Chapter 20

Can all of the terms enantiomer, acbiral, and optically active be used to describe a single compound? Explain.

Two compounds have the same structure and the same physical properties but also have the same optical activity. Are they enantiomers or the same molecule?

How do constitutional isomers differ from stereoisomers?

Could a racemic mixture be distinguished from an achiral compound on the basis of optical activity? Explain your answer.

Can stereoisomers of molecules such as cis and trans \(\mathrm{RHC}=\mathrm{CHR}\) also have optical isomers? (R may be any of the functional groups we have encountered in this textbook.) Explain your answer.

Which type of hybrid orbitals on a carbon atom, \(s p, s p^{2},\) or \(s p^{3},\) can give rise to enantiomers?

Could an oxygen atom in an alcohol, ketone, or ether ever be a chiral center in the molecule?

In living cells, amino acids combine to make peptides and proteins. Are these processes accompanied by increases or decreases in entropy of the reaction system?

What is the difference between a peptide bond and an amide bond?

Meteorites contain more L-amino acids, which are the forms that make up the proteins in our bodies, than D-amino acids. What do the prefixes \(\mathrm{L}\) - and \(\mathrm{D}\) - mean?

Why do most amino acids exist in the zwitterionic form at physiological \(\mathrm{pH}(\mathrm{pH} \approx 7.4) ?\)

Draw the condensed structural formulas of the amino acid tyrosine that you would expect to predominate in aqueous solution under the following conditions: a. in strongly acidic solution b. in strongly basic solution c. in a solution in which \(\mathrm{pH}=\mathrm{p} I\)

When protein strands fold back on themselves in forming stable tertiary structures, lysine residues are often paired up with glutamic acid residues. Why?

Ion-ion interactions are particularly effective at stabilizing tertiary structures of proteins. Suggest a pair of amino acid residues that would be attracted to each other via ion-ion interactions at pH 7.4

The molecular formula for glycine is \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NO}_{2} .\) What is the molecular formula of the linear peptide formed when ten glycine molecules are linked together in peptide bonds?

What are the structural differences between starch and cellulose?

Why is the discovery of enzymes that catalyze cellulose hydrolysis a worthwhile objective?

Is the fuel value (see Chapter 5 ) of glucose in the linear form the same as that in the cyclic form?

The second step in glycolysis converts glucose 6 -phosphate into fructose 6 -phosphate. Can you think of a reason why \(\Delta G^{\circ}\) for this reaction is close to zero?

Which of the following statements are correct about glycosidic bonds in carbohydrates? a. The glycosidic bond in maltose is hydrolyzed by people who are lactose intolerant. b. A glycosidic bond links glucose and fructose together to form sucrose. c. A glycosidic bond is an ether linkage, but all ether linkages are not glycosidic bonds.

How do we calculate the overall free-energy change of a process consisting of two steps?

During glycolysis a monosaccharide is converted to pyruvate. Do you think this process produces an increase or decrease in the entropy of the system? Explain your answer.

What is the difference between a saturated and an unsaturated fatty acid?

Which of the following lipids would have the lowest energy value in terms of human nutrition: olive oil, margarine, olestra, or butter? Explain your answer.

Some Arctic explorers have eaten sticks of butter on their explorations. Give a nutritional reason for this unusual cuisine.

If you agitate a mixture of fatty acids in water, an emulsion forms, in which spherical structures called micelles are dispersed throughout the water. Micelles form when the carboxylic acid groups of the fatty acids face the solvent and their hydrocarbon tails are directed toward the inside of the sphere. a. Explain why these structures form with this orientation. b. It is sometimes possible to "break" an emulsion, destroying the micelles by adding a strong acid to the mixture. Why would this destroy the micelles? c. One can also sometimes break an emulsion by adding salt (NaCl) to the mixture. Why would this destroy the micelles?

Do triglycerides have a chiral center? Explain your answer.

Using your knowledge of molecular geometry and intermolecular forces, why might polyunsaturated triglycerides be more likely to be liquid than saturated triglycerides?

Draw the structures of the three fats formed by reaction of glycerol with (a) octanoic acid \(\left(\mathrm{C}_{7} \mathrm{H}_{15} \mathrm{COOH}\right)\) (b) decanoic acid \(\left(\mathrm{C}_{9} \mathrm{H}_{19} \mathrm{COOH}\right),\) and (c) dodecanoic acid \(\left(\mathrm{C}_{11} \mathrm{H}_{23} \mathrm{COOH}\right)\)

What are the three kinds of molecular subunits in DNA? Which two form the "backbone" of DNA strands?

How do DNA and RNA differ: a. in molecular composition? b. in structure? c. in function?

What kind of intermolecular force holds together the strands of DNA in the double-helix configuration?

DNA is a highly charged polyanion. If a solution of DNA is heated, the DNA will separate into individual strands, a process called denaturation. If the salt concentration of the solution is increased, the temperature at which denaturation occurs increases. Suggest a reason why.

In the replication of DNA, a segment of an original strand has the sequence T-C-G. a. What is the sequence of the opposite strand? b. Draw the structure of this section of the double helix, clearly showing all hydrogen bonds.

If the sequence of one strand of DNA is \(5^{\prime}\) ATTGCCA \(3^{\prime}\) what is the sequence (in the \(\left.5^{\prime} \text { to } 3^{\prime} \text { direction }\right)\) of the other strand?

In Section 20.2 we discussed the rotation of a beam of polarized light when it passes through a solution containing an optically active molecule. Equimolar solutions of \(\alpha\) -glucose and \(\beta\) -glucose rotate plane-polarized light by \(+112^{\circ}\) and \(+18.7^{\circ},\) respectively. If these two solutions are then mixed and allowed to reach equilibrium, the solution then rotates the polarized light by \(+53.4 "\) Calculate the percent glucose in the \(\alpha\) and \(\beta\) forms in this solution.

Three amino acids- -glutamic acid, arginine, and tryptophan-are dissolved in a gel that is buffered at a pH of \(5.9 .\) Two electrodes are placed in the gel and an electric current is applied. a. Toward which electrode does each amino acid migrate? b. Draw the forms of each amino acid present in the gel at a pH of 5.9

Without doing the actual calculation, estimate the fuel values of leucine and isoleucine by considering average bond energies. Should the fuel values of the two amino acids be the same? Actual calorimetric measurements show that isoleucine has a lower fuel value than leucine. Explain why.

A newspaper article contains the wording, "Made primarily by the liver, cholesterol begins with tiny pieces of sugar..." What does this statement mean at the molecular level?