Step 1: Understanding the Concept:
Frederick Sanger used 2,4-dinitrofluorobenzene (DNFB Sanger's reagent) to label and identify N-terminal amino acid residues during the landmark sequencing of insulin. Key Formula or Approach:
\[ \text{DNFB} + \text{H}_2\text{N-Peptide} \xrightarrow{\text{Mild Alkali}} \text{DNP-Peptide} \xrightarrow{6\text{M HCl Hydrolysis}} \text{DNP-Amino Acid (Yellow, N-terminal)} + \text{Free Amino Acids} \] Step 2: Detailed Explanation: 2,4-Dinitrofluorobenzene (DNFB Sanger's Reagent): - Reacts with the free unprotonated $\alpha$-amino group of the N-terminal amino acid in a peptide under mildly alkaline conditions to form a stable, bright yellow dinitrophenyl (DNP) derivative. - Subsequent complete acid hydrolysis ($6\text{M HCl}$) hydrolyzes all internal peptide bonds, but the covalent DNP-amino acid bond remains intact and is extracted into organic solvents and identified by chromatography. - (Ninhydrin stains all free $\alpha$-amino acids; Mercaptoethanol cleaves disulfide bonds). Step 3: Final Answer:
Hence, Sanger's reagent (DNFB) is used for N-terminal identification, corresponding to option (D).
