Chapter 24

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.

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.

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.

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.

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?

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.

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

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?

(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.

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

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 \(\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O}_{2} .\)

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.

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.

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

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.

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

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 \(78^{\circ} \mathrm{C} ?\)

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.

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.

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?

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?

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?

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 \(\mathrm{pH} 7.4\) ?