Chemistry The Central Science

Theodore L. Brown

Chapter 24

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

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Chapter Questions

Problem 1

All the structures shown here have the molecular formula $\mathrm{C}_{8} \mathrm{H}_{18} .$ Which structures are the same molecule? (Hint: One way to answer this question is to determine the chemical name for each structure.) [Section 24.2$]$

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Problem 2

Which of these molecules is unsaturated? [Section 24.3]

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Problem 3

(a) Which of these molecules most readily undergoes an addition reaction? (b) Which of these molecules is aromatic? (c) Which of these molecules most readily undergoes a substitution reaction? [Section 24.3$]$

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Problem 4

(a) Which of these compounds below would you expect to have the highest boiling point? (b) Which of these compounds is the most oxidized? (c) Which of these compounds, if any, is an ether? (d) Which of these compounds, if any, is an ester? (e) Which of these compounds, if any, is a ketone? [Section 24.4]

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Problem 5

For each of the compounds below, state its systematic name, its functional group(s), and whether or not it is chiral. [Sections $24.2,24.4 ]$

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Problem 6

From examination of the molecular models i-v, choose the substance that (a) can be hydrolyzed to form a solution containing glucose, ( b) is capable of forming a zwitterion,(c) is one of the four bases present in DNA, (d) reacts with an acid to form an ester, (e) is a lipid. [ Sections $24.6-24.10 ]$

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Problem 7

Indicate whether each statement is true or false. (a) Butane contains carbons that are $s p^{2}$ hybridized. (b) Cyclohexane is another name for benzene. (c) The isopropyl group contains three $s p^{3}$ -hybridized carbons. (d) Olefin is another name for alkyne.

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Problem 8

Indicate whether each statement is true or false. (a) Pentane has a higher molar mass than hexane. (b) The longer the linear alkyl chain for straight-chain hydrocarbons, the higher the boiling point. (c) The local geometry around the alkyne group is linear. (d) Propane has two structural isomers.

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Problem 9

Predict the ideal values for the bond angles about each carbon atom in the following molecule. Indicate the hybridization of orbitals for each carbon.
$$\mathrm{CH}_{3} \mathrm{CCCH}_{2} \mathrm{COOH}$$

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Problem 10

Identify the carbon atom(s) in the structure shown that has (have) each of the following hybridizations: (a) $s p^{3}$ (b) $s p,(\mathbf{c}) s p^{2}.$

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Problem 11

For each of the following hydrocarbons, state how many carbon atoms are in each molecule:
\begin{equation}\begin{array}{l}{\text { (a) methane }} \\ {\text { (b) decane }} \\ {\text { (c) } 2 \text { -methylhexane }} \\ {\text { (d) neopentane }} \\ {\text { (e) acetylene }}\end{array}\end{equation}

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Problem 12

True or false: The weaker a single bond in a molecule, the greater the chance it will be the site of a reaction (compared to stronger single bonds in the molecule).

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Problem 13

Indicate whether each statement is true or false. (a) Alkanes do not contain any carbon-carbon multiple bonds. (b) Cyclobutane contains a four-membered ring. (c) Alkenes contain carbon-carbon triple bonds. (d) Alkynes contain carbon-carbon double bonds. (e) Pentane is a saturated hydrocarbon but 1-pentene is an unsaturated hydrocarbon. (f) Cyclohexane is an aromatic hydrocarbon. (g) The methyl group contains one less hydrogen atom than methane.

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Problem 14

What structural features help us identify a compound as (a) an alkane, (b) a cycloalkane, (c) an alkene, (d) an alkyne, (e) a saturated hydrocarbon, (f) an aromatic hydrocarbon?

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Problem 15

Give the the name or condensed structural formula, as appropriate:

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Problem 16

Give the name or condensed structural formula, as appropriate:

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Problem 17

Give the name or condensed structural formula, as appropriate:

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Problem 18

Give the name or condensed structural formula, as appropriate:

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Problem 19

What is the octane number of a mixture that is 35$\%$ heptane and 65$\%$ isooctane?

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Problem 20

Describe two ways in which the octane number of a gasoline consisting of alkanes can be increased.

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Problem 21

\begin{equation}\begin{array}{l}{\text { (a) Is } \mathrm{C}_{4} \mathrm{H}_{6} \text { a saturated or unsaturated hydrocarbon? }} \\ {\text { (b) Are all alkynes unsaturated? }}\end{array}\end{equation}

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Problem 22

(a) Is the compound $\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}$ saturated or unsaturated? Explain. (b) What is wrong with the formula $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}_{3} ?$

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Problem 23

Give the molecular formula of a hydrocarbon containing five carbon atoms that is (a) an alkane, (b) a cycloalkane, (c) an alkene, (d) an alkyne.

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Problem 24

Give the molecular formula of a hydrocarbon containing five carbon atoms that is (a) an alkane, (b) a cycloalkane, (c) an alkene, (d) an alkyne.

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Problem 25

Enediynes are a class of compounds that include some antibiotic drugs. Draw the structure of an "enediyne" fragment that contains six carbons in a row.

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Problem 26

Give the general formula for any cyclic alkene, that is, a cyclic hydrocarbon with one double bond.

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Problem 27

Write the condensed structural formulas for two alkenes and one alkyne that all have the molecular formula $\mathrm{C}_{6} \mathrm{H}_{10}$ .

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Problem 28

Draw all the possible noncyclic structural isomers of $\mathrm{C}_{5} \mathrm{H}_{10}$ . Name each compound.

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Problem 29

Name or write the condensed structural formula for the following compounds:

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Problem 30

Name or write the condensed structural formula for the following compounds:
\begin{equation}\begin{array}{l}{\text { (a) } 4 \text { -methyl-2-pentene }} \\ {\text { (b) } \text {cis}-2,5 \text { -dimethyl-3-hexene }} \\ {\text { (c) ortho-dimethylbenzene }}\\{\text { (d) } \mathrm{HC}=\mathrm{CCH}_{2} \mathrm{CH}_{3}} \\ {\text { (e) } \text {trans}-\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}}\end{array}\end{equation}

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Problem 31

Indicate whether each statement is true or false. (a) Two geometric isomers of pentane are $n$-pentane and neopentane. (b) Alkenes can have cis and trans isomers around the CC double bond. (c) Alkynes can have cis and trans isomers around the CC triple bond.

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Problem 32

Draw all structural and geometric isomers of butene and name them.

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Problem 33

Indicate whether each of the following molecules is capable of geometrical isomerism. For those that are, draw the structures: (a) $1,1$ -dichloro-1-butene, (b) $2,4$ -dichloro-2-butene, (c) $1,4$ -dichlorobenzene, (d)$4,4$-dimethyl-2-pentyne.

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Problem 34

Draw the three distinct geometric isomers of $2,4$-hexadiene.

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Problem 35

(a) True or false: Alkenes undergo addition reactions and aromatic hydrocarbons undergo substitution reactions. (b) Using condensed structural formulas, write the balanced equation for the reaction of 2 -pentene with Br$_{2}$ and name the resulting compound. Is this an addition or a substitution reaction? (c) Write a balanced chemical equation for the reaction of $\mathrm{Cl}_{2}$ with benzene to make paradichlorobenzene in the presence of $\mathrm{FeCl}_{3}$ as a catalyst. Is this an addition or a substitution reaction?

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Problem 36

Using condensed structural formulas, write a balanced chemical equation for each of the following reactions: (a) hydrogenation of cyclohexene, (b) addition of $\mathrm{H}_{2} \mathrm{O}$ to trans-2-pentene using $\mathrm{H}_{2} \mathrm{SO}_{4}$ as a catalyst (two products), (c) reaction of 2-chloropropane with benzene in the presence of $\mathrm{AlCl}_{3}$ .

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Problem 37

(a) When cyclopropane is treated with HI, 1 -i-iodopropane is formed. A similar type of reaction does not occur with cyclopentane or cyclohexane. Suggest an explanation for cyclopropane's reactivity. (b) Suggest a method of preparing ethylbenzene, starting with benzene and ethylene as the only organic reagents.

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Problem 38

(a) One test for the presence of an alkene is to add a small amount of bromine, which is a red-brown liquid, and look for the disappearance of the red-brown color. This test does not work for detecting the presence of an aromatic hydrocarbon. Explain. (b) Write a series of reactions leading to para-bromoethylbenzene, beginning with benzene and using other reagents as needed. What isomeric side products might also be formed?

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Problem 39

The rate law for addition of $\mathrm{Br}_{2}$ to an alkene is first order in $\mathrm{Br}_{2}$ and first order in the alkene. Does this information suggest that the mechanism of addition of $\mathrm{Br}_{2}$ to an alkene proceeds in the same manner as for addition of HBr? Explain.

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Problem 40

Describe the intermediate that is thought to form in the addition of a hydrogen halide to an alkene, using cyclohexene as the alkene in your description.

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Problem 41

The molar heat of combustion of gaseous cyclopropane is $-2089 \mathrm{kJ} / \mathrm{mol};$ that for gaseous cyclopentane is $-3317 \mathrm{kJ} / \mathrm{mol} .$ Calculate the heat of combustion per $\mathrm{CH}_{2}$ group in the two cases, and account for the difference.

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Problem 42

The heat of combustion of decahydronaphthalene $\left(\mathrm{C}_{10} \mathrm{H}_{18}\right)$ is $-6286 \mathrm{kJ} / \mathrm{mol} .$ The heat of combustion of naphthalene $\left(\mathrm{C}_{10} \mathrm{H}_{8}\right)$ is $-5157 \mathrm{kJ} / \mathrm{mol} .$ (In both cases $\mathrm{CO}_{2}(g)$ and $\mathrm{H}_{2} \mathrm{O}(l)$ are the products. $.$ Using these data and data in Appendix $\mathrm{C},$ calculate the heat of hydrogenation and the resonance energy of naphthalene.

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Problem 43

(a) Which of the following compounds, if any, is an ether? (b) Which compound, if any, is an alcohol? (c) Which compound, if any, would produce a basic solution if dissolved in water? (Assume solubility is not a problem). (d) Which compound, if any, is a ketone? (e) Which compound, if
any, is an aldehyde?

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Problem 44

Identify the functional groups in each of the following compounds:

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Problem 45

Draw the molecular structure for (a) an aldehyde that is an isomer of acetone, (b) an ether that is an isomer of 1-propanol.

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Problem 46

(a) Give the empirical formula and structural formula for a cyclic ether containing four carbon atoms in the ring. (b) Write the structural formula for a straight-chain compound that is a structural isomer of your answer to part (a).

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Problem 47

The IUPAC name for a carboxylic acid is based on the name of the hydrocarbon with the same number of carbon atoms. The ending-oic is appended, as in ethanoic acid, which is the IUPAC name for acetic acid. Draw the structure of the following acids: (a) methanoic acid, (b) pentanoic acid, (c) 2-chloro-3-methyldecanoic acid.

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Problem 48

Aldehydes and ketones can be named in a systematic way by counting the number of carbon atoms (including the carbonyl carbon) that they contain. The name of the aldehyde or ketone is based on the hydrocarbon with the same number of carbon atoms. The ending -al for aldehyde or -one for ketone
is added as appropriate. Draw the structural formulas for the following aldehydes or ketones: (a) propanal, (b) 2-penta-none, (c) 3-methyl-2-butanone, (d) 2-methylbutanal.

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Problem 49

Draw the condensed structure of the compounds formed by condensation reactions between (a) benzoic acid and ethanol, (b) ethanoic acid and methylamine, (c) acetic acid and phenol. Name the compound in each case.

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Problem 50

Draw the condensed structures of the compounds formed from (a) butanoic acid and methanol, ( b) benzoic acid and 2-propanol, (c) propanoic acid and dimethylamine. Name the compound in each case.

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Problem 51

Write a balanced chemical equation using condensed structural formulas for the saponification (base hydrolysis) of (a) methyl propionate, (b) phenyl acetate.

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Problem 52

Write a balanced chemical equation using condensed structural formulas for (a) the formation of butyl propionate from the appropriate acid and alcohol, (b) the saponification (base hydrolysis) of methyl benzoate.

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Problem 53

Pure acetic acid is a viscous liquid, with high melting and boiling points ($16.7^{\circ} \mathrm{C}$ and $118^{\circ} \mathrm{C}$) compared to compounds of similar molecular weight. Suggest an explanation.

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Problem 54

Acetic anhydride is formed from two acetic acid molecules, in a condensation reaction that involves the removal of a molecule of water. Write the chemical equation for this process, and show the structure of acetic anhydride.

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Problem 55

Write the condensed structural formula for each of the following compounds: (a) 2-pentanol, (b) $1,2$-propanediol,(c) ethyl acetate, (d) diphenyl ketone, (e) methyl ethyl ether.

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Problem 56

Write the condensed structural formula for each of the following compounds: (a) 2 -ethyl-1-hexanol, (b) methyl phenyl ketone, (c) para-bromobenzoic acid, (d) butyl ethyl ether, (e) $N,N$-dimethylbenzamide.

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Problem 57

How many chiral carbons are in 2 -bromo-2-chloro-3-methylpentane? (a) $0,(\mathbf{b}) 1,(\mathbf{c}) 2,(\mathbf{d}) 3,(\mathbf{e}) 4$ or more.

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Problem 58

Is 3 -chloro-3-methylhexane chiral?

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Problem 59

(a) Draw the chemical structure of a generic amino acid, using $R$ for the side chain. (b) When amino acids react to form proteins, do they do so via substitution, addition, or condensation reactions? (c) Draw the bond that links amino acids together in proteins. What is this called?

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Problem 60

Indicate whether each statement is true or false. (a) Tryptophan is an aromatic amino acid. (b) Lysine is positively charged at pH $7 .(\mathbf{c})$ Asparagine has two amide bonds. (d) Isoleucine and leucine are enantiomers. (e) Valine is probably more water-soluble than arginine.

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Problem 61

Draw the two possible dipeptides formed by condensation reactions between histidine and aspartic acid.

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Problem 62

Write a chemical equation for the formation of methionyl-glycine from the constituent amino acids.

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Problem 63

(a) Draw the condensed structure of the tripeptide Gly-Gly-His. (b) How many different tripeptides can be made from the amino acids glycine and histidine? Give the abbreviations for each of these tripeptides, using the three-letter and one-letter codes for the amino acids.

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Problem 64

\begin{equation}\begin{array}{l}{\text { (a) What amino acids would be obtained by hydrolysis of }} \\ {\text { the following tripeptide? }}\\ {\text { (b) How many different tripeptides can be made from glycine, }} \\ {\text { serine, and glutamic acid? Give the abbreviation for }} \\ {\text { each of these tripeptides, using the three-letter codes and }} \\ {\text { one-letter codes for the amino acids. }}\end{array}\end{equation}

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Problem 65

Indicate whether each statement is true or false. (a) The sequence of amino acids in a protein, from the amine end to the acid end, is called the primary structure of the protein. (b) Alpha helix and beta sheet structures are examples of quaternary protein structure. (c) It is impossible for more than one protein to bind to another and make a higher order structure.

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Problem 66

Indicate whether each statement is true or false: (a) In the alpha helical structure of proteins, hydrogen bonding occurs between the side chains (R groups). (b) Dispersion forces, not hydrogen bonding, holds beta sheet structures together.

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Problem 67

Indicate whether each statement is true or false: (a) Disaccharides are a type of carbohydrate. (b) Sucrose is a monosaccharide. (c) All carbohydrates have the formula $\mathrm{C}_{n} \mathrm{H}_{2 m} \mathrm{O}_{m} .$

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Problem 68

(a) Are $\alpha$ -glucose and $\beta$ -glucose enantiomers? (b) Show the condensation of two glucose molecules to form a disaccharide with an $\alpha$ linkage. (c) Repeat part (b) but with a $\beta$ linkage.

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Problem 69

(a) What is the empirical formula of cellulose? (b) What is the monomer that forms the basis of the cellulose polymer? (c) What bond connects the monomer units in cellulose: amide, acid, ether, ester, or alcohol?

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Problem 70

(a) What is the empirical formula of starch? (b) What is the monomer that forms the basis of the starch polymer? (c) What bond connects the monomer units in starch: amide, acid, ether, ester, or alcohol?

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Problem 71

The structural formula for the open-chain form of D-mannose is
(a) Is this molecule a sugar? (b) How many chiral carbons are present in the molecule? (c) Draw the structure of the six-member-ring form of this molecule.

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Problem 72

The structural formula for the open-chain form of galactose is
(a) Is this molecule a sugar? (b) How many chiral carbons are present in the molecule? (c) Draw the structure of the six-member-ring form of this molecule.

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Problem 73

Indicate whether each statement is true or false: (a) Fat molecules contain amide bonds. (b) Phosphoplipids can be zwitterions. (c) Phospholipids form bilayers in water in order to have their long hydrophobic tails interact favorably with each other, leaving their polar heads to the aqueous environment.

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Problem 74

Indicate whether each statement is true or false: (a) If you use data from Table 8.3 on bond enthalpies, you can show that the more $\mathrm{C}-\mathrm{H}$ bonds a molecule has compared to $\mathrm{C}-\mathrm{O}$ and $\mathrm{O}-\mathrm{H}$ bonds, the more energy it can store. (b) Trans fats are saturated. ( c) Fatty acids are long-chain carboxylic acids. (d) Monounsaturated fatty acids have one CC single bond in the chain, while the rest are double or triple bonds.

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Problem 75

Adenine and guanine are members of a class of molecules known as purines; they have two rings in their structure. Thymine and cytosine, on the other hand, are pyrimidines,and have only one ring in their structure. Predict which have larger dispersion forces in aqueous solution, the purines or the pyrimidines.

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Problem 76

A nucleoside consists of an organic base of the kind shown in Section $24.10,$ bound to ribose or deoxyribose. Draw the structure for deoxyguanosine, formed from guanine and deoxyribose.

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Problem 77

What is the DNA sequence for the molecule shown here?

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Problem 78

You are working in a biotechnology lab and are analyzing DNA. You obtain a sample of a short dodecamer of DNA that contains 12 base pairs. (a) What must the ratio of adenine to thymine be in your sample? (b) What must the ratio of cytosine to guanine be in your sample? (c) Assume the counterions present in your DNA solution are sodium ions. How many sodium ions must there be per dodecamer? Assume the $5^{\prime}$ end phosphates each bear $a-1$ charge.

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Problem 79

Imagine a single DNA strand containing a section with the following base sequence: $5^{\prime}-$ GCATTGG $C-3^{\prime}$ . What is the base sequence of the complementary strand?

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Problem 80

Which statement best explains the chemical differences between DNA and RNA? (a) DNA has two different sugars in its sugar-phosphate backbone, but RNA only has one. (b) Thymine is one of the DNA bases, whereas RNA's corresponding base is thymine minus a methyl group. (c) The RNA sugar-phosphate backbone contains fewer oxygen atoms than DNA's backbone. (d) DNA forms double helices but RNA cannot.

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Problem 81

Draw the condensed structural formulas for two different molecules with the formula $\mathrm{C}_{3} \mathrm{H}_{4} \mathrm{O}$ .

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Problem 82

How many structural isomers are there for a five-member straight carbon chain with one double bond? For a six-member straight carbon chain with two double bonds?

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Problem 83

(a) Draw the condensed structural formulas for the cis and trans isomers of 2 -pentene. (b) Can cyclopentene exhibit cis-trans isomerism? Explain. (c) Does 1 -pentyne have enantiomers? Explain.

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Problem 84

If a molecule is an "ene-one," what functional groups must it have?

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Problem 85

Identify each of the functional groups in these molecules:

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Problem 86

For the molecules shown in $24.85,$ (a) Which one(s) of them, if any, would produce a basic solution if dissolved in water? (b) Which one(s) of them, if any, would produce an acidic solution if dissolved in water? (c) Which of them is the most water-soluble?

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Problem 87

Write a condensed structural formula for each of the following: (a) an acid with the formula $\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2},$ (b) a cyclic ketone with the formula $\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O},(\mathbf{c})$ a dihydroxy compound with the formula $\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}_{2},(\mathbf{d})$ a cyclic ester with the formula $\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O}_{2} .$

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Problem 88

Draw each molecule given its name and the following information. (a) Nitroglycerin, also known as $1,2,3$ -trinitroxypropane, the active ingredient in dynamite and a medication administered to people having a heart attack, (Hint: The nitroxy group is the conjugate base of nitric acid.) (b) Putrescine, also known as $1,4$ -diamino-butane, the compound responsible for the odor of putre-fying fish, ( c) Cyclohexanone, the precursor to Nylon, (a) $1,1,2,2$ -tetrafluoroethene, the precursor to Teflon,
(e) Oleic acid, also known as cis-9-octanedecenoic acid, a monounsaturated fatty acid found in many fats and oils.Draw the correct isomer.

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Problem 89

Indole smells terrible in high concentrations but has a pleasant floral-like odor when highly diluted. Its structure is
The molecule is planar, and the nitrogen is a very weak base, with $K_{b}=2 \times 10^{-12}$ . Explain how this information indicates that the indole molecule is aromatic.

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Problem 90

For each molecule below, identify its functional groups and how many chiral centers it contains.

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Problem 91

Which of the following peptides have a net positive charge at $\mathrm{pH} 7 ?$ (a) Gly-Ser-Lys, (b) Pro-Leu-lle, (c) Phe-Tyr-Asp.

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Problem 92

Glutathione is a tripeptide found in most living cells. Partial hydrolysis yields Cys-Gly and Glu-Cys. What structures are possible for glutathione?

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Problem 93

Monosaccharides can be categorized in terms of the number of carbon atoms (pentoses have five carbons and hexoses have six carbons) and according to whether they contain an aldehyde (aldo-prefix, as in aldopentose) or ketone group (keto-prefix, as in ketopentose). Classify glucose and fructose in this way.

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Problem 94

Can a DNA strand bind to a complementary RNA strand? Explain.

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Problem 95

You have samples of four compounds: dimethyl ether, methane, difluoromethane, and ethanol. You measure the boiling points of the compounds as $-128^{\circ} \mathrm{C},-52^{\circ} \mathrm{C}$
$-25^{\circ} \mathrm{C},$ and $78^{\circ} \mathrm{C},$ but then lose the labels for each sample. Make the following predictions: (a) Which compound boils at $-128^{\circ} \mathrm{C} ?(\mathbf{b})$ Which boils at $-52^{\circ} \mathrm{C} ?(\mathbf{c})$ Which boils at $-25^{\circ} \mathrm{C} ?(\mathbf{d})$ Which boils at $78^{\circ} \mathrm{C} ?$

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Problem 96

An unknown organic compound is found on elemental analysis to contain 68.1$\%$ carbon, 13.7$\%$ hydrogen, and 18.2$\%$ oxygen by mass. It is slightly soluble in water. Upon careful oxidation it is converted into a compound that behaves chemically like a ketone and contains 69.7$\%$ carbon, 11.7$\%$ hydrogen, and 18.6$\%$ oxygen by mass. Indicate two or more reasonable structures for the unknown.

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Problem 97

An organic compound is analyzed and found to contain 66.7$\%$ carbon, 11.2$\%$ hydrogen, and 22.1$\%$ oxygen by mass. The compound boils at $79.6^{\circ} \mathrm{C} .$ At $100^{\circ} \mathrm{C}$ and 0.970 atm, the vapor has a density of 2.28 $\mathrm{g} / \mathrm{L} .$ The compound has a carbonyl group and cannot be oxidized to a carboxylic acid. Suggest a structure for the compound.

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Problem 98

An unknown substance is found to contain only carbon and hydrogen. It is a liquid that boils at $49^{\circ} \mathrm{C}$ at 1 atm pressure. Upon analysis it is found to contain 85.7$\%$ carbon and 14.3$\%$ hydrogen by mass. At $100^{\circ} \mathrm{C}$ and 735 torr, the vapor of this unknown has a density of 2.21 $\mathrm{g} / \mathrm{L} .$ When it is dissolved in hexane solution and bromine water is added, no reaction occurs. What is the identity of the unknown compound?

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Problem 99

The standard free energy of formation of solid glycine is $-369 \mathrm{kJ} / \mathrm{mol}$ , whereas that of solid glycylglycine is $-488 \mathrm{kJ} / \mathrm{mol}$ . What is $\Delta G^{\circ}$ for the condensation of glycine to form glycylglycine?

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Problem 100

A typical amino acid with one amino group and one carboxylic acid group, such as serine, can exist in water in several ionic forms. (a) Suggest the forms of the amino acid at low pH and at high pH. (b) Amino acids generally have two pK$_{a}$ values, one in the range of 2 to 3 and the other in the range of 9 to $10 .$ Serine, for example, has pK$_{a}$ values of 2.19 and $9.21 .$ Using species such as acetic acid and ammonia as models, suggest the origin of the two pK$_{a}$ values. (c) Glutamic acid is an amino acid that has three $\mathrm{p} K_{a}^{\prime} \mathrm{s} : 2.10,4.07,$ and $9.47 .$ Draw the structure of glutamic acid, and assign each $\mathrm{pK}_{a}$ to the appropriate part of the
molecule. (d) An unknown amino acid is titrated with strong base, producing the following titration curve. Which amino acids are likely candidates for the unknown?

Aadit Sharma

Numerade Educator

Problem 101

The protein ribonuclease $A$ in its native, or most stable, form is folded into a compact globular shape:
(a) Does the native form have a lower or higher free energy than the denatured form, in which the protein is an extended chain? (b) What is the sign of the system's entropy change in going from the denatured to the folded form? (c) In the native form, the molecule has four $-\mathrm{S}-\mathrm{S}-$ bonds that bridge parts of the chain. What effect do you predict these four linkages to have on the free energy and entropy of the native form relative to the free energy and entropy of a hypothetical folded structure that does not have any $-\mathrm{S}-\mathrm{S}-$ linkages? Explain. (d) A gentle reducing agent converts the four $-\mathrm{S}-\mathrm{S}-$ linkages in ribonuclease $\mathrm{A}$ to eight $-\mathrm{S}-\mathrm{H}$ bonds. What effect do you predict this conversion to have on the tertiary structure and entropy of the protein? (e) Which amino acid must be present for $-\mathrm{SH}$ bonds to exist in ribonuclease A?

Aadit Sharma

Numerade Educator

Problem 102

The monoanion of adenosine monophosphate (AMP) is an intermediate in phosphate metabolism:
where $A=$ adenosine. If the $\mathrm{p} K_{a}$ for this anion is 7.21 what is the ratio of $\left[\mathrm{AMP}-\mathrm{OH}^{-}\right]$ to $\left[\mathrm{AMP}-\mathrm{O}^{2-}\right]$ in blood at $\mathrm{pH} 7.4$ ?

Aadit Sharma

Numerade Educator