Question

Tetrapeptide(s) that gives(give) the following product on reaction with Sanger's reagent followed by hydrolysis is(are) 

• A. Ala-Gly-Leu-Phe
• B. Asp-Phe-Leu-Pro
• C. Asp-Gly-Tyr-Phe
• D. Ala-Phe-Tyr-Pro

Hint:

Sanger's reagent reacts with the N-terminal amine group of peptides to identify the first amino acid in the sequence. Hydrolysis then breaks down the peptide bond.

Answer 1

The Correct Option is A, D

1. Sanger's Reagent Reaction

Sanger's method is a classic technique used for the determination of the N-terminal amino acid of a peptide or protein.

Reagent: Sanger's reagent (DNFB) reacts specifically with the free amino group ($\text{NH}_2$) of the N-terminal amino acid residue.

Mechanism: It forms a stable yellow derivative called a Dinitrophenyl (DNP)-peptide.

The general reaction is:

$$\text{Peptide} \quad \xrightarrow{\text{Sanger's Reagent}} \quad \text{DNP-Peptide}$$

2. Hydrolysis and Product Analysis

After the reaction with Sanger's reagent, the DNP-peptide is subjected to complete acid hydrolysis (e.g., $6M$ $\text{HCl}$ at $100^\circ \text{C}$).

Hydrolysis breaks all the peptide bonds, but the bond between the DNP group and the N-terminal amino acid is stable under these conditions.

The products of hydrolysis are a mixture of free amino acids (from the rest of the peptide chain) and the DNP-N-terminal amino acid (the yellow colored derivative).

$$\text{DNP-Peptide} \quad \xrightarrow{\text{Hydrolysis}} \quad \text{DNP-(N-terminal amino acid)} + \text{Free amino acids}$$

3. Identifying the Product

The structure shown in the image is the product obtained after reaction with Sanger's reagent followed by hydrolysis:

$$\text{Product} = \text{2,4-Dinitrophenyl-N-amino acid}$$

By analyzing the structure, we can identify the original N-terminal amino acid:

The molecule consists of a 2,4-dinitrophenyl group bonded to the nitrogen atom ($\text{N}$) of an amino acid.

The amino acid part has a side chain ($\text{R}$) of a methyl group ($\text{-CH}_3$) attached to the $\alpha$-carbon.

The amino acid with a $\text{-CH}_3$ side chain is Alanine (Ala).

The product shown is thus Dinitrophenyl-Alanine (DNP-Ala).

4. Selecting the Correct Tetrapeptide

Since the reaction product is DNP-Alanine, the original tetrapeptide must have Alanine (Ala) as its N-terminal residue. We look for a tetrapeptide starting with Ala:

(A) Ala-Gly-Leu-Phe

(B) Asp-Phe-Leu-Pro

(C) Asp-Gly-Tyr-Phe

(D) Ala-Phe-Tyr-Pro

Both options (A) and (D) begin with Alanine (Ala).

Therefore, the tetrapeptide(s) that give this product are (A) Ala-Gly-Leu-Phe and (D) Ala-Phe-Tyr-Pro.

The correct answer is (A) and (D).