Protein backbone

The protein backbone is the repeating chain of chemical atoms that connects successive amino acid residues, including their peptide bonds. The attached groups that distinguish amino acids project from this shared framework. Protein folding changes the framework's spatial arrangement while ordinarily preserving its sequence connectivity. See Cooper: polypeptide backbone and attached groups.

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.