Chemistry The Science in Context

Thomas R. Gilbert

Chapter 20

Organic and Biological Molecules: The Compounds of Life - all with Video Answers

Educators

Chapter Questions

Problem 1

The nucleotides in DNA contain the bases with the structures shown in Figure P20.1. Identify the basic functional groups in the structures.
(FIGURE CAN'T COPY)

Jil Modi

Jil Modi

Rutgers, The State University of New Jersey

Problem 2

Bentiromide (Figure $\mathrm{P} 20.2$ ) is a peptide that was once evaluated as an agent to monitor the function of the pancreas during therapy. Draw the structures of the amino acids that form bentiromide, and indicate whether they are $\alpha$ -amino acids.
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 3

Olive oil contains triglycerides such as those shown in Figure P20.3. Which of the fatty acids in these triglycerides is/are saturated?
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 4

The major component of the antibiotic ointment bacitracin is a cyclic polypeptide called bacitracin
A. It was first isolated in 1943 from a knee scrape from
a girl named Margaret Tracy, after whom it is named. Bacitracin is effective topically and is used to treat skin, eye, and wound infections. Figure $\mathrm{P} 20.4$ shows the structure of bacitracin A. Identify the amino acids found in human proteins that are also part of the structure of bacitracin A.
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 5

The human brain produces polypeptides called endorpbins that help in controlling pain. The pentapeptide in Figure $\mathrm{P} 20.5$ is called enkephalin. Identify the five amino acids that make up enkephalin.
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 6

Cocoa butter (Figure $\mathrm{P} 20.6$ ) is a key ingredient in chocolate. Cocoa butter is a triglyceride that results from esterification of glycerol with three fatty acids. Identify the fatty acids produced by hydrolysis of cocoa butter(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 7

The role of "trans fats" in human health has been extensively debated both in the scientific community and in the popular press. What type of isomerism does the word "trans fat" refer to? Which of the molecules in Figure $P 20.7$ are considered trans fats?
(FIGURE CAN'T COPY)

Jil Modi

Rutgers, The State University of New Jersey

Problem 8

Figure $P 20.8$ shows the titration curve of which of these amino acids: leucine, histidine, or lysine?
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 9

The molecular structure of the artificial sweetener sucralose (trade name Splenda) is shown in Figure $\mathrm{P} 20.9 .$ Advertising for this product claims that it is made from sugar, implying that it is a natural product. What sugar might it be made from? Comment on the implication that it is a "natural" product.
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 10

D. Covalent bonding leads to primary structure and intermolecular forces, both of which ultimately lead to secondary, tertiary, and quaternary structure. Use representations [A] through [I] in Figure $\mathrm{P} 20.10$ to answer questions a-f.
a. $[\mathrm{A}]$ depicts two strands of DNA in a double helix. Are the two strands held together by covalent bonds, intermolecular forces, or both? [B] depicts two alpha helices linked together. Are the two helices held together by covalent bonds, intermolecular forces, or both?
b. $[\mathrm{C}]$ depicts a phospholipid bilayer. Is the bilayer held together by covalent bonds, intermolecular forces, or both?
c. [D] depicts a disaccharide. Are the two sugars each held together by covalent bonds, intermolecular forces, or both?
d. Which of the single molecules depicted are an important structural component of
$[\mathrm{C}] ?$
e. Which of the single molecules depicted react to form [E]?
f. What intermolecular forces are typically involved for the amino acid depicted
in [F] when it is incorporated into a protein?
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 11

Can all of the terms enantiomer, acbiral, and optically active be used to describe a single compound? Explain.

Jil Modi

Rutgers, The State University of New Jersey

Problem 12

Two compounds have the same structure and the same physical properties but also have the same optical activity. Are they enantiomers or the same molecule?

Charles Thomas

Numerade Educator

Problem 13

How do constitutional isomers differ from stereoisomers?

Jil Modi

Rutgers, The State University of New Jersey

Problem 14

Could a racemic mixture be distinguished from an achiral compound on the basis of optical activity? Explain your answer.

Charles Thomas

Numerade Educator

Problem 15

Why is the amino acid glycine (Figure $\mathrm{P} 20.15$ ) achiral?
(FIGURE CAN'T COPY)

Jil Modi

Rutgers, The State University of New Jersey

Problem 16

Can stereoisomers of molecules such as cis and trans $\mathrm{RHC}=\mathrm{CHR}$ also have optical isomers? (R may be any of the functional groups we have encountered in this textbook.) Explain your answer.

Charles Thomas

Numerade Educator

Problem 17

Which type of hybrid orbitals on a carbon atom, $s p, s p^{2},$ or $s p^{3},$ can give rise to enantiomers?

Jil Modi

Rutgers, The State University of New Jersey

Problem 18

Could an oxygen atom in an alcohol, ketone, or ether ever be a chiral center in the molecule?

Charles Thomas

Numerade Educator

Problem 19

Which of the molecules in Figure $\mathrm{P} 20.19$ are constitutional isomers of octane $\left(\mathrm{C}_{8} \mathrm{H}_{18}\right) ?$
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 20

Which of the molecules in Figure $\mathrm{P} 20.20$ are constitutional isomers of heptane $\left(\mathrm{C}_{7} \mathrm{H}_{16}\right) ?$
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 21

Cinnamon Label the isomers of cinnamaldehyde (oil of cinnamon) in Figure $\mathrm{P} 20.21$ as cis or trans and $E$ or $Z$
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 22

Figure $P 20.22$ shows the carbon-skeleton structure of carvone, which is found in oil of spearmint. Why doesn't the molecule carvone have cis and trans isomers?
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 23

Which of the molecules in Figure $\mathrm{P} 20.23$ are chiral?
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 24

Which, if any, of the molecules shown in Figure $\mathrm{P} 20.24$ contains a chiral center?
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 25

Artificial Sweeteners Artificial sweeteners are fundamental to the diet food industry. Figure $\mathrm{P} 20.25$ shows three artificial sweeteners that have been used in food. Saccharin is the oldest, dating to $1879 .$ Cyclamates were banned in the United States in 1969 after research suggested they led to tumors. Aspartame may be more familiar to you under the name NutraSweet. Each of these sweeteners contains between zero and two chiral carbon atoms. Circle the chiral center(s) in each compound.
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 26

Researchers at Tufts University have found that a fungal toxin called cytochalasin B (Figure P20.26) can restore elasticity and shrink skin cells in mice without causing any ill effects. Mice treated with a cream containing this molecule had smoother skin than mice treated with a plain cream. Circle the chiral centers in the molecule.
(FIGURE CAN'T COPY)

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 27

Crestor (shown as the carboxylic acid anion in Figure $\mathrm{P} 20.27$ ) is one of the top-ten best-selling pharmaceutical agents in the world.
a. Is the anion chiral? If so, how many chiral centers does
it have?
b. Are any other kinds of constitutional isomers possible for the anion? If so, draw them.
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 28

In 2009 Plavix (Figure P20.28) was the second-highest-selling pharmaceutical in the world. It is prescribed to prevent blood clots in heart attack and stroke patients.
Is Plavix optically active? If so, identify the chiral center(s) in the structure.
b. Plavix contains an ester group. Identify it. Draw the carboxylic acid and the alcohol you could react together to make Plavix.
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 29

In living cells, amino acids combine to make peptides and proteins. Are these processes accompanied by increases or decreases in entropy of the reaction system?

Kevin Zaborsky

Numerade Educator

Problem 30

What is the difference between a peptide bond and an amide bond?

Charles Thomas

Numerade Educator

Problem 31

The author of the book Into the Wild postulated that the main character in the book died in the Alaskan wilderness because he accidentally ate seeds containing a toxin called $\beta$ -ODAP (Figure $\mathrm{P} 20.31$ ). This substance is a neurotoxin; its mechanism of action is not fully understood, but it is referred to as a "glutamic acid mimic." Compare the structure of $\beta$ - ODAP to that of glutamic acid and describe the similarities between the two
molecules.

(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 32

Cystinuria is a metabolic disease that occurs in some breeds of dogs. It is characterized by the presence of kidney stones made of cystine, a dimer formed when two cysteine residues covalently link through an $-\mathrm{S}-\mathrm{S}$ - bond (Figure 20.27 ).
a. Draw the structure of the dipeptide Cys-Cys.
b. Is cystine a dipeptide? Why or why not?

(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 33

Meteorites contain more L-amino acids, which are the forms that make up the proteins in our bodies, than D-amino acids. What do the prefixes $\mathrm{L}$ - and $\mathrm{D}$ - mean?

Kevin Zaborsky

Numerade Educator

Problem 34

Do any of the amino acids in Table 20.2 have more than one chiral carbon atom per molecule?

Charles Thomas

Numerade Educator

Problem 35

Which of the compounds in Figure $\mathrm{P} 20.35$ is not an $\alpha$ -amino acid?
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 36

Which of the compounds in Figure $\mathrm{P} 20.36$ are $\alpha$ -amino acids?
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 37

Why do most amino acids exist in the zwitterionic form at physiological $\mathrm{pH}(\mathrm{pH} \approx 7.4) ?$

Kevin Zaborsky

Numerade Educator

Problem 38

Draw the condensed structural formulas of the amino acid tyrosine that you would expect to predominate in aqueous solution under the following conditions:
a. in strongly acidic solution
b. in strongly basic solution
c. in a solution in which $\mathrm{pH}=\mathrm{p} I$

Charles Thomas

Numerade Educator

Problem 39

When protein strands fold back on themselves in forming stable tertiary structures, lysine residues are often paired up with glutamic acid residues. Why?

Kevin Zaborsky

Numerade Educator

Problem 40

Ion-ion interactions are particularly effective at stabilizing tertiary structures of proteins. Suggest a pair of amino acid residues that would be attracted to each other via ion-ion interactions at pH 7.4

Charles Thomas

Numerade Educator

Problem 41

Draw structures and name all possible dipeptides produced from condensation reactions of the following L-amino acids:
a. alanine $+$ isoleucine
b. serine $+$ tyrosine
c. valine $+$ phenylalanine

Vishal Sharma

Numerade Educator

Problem 42

Draw structures of the peptides produced from condensation reactions of the following L-amino acids. Assume the amino acids bond in the order given.
a. glycine $+$ tyrosine $+$ aspartic acid
b. serine $+$ threonine $+$ leucine
c. asparagine $+$ lysine $+$ histidine

Charles Thomas

Numerade Educator

Problem 43

Identify the amino acids in the dipeptides shown in Figure $\mathrm{P} 20.43$
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 44

Identify the amino acids in the tripeptides in Figure $\mathrm{P} 20.44$
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 45

Identify the missing product in the metabolic reaction shown in Figure P20.45.
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 46

The molecular formula for glycine is $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NO}_{2} .$ What is the molecular formula of the linear peptide formed when ten glycine molecules are linked together in peptide bonds?

Charles Thomas

Numerade Educator

Problem 47

What are the structural differences between starch and cellulose?

Kevin Zaborsky

Numerade Educator

Problem 48

Why is the discovery of enzymes that catalyze cellulose hydrolysis a worthwhile objective?

Charles Thomas

Numerade Educator

Problem 49

Is the fuel value (see Chapter 5 ) of glucose in the linear form the same as that in the cyclic form?

Kevin Zaborsky

Numerade Educator

Problem 50

Without doing the actual calculation, estimate the fuel values of glucose and starch by considering average bond energies. Do you predict the fuel values of the two substances to be the same or different?

Charles Thomas

Numerade Educator

Problem 51

The second step in glycolysis converts glucose 6 -phosphate into fructose 6 -phosphate. Can you think of a reason why $\Delta G^{\circ}$ for this reaction is close to zero?

Kevin Zaborsky

Numerade Educator

Problem 52

Which of the following statements are correct about glycosidic bonds in carbohydrates?
a. The glycosidic bond in maltose is hydrolyzed by people who are lactose intolerant.
b. A glycosidic bond links glucose and fructose together to form sucrose.
c. A glycosidic bond is an ether linkage, but all ether linkages are not glycosidic bonds.

Charles Thomas

Numerade Educator

Problem 53

How do we calculate the overall free-energy change of a process consisting of two steps?

Kevin Zaborsky

Numerade Educator

Problem 54

During glycolysis a monosaccharide is converted to pyruvate. Do you think this process produces an increase or decrease in the entropy of the system? Explain your answer.

Charles Thomas

Numerade Educator

Problem 55

Describe the similarities and differences in the structures of the $\alpha$ and $\beta$ isomers formed when galactose (Figure $\mathrm{P} 20.55$ ) forms a six-membered ring.
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 56

Describe the similarities and differences in the structures of the $\alpha$ and $\beta$ isomers formed when ribose (Figure $\mathrm{P} 20.56$ ) forms a five-membered ring.
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 57

Which, if any, of the structures in Figure $\mathrm{P} 20.57$ are $\beta$ isomers of a monosaccharide?
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 58

Which, if any, of the structures in Figure P20.58 are $\alpha$ isomers?
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 59

Which of the saccharides in Figure $\mathrm{P} 20.59$ is digestible by humans?
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 60

For any of the disaccharides in Problem 20.59 that are not digestible by humans, draw an isomer that would be digested.

Charles Thomas

Numerade Educator

Problem 61

The structure of the disaccharide maltose appears in Figure $\mathrm{P} 20.61 .$ Hydrolysis of 1 mole of maltose $\left(\Delta G_{f}^{\circ}=-2246.6 \mathrm{kJ} / \mathrm{mol}\right)$ produces 2 moles of glucose $\left(\Delta G_{f}^{\circ}=-1274.4 \mathrm{kJ} / \mathrm{mol}\right)$
$$\text { Maltose }+\mathrm{H}_{2} \mathrm{O} \rightarrow 2 \text { glucose }$$
If the value of $\Delta G_{f}^{\circ}$ for water is $-285.8 \mathrm{kJ} / \mathrm{mol},$ what is the change in free energy of the hydrolysis reaction?
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 62

If the maltose in Problem 20.61 were replaced by another disaccharide, would you expect the free-energy change for the hydrolysis to be exactly the same or just similar in value? Explain your answer.

Charles Thomas

Numerade Educator

Problem 63

What is the difference between a saturated and an unsaturated fatty acid?

Kevin Zaborsky

Numerade Educator

Problem 64

Which of the following lipids would have the lowest energy value in terms of human nutrition: olive oil, margarine, olestra, or butter? Explain your answer.

Charles Thomas

Numerade Educator

Problem 65

Some Arctic explorers have eaten sticks of butter on their explorations. Give a nutritional reason for this unusual cuisine.

Kevin Zaborsky

Numerade Educator

Problem 66

If you agitate a mixture of fatty acids in water, an emulsion forms, in which spherical structures called micelles are dispersed throughout the water. Micelles form when the carboxylic acid groups of the fatty acids face the solvent and their hydrocarbon tails are directed toward the inside of the sphere.
a. Explain why these structures form with this orientation.
b. It is sometimes possible to "break" an emulsion, destroying the micelles by adding a strong acid to the mixture. Why would this destroy the micelles?
c. One can also sometimes break an emulsion by adding salt (NaCl) to the mixture. Why would this destroy the micelles?

Charles Thomas

Numerade Educator

Problem 67

Do triglycerides have a chiral center? Explain your answer.

Kevin Zaborsky

Numerade Educator

Problem 68

Using your knowledge of molecular geometry and intermolecular forces, why might polyunsaturated triglycerides be more likely to be liquid than saturated triglycerides?

Charles Thomas

Numerade Educator

Problem 69

Oleic acid and $\alpha$ -linolenic acid (Table 20.4 ) are both unsaturated fats and are liquids at room temperature. They can be converted into solid saturated fats by hydrogenation.
a. Which would consume more hydrogen: $1.0 \mathrm{kg}$ of oleic acid or $0.50 \mathrm{kg}$ of $\alpha$ -linolenic acid?
b. Could you distinguish between the two fatty acids by determining the identity of their hydrogenation products? Explain your answer.

Kevin Zaborsky

Numerade Educator

Problem 70

For each of the pairs of fatty acids in Figure $\mathrm{P} 20.70$ indicate whether they are constitutional isomers, stereoisomers, or unrelated compounds.
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 71

Draw the structures of the three fats formed by reaction of glycerol with (a) octanoic acid $\left(\mathrm{C}_{7} \mathrm{H}_{15} \mathrm{COOH}\right)$
(b) decanoic acid $\left(\mathrm{C}_{9} \mathrm{H}_{19} \mathrm{COOH}\right),$ and (c) dodecanoic acid $\left(\mathrm{C}_{11} \mathrm{H}_{23} \mathrm{COOH}\right)$

Kevin Zaborsky

Numerade Educator

Problem 72

Oil-based paints contain linseed oil, a triglyceride formed by esterification of glycerol with linolenic acid (Figure $\mathrm{P} 20.72$ ).
(FIGURE CAN'T COPY)
a. Draw the line structure of linolenic acid.
"b. Are the double bonds in linolenic acid conjugated?

Charles Thomas

Numerade Educator

Problem 73

What are the three kinds of molecular subunits in DNA? Which two form the "backbone" of DNA strands?

Kevin Zaborsky

Numerade Educator

Problem 74

How do DNA and RNA differ:
a. in molecular composition?
b. in structure?
c. in function?

Charles Thomas

Numerade Educator

Problem 75

What kind of intermolecular force holds together the strands of DNA in the double-helix configuration?

Kevin Zaborsky

Numerade Educator

Problem 76

DNA is a highly charged polyanion. If a solution of DNA is heated, the DNA will separate into individual strands,
a process called denaturation. If the salt concentration of the solution is increased, the temperature at which denaturation occurs increases. Suggest a reason why.

Charles Thomas

Numerade Educator

Problem 77

Draw the structure of adenosine $5^{\prime}$ -monophosphate, one of the four ribonucleotides in a strand of RNA.

Kevin Zaborsky

Numerade Educator

Problem 78

Draw the structure of deoxythymidine $5^{\prime}$ -monophosphate, one of the four nucleotides in a strand of DNA.

Charles Thomas

Numerade Educator

Problem 79

In the replication of DNA, a segment of an original strand has the sequence T-C-G.
a. What is the sequence of the opposite strand?
b. Draw the structure of this section of the double helix, clearly showing all hydrogen bonds.

Kevin Zaborsky

Numerade Educator

Problem 80

If the sequence of one strand of DNA is $5^{\prime}$ ATTGCCA $3^{\prime}$ what is the sequence (in the $\left.5^{\prime} \text { to } 3^{\prime} \text { direction }\right)$ of the other strand?

Charles Thomas

Numerade Educator

Problem 81

Salsa has antibacterial properties because it contains dodecenal (Figure $\mathrm{P} 20.81$ ), a compound found in the cilantro used to make salsa.
a. How many carbon atoms are in dodecenal?
b. What functional groups are present in dodecenal?
c. What types of isomerism are possible in dodecenal?
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 82

Turmeric is commonly used as a spice in Indian and Southeast Asian dishes. Turmeric contains a high concentration of curcumin (Figure $\mathrm{P} 20.82$ ), a potential anticancer drug and a possible treatment for cystic fibrosis.
a. Are the substituents on the $\mathrm{C}=\mathrm{C}$ double bonds highlighted in red in cis or trans configurations?
b. Draw two other stereoisomers of this compound.
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 83

Olestra is a calorie-free fat substitute. The core of the olestra molecule (Figure $\mathrm{P} 20.83$ ) is a disaccharide that has reacted with a carboxylic acid; this results in the conversion of hydroxyl groups on the disaccharide into the depicted structure.
a. What is the name of the disaccharide core of the olestra molecule?
b. What functional group has replaced the hydroxyl groups on the disaccharide?
c. What is the formula of the carboxylic acid used to make olestra?
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 84

When scientists at UC Santa Cruz directed UV radiation at an ice crystal containing methanol, ammonia, and hydrogen cyanide, three amino acids (glycine, alanine, and serine) were detected among the products of photochemical reactions. The formation of these amino acids suggests that they may also be synthesized in comets approaching the Sun (and Earth). Determine the standard free-energy change of the hypothetical formation of glycine
in comets, using standard free energies of formation of the reactants and products in this reaction $$\left[\Delta G_{\mathrm{f}}^{\circ} \text { for } \mathrm{HCN}(g)\right.$$is \right.$+125 \mathrm{kJ} / \mathrm{mol}$ and for solid glycine is $-368.4 \mathrm{kJ} / \mathrm{mol} ;$ other $\left.\Delta G_{f}^{\circ} \text { values are in Appendix } 4\right]$
$$\mathrm{CH}_{3} \mathrm{OH}(\ell)+\mathrm{HCN}(g)+\mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow \mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{COOH}(s)+\mathrm{H}_{2}(g)$$

Charles Thomas

Numerade Educator

Problem 85

Homocysteine (Figure $\mathrm{P} 20.85$ ) is formed during the metabolism of amino acids. A mutation in some people's genes leads to high concentrations of homocysteine in the blood and a consequent increase in their risk of heart disease and incidence of bone fractures in old age.
a. What is the structural difference between homocysteine and cysteine?
b. Cysteine is a chiral compound. Is homocysteine chiral?
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 86

Some scientists believe life on Earth can be traced to amino acids and other molecules brought to Earth by comets and meteorites. In 2004 , a new class of amino acids called diamino acids (Figure $\mathrm{P} 20.86$ ) were found in the Murchison meteorite.
a. Which of these diamino acids is not an $\alpha$ -amino acid?
b. Which of these amino acids is chiral?
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 87

Ackee, the national fruit of Jamaica, is a staple in many Jamaican diets. Unfortunately, a potentially fatal sickness known as Jamaican vomiting disease is caused by the consumption of unripe ackee fruit, which contains the amino acid hypoglycin (Figure $\mathrm{P} 20.87$ ). Is hypoglycin an $\alpha$ -amino acid?
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 88

In Section 20.2 we discussed the rotation of a beam of polarized light when it passes through a solution containing an optically active molecule. Equimolar solutions of $\alpha$ -glucose and $\beta$ -glucose rotate plane-polarized light by $+112^{\circ}$ and $+18.7^{\circ},$ respectively. If these two solutions are then mixed and allowed to reach equilibrium, the solution then rotates the polarized light by $+53.4 "$ Calculate the percent glucose in the $\alpha$ and $\beta$ forms in this solution.

Charles Thomas

Numerade Educator

Problem 89

Supplements containing BCAAs (branched-chain amino acids) have been studied for their effects on athletic performance (in terms of the perception of exertion) and mental fatigue. Current research does not support that they have either of these effects, but they are used medically to slow muscle wasting in bedridden patients. BCAAs are amino acids having hydrocarbon side chains with a branch, a carbon atom bound to more than two other carbon atoms. Consult Table 20.2 and draw the structures of the BCAAs among the common amino acids.

Kevin Zaborsky

Numerade Educator

Problem 90

In response to specific neural messages, the human hypothalamus may secrete several polypeptides, including the tripeptide shown in Figure $\mathrm{P} 20.90$
a. Sketch the structures of the three amino acids that combine to make this tripeptide.
b. Which, if any, of the constituent amino acids are among the $20 \mathrm{\alpha}$ -amino acids in proteins?
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 91

Glutathione (Figure $\mathrm{P} 20.91$ ) is an essential molecule in the human body. It acts as an activator for enzymes and protects lipids from oxidation. Which three amino acids combine to make glutathione?
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 92

Three amino acids- -glutamic acid, arginine, and tryptophan-are dissolved in a gel that is buffered at a pH of $5.9 .$ Two electrodes are placed in the gel and an electric current is applied.
a. Toward which electrode does each amino acid migrate?
b. Draw the forms of each amino acid present in the gel at a pH of 5.9

Charles Thomas

Numerade Educator

Problem 93

Without doing the actual calculation, estimate the fuel values of leucine and isoleucine by considering average bond energies. Should the fuel values of the two amino acids be the same? Actual calorimetric measurements show that isoleucine has a lower fuel value than leucine. Explain why.

Kevin Zaborsky

Numerade Educator

Problem 94

Sucralose (see Figure $\mathrm{P} 20.9$ ) is about 600 times sweeter than sucrose (see Figure 20.35 ). All substances that taste sweet have functional groups that form hydrogen bonds with "sweetness" receptor sites on the tongue. What does the difference in sweetness between sucralose and sucrose tell you about additional intermolecular interactions between sweet compounds and receptor sites that contribute to their sweet taste?
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 95

Prostaglandins, naturally occurring compounds in our bodies that cause inflammation and other physiological responses, are formed from arachidonic acid, an unsaturated hydrocarbon containing four $\mathrm{C}=\mathrm{C}$ double bonds and a carboxylic acid functional group. The stereoisomer containing all cis double bonds is shown in Figure P20.95. How many stereoisomers other than this one are possible? Draw the isomer containing all trans double bonds.(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 96

A newspaper article contains the wording, "Made primarily by the liver, cholesterol begins with tiny pieces of sugar..." What does this statement mean at the molecular level?

Charles Thomas

Numerade Educator

Problem 97

Which amino acids are zwitterions at physiological pH $(-7.4),$ but exist as neutral molecules?

Kevin Zaborsky

Numerade Educator

Problem 98

The structures of morphine and demerol, two powerful pain medicines, are shown in Figure P20.98. They interact similarly with receptors because they share similar structural features, including one part of the molecule that is flat or planar and another part that can bind to the receptor site. Identify these two structural features.
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator

Problem 99

Figure $\mathrm{P} 20.99$ shows the structure of the histamine molecule that causes sneezing and itching in allergy sufferers. Antihistamines are a class of molecules that reduce allergy symptoms by preferentially binding to the same receptor sites as histamines. What structural features would permit these molecules to bind to the same receptor?
(FIGURE CAN'T COPY)

Kevin Zaborsky

Numerade Educator

Problem 100

The structures of aspirin, acetaminophen, and ibuprofen are shown in Figure P20.100. What structural features do these molecules share?
(FIGURE CAN'T COPY)

Charles Thomas

Numerade Educator