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GATE 2022 CY – Question 14

Organic Chemistry · Biomolecules: Structure, properties and reactions of mono- and di-saccharides, physicochemical properties of amino acids, chemical synthesis of peptides, chemical structure determination of peptides and proteins, structural features of proteins, nucleic acids, lipids, steroids, terpenoids, carotenoids, and alkaloids. · 1 mark · Multiple choice

A tripeptide on treatment with PhNCS (pH = 8.0) followed by heating with dilute HCl afforded a cyclic compound M and a dipeptide. The dipeptide on treatment with PhNCS (pH = 8.0) followed by heating with dilute HCl afforded a cyclic compound N and an acyclic compound O. The CORRECT sequence (from N- to C-terminus) of the tripeptide is

Diagram for GATE 2022 CY question 14
  1. glycine-phenylalanine-valine
  2. valine-phenylalanine-glycine
  3. glycine-tyrosine-valine
  4. glycine-phenylalanine-alanine

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Correct answer: (A) glycine-phenylalanine-valine

Explanation

Phenylisothiocyanate removes one N-terminal residue per cycle as its phenylthiohydantoin. M has no side chain beyond H, identifying glycine; N has CH2Ph, identifying phenylalanine. The remaining acyclic amino acid O has an isopropyl side chain, identifying valine. Sequence **Gly–Phe–Val, A**.