The mysterious peptide X is a nonapeptide that cannot be sequenced by Edman degradation. However, it is susceptible to cleavage by proteases, and the resulting peptide fragments can be sequenced. Treatment of peptide X with trypsin yields the tripeptide phe-ala-arg while treatment with chymotrypsin yields the tripeptide ala-arg-phe. Propose a strcture for peptide X and explain why it cannot be sequenced by Edman degradation.