Question

Vasopressin, a nonapeptide hormone secreted by the pituitary gland, functions by stimulating the kidney to retain water. Its sequence was determined by the following evidence:
1. Vasopressin is a cyclic compound containing a disulfide bridge between two cysteine residues.
2. When the disulfide bridge is reduced, vasopressin has the constitution Arg, Asn, Cys₂, Gln, Gly, Ile, Phe, Pro.
3. Partial hydrolysis of vasopressin yields seven fragments: Asp-Cys, Ile-Glu, Cys-Phe, Arg-Gly, Phe-Ile-Glu, Glu-Asp-Cys, Cys-Pro-Arg.
4. Gly is the C-terminal group.
5. Both Glu and Asp are present as their side chain amides (Gln and Asn) rather than as free side-chain acids.

From the above information, determine the primary sequence of vasopressin and then draw the structure of the N-terminal amino acid.