Fig. 1.

The UNRES model of polypeptide chains. The interaction sites are united peptide groups (p, represented by dark spheres), each located halfway between two consecutive α-carbon (C^α) atoms (represented by white spheres), and side-chains (SC, represented by ellipsoids with different dimensions) that are attached to the corresponding C^αs, which define backbone geometry and are not the interaction centers. The equilibrium length of a C^α…C^α virtual bond is 3.8 Å for the trans and 2.8 Å for the cis peptide group, respectively. For the $i$th residue, the virtual-bond angle ${\theta }_i$, virtual-bond-dihedral angle ${\gamma }_i$, and the polar angles ${\alpha }_i$ and ${\beta }_i$ are also indicated in the figure