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Chapter 24: Q13P (page 1271)
Suggest how you would separate the free L-amino acid from its acylated D enantiomer in Figure 24-5.
Water extraction or ion-exchange chromatography would be practical technique to separate free L-amino acid from its acylated D enantiomer.
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Draw the electrophoretic separation of Ala, Lys, and Asp at pH 9.7.
Aspartame(Nutrasweet®) is a remarkably sweet-tasting dipeptide ester. Complete hydrolysis of aspartame gives phenylalanine, aspartic acid, and methanol. Mild incubation with carboxypeptidase has no effect on aspartame. Treatment of aspartame with phenyl isothiocyanate, followed by mild hydrolysis, gives the phenylthiohydantoin of aspartic acid. Propose a structure for aspartame.
A student took the proton NMR spectrum of phenylalanine in D2Osolution, and had the instrument suppress the DOHsolvent peak. The spectrum is shown below. The integrated relative areas of the peaks are 5:1:1:1.
(a) Draw the structure of phenylalanine as it exists in D2Osolution. (There is a large excess of D2O, and any exchangeable protons in phenylalanine will exchange with the solvent.)
(b) Assign the peaks in the spectrum to the protons in the structure.
(c) Why don’t we see the -NH2or -COOHprotons in the spectrum?
(d) What is the relationship between the two protons that generate nearly mirror-image multiplets at 3.1 and 3.3?
Most naturally occurring amino acids have chiral centers (the asymmetric α carbon atoms) that are named (S) by the Cahn-Ingold-Prelog convention (Section 5-3). The common naturally occurring form of cysteine has a chiral center that is named (R), however.
(a) What is the relationship between (R)-Cysteine and (S)-alanine? Do they have the opposite three-dimensional configuration (as the names might suggest) or the same configuration?
(b) (S)-Alanine is an L-amino acid (Figure 24-2). Is (R)-cysteine a D-amino acid or an L-amino acid?
Draw the structure of the predominant form of
(a)Isoleucine at pH 11 (b) Proline at pH 2
(c)Arginine at pH 7 (d) Glutamic acid at pH 7
A mixture of alanine, lysine, and aspartic acid at 1). pH 6 ; 2). pH 11; 3). pH 2
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