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Chapter 29: Q34P (page 1192)

Tryptophan is not classified as a basic amino acid even though it has a heterocycle containing a nitrogen atom. Why is the N atom in the five-membered ring of tryptophan not readily protonated by acid?

Short Answer

Expert verified

The protonation of the nitrogen atom disrupts the aromaticity, making it a less favorable reaction.

Step by step solution

01

Step 1:Basic amino acid

Amino acid comprising a second basic group, an amino acid, is termed a basic amino acid. Lysine and arginine are examples of a few basic amino acids.

02

 Step 2: Tryptophan

Tryptophan comes under the category of essential amino acids. The body cannot generate this amino acid, and it must be acquired via diet.

03

Reason for the nitrogen atom in the five-membered ring of tryptophan not readily protonated by acid

The electron pair indicated by the arrow in the first figure is delocalized on the bicyclic ring system, providing electrons. This makes the electron pair less available for donation and less basic.

The ring structure on tryptophan is aromatic as each atom comprises a p orbital. The aromaticity is disrupted due to the protonation of the nitrogen atom, thereby making it a less favorable reaction.

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Most popular questions from this chapter

a.(S)-Penicillamine, an amino acid that does not occur in proteins, is used as a copper chelating agent to treat Wilson’s disease, an inherited defect in copper metabolism.

(R)-Penicillamine is toxic, sometimes causing blindness. Draw the structures of (R) and (S)-penicillamine.

b. What disulfide is formed from oxidation of L-penicillamine?

What aldehyde is needed to synthesize each amino acid by the Strecker synthesis?

(a) Valine;

(b) Leucine;

(c) Phenylalanine.

Give the amino acid sequence of each peptide using the fragments obtained by partial hydrolysis of the peptide with acid.

  1. A tetrapeptide that contains Ala, Gly, His, and Tyr, which is hydrolyzed to the dipeptides His-Tyr, Gly-Ala, and Ala-His.
  2. A pentapeptide that contains Glu, Gly, His, Lys, and Phe, which is hydrolyzed to His-Gly-Glu, Gly-Glu-Phe, and Lys-His.

Deduce the sequence of a heptapeptide that contains the amino acids Ala, Arg, Glu, Gly, Leu, Phe, and Ser, from the following experimental data. Edman degradation cleaves Leu from the heptapeptide, and carboxypeptidase forms Glu and a hexapeptide. Treatment of the heptapeptide with chymotrypsin forms a hexapeptide and a single amino acid. Treatment of the heptapeptide with trypsin forms a pentapeptide and a dipeptide. Partial hydrolysis forms Glu, Leu, Phe, and the tripeptidesGly–Ala–Ser and Ala–Ser–Arg

L-thyroxine, a thyroid hormone and oral medication used to treat thyroid hormone deficiency, is an amino acid that does not exist in proteins. Draw the zwitterionic form of L-thyroxine.
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