D-ribose to a primary alcohol.
CH2 OH
H
C
OH
H
C
OH
H
C
OH
CH2 OH
28.
H2/Pt reduces the aldehyde group on D-mannose to a primary alcohol.
CH2 OH
HO
C
H
HO
C
H
H
C
OH
H
C
OH
CH2 OH
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29.
Warm, dilute nitric acid oxidizes both primary alcohols and aldehyde groups to
carboxylic acids.
COOH
HO
C
H
HO
C
H
H
C
OH
H
C
OH
COOH
30.
Warm, dilute nitric acid oxidizes both primary alcohols and aldehyde groups to
carboxylic acids.
COOH
H
C
OH
H
C
OH
H
C
OH
COOH
31.
(a)
(b)
(c)
32.
(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 C-4 chiral carbon.
Lactose and maltose are both disaccharides with a 1,4 linkage. Each has one
α-D-glucopyranose unit. Lactose has a β-D-galactopyranose for the other unit while
maltose has an α-D-glucopyranose.
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).
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(b)
(c)
33.
Maltose and isomaltose share the same structure except that maltose contains a
(1,4) acetal bond while isomaltose contains a (1,6) acetal bond.
Cellulose and amylose share the same structure except that cellulose has β-anomer
glucose units while amylose has α-anomer glucose units.
CH 2OH
O
CH 2OH
O
O
OH
maltose
OH
OH
HO
OH
OH
34.
CH 2OH
O
CH 2OH
O
O
OH
OH
cellobiose
OH
HO
OH
OH
35.
CH 2OH
O
OH
CH 2OH
O
O
CH2OH
O
OH
OH
OH
HO
OH
OH
HO
O
OH
CH2
O
OH
HO
OH
OH
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36.
CH 2OH
O
OH
CH 2OH
O
O
CH 2OH
O
OH
OH
OH
HO
OH
OH
HO
O
OH
CH2
O
OH
HO
OH
OH
37.
(a)
(b)
(c)
D-galactose
38.
(a)
(b)
(c)
D-ribose
39.
lactose, b -D-galactopyranosyl-(1,4)- -D-glucopyranose
40.
maltose, -D-glucopyranosyl-(1,4)- -D-glucopyranose
41.
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; (b) 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; (b) 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 +18.7º contains
primarily the β-anomer of glucose and, as the optical rotation becomes more positive,
more of the α-anomer is forming.
-D-glucopyranose
b -D-ribofuranose
b -D-glucopyranose
-D-fructofuranose
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44.
When D-galactose reacts to form a cyanohydrin, the carbonyl group reacts with HCN.
This reaction is called addition because cyanide adds to the carbonyl carbon while H adds
to the carbonyl oxygen. The double bond is lost in this process. Two isomers form in this
reaction.
CN
CN
H
C
OH
HO
C
H
H
C
OH
H
C
OH
HO
C
H
HO
C
H
HO
C
H
HO
C
H
H
C
OH
H
C
OH
CH2 OH
45.
(a)
CH2 OH
D-2-deoxyribose
has two chiral carbons (marked with asterisks in the following
structure).
O
CH
CH2
H
*C
OH
H
*C
OH
CH2 OH
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(b)
When D-2-deoxyribose forms the ringed structure, α-D-2-deoxyribofuranose, a new
chiral carbon forms at the hemiacetal position. α-D-2-deoxyribofuranose has three
chiral carbons as marked by asterisks in the following structure:
HOCH2 O
*
*
OH
*
OH
46.
(a)
(b)
The sugar acid could be most easily derived from -D-mannopyranose.
COOH
COOH
COOH
O
O
OH HO
O
OH HO
O
O
OH HO
O
n
47.
Compound A must be a reducing disaccharide because it produces a reddish color in the
Benedict test. Sucrose is nonreducing so compound A must be maltose.
48.
A nonreducing disaccharide composed of two molecules of -D-galactopyranose can
have no hemiacetal structures. Thus, the hemiacetal structure of one -D-galactopyranose
must be used to form the glycosidic link to the hemiacetal structure of the other
-D-galactopyranose unit.
CH2OH
CH2OH
O
HO
O
HO
OH
OH
OH
OH
O
49.
Cellulose, amylose, and amylopectin are polymers of glucose. Cellulose 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.
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50.
(a)
(b)
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.
(a)
CH2OH
O
OH
HO
OH
O
O
HO CH2
HO
CH2OH
HO
(b)
CH2OH
CH2OH
O
O
OH
OH
HO
OH
HO
OH
OH
O
HO CH2
acid
H2O
O
HO CH2
HO
OH
HO
CH2OH
HO
(c)
O
CH2OH
HO
Fructose is sweeter than sucrose. When sucrose is broken down to its two
component monosaccharides, glucose and fructose, the candy becomes sweeter.
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