In a random (amino acid sequence) polypeptide chain, rotation is severely restricted about the following bonds:
A. Bond from backbone α-C to α-NH2 or to α-COOH groups
B. Peptide bond
C. Backbone α-C to theside chain carbons
D. Terminal α-NH2 to α-COOH groups
E. None of the above
Generally, the covalent bonds in a molecule gives freedom for rotation of substituents linked by them. This will only be restricted by connections involving pi bonds, as these bonds share the p orbitals with each other and its rotation would require a 180º twist of the shared orbitals, which would either require a lot of energy and would eventually break the π bond.