Chapter 24

What is the empirical formula of cellulose? What is the unit that forms the basis of the cellulose polymer? What form of linkage joins these monomeric units?

What is the empirical formula of glycogen? What is the unit that forms the basis of the glycogen polymer? What form of linkage joins these monomeric units?

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

Describe the chemical structures of lipids and phospholipids. Why can phospholipids form a bilayer in water?

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.

When samples of double-stranded DNA are analyzed, the quantity of adenine present equals that of thymine. Similarly, the quantity of guanine equals that of cytosine. Explain the significance of these observations.

Imagine a single DNA strand containing a section with the following base sequence: \(5^{\prime}\) -GCATTGGC-3'. What is the base sequence of the complementary strand? (The two strands of DNA will come together in an antiparallel fashion; that is, \(5^{\prime}-\mathrm{TAG}-3^{\prime}\) will bind to \(3^{\prime}-\) ATC-5'.)

Explain the chemical differences between DNA and RNA.

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?

Draw the condensed structural formulas for the cis and trans isomers of 2 -pentene. Can cyclopentene exhibit cis-trans isomerism? Explain.

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

Write the structural formulas for as many alcohols as you can think of that have empirical formula \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}\).

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 \(\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O},\) (c) a dihydroxy compound with the formula \(\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}_{2},\) (d) a cyclic ester with the formula \(\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O}_{2}\).

Although carboxylic acids and alcohols both contain an \(-\mathrm{OH}\) group, one is acidic in water and the other is not. Explain the difference.

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 pH 7? (a) Gly- Ser-Lys, (b) Pro-Leu-Ile, (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.

Explain why the boiling point of ethanol \(\left(78^{\circ} \mathrm{C}\right)\) is much higher than that of its isomer, dimethyl ether \(\left(-25^{\circ} \mathrm{C}\right),\) and why the boiling point of \(\mathrm{CH}_{2} \mathrm{~F}_{2}\left(-52{ }^{\circ} \mathrm{C}\right)\) is far above that of \(\mathrm{CH}_{4}\left(-128^{\circ} \mathrm{C}\right)\)

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?

A typical amino acid with one amino group and one carboxylic acid group, such as serine (Figure 24.18 ), can exist in water in several ionic forms. (a) Suggest the forms of the amino acid at low \(\mathrm{pH}\) and at high \(\mathrm{pH}\). (b) Amino acids generally have two \(\mathrm{p} K_{a}\) values, one in the range of 2 to 3 and the other in the range of 9 to \(10 .\) Serine, for example, has \(\mathrm{p} K_{a}\) values of 2.19 and \(9.21 .\) Using species such as acetic acid and ammonia as models, suggest the origin of the two \(\mathrm{p} K_{a}\) values. (c) Glutamic acid is an amino acid that has three \(\mathrm{p} K_{a}\) 's: 2.10 , 4.07, and 9.47. Draw the structure of glutamic acid, and assign each \(\mathrm{p} K_{a}\) to the appropriate part of the molecule.

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 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. \((\mathrm{d})\) A gentle reducing agent converts the four \(-\mathrm{S}-\mathrm{S}-\) linkages in ribonuclease \(A\) to eight \(-S-H\) 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 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 \(\mathrm{pH} 7.4 ?\)