Solving the sequence of an oligopeptide from sequence analysis dataAmino acid analysis of...

Solving the sequence of an oligopeptide from sequence analysis data

Amino acid analysis of an octapeptide gave the following results:

The following facts were observed:

a. Trypsin treatment yielded a pentapeptide and a tripeptide.

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b. Chemical reduction of the free-COOH and subsequent acid hydrolysis yielded 2- aminopropanol.

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c. Partial acid hydrolysis of the tryptic pentapeptide yielded, among other products, two dipeptides, each of which contained C-terminal isoleucine. One of these dipeptides migrated as an anionic species upon electrophoresis at neutral pH.

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d. The tryptic tripeptide was degraded in an Edman sequenator, yielding first A, then B:

What is an amino acid sequence of the octapeptide? Four sequences are possible, but only one suits the authors. Why?