2018 H2 Chem P1 Q30 — Nitrogen Compound Solution

The answer is D.
Each fragment shows two amino acids that are next to each other in the chain, read from the \(\text{NH}_2\) end (N-terminus) to the \(\text{CO}_2\text{H}\) end:
• Fragment 1: Ser-Gly (serine, \(\text{CH}_2\text{OH}\) side chain, then glycine)
• Fragment 2: Ala-Ser (alanine, \(\text{CH}_3\) side chain, then serine)
• Fragment 3: Ser-Ala
• Fragment 4: Gly-Lys (lysine, \((\text{CH}_2)_4\text{NH}_2\) side chain)
Linking the overlaps: Ser-Ala (3), Ala-Ser (2), Ser-Gly (1), Gly-Lys (4) gives Ser-Ala-Ser-Gly-Lys, which is pentapeptide D. Its four adjacent pairs are exactly the four fragments.
• A is wrong: A is Ala-Ser-Ser-Gly-Lys. Its adjacent pairs are Ala-Ser, Ser-Ser, Ser-Gly and Gly-Lys, so it cannot give fragment 3 (Ser-Ala).
• B is wrong: B is Ser-Gly-Lys-Ala-Gly. Its pairs are Ser-Gly, Gly-Lys, Lys-Ala and Ala-Gly, so it cannot give fragment 2 (Ala-Ser) or fragment 3 (Ser-Ala).
• C is wrong: C is Ala-Ser-Gly-Lys-Gly. Its pairs are Ala-Ser, Ser-Gly, Gly-Lys and Lys-Gly, so it cannot give fragment 3 (Ser-Ala).