Log out Go to App
Go to App

Learning Materials

Features

Discover

Chapter 22: Problem 35

(a) A peptide of unknown structure is treated with the Sanger reagent, 2,4 -dinitrofluorobenzene, followed by acid hydrolysis. The result is formation of a compound containing signals integrating for eight aromatic hydrogens in its \({ }^{1} \mathrm{H}\) NMR spectrum. What information can you derive about the structure of this peptide? (b) A second peptide, similarly treated, gives a compound in which a single hydrogen appears in the high-resolution \({ }^{1} \mathrm{H}\) NMR spectrum as an eight-line signal at about \(\delta 3.5 \mathrm{ppm}\). What information can you derive about the structure of this peptide?

Short Answer

Expert verified
Peptide (a) has phenylalanine as its N-terminal amino acid. Peptide (b) has glycine as its N-terminal amino acid.

Step by step solution

01

- Analyze the results for peptide (a)

When treated with the Sanger reagent (2,4-dinitrofluorobenzene) and followed by acid hydrolysis, the resulting compound shows signals integrating for eight aromatic hydrogens in its \({ }^{1} \mathrm{H}\) NMR spectrum. This suggests that the peptide has reacted with the Sanger reagent to form a dinitrophenyl (DNP) derivative. The eight aromatic hydrogens indicate the presence of two aromatic rings, each contributing four aromatic hydrogens, consistent with a DNP-derivative.
02

- Identify the terminal amino acid in peptide (a)

The presence of DNP indicates the reaction with the N-terminal amino group of the peptide. Given that the signal integrates for eight aromatic hydrogens, one aromatic ring comes from the DNP group, and the second aromatic ring from the terminal amino acid. This information suggests that the N-terminal amino acid is phenylalanine, which has an aromatic ring contributing four hydrogens.
03

- Analyze the results for peptide (b)

For the second peptide, the treatment results in a single hydrogen appearing as an eight-line signal at about \(\delta 3.5 \text{ppm}\). An eight-line signal indicates the presence of a proton that is split by seven equivalent protons in the \({ }^{1} \mathrm{H}\) NMR spectrum, suggestive of a highly symmetric or repetitive splitting pattern. This splitting pattern corresponds to the N-terminal amino acid being glycyl (glycine) after incorporation of the DNP group.
04

- Interpret the splitting pattern for peptide (b)

The eight-line signal is characteristic of a lone hydrogen adjacent to an environment with several equivalent protons, such as the methylene group in glycine (post DNP treatment). This splitting matches the N-terminal amino acid being a glycyl group as it has a single hydrogen on the alpha carbon next to the amino group, and when derivatized to DNP-glycine, it will exhibit this splitting pattern.

Unlock Step-by-Step Solutions & Ace Your Exams!

  • Full Textbook Solutions

    Get detailed explanations and key concepts

  • Unlimited Al creation

    Al flashcards, explanations, exams and more...

  • Ads-free access

    To over 500 millions flashcards

  • Money-back guarantee

    We refund you if you fail your exam.

Start your free trial

Over 30 million students worldwide already upgrade their learning with Vaia!

Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Sanger Reagent
The Sanger reagent, also known as 2,4-dinitrofluorobenzene (DNFB), is a chemical used for identifying the N-terminal amino acid of a peptide. When DNFB reacts with the terminal amino group, it forms a stable dinitrophenyl (DNP) derivative. This helps in chemically labeling the N-terminal for further analysis. The reagent was developed by Frederick Sanger, who earned a Nobel Prize for his work on the structure of proteins.
Dinitrofluorobenzene
Dinitrofluorobenzene (DNFB) is pivotal in peptide analysis. When DNFB reacts with the free amino group of the peptide, it forms a DNP-derivative. This reaction is specific and forms a stable complex which aids in subsequent analysis, often leading to the identification of the N-terminal amino acid.
NMR Spectroscopy
Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful technique that provides detailed information about the physical and chemical properties of atoms in a molecule. In the context of peptides:
  • The aromatic hydrogens provide insight into aromatic rings.
  • Splitting patterns in the NMR spectrum help deduce the neighboring atoms and the structure.
For peptides treated with the Sanger reagent, NMR can reveal the nature and environment of the aromatic hydrogens, helping to identify the N-terminal amino acid.
DNP-Derivative
A DNP-derivative is formed when a peptide reacts with DNFB, tagging the N-terminal amino group. This stable complex allows the distinction of the N-terminal amino acid through various analytical techniques:
The identification process often involves:
  • Acid hydrolysis of the peptide.
  • NMR spectroscopy to analyze the chemical environment of hydrogens.
The formation of this derivative is essential for determining the structure and sequence of peptides.
N-terminal Amino Acid
The N-terminal amino acid in a peptide chain is the amino acid at the beginning of the chain with a free amino group. Identification of the N-terminal amino acid is crucial for understanding the primary structure of peptides. When a peptide is treated with DNFB, the N-terminal amino acid reacts to form a DNP-derivative:
  • Peptide (a) shows eight aromatic hydrogens, indicating the presence of an aromatic ring (likely Phenylalanine with four hydrogens).
  • Peptide (b) has an eight-line signal due to splitting from adjacent protons, characteristic of glycine’s environment.
This analysis helps in decoding the structure and properties of peptides.

One App. One Place for Learning.

All the tools & learning materials you need for study success - in one app.

Get started for free

Most popular questions from this chapter

(a) What mRNA sequence does the following DNA sequence produce: T A C G G G T T T A T C? (b) What message does that mRNA sequence produce?

A dodecapeptide \(\mathbf{1}\) is hydrolyzed in acid to give two Arg, Asp, Cys, and His, two Leu and Lys, two Phe, and two Val. The Edman procedure yields a phenylthiohydantoin that shows two \(3 \mathrm{H}\) doublets and a multiplet integrating for \(1 \mathrm{H}\) at about \(\delta 4.0 \mathrm{ppm}\) in the \({ }^{1} \mathrm{H}\) NMR spectrum. Treatment of 1 with trypsin leads to Val- His'Phe-Leu*Arg, Asp.Cys.Leu.Phe Lys, and Val-Arg. Treatment of 1 with chymotrypsin leads to Val-His.Phe, Leu.Arg.Asp* Cys.Leu.Phe, and Lys.Val-Arg. Deduce the structure of 1 and explain your reasoning.

(a) Amides protonate on oxygen rather than nitrogen. Why? (b) Amides \(\left(\mathrm{p} K_{\mathrm{b}} \sim 14\right)\) are weaker bases than amines \(\left(\mathrm{p} K_{\mathrm{b}} \sim 5\right)\). Why?

A tripeptide of unknown structure is hydrolyzed in acid to one molecule of Ala, one of Cys, and one of Met. The tripeptide reacts with phenyl isothiocyanate, followed by treatment with acid, to give a phenylthiohydantoin that shows only a three-hydrogen doublet at about \(\delta 2 \mathrm{ppm}\) in the \({ }^{1} \mathrm{H}\) NMR spectrum. The tripeptide is not cleaved by \(\mathrm{BrCN}\). Deduce the structure from these data.

Reaction of \(N\)-phenylalanine (1) with nitrous acid affords \(N\)-nitroso amino acid (2), which on treatment with acetic anhydride yields "sydnone" (3). Sydnones belong to a class of compounds known as mesoionic compounds. These compounds cannot be satisfactorily represented by Lewis structures not involving charge separation. The name sydnone derives from the University of Sydney where the first examples were prepared in 1935. Reaction of 3 and dimethyl acetylenedicarboxylate (DMAD) gives pyrazole (4). Propose mechanisms for the formations of 2,3 , and 4. Hint: For the formation of 4 , it may be helpful to consider the other possible Lewis structures for sydnone 3 .
See all solutions

Recommended explanations on Chemistry Textbooks

Chemical Analysis

Read Explanation

Inorganic Chemistry

Read Explanation

The Earths Atmosphere

Read Explanation

Making Measurements

Read Explanation

Chemical Reactions

Read Explanation

Organic Chemistry

Read Explanation
View all explanations

What do you think about this solution?

We value your feedback to improve our textbook solutions.

Study anywhere. Anytime. Across all devices.

Sign-up for free