Which description best defines a peptide bond?

• A. A covalent linkage between the carboxyl group of one amino acid and the amino group of another, forming an amide bond
• B. A bond between amino acids via an ester linkage
• C. A bond linking nucleotides through a phosphate-sugar bond
• D. A hydrogen bond between amino acid side chains

Answer: (A)

A peptide bond is the covalent amide linkage that connects amino acids in proteins. It forms when the carboxyl group of one amino acid reacts with the amino group of the next amino acid, releasing a water molecule in a dehydration synthesis. This creates a C–N bond between the carbonyl carbon and the amide nitrogen. Because of resonance, the bond has partial double-bond character, making the backbone planar and restricting rotation around that bond—an important feature for the overall shape of proteins. This description fits the actual chemistry of peptide bonds, unlike an ester linkage (which involves a hydroxyl and carboxyl), a phosphate-sugar bond (found in nucleic acids), or a hydrogen bond (a noncovalent interaction).