A biochemist observes that an internal residue in a polypeptide lacks an observable free amine peak in a spectrum that free amino acids normally show. Analyze why this spectral difference occurs and which chemical feature has changed.

• A. Backbone amine groups are always invisible in spectroscopic methods regardless of bonding, so the observed change does not report on peptide bond formation.
• B. The internal residue has lost its amino nitrogen through decarboxylation to become a fatty acid, which explains the missing amine peak.
• C. The missing amine peak indicates that the residue has been glycosylated at the backbone nitrogen, forming a polysaccharide chain, which is the standard outcome of peptide bond formation.
• D. The internal residue's amine is covalently linked in a peptide (amide) bond to the previous residue's carbonyl, eliminating the free —NH2 peak and producing the characteristic amide/peptide bond spectral signatures.

Answer: (D)