Figure 1.

The segment with six ϕ,ψ angles extends from the C_α of residue 1 to the C^α of residue 4 ($N_{\text{res}}=4$), covering three peptide units. (a) Definition of torsion and bond angle numbering showing the coordinate system on atom 0 and atom 9 (atom numbering as defined in b). Bonds, and consequently bond lengths and torsion angles, are indexed with the number of the atom on the C-terminal side of the bond. The torsions shown determine the relative position and orientation of the coordinate systems and consequently everything flanking atoms 0 and 9. The x and y axes of the coordinate systems on the first and last atoms are shown. (b) Atom numbering and ϕ,ψ angle numbering.