2023 H2 Chem P1 Q27 — Nitrogen Compound Solution

The answer is D (2 and 3 only).
An amino acid is \(\text{H}_2\text{NCHRCO}_2\text{H}\); in a peptide each amino acid becomes a residue \(-\text{NHCHRCO}-\), joined to the next by an amide (peptide) link. A tripeptide is three such residues, with a free \(-\text{NH}_2\) at one end and a free \(-\text{CO}_2\text{H}\) at the other.
• Structure 1, not a tripeptide. It splits into \(\text{H}_2\text{NCHRCO}\), \(\text{NHCHR'CO}\) and \(\text{NHCHR''NH}_2\). The last unit has no C=O and ends in \(\text{NH}_2\), so it comes from a diamine, not an amino acid.
• Structure 2, a tripeptide. \(\text{H}_2\text{NCHRCO}\), \(\text{NHCHR'CO}\) and \(\text{NHCHR''CO}_2\text{H}\) are three amino acid residues joined by two amide links.
• Structure 3, a tripeptide. It is written from the \(-\text{CO}_2\text{H}\) end: \(\text{HO}_2\text{CCHRNH}\), \(\text{COCHR'NH}\) and \(\text{COCHR''NH}_2\) are again three amino acid residues joined by two amide links.
A, B and C: all include structure 1, which contains a diamine unit and so is not a tripeptide.