See image — Biomolecules Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the amino acids in the abbreviation Phe-Val-Ala: - Phe (Phenylalanine): side chain = -CH2-C6H5 (benzyl group) - Val (Valine): side chain = -CH(CH3)2 (isopropyl, i.e., -CHCH3 with a CH3 branch) - Ala (Alanine): side chain = -CH3 Step 2 - Determine the correct sequence: The peptide is written N-terminus to C-terminus as Phe-Val-Ala, meaning the first amino acid residue (at the H2N- end) must be Phe, the second must be Val, and the third (at the -OH end) must be Ala. Step 3 - Analyze option (a): - First residue side chain: -CH2-phenyl → Phe ✓ - Second residue side chain: -CHCH3-CH3 (isopropyl) → Val ✓ - Third residue side chain: -CH3 → Ala ✓ Sequence: Phe-Val-Ala ✓ Step 4 - Analyze why other options fail: - Option (b): Third residue has -CH2-phenyl (Phe) instead of Ala ✗ - Option (c): Second residue has -CH3 (Ala) and third has -CHCH3-CH3 (Val), giving Phe-Ala-Val ✗ - Option (d): First residue has -CH3 (Ala), giving Ala-Phe-Val ✗ Therefore, the correct answer is A.