Complete hydrolysis of an unknown basic decapeptide gives Gly, Ala, Leu, Ile, Phe, Tyr, Glu, Arg, Lys, and Ser. Terminal residue analysis shows that the N terminus is Ala and the C terminus is Ile. Incubation of the decapeptide with chymotrypsin gives two tripeptides, A and B, and a tetrapeptide, C. Amino acid analysis shows that peptide A contains Gly, Glu, Tyr, and NH3, ˆ™ peptide B contains Ala, Phe, and Lys; and peptide C contains Leu, Ile, Ser, and Arg. Terminal residue analysis gives the following results.

Incubation of the decapeptide with trypsin gives a dipeptide D, a pentapeptide E, and a tripeptide F. Terminal residue analysis of F shows that the N terminus is Ser, and the C terminus is Ile. Propose a structure for the decapeptide and for fragments A through F.

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C terminus N terminus Gln Ala Arg Tyr Phe Ile