So we have torsion angle Phi and torsion angle, Phi. The Phi angle is the angle of rotation about the single bond between the nitrogen and the alpha carbon, while the PSI angle is the angle of rotation about the single bond between the alpha carbon and the carbon of the carbon dill. Now, why is it that these can rotate and the peptide bond can? Well, the peptide bond is rather than stabilized. It is not a single bond. It is more of a double bond.