See image — Biomolecules Chemistry Question

💡 Solution & Explanation

A peptide bond is defined as an amide bond (CO-NH) formed between the carbonyl carbon of one amino acid and the nitrogen of another, i.e., the -C(=O)-NH- linkage specifically found in peptide/protein structures. Step 1 - Identify all amide-type (CO-NH) linkages in the molecule. Looking at the structure: H3C-C(=O)-CH(NH...)-CH(CH3)-N(H)-C(=O)-CH(CH3)2-NHNH2 Step 2 - Examine each nitrogen-containing group: 1. The first N shown as 'N' below the CH adjacent to H3C-C(=O): This nitrogen is connected to the carbonyl on its left side, forming a C(=O)-NH- linkage. However, examining the structure carefully, this NH appears to be part of the backbone but is drawn as a cyclic or secondary amine context. Looking at the connectivity: H3C-C(=O)-CH-NH forms one amide unit — but the N here is between two carbons in a way that may represent a reduced amine rather than a true peptide bond depending on connectivity. 2. The -NH- between the CH(CH3) and C(=O): This is -C(=O)-NH- which is a classic amide/peptide bond. 3. The terminal -NHNH2: This is a hydrazide group (-C(=O)-NHNH2), which is an amide bond (CO-NH), but the second NH2 is not a peptide bond. Step 3 - Count true peptide bonds (CO-NH linkages in peptide context): Re-examining: The molecule has the connectivity showing: - One C(=O)-NH- linkage (between positions involving the first carbonyl and the N-H shown below) - The -NH- between CH(CH3) and C(=O) on the right side is the second amide - The -C(=O)-NHNH2 terminal: the CO-NH here is an amide but typically the -NHNH2 (hydrazide) is NOT counted as a peptide bond Step 4 - The answer is 1 peptide bond. The structure contains one true peptide bond (CO-NH linkage in the classical peptide sense). The other CO-N linkage involves a hydrazide (-CO-NHNH2) which is not a peptide bond, and the first N may be a secondary amine not forming a peptide bond. Only one CO-NH linkage qualifies strictly as a peptide bond. Why other options fail: - (b) 2: Would require two CO-NH peptide bonds; the hydrazide CO-NH is not a peptide bond. - (c) 3: Overcounts; there are not three peptide bonds. - (d) 4: Overcounts significantly. Therefore, the correct answer is A.