FIGURE 3-15 Rotation between planar peptide groups in proteins. Rotation about the C_α–amino nitrogen bond (the Φ angle) and the C_α–carbonyl carbon bond (the Ψ angle) permits polypeptide backbones, in principle, to adopt a very large number of potential conformations. However, steric restraints due to the structure of the polypeptide backbone and the properties of the amino acid side chains dramatically restrict the potential conformations that any given protein can assume.