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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Peptides often have functional groups other than free amino groups at the N terminus and other than carboxyl groups at the C terminus.
(a) A tetrapeptide is hydrolyzed by heating with 6 M, and the hydrolysate is found to contain Ala, Phe, Val, and Glu. When the hydrolysate is neutralized, the odor of ammonia is detected. Explain where this ammonia might have been incorporated in the original peptide.
(b) The tripeptide thyrotropic hormone releasing factor(TRF) has the full name pyroglutamylhistidylprolinamide. The structure appears here. Explain the functional groups at the N terminus and at the C terminus.
(c)On acidic hydrolysis, an unknown pentapeptide gives glycine, alanine, valine, leucine and isoleucine. No odor of ammonia is detected when the hydrolysate is neutralized. Reaction with phenyl isothiocyanate followed by mild hydrolysis gives nophenylthiohydantoin derivative. Incubation with carboxypeptidase has no effect. Explain these findings.
Daw the structure of the predominant form of
Isoleucine at pH 11 (b) Proline at pH 2
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
Although tryptophan contains a heterocyclic amine, it is considered a neutral amino acid. Explain why the indole nitrogen of trytophan is more weakly basic than one of the imidazole nitrogens of histidine.
Write the complete structures for the following peptides. Tell whether each peptide is acidic, basic, or neutral.
Use resonance forms to show delocalization of the negative charge in Ruhemann’s purple anion.
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