Final answer:
The peptide bond in proteins does not rotate due to its partial double bond character stemming from resonance.
Step-by-step explanation:
The bond in a protein that does not rotate is the peptide bond. Due to resonance, the peptide bond acquires partial double bond character, preventing free rotation around it. This rigidity contributes to the protein's secondary and tertiary structures through limited possible orientations of the peptide backbone. The phi and psi bonds are the rotational angles about the C-N (alpha carbon to amide nitrogen) and C-C (alpha carbon to carbonyl carbon) bonds, respectively, and they do allow rotation. Bonds between nonpolar amino acids often lay in the interior of proteins, while hydrophilic R groups are on the exterior. Interactions such as van der Waals forces, hydrogen bonds, and disulfide linkages between cysteine side chains are critical for the three-dimensional folding of proteins.