Chapter 24

Indicate whether each statement is true or false: (a) Maltose is a monosaccharide. (b) Polysaccharides are a type of carbohydrate. (c) All carbohydrates have an oxygen to hydrogen ratio of 1: 2 .

(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.

(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?

(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?

Indicate whether each statement is true or false: (a) Fat molecules must contain \(\mathrm{C}=\mathrm{C}\) bonds. (b) Phosphoplipids have overall net charge. (c) Cis-monounsaturated fat molecules are bent in shape.

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: \(5^{\prime}\) -AGTTACTGG-3'. 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 3-methyl-3-hexene. (b) Can cyclohexene exhibit cis-trans isomerism? Explain. (c) Does 2 -hexyne have geometric isomers? Explain.

If a molecule is an "ene-ol," what functional groups must it have? The suffix -ene signifies an alkene, \(-\) ol an alcohol. The molecule has alkene and alcohol functional groups.

Write a condensed structural formula for each of the following: \((\mathbf{a})\) an amide with the formula \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{NO},(\mathbf{b})\) a cyclic ether with the formula \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O},(\mathbf{c})\) a diketone compound with the formula \(\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O}_{2}\), (d) a phenyl carboxylic acid with the formula \(\mathrm{C}_{7} \mathrm{H}_{6} \mathrm{O}_{2}\)

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-diaminobutane, the compound responsible for the odor of putrefying fish, (c) Cyclohexanone, the precursor to Nylon, (d) 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.

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.

Which of the following peptides have a net positive charge at \(\mathrm{pH} 7 ?\) (a) Asn-His-Arg, (b) Glu-Lys-Phe, (c) Thr-Leu-Ala.

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: diethyl ether, butane, 1 -chlorobutane, and ethylene glycol. You measure the boiling points of the compounds as \(-0.5^{\circ} \mathrm{C}, 35^{\circ} \mathrm{C}\), \(77^{\circ} \mathrm{C}\), and \(195^{\circ} \mathrm{C}\), but then lose the labels for each sample. Make the following predictions: (a) Which compound boils at \(-0.5^{\circ} \mathrm{C} ?(\mathbf{b})\) Which boils at \(35^{\circ} \mathrm{C} ?(\mathbf{c})\) Which boils at \(77^{\circ} \mathrm{C} ?(\mathbf{d})\) Which boils at \(195^{\circ} \mathrm{C} ?\)

An unknown organic compound is found on elemental analysis to contain \(72.0 \%\) carbon, \(12.0 \%\) hydrogen, and \(16.0 \%\) oxygen by mass. It has a molecular weight of \(100 \mathrm{~g} / \mathrm{mol}\) and is slightly soluble in water. Upon careful oxidation it is converted into a ketone which contains \(73.4 \%\) carbon, \(10.3 \%\) hydrogen, and \(16.3 \%\) oxygen by mass. Indicate two or more reasonable structures for the unknown.

An unknown substance is found to contain only carbon and hydrogen. It is a liquid that boils at \(49^{\circ} \mathrm{C}\) at \(101.3 \mathrm{kPa}\) pressure. Upon analysis it is found to contain \(85.7 \%\) carbon and \(14.3 \%\) hydrogen by mass. At \(100^{\circ} \mathrm{C}\) and \(97.99 \mathrm{kPa}\), 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?

The standard free energy of formation of solid maltose is \(-1334.42 \mathrm{~kJ} / \mathrm{mol},\) whereas that of solid glucose is \(-910.56 \mathrm{~kJ} / \mathrm{mol}\). What is \(\Delta G^{\circ}\) for the condensation of two glucose molecules to form maltose?

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 hasfour \(-\mathrm{S}-\mathrm{S}-\) 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 \(\mathrm{A}\) ?

The amino acid glycine can act as a weak acid: If the \(1^{\text {st }} \mathrm{p} K_{a}\) for the protonated amino group of glycine is \(9.8,\) what is the ratio of the neutral to anionic form of glycine in blood at \(\mathrm{pH} 7.4 ?\)