Ch. 24 - Amino Acids, Peptides, and Proteins

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 24 - Amino Acids, Peptides, and Proteins

Problem 33

Chapter 24, Problem 33

Show how you would use the Strecker synthesis to make tryptophan. What stereochemistry would you expect in your synthetic product?

Verified step by step guidance

The Strecker synthesis is a method for synthesizing α-amino acids by reacting an aldehyde or ketone with ammonium chloride (NH₄Cl) and potassium cyanide (KCN), followed by hydrolysis. To synthesize tryptophan, start by identifying the appropriate aldehyde precursor. For tryptophan, the precursor is indole-3-acetaldehyde, which contains the indole ring structure characteristic of tryptophan.

React indole-3-acetaldehyde with ammonium chloride (NH₄Cl) and potassium cyanide (KCN). This step forms an α-aminonitrile intermediate. The reaction mechanism involves the formation of an imine (Schiff base) between the aldehyde group and ammonia, followed by nucleophilic addition of the cyanide ion (CN⁻) to the imine carbon.

Hydrolyze the α-aminonitrile intermediate under acidic or basic conditions to convert the nitrile group (-C≡N) into a carboxylic acid (-COOH). This step yields the α-amino acid structure of tryptophan.

Consider the stereochemistry of the product. The Strecker synthesis typically produces a racemic mixture of both L- and D-enantiomers of the amino acid because the reaction does not control the stereochemistry at the α-carbon. Tryptophan synthesized via this method will therefore be a 50:50 mixture of L-tryptophan and D-tryptophan.

To obtain a specific enantiomer, such as L-tryptophan (the biologically active form), additional steps like chiral resolution or the use of a chiral catalyst during synthesis would be required. These methods can separate or selectively produce one enantiomer from the racemic mixture.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.

Video duration:

Was this helpful?

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Strecker Synthesis

Strecker synthesis is a method for synthesizing amino acids through the reaction of an aldehyde with ammonia and hydrogen cyanide. This process involves the formation of an α-amino nitrile intermediate, which can be hydrolyzed to yield the corresponding amino acid. Understanding this reaction is crucial for synthesizing tryptophan, as it provides the foundational steps to construct the amino acid's structure.

Tryptophan Structure

Tryptophan is an essential amino acid characterized by its indole side chain, which contributes to its unique properties and biological functions. The structure includes an α-amino group, a carboxylic acid group, and the indole ring, making it a complex molecule. Recognizing the specific functional groups and their arrangement is vital for predicting the outcomes of the synthesis and the stereochemistry of the product.

Stereochemistry

Stereochemistry refers to the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In the context of amino acids, chirality is significant, as they typically exist in L- and D- forms. For tryptophan, the expected product from Strecker synthesis would be the L-form, which is biologically active, highlighting the importance of stereochemical considerations in organic synthesis.

Recommended video:

1:38

Polymer Stereochemistry Concept 1

Related Practice

Textbook Question

Show how you would convert alanine to the following derivatives. Show the structure of the product in each case.

(d) tert-butyloxycarbonyl alanine

836

views

Textbook Question

Suggest a method for the synthesis of the unnatural D enantiomer of alanine from the readily available L enantiomer of lactic acid.

1036

views

Textbook Question

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.

1358

views

Textbook Question

Aspartame (Nutrasweet®) is a remarkably sweet-tasting dipeptide ester. Complete hydrolysis of aspartame gives phenyl alanine, 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.

1420

views

Textbook Question

Show how you would convert alanine to the following derivatives. Show the structure of the product in each case.

(a) alanine isopropyl ester

(b) N-benzoylalanine

1122

views

Textbook Question

Histidine is an important catalytic residue found at the active sites of many enzymes. In many cases, histidine appears to remove protons or to transfer protons from one location to another.

(a) Show which nitrogen atom of the histidine heterocycle is basic and which is not.

(b) Use resonance forms to show why the protonated form of histidine is a particularly stable cation.

(c) Show the structure that results when histidine accepts a proton on the basic nitrogen of the heterocycle and then is deprotonated on the other heterocyclic nitrogen. Explain how histidine might function as a pipeline to transfer protons between sites within an enzyme and its substrate.

1306

views