Peptide backbone dihedral angles ($\varphi ,\psi$)

The peptide backbone dihedral angles are $\varphi =\angle (C_{i-1}^{\prime },N_i,C_{\alpha i},C_i^{\prime })$ and $\psi =\angle (N_i,C_{\alpha i},C_i^{\prime },N_{i+1})$. Rotation is restricted by ring closure, peptide geometry and steric contacts.