Where is rotation allowed in a polypeptide chain?

The correct option indicates that rotation is allowed between the amide bond (the bond between the nitrogen of the amine group and the carbonyl carbon) and the alpha carbon, as well as between the carbonyl group and the alpha carbon. This is because these regions in the polypeptide chain are connected by single bonds, which allow free rotation under normal physiological conditions.

In a polypeptide, the backbone consists of repeated sequences of atoms including nitrogen, carbon, and oxygen. The amide bond (or peptide bond) itself has partial double-bond character due to resonance, which restricts rotation around this bond. However, the bonds connecting these amide groups to the alpha carbon (Cα) and the carbonyl carbon do allow for rotation, giving rise to the flexibility needed for proteins to adopt various conformations necessary for their functionality.

By contrast, other regions mentioned, like hydroxyl groups and terminal amino acids, do not describe the primary sources of freedom for rotation. Therefore, rotation occurs primarily between the alpha carbon and the amide bonds, which is critical for the structure and function of proteins.