Solution Breakdown

The correct octapeptide sequence is determined by analyzing the outcomes of the different analytical methods:

1. N-terminal Analysis (Sanger's Method)

• The method identified 'Ser' as the N-terminal amino acid. This establishes the first amino acid of the sequence.
• Sequence starts as: $Ser - ? - ? - ? - ? - ? - ? - ?$

2. Chymotrypsin Treatment Outcome

• Chymotrypsin cleaves peptide bonds following aromatic amino acids (Tyrosine - Tyr, Tryptophan - Trp, Phenylalanine - Phe).
• The result was a pentapeptide, a 'Tyr'-containing dipeptide, and a free 'Glu' (Glutamic acid).
• This implies 8 amino acids were cleaved into fragments of size 5, 2, and 1. The sum (5 + 2 + 1 = 8) confirms this.
• The free 'Glu' suggests it is the C-terminal amino acid (AA8 = Glu).
• The cleavage sites must be after the 5th and 7th amino acids. Thus, AA5 and AA7 must be aromatic residues (Tyr or Trp).
• If the sequence is $Ser-Tyr-Arg-Met-Lys-Thr-Trp-Glu$:
  • Aromatic residues are Tyr (AA2) and Trp (AA7).
  • Cleavage after Tyr (AA2) yields: $Ser-Tyr$ (a Tyr-containing dipeptide).
  • Cleavage after Trp (AA7) yields: $Arg-Met-Lys-Thr-Trp$ (a pentapeptide).
  • The remaining amino acid is $Glu$ (a free C-terminal residue).
• This fragmentation pattern perfectly matches the outcome described in point Q.

3. Cyanogen Bromide (CNBr) Treatment

• CNBr specifically cleaves peptide bonds following Methionine (Met).
• The outcome was two tetrapeptides, indicating one Met residue that splits the octapeptide exactly in half.
• This means Met must be at position 4 or 5.
• In the sequence $Ser-Tyr-Arg-Met-Lys-Thr-Trp-Glu$, Met is at position 4 (AA4).
• CNBr cleavage yields:
  • $Ser-Tyr-Arg-Met$ (Tetrapeptide)
  • $Lys-Thr-Trp-Glu$ (Tetrapeptide)
• This aligns with the observation of two tetrapeptides.

4. Trypsin Treatment

• Trypsin cleaves peptide bonds following basic amino acids (Lysine - Lys, Arginine - Arg), unless they are followed by Proline.
• The outcome was two tripeptides and a dipeptide (3 + 3 + 2 = 8 amino acids). This implies two cleavage sites.
• In the sequence $Ser-Tyr-Arg-Met-Lys-Thr-Trp-Glu$:
  • Arg is at position 3 (AA3), followed by Met. Trypsin cleaves after Arg. Fragment 1: $Ser-Tyr-Arg$ (Tripeptide).
  • Lys is at position 5 (AA5), followed by Thr. Trypsin cleaves after Lys. Fragment 2: $Met-Lys-Thr$ (Tripeptide).
  • The remaining sequence is $Trp-Glu$ (Dipeptide).
• The resulting fragments (two tripeptides and one dipeptide) match the experimental outcome described in point S.

Conclusion

Based on the consistent results from all four analytical methods (P, Q, R, and S), the correct octapeptide sequence is determined to be Ser-Tyr-Arg-Met-Lys-Thr-Trp-Glu.