Discussion
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
Answer: A
The tripeptide is glycine-phenylalanine-valine.
PhNCS at pH 8 followed by heating in dilute acid is the Edman degradation: it removes the N-terminal residue as a cyclic phenylthiohydantoin (PTH) and leaves the rest of the chain intact.
M is the PTH with no side chain at C5, so the first residue is glycine. N carries a benzyl group (CH2Ph) at C5, so the second residue is phenylalanine. O is the free amino acid left over, 2-amino-3-methylbutanoic acid, which is valine, the C-terminal residue.
Valine-phenylalanine-glycine reverses the order. Tyrosine would give a PTH with a 4-hydroxybenzyl group, and alanine would be left as a 2-aminopropanoic acid, so those options do not match N and O.
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