Chapter 24

The structural formula for the linear form of galactose is O=CC(O)C(O)C(O)C(O)CO (a) Is this molecule a sugar? (b) How many chiral carbons are present in the molecule? (c) Draw the structure of the sixmember-ring form of this molecule.

Indicate whether each statement is true or false: (a) Fat molecules contain amide bonds. (b) Phosphoplipids can be zwitterions. (c) Phespholipids 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 \(8.4\) 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. Nucleic Acids (Section 24.10)

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'-TAG-3' will bind to \(3^{\prime}\)-ATC-5'.)

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 molecules with the formula \(\mathrm{C}_{3} \mathrm{H}_{4} \mathrm{O}\).

How many structural isomers are there for a five straight carbon chain with one double bond? member straight carbon chain with two double bon.

(a) Draw the condensed structural formulas for th trans isomers of 2-pentene. (b) Can cyclopentene ex trans isomerism? Explain. (c) Does 1-pentyne have mers? Explain.

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

Carbexylic acids generally have \(\mathrm{p} K_{2}\) 's of \(-5\), but alcohols have \(\mathrm{pK}_{\mathrm{y}}\) 's of \(\sim 16\). (a) Write the acid- dissociation chemical equation for the generic alcohol ROH in water. (b) Which compounds will produce more acidic solutions upon dissolution in water, acids or alcohols? (c) Suggest an explanation for the difference in \(\mathrm{pK}_{\mathrm{a}}\) 's for acids compared to alcohols.

Locate the chiral carbon atoms, if any, in each molecule: (a) \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{CCH}_{2} \mathrm{OH}\) (b) O=C(O)CO (c) CCC(C)C(N)=O

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

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 \(\mathrm{CH}_{2} \mathrm{~F}_{1}\left(-52^{\circ} \mathrm{C}\right)\) is far above that of \(\mathrm{CH}_{4}\left(-128^{\circ} \mathrm{C}\right)\).

An unknown organic compound is found on elemental analysis to contain \(68.1 \%\) carben, \(13.7 \%\) 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 twe or more reasonable structures for the unknown.

The standard free energy of formation of solid glycine is \(-369 \mathrm{~kJ} / \mathrm{mol}\), whereas that of solid glycylglycine is \(-488 \mathrm{kl} / \mathrm{mol}\). What is \(\Delta G^{4}\) for the condensation of glycine to form glycylglycine?

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 \(\mathrm{pH}\). (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 \(\mathrm{pK} K_{4}\) values of \(2.19\) and \(9.21\). Using species such as acetic acid and ammonia as models, suggest the origin of the two \(\mathrm{pK}_{\mathrm{a}}\) values. (c) Glutamic acid is an amino acid that has three \(\mathrm{pK}_{a}\) ' \(\times 2.10,4.07\), and 9.47. Draw the structure of glutamic acid, and assign each \(\mathrm{p} K_{a}\) to the appropriate part of the molecule. (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: Native ribonuclease A (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 \(-5-5\) - 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 - \(S-S\) - linkages in ribonuclease 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 - SH bonds to exist in ribonuclease A?

The monoanion of adenosine monophosphate (AMP) is an intermediate in phosphate metabolism: where \(\mathrm{A}=\) adenosine. If the \(\mathrm{p} K_{\mathrm{a}}\) 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?