Question

A peptide containing only aromatic amino acids (tryptophan, phenylalanine, and tyrosine) has a molecular weight of 11,000. The likely number of amino acid residues in the peptide is:

• A. 60
• B. 100
• C. 125
• D. 150

Hint:

Standard residue = 110 Da; Aromatic residue = ~165 Da. Heavy residues = fewer total residues for the same weight.

Answer 1

The Correct Option is A

Step 1: Concept
The average molecular weight of an amino acid residue in a protein is approximately 110 Da.

Step 2: Meaning
Aromatic amino acids (Trp, Phe, Tyr) are significantly bulkier and heavier than average amino acids. Their individual molecular weights are: Tryptophan (~204 Da), Phenylalanine (~165 Da), and Tyrosine (~181 Da).

Step 3: Analysis
Accounting for the loss of a water molecule (18 Da) during peptide bond formation, the average residue weight for these three is roughly $(186 + 147 + 163) / 3 \approx 165$ Da. Calculation: $11,000 / 165 \approx 66.6$. Among the choices, 60 is the most likely estimate, as the standard "110 Da" rule (which would give 100) applies to a mix of all 20 amino acids, not a heavy aromatic-only peptide.

Step 4: Conclusion
Because the peptide consists exclusively of heavy aromatic residues, the residue count is lower than a standard protein of the same weight. Final Answer: (A)