Chapter 25

Indicate the approximate positions of \(\mathrm{C}=\mathrm{O}\) and \(\mathrm{N}-\mathrm{H}\) absorptions you would expect in the infrared spectra of (a) \(\mathrm{ClH}_{3}^{\oplus} \mathrm{NCH}_{2} \mathrm{CO}_{2} \mathrm{H}\) and (b) \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CO}_{2} \mathrm{Na}\)

Explain why arginine elutes from an ion-exchange column using a buffer at \(\mathrm{pH} 5-6\), whereas glutamic acid elutes at \(\mathrm{pH} 3\).

A cation-exchange resin can be prepared by radical-addition polymerization of phenylethene (styrene, Section 10-8) in the presence of about \(2-10 \%\) 1,4-diethenylbenzene (1,4-divinylbenzene), \(\quad\), followed by electrophilic sulfonation of the resulting polymer with \(\mathrm{H}_{2} \mathrm{SO}_{4}-\mathrm{SO}_{3}\) (see Section \(22-4 \mathrm{G}\) ). Explain how these reactions lead to a three-dimensional insoluble polymer with linkages as shown below. Indicate the reaction mechanisms involved.

Consider a "hard" water comprised of dilute \(\mathrm{MgCl}_{2}\). Ion exchange with resin \(-\mathrm{SO}_{3}\) Na replaces \(\mathrm{Mg}^{2+}\) with \(\mathrm{Na}^{+}\), and with resin \(-\mathrm{SO}_{3} \mathrm{H}, \mathrm{Na}^{+}\) is replaced by \(\mathrm{H}^{+}\), thereby producing a dilute \(\mathrm{HCl}\) solution. What kind of an ion-exchange resin would you need to remove the \(\mathrm{Cl}^{-}\) from the HCl solution and produced "deionized" water? (Consider exchanging \(\mathrm{Cl}^{-}\) for \(-\mathrm{OH}\).)

a. Draw the structure of the azlactone derived from \(L\) -phenylalanine and ethanoic anhydride. b. Which of the hydrogens in this azlactone would you expect to be the most acidic? Explain. c. Why do chiral azlactones derived from amino acids such as \(L\) -phenylalanine racemize easily on heating in ethanoic acid in the presence of ethanoate ion?

Show how the following amino acids may be prepared from the indicated method and starting materials: a. glutamic acid from 2 -oxopentanedioic acid ( \(\alpha\) -ketoglutaric acid) by the Strecker method b. leucine from 2 -methyl-1-propanol by the phthalimidomalonic ester synthesis c. aspartic acid from ethyl chloroethanoate by the \(\mathrm{N}\) -formylaminomalonic ester synthesis

Suggest a synthetic route to proline from hexanedioic acid (adipic acid) that involves the transformations \(-\mathrm{CO}_{2} \mathrm{H}-\rightarrow-\mathrm{NH}_{2}\), and \(-\mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}\) to \(-\mathrm{CHBrCO}_{2} \mathrm{H}\). Specify the reagents required to accomplish each step.

What problems might be encountered in using the 2,4 -dinitrofluorobenzene method for determination of end groups on Gly-Lys-Ala? Explain.

The tripeptide, eisenine, has only one free carboxyl group, does not react with 2,4 -dinitrofluorobenzene, and on complete hydrolysis yields 2 moles of \(L\) -glutamic acid, 1 mole of \(L\) -alanine, and 1 mole of ammonia. Alanine is indicated to be the C-terminal amino acid. Write a structure for eisenine that is in accord with the above facts.

Eledoisin is a peptide isolated from the salivary glands of eledone, a Mediterranean eight-armed cephalopod. The peptide is a powerful hypotensive agent. Deduce a possible structure from the following information: (1) Complete hydrolysis gives equal amounts of ammonia, Ala, Asp, Glu, Gly, Ile, Leu, Lys, Met, Phe, Pro, and Ser. (2) No free amino Nterminal group or free carboxyl C-terminal group can be detected. (3) Chymotrypsin hydrolysis forms two peptides, \(\mathrm{L}\) and \(\mathrm{M}\). Their compositions are \(\mathrm{L}=\) Ala, Asp, Glu, Lys, Phe, Pro, Ser (unsequenced) \(\mathrm{M}=\) lle-Gly-Leu-MetNH \(_{2}\) (sequenced) (At this point you should be able to deduce the sequence of five amino acids at the C-terminus of eledoisin.) (4) Trypsin hydrolysis gives two peptides, \(\mathrm{P}\) and \(\mathrm{Q}\), with the indicated compositions: \(\mathrm{P}=\) Glue, Lys, Pro, Ser \(\mathrm{Q}=\) Ala, Asp, Gly, Ile, Leu, Met, Phe (At this point, you can deduce two possible sequences for Q.) (5) Trypsin hydrolysis of L gives a peptide of composition Ala, Asp, Phe which, with 2,4 -dinitrofluorobenzene, gives the 2,4 -dinitrophenyl derivative of aspartic acid. (6) Partial acid hydrolysis of eledoisin gives several dipeptides, among them Ser-Lys and Pro-Ser.

Suppose there is \(1 \%\) formation in each step of the wrong isomer of the acylating component in an otherwise quantitative 100 -step peptide synthesis. What is the yield of the desired polypeptide isomer?

Indicate the steps that would be necessary to attach each of the amino acids listed to the N-terminus of a peptide chain. Assume that any side-chain functions in the peptide are suitably protected, but do not assume that the amino acids will couple with the peptide without suitable protection of their functional groups. a. lysine b. aspartic acid c. cystine d. serine

Hemoglobin, the protein responsible for carrying oxygen from the lungs to the body tissues, contains \(0.355 \%\) iron. Hydrolysis of \(100 \mathrm{~g}\) of hemoglobin gives \(1.48 \mathrm{~g}\) of tryptophan. Calculate the minimum molecular weight of hemoglobin that is consistent with these results.

Devise a way to use a stereospecific hydrolytic enzyme for resolution of \(D, L\) -alanine.