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Chapter 24: Q17P. (page 1273)

Use resonance forms to show delocalization of the negative charge in Ruhemann’s purple anion.

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Formation of Ruhemann’s purple anion

Resonance form of Ruhemann’s purple anion

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01

Formation of Ruhemann’s purple anion

When ninhydrin reacts with an amino acid, the product formed is a deep violet resonance-stabilized anion known as Ruhemann’s purple. The reaction of an amino acid with ninhydrin is shown below:

Formation of Ruhemann’s purple anion

02

Resonance form of Ruhemann’s purple anion

The resonance form of Ruhemann’s purple anion is as shown below:

Resonance form of Ruhemann’s purple anion

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Most popular questions from this chapter

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?

There are many methods for activating a carboxylic acid in preparation for coupling with an amine. The following method converts the acid to an N-hydroxysuccinimide (NHS) ester.

(a) Explain why an NHS ester is much more reactive than a simple alkyl ester.

(b) Propose a mechanism for the reaction shown.

(c) Propose a mechanism for the reaction of the NHS ester with an amine, R-NH2
  1. Show how you would use a Strecker synthesis to make phenylalanine.
  2. Propose a mechanism for each step in the synthesis in part (a).

The Sanger method for N-terminus determination is a less common alternative to the Edman degradation. In the Sanger method, the peptide is treated with the Sanger reagent, 2,4-dinitrofluorobenzene, and then hydrolyzed by reaction with 6 M aqueous HCl. The N-terminal amino acid is recovered as its 2,4-dinitrophenyl derivative and identified.

(a)Propose a mechanism for the reaction of the N terminus of the peptide with 2,4-dinitrofluorobenzene.

(b) Explain why the Edman degradation is usually preferred over the Sanger method.

Lipoic acid is often found near the active sites of enzymes, usually bound to the peptide by a long, flexible amide linkage with a lysine residue.

(a) Is lipoic acid a mild oxidizing agent or a mild reducing agent? Draw it in both its oxidized and reduced forms.

(b) Show how lipoic acid might react with two Cys residues to form a disulfide bridge.

(c) Give a balanced equation for the hypothetical oxidation or reduction, as you predicted in part (a), of an aldehyde by lipoic acid.

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