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JEE Advanced levelSingle correct optionDifficulty 3 of 3o3202

The tripeptide Gly-Ala-Phe (glycine is the N-terminal residue) is treated with 1-fluoro-2,4-dinitrobenzene under mild basic conditions and the product is then completely hydrolysed with HCl. Which labelled amino acid is obtained, together with the unlabelled amino acids?

  1. AN-(2,4-dinitrophenyl)phenylalanine, together with free glycine and alanine
  2. BN-(2,4-dinitrophenyl)alanine, together with free glycine and phenylalanine
  3. CN-(2,4-dinitrophenyl)glycine, together with free alanine and phenylalanine
  4. DN-(2,4-dinitrophenyl) derivatives of all three amino acids

Answer: C

The free amino group of the N-terminal residue displaces fluoride from the reagent by SNAr and gives a stable aryl-nitrogen bond. The amide (peptide) nitrogens are not nucleophilic enough to react. Acid hydrolysis breaks the peptide bonds but not the aryl C-N bond, so only the N-terminal glycine is released as its dinitrophenyl derivative.
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